Main The Space Book: From the Beginning to the End of Time, 250 Milestones in the History of Space &...
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185 Chinese Observe “Guest Star” Ancient Chinese astronomers were meticulous observers of the skies. As historians have pointed out, because official Chinese astronomy research was often conducted by cadres of full-time, court-appointed civil servants as opposed to individual scholars, they were much more systematic and thorough about surveying the skies for changes than their Roman, Greek, or Babylonian counterparts and predecessors were. Thus, when something new happened in the sky, the Chinese noticed and recorded the observation, and the recording became part of the imperial dynastic records, many of which are still preserved. A prime example was the sudden appearance of what they called a guest star in the southern skies in the year 185. This appearance was recorded by Chinese astronomers as a notable event in the surviving annals of the Eastern Han dynasty (25–220 CE). Although no drawings were included, the description of the location of the guest star and the fact that it faded from view over a period of about six months convinced modern astronomers that the Chinese had made the first recorded observations of a supernova. Modern optical, radio, and X-ray telescopes trained on that location reveal a semispherical gaseous nebula called RCW 86, which appears to be the expanded remains of that stellar explosion from more than 1,800 years ago. [image: ] A modern view of the remains of the supernova explosion of 185. Many other guest stars were recorded in ancient Chinese astronomical drawings that are still preserved today. Among the most interesting are drawings that show objects with a bright, round “head” and one or more feathery or spiky “tails.” These objects were referred to as “broom stars” by the Chinese, and they are now widely interpreted as bright comets with long tails of gas and dust. In fact, prominent comets observed by Chinese astronomers in 240 BCE and 12 BCE, and in 141, 684, and 837 CE, are all likely to have been observations of the same comet, eventually recognized in 1682 ; as the 76-year periodic Halley’s Comet. The careful, methodical sky watching and record keeping of early Chinese astronomers has proven to be a rich treasure trove of data for studies by both historians and astronomers. SEE ALSO Astronomy in China (c. 2100 BCE), “Daytime Star” Observed (1054), Halley’s Comet (1682), Miss Mitchell’s Comet (1847), Tunguska Explosion (1908). [image: ] Ancient Chinese drawings of the changing appearances of several comets, as recorded in the Bamboo Annals, a historical accounting of China covering the period from about 2400 BCE to 300 BCE. 2004 Spirit and Opportunity on Mars More than three decades of successful orbital and landed investigations of Mars by scientists involved in the Mariner and Viking missions painted a compelling picture of major past climate changes on the Red Planet. The Martian surface today is extremely cold, bone dry, and inhospitable to life as we know it. But ancient Mars, as revealed by these missions, appears to have been a warmer, wetter, and potentially more Earthlike place. If so, then early Mars (during the first billion years or so after its formation) may have been a habitable environment where, as on our own planet, life could have thrived. Planetary scientists wanted to move beyond photographic evidence of a potentially habitable early Mars, however, and make quantitative geologic, geochemical, and mineralogic measurements that could provide smoking-gun proof. Experience gained from the 1997 Mars Pathfinder mission proved the value of mobility in doing geologic field work with robots in distant locations, leading to the choice to embark on an even longer-range rover mission. Because of two Mars missions failures in 1999, NASA decided to reduce its risk: instead of just one rover, it would launch twin rovers—named Spirit and Opportunity—in 2003. [image: ] Both rovers landed safely in early 2004 and began their separate adventures on opposite sides of the planet: Spirit in an ancient crater named Gusev, which may once have hosted a lake, and Opportunity in the cratered area Meridiani Planum, where Mars Global Surveyor data showed evidence for water-formed minerals. After several years of virtually roving around Gusev with Spirit, mission scientists discovered evidence of water-bearing minerals in an ancient hydrothermal system that provided smoking-gun evidence for past habitability in Gusev. At Meridiani, the team immediately found other water-formed minerals like hematite, clays, and sulfates that provided smoking-gun evidence for past habitability there as well. Despite their expected lifetimes of just ninety days, Spirit operated until early 2010, and as of mid-2018 Opportunity continues to roll on and make new discoveries. SEE ALSO Mars (c. 4.5 Billion BCE), Mars and Its Canals (1906), First Mars Orbiters (1971), Vikings on Mars (1976), First Rover on Mars (1997), Mars Global Surveyor (1997), Life on Mars? (1996), Mars Science Laboratory Curiosity Rover (2012), First Humans on Mars? (~2035–2050). [image: ] A computer-generated version of the NASA Mars rover Opportunity placed into an actual Opportunity Pancam mosaic of finely layered rocks inside Endurance crater. These rocks contain evidence of past liquid water on Mars, including millimeter-size iron-rich spheres (inset) called concretions. 1992 First Extrasolar Planets Are there planets around other stars? For most of the history of astronomy, the question has been either too heretical to ask (Giordano Bruno was burned at the stake in 1600 because of it) or too technologically impossible to address. Recently, though, astronomers have discovered that the answer is a resounding yes. By the late twentieth century, telescope and observational technology had advanced to the level where astronomers could detect the presence of planets around other stars using a variety of methods. One method exploits the fact that planets make their parent stars “wobble” in the sky; Jupiter’s gravity, for example, makes our own Sun’s path appear to wobble slightly as it orbits the center of the galaxy. In 1992, a team of astronomers discovered that such slight wobbles could also be detected as very slight changes in the rotational speed of rapidly spinning neutron stars known as pulsars. In 1990, astronomers using the Arecibo radio telescope discovered a millisecond pulsar named PSR B1257+12 in the constellation Virgo. Monitoring the pulses every 6.22 milliseconds (msec) from this collapsed neutron star’s supernova remnant revealed small regular variations in the pulse rate. In 1992 researchers explained that this was caused by the gravitational pull of at least three planets in orbit around the pulsar. Mathematical modeling showed that two of the planets were likely around four times Earth’s mass, and a third was around 2 percent of Earth’s mass; all appear to orbit within 0.5 astronomical units of the pulsar. This first confirmed evidence for the existence of extrasolar planets came as a surprise to most astronomers, because the expectation had been that planets would be found around other normal, main sequence stars like the Sun rather than around exotic objects like neutron stars. There is much speculation, then, about the nature of these particular pulsar planets. Perhaps they are the rocky and metallic cores of previous gas or ice giants that had their outer volatile layers stripped away by the supernova explosion that created the pulsar. Or perhaps they represent the results of a second round of solar nebula planet formation using remnant materials ejected by the supernova explosion. Whatever the origin of these worlds, their detection appears to be robust, and so astronomers and planetary scientists now discovering and characterizing planets around other suns must also consider an even wider range of ways that “extreme” extrasolar planets can form and evolve in a variety of environments. SEE ALSO Solar Nebula (c. 5 Billion BCE), Bruno’s On the Infinite Universe and Worlds (1600), First Astronomical Telescopes (1608), Neutron Stars (1933), Arecibo Radio Telescope (1963), Pulsars (1967), Planets Around Other Suns (1995). [image: ] Artist’s conception of the planetary system detected around the pulsar PSR B1257+12 (lower left). c. 13.7 Billion BCE Big Bang Edwin Hubble (1889–1953) There’s no better place to start considering the broad sweep of astronomical history than the beginning—that is, the actual beginning of both space and time. Twentieth-century astronomers such as Edwin Hubble discovered that the universe is expanding by observing that large-scale structures like galaxies are all moving away from each other, in any direction that we look. This means that, in the past, the universe was smaller and that, at some point in the far distant past, everything started out as a single point of space and time: a singularity. Years of careful observations by the Hubble Space Telescope and other facilities have revealed that the universe was born in a violent explosion of this singularity about 13.7 billion years ago. The details of big bang theory—as it was initially dubbed by astronomers in the 1930s—have been rigorously tested with decades of astronomical observations, laboratory experiments, and mathematical modeling by cosmologists and astronomers who specifically focus their research on the origin and evolution of the universe. What we have learned about the early history of our universe from these studies is impressive: within the first second of the universe’s existence, the temperature dropped from a million billion degrees to “only” 10 billion degrees, and all of the universe’s present supply of protons (hydrogen atoms) and neutrons formed out of this primordial plasma. By the time the universe was only three minutes old, helium and other light elements had been formed from hydrogen in the same kind of nuclear fusion process that still occurs today deep inside of stars. It’s mind-blowing to think about both space and time being created at a single instant, 13.7 billion years ago. What caused the explosion? What was there before the big bang? Cosmologists tell us that we can’t really ask that question because time itself was created in the big bang. It’s also humbling to realize that the most abundant element within each of our bodies—hydrogen—was created in the very first second that ever was. We are ancient! SEE ALSO Hubble’s Law (1929), Nuclear Fusion (1939), Hubble Space Telescope (1990). [image: ] Graphically depicting the beginning of the universe is just as challenging as trying to understand it! Here, an artist has fancifully captured the idea that the big bang was triggered by a collision with another three-dimensional universe that had been hidden in higher dimensions. c. 3.8 Billion BCE Life on Earth No one knows exactly how, when, or why life first appeared on planet Earth, but we know that almost as soon as it could, it did. The oldest signs of life on Earth are chemical, not fossil, and are inferred as evidence because all known life on this planet is based on a common chemical architecture. Specifically, certain biogeochemical processes and reactions that are common to all life on Earth—reactions involving certain amino acids commonly associated with DNA or RNA, for example—create recognizable patterns in the isotopes of carbon and some other elements. Life prefers to use (and create) certain materials, in essence, and anomalous chemistries, like the occurrence of extra carbon-12 (12C) compared to carbon-13 (13C) in some 3.8-billion-year-old rocks from Greenland, provide circumstantial but controversial chemofossil evidence for life very early in our planet’s history. The oldest known fossil evidence of microbial life on our planet is dated at around 3.5 billion years old and is preserved in the layers of ancient stromatolites, which are rock and mineral structures built up by colonies of simple organisms such as blue-green algae. Stromatolites still form in places such as Shark Bay in Western Australia, making them among the oldest life forms on our planet. Recent studies of the very earliest period of Earth’s history, the Hadean (4.5–3.8 billion years ago), provide evidence that oceans and continents may have formed much earlier than previously thought, and that conditions may have been suitable for life just a few hundred million years after our planet formed. The Late Heavy Bombardment of 3.8 to 4.1 billion years ago may have killed off earlier life forms, or perhaps just frustrated their attempts to flourish. Whatever the case may be, soon after Earth’s crust cooled, the oceans formed, the late heavy bombardment ended, and Earth became stable enough to support life. The fact that it thrived and began to evolve into so many niches is remarkable. Now astronomers, planetary scientists, and astrobiologists are searching for evidence of life on other Earthlike worlds. SEE ALSO Earth (c. 4.5 Billion BCE), Late Heavy Bombardment (c. 4.1 Billion BCE). [image: ] Cross-sectional view of a stromatolite fossil; the reddish layers are thought to be fossilized remains of the ancient blue-green algae that are some of the oldest preserved evidence for life on Earth. This particular piece, from the Ord Range of Western Australia, is about 2.4 inches (6 centimeters) tall. c. 825 Early Islamic Astronomy Habash al-Hāsīb (c. 770–870), Muhammad ibn Mūsā al-Khwārizmī (c. 780–c. 850), Muhammad ibn Jābir al-Harrānī al-Battānī (c. 858–929), Abū ar-Rayhān al-Bīrūnī (973–1048) Much of the modern language and methodology of astronomy and mathematics can be directly traced back to a several-centuries-long burst of genius and creativity in the arts and sciences in medieval Islam. Scientific development stagnated in Europe during this period, and so it was primarily the Persian and Arab world that became the heir to the Greco-Roman legacy of astronomy and mathematics. Among the many early Islamic astronomers and mathematicians who made important new contributions were al-Khwārizmī, who founded modern algebra (al-jabr, or “completion” in Arabic) and developed new methods for calculating the positions of the Sun, Moon, and planets; al-Hāsīb, who calculated the best estimates yet made for the diameter and distance of the Moon and the diameter of the sun, and compiled his observations in The Book of Bodies and Distances; al-Battānī, who refined results from Ptolemy’s Almagest and developed new methods of timing the first appearance of the Moon’s crescent; and al-Bīrūnī, who invented new astronomical instruments and observing methods, and who (along with a number of other Persian and Arabic astronomers) hypothesized that a Sun-centered model of the solar system could fit the available observational data as well as the widely accepted Earth-centered model. Indeed, the work of these and other medieval Islamic astronomers went on to influence Renaissance Western astronomers such as Brahe, Kepler, Copernicus, and Galileo, and the eventual overthrow of Ptolemaic geocentrism in favor of a heliocentric cosmology. In addition, almost all the noted astronomers and mathematicians of early Islam were working as part of teams in what were essentially the world’s first research groups—part of the world’s first system of state-run observatories and research institutes. This kind of collaborative environment enabled Islamic scientists to achieve significant advances in astronomy and other fields, and it is the basis for the way most science is done today. SEE ALSO Greek Geocentrism (c. 400 BCE), Sun-Centered Cosmos (c. 280 BCE), Ptolemy’s Almagest (c. 150), Andromeda Sighted (964), Experimental Astrophysics (c. 1000), Copernicus’s De Revolutionibus (1543), Brahe’s “Nova Stella” (1572), Galileo’s Starry Messenger (1610), Three Laws of Planetary Motion (1619). [image: ] Illustration and text (in Persian) by al-Bīrūnī of different phases of the Moon, from his astrological treatise Kitāb al-tafhīm. 1848 Doppler Shift of Light Christian Doppler (1803–1853), Armand Hippolyte Fizeau (1819–1896), Vesto Slipher (1875–1969), Edwin Hubble (1889–1953) Most of us are familiar with the dramatic change in the sound of an ambulance or a train whistle or a race car as it approaches us and then speeds past. As the vehicle recedes, its sound changes to a distinctly lower pitch or frequency than when it was approaching. This change is known as the Doppler effect, named after the Austrian physicist Christian Doppler, who first proposed in 1842 the idea that the observed frequency of any kind of wave should depend on the relative difference in speeds between the wave’s source and the observer. In 1845 Doppler’s hypothesis was verified for sound waves in some clever experiments by the Dutch meteorologist C. H. D. Buys Ballot, who hired musicians to play notes on a moving train and then had stationary observers report the pitches that they heard as the musicians approached and receded. In 1848, the French physicist Armand Hippolyte Fizeau showed how Doppler’s hypothesis applied to light waves by noting slight changes in frequency or shifts of absorption lines in the spectra of stars. Astronomers call these frequency changes Doppler shifts, and the size and direction of the frequency change can be used to determine the speed at which astronomical bodies are approaching or receding from each other. Objects approaching us have their spectra shifted to higher frequencies, or shorter (bluer) wavelengths; conversely, objects receding from us have spectra that are red-shifted. In the 1860s the first accurate relative stellar velocities were measured, and in the 1870s it became possible to detect the Doppler shift of the stars caused by the Earth’s annual motion around the Sun. In the early twentieth century, the American astronomer Vesto Slipher made observations showing that most of the known nebulae (such as those in Messier’s list) were red-shifted—or moving away from us. Soon after, Edwin Hubble, another American, showed that many of these nebulae were actually other galaxies, enormously far from the Milky Way. Hubble’s work led directly to the concept of an expanding universe and the Big Bang theory. SEE ALSO Big Bang (c. 13.7 Billion BCE), Messier Catalog (1771), Birth of Spectroscopy (1814), Hubble’s Law (1929). [image: ] A graphical representation of the Doppler effect: waves are being emitted by a source moving here from right to left. To the observer, waves in front of the source are compressed to a higher frequency (shorter wavelength, or bluer); waves behind the source have a longer wavelength (redder). 1967 Pulsars Antony Hewish (b. 1924), Samuel Okoye (1939–2009), Jocelyn Bell (b. 1943) The astrophysicists Walter Baade and Fritz Zwicky proposed the concept of neutron stars—highly dense, compact stellar remnants from supernova explosions—back in 1933. However, it wasn’t until 1965 that radio astronomers Antony Hewish and Samuel Okoye discovered the first observational evidence for a neutron star—a powerful but very small source of intense radio energy coming from the center of the Crab Nebula, the explosive remains of the famous “Daytime Star” supernova of 1054. Hewish and colleagues at the University of Cambridge continued searching for new neutron stars and other radio sources. Just two years later, using the new, more sensitive four-acre radio telescope west of Cambridge, Hewish’s student Jocelyn Bell discovered the first rapidly pulsating radio star (“pulsar”) in the constellation Vulpecula, with a constant pulse rate of every 1.3373 seconds. Bell and Hewish considered the possibility that the pulsar’s eerily regular radio signal might be a sign of extraterrestrial intelligence (they had jokingly named the source LGM-1, for “Little Green Men-1”). However, by 1968 they and other astronomers had come up with a more plausible explanation, partly because the neutron star in the center of the Crab Nebula had also been discovered to be a radio pulsar, with a pulse rate of every 33 milliseconds. Pulsars were found to be rapidly spinning neutron stars with strong magnetic fields that “beam” some of their energy in specific directions (usually along or close to their rotation axis). If the beamed electromagnetic radiation from the spinning pulsar is aligned so that it sweeps past the Earth, it can “light up” radio telescopes like the spinning beacon of a lighthouse. Several thousand pulsars have since been discovered, including several hundred-millisecond pulsars like the one in the Crab Nebula. Amazingly, variations in the timing of signals from the pulsar named PSR B1257+12 were interpreted in 1992 to be caused by the presence of planets orbiting the pulsar—the first examples of extrasolar planets. SEE ALSO “Daytime Star” Observed (1054), Neutron Stars (1933), SETI (1960), Arecibo Radio Telescope (1963), First Extrasolar Planets (1992). [image: ] A high-resolution composite Hubble Space Telescope (red) and Chandra X-ray Observatory (blue) image of the central region of the Crab Nebula (Messier 1), a remnant from a supernova explosion in 1054. The central energy source is a pulsar—a rapidly rotating neutron star—with a 33-millisecond rotation period. ~2050? Breakthrough Starshot Yuri Milner (b. 1961) What kinds of technological advances would it take to dramatically decrease the travel time of space probes to the nearest stars? That and other cutting-edge questions related to the search for extraterrestrial intelligence are at the heart of a new set of $100 million (each) Breakthrough Initiatives in science and technology that were started in 2015 by Russian entrepreneur, tech sector venture capitalist, and physicist Yuri Milner. The project, focused on rapid travel to nearby stars, is called Breakthrough Starshot. Specifically, Breakthrough Starshot aims to demonstrate that a swarm of tiny space probes can be accelerated to roughly 20 percent the speed of light and complete a scientific flyby survey of the Alpha Centauri star system (about 4.3 light-years away) in just twenty years. Working with physicist Stephen Hawking and others, Milner’s team proposes to build a fleet of a thousand “StarChip” spacecraft, each only postage-stamp size, that use lightsail technology to achieve extremely high speeds. The sails would be propelled by extremely high-energy (gigawatt) lasers on Earth. Transfer of the momentum of the laser light to the probes would, in theory, produce dramatic accelerations. In theory is the operative phrase, because Milner and others on the project realize that there are many significant challenges to overcome before such a plan could succeed. New materials would need to be developed, new laser-beaming methods would need to be devised and perfected, new miniature instruments (cameras and magnetometers, for example) would need to be invented. The significant seed funding for the project is motivating research in these and other areas, however. For example, small prototype probes have been developed and launched into low Earth orbit, validating some aspects of their expected in-space performance. The idea may seem far-fetched, but should even a tiny fraction of such probes succeed, the payoff could be enormous. For example, the star Proxima Centauri, one of the stars in the Alpha Centauri system, was recently discovered to have an Earth-sized exoplanet orbiting in the star’s habitable zone. Relatively rapidly acquired images or other remote sensing data about that world could help to motivate more extensive, traditional missions to that system—or perhaps even, eventually, a human journey to the Sun’s nearest neighbor. SEE ALSO Proper Motion of Stars (1718), SETI (1960), Habitable Super Earths? (2007), Kepler Mission (2009), LightSail-1 (2015), Planets Around TRAPPIST-1 (2017). [image: ] Artist’s rendering of a tiny, postage-stamp-sized StarChip solar sail spacecraft being launched from Earth toward the Alpha Centauri star system. 2011 ALMA Radio astronomers are used to working with enormous telescopes that are designed to capture faint signals. Pioneers in the early history of that field were among the first to discover that individual, separate radio telescopes could be linked together in an array—a technique called interferometry—to simulate the resolving power of a single telescope as large as the width of the array. One of the first examples of this technique employed on a large scale is the 27-telescope Very Large Array (VLA) facility outside of Socorro, New Mexico, which came online in 1980, and which was most recently upgraded in 2011. Another even more sensitive array of radio telescopes also came online in 2011, called the Atacama Large Millimeter Array, or ALMA. Sixty-six radio telescopes were deployed at high altitude (an elevation of 16,600 feet [5,060 meters]) in the super-dry Atacama Desert of northern Chile, positioned above most of the water vapor in the Earth’s atmosphere. Without the blocking effects of water vapor, ALMA can detect long-wave (millimeter-wave) infrared radiation from distant galaxies, stars, and even solar system objects, obtaining compositional information that would otherwise require a space telescope. An international consortium of countries from North America, Europe, and Asia built ALMA at a cost of around $1.4 billion, making it the most expensive ground-based telescope facility ever constructed. [image: ] ALMA reconstructed view of a protoplanetary disk of gas and dust around the young star HL Tauri. Results from ALMA have been stunning, especially because the maximum width of the array (10 miles [16 kilometers) provides the ability to resolve finer details than ever before with radio telescopes. A prime early example comes from ALMA images of a nascent solar system forming around HL Tauri, a star in the Taurus constellation: cleared lanes in the dusty disk reveal the presence of newly forming planets around the hot young star. A steady stream of other discoveries continues to come from ALMA, from the nature of hydrocarbons in nearby comets to the detailed interactions of distant starburst galaxies. SEE ALSO Solar Nebula (5 Billion BCE), Radio Astronomy (1931), NASA and the Deep Space Network (1958), Circumstellar Disks (1984). [image: ] A subset of the sixty-six antennae in the Atacama Large Millimeter Array, or ALMA, in the high and dry Chilean desert. 1990 Hubble Space Telescope Lyman Spitzer (1914–1997) The first astronomical telescopes, developed in the early seventeenth century, opened the skies to astronomers; they and subsequent larger and more advanced instruments enabled amazing discoveries about the solar system, galaxy, and universe. But astronomers have always known that even the largest telescopes on Earth are fundamentally limited in two important ways: first, the unavoidable shimmering and twinkling of our atmosphere limit the resolution to much less than a large telescope’s theoretical limit; and second, our atmosphere blocks many parts of the spectrum—especially in the ultraviolet and infrared ranges—making ground-based observations difficult or impossible at key wavelengths. With the advent of space satellites in the 1960s, astronomers began advocating within NASA for a dedicated, space-based telescope to overcome those limitations. A chief champion of an orbiting space telescope was the American astronomer Lyman Spitzer, who led a critical grassroots lobbying campaign for the necessary support and funding of the project. After many bureaucratic hurdles and a forged partnership with the European Space Agency, the Large Space Telescope, later named the Hubble Space Telescope (HST) after astronomer Edwin Hubble, was approved in 1978. HST was eventually launched into low Earth orbit—about 350 miles (570 kilometers) above the surface—by the Space Shuttle Discovery in April 1990. Shortly after launch, HST was discovered to have a major flaw in its primary mirror design. Fortunately, the telescope could be serviced by space shuttle astronauts, and five shuttle missions between 1993 and 2009 fixed the telescope and upgraded key instruments and components. As a result, HST serves as a cosmic time machine that uses CCD imaging and spectroscopy to determine the nature and even the age of our universe. By today’s giant telescope standards, HST is only a medium-size telescope, but its constant clear-sky and full-spectrum view of the cosmos have enabled it to realize the dreams of Spitzer and other early supporters, and fundamentally revolutionize modern astronomy and astrophysics. SEE ALSO First Astronomical Telescopes (1608), Hubble’s Law (1929), Space Shuttle (1981), Age of the Universe (2001). [image: ] The Hubble Space Telescope floats freely about 350 miles (560 kilometers) above Earth’s surface after being released by the space shuttle Discovery during a servicing mission in February 1997. The telescope is about 8.2 feet (2.5 meters) in diameter and about 43 feet (13.1 meters) long, or slightly longer than an average school bus. 2016 ExoMars Trace Gas Orbiter Numerous robotic flyby, orbiter, lander, and rover missions have been sent by the world’s space agencies to study the interesting and enigmatic history of Mars. A hallmark of this global program of Mars exploration has been that new missions follow up on previous puzzles and discoveries. In March 2016, the European Space Agency (ESA) launched the ExoMars Trace Gas Orbiter (TGO) mission, focusing its scientific studies on the detection and mapping of minor atmospheric gases (like methane) that could potentially provide diagnostic information about the past geologic and potentially biologic history of Mars. The spacecraft successfully went into an elliptical orbit in October 2016, and began using aerobraking to get to its eventual low-altitude near-polar circular orbit. The detection and monitoring/mapping of methane on Mars has had a complicated history. Missions to the surface have measured only very small amounts—typically from zero to ten parts per billion, or ppb (Earth’s atmosphere typically has about 1,800 ppb). However, some ground-based telescopic observations have detected much higher methane levels—30 ppb or more—that vary over time on different parts of the planet. Atmospheric chemists have realized for decades that methane is relatively quickly broken down by ultraviolet radiation in the Martian atmosphere, so detection of significant amounts would imply that there is some active source that resupplies it. Potential sources include weathering of subsurface rocks, breakdown of other more complex surface or atmospheric organic molecules, or biologic activity of some kind. While it may be a long shot, the possibility of deep, subsurface microbes or other biologic activity creating methane on Mars is a strong driver for TGO’s measurements. Planetary scientists and astrobiologists worldwide will be watching with great interest as the mission’s data sets continue to roll in and get analyzed and interpreted. TGO also carried and deployed a small lander called Schiaparelli, designed to test ESA technologies for entry, descent, and landing vehicles to Mars, and pave the way for a more ambitious European rover mission in the early 2020s. The lander carried cameras and meteorology equipment, but, unfortunately, it crashed onto the surface because of software problems during the landing process. SEE ALSO Mars (c. 4.5 Billion BCE), Mars and Its Canals (1906), First Mars Orbiters (1971), Vikings on Mars (1976), Life on Mars? (1996), First Rover on Mars (1997), Mars Global Surveyor (1997), Spirit and Opportunity on Mars (2004), Mars Science Laboratory Curiosity Rover (2012). [image: ] Artist’s rendering of the European Space Agency’s ExoMars Trace Gas Orbiter. 1945 Geosynchronous Satellites Hermann Oberth (1894–1989), Herman Potocˇnik (1892–1929), Arthur C. Clarke (1917–2008) Newton’s Laws of Gravity and Motion and Kepler’s Laws of Planetary Motion in particular apply to artificial satellites just as well as to planets in orbit around a star or moons in orbit around a planet. Rocket technology and astronautics—the study of navigating through space—advanced quickly after the first liquid-fueled rockets capable of reaching high altitudes were developed in the 1920s by Robert Goddard. Several of Goddard’s contemporaries were already beginning to think about the mechanics and dynamics of orbital (and beyond) rocket flights. Two of those contemporaries were the Hungarian-German physicist Hermann Oberth and the Austro-Hungarian rocket engineer Herman Potocˇnik, who expanded on and worked out the details of concepts first described by the Russian mathematician Konstantin Tsiolkovsky. One of those concepts was the idea of a geostationary, or geosynchronous, orbit. A satellite in a geosynchronous orbit will complete one orbital revolution in the same amount of time that it takes the Earth to spin once on its axis. From the vantage point of an observer on the Earth’s surface, such a satellite would appear to be parked high in the sky, never moving. Given the Earth’s mass and rotation rate, Newton’s second law can be used to derive the orbital altitude of a geosynchronous satellite—it turns out to be about 22,000 miles (36,000 kilometers) above the surface. [image: ] Space shuttle Discovery deploying the AUSSAT-1 communications satellite in 1985. The British science fiction author and futurist Arthur C. Clarke was one of the first to grasp what would ultimately be one of the most practical applications of such satellite orbits: global telecommunications, described in a 1945 magazine article called “Extra-Terrestrial Relays—Can Rocket Stations Give Worldwide Radio Coverage?” Clark’s popularization of the idea helped it to gain wide attention and support. Starting in 1964, the actual use of geosynchronous satellites has now gone far beyond just radio relays. Today they also relay TV, Internet, and global positioning system (GPS) signals and help us to monitor Earth’s weather and climate. SEE ALSO Three Laws of Planetary Motion (1619), Newton’s Laws of Gravity and Motion (1687), Liquid-Fueled Rocketry (1926). [image: ] A snapshot of satellites currently being tracked by the NASA Orbital Debris Program Office; Earth’s ring of geosynchronous satellites can be clearly seen. 45 BCE Julian Calendar Julius Caesar (100–44 BCE) Like other past civilizations that were tuned in to the skies, the Romans had developed a calendar system that had strong astronomical connections. The calendar system that they originally designed in the eighth century BCE was the source of constant confusion, however, partly because it was cobbled together from pieces borrowed from the Greeks and others before them. For example, the year had 10 months of 30 or 31 days for a total of 304 days—the remaining 61-plus days needed to make up an actual trip around the Sun were brushed under the rug as “winter.” A later change added two new winter months (January and February) but still came to only 355 days total per year. To keep the calendar lined up with the seasons, a leap month was occasionally added in by the high priests, but the decision to add extra days to a given year was often arbitrary and politically motivated. The situation got so muddled that many ordinary Romans had no idea what day, year, or month it was. In fact, the Roman calendar system was such a confused mess when Julius Caesar came to power in 49 BCE that he ordered a reform that would align the calendar more with the motions of the Sun rather than with the affairs of men. Days were added to some of the 12 months to bring the total number of days in a year to 365, and he decreed that every fourth year an extra leap day would be added to the end of February, making the average length of the year 365.25 days, which is close to the actual length of a solar year—365.242 days. Caesar’s reform of the calendar took effect on January 1 in the year 45 BCE (or, to the Romans, 709 years after the founding of Rome), after the priests had to make the year 46 BCE 445 days long to try to fix all the problems that had accumulated before the reform. The Julian calendar worked well for a long time because it was only 0.008 days (about 11 minutes) per year different from a true solar year. By the sixteenth century, however, those 11 minutes per year had added up to a significant shift between the calendar year and the solar year, and so a further tweak, called the Gregorian Calendar reform, was needed to resync the calendar with the seasons. SEE ALSO Egyptian Astronomy (c. 2500 BCE), Gregorian Calendar (1582). [image: ] Ancient Roman calendars were sometimes carved into stone blocks, as in this marble version that shows day names and astrological symbols for the months of April through September. c. 250 BCE Eratosthenes Measures the Earth Plato (427–347 BCE), Aristotle (384–322 BCE), Eratosthenes (c. 276–195 BCE) The Greeks had generally accepted the fact that the Earth is round at least as far back as the time of Pythagoras, but estimates of the actual size of the Earth varied widely. Plato had guessed the Earth’s circumference to be around 44,000 miles (70,000 kilometers), corresponding to a diameter of about 14,000 miles (22,000 kilometers), and Archimedes had estimated a circumference of about 34,000 miles (55,000 kilometers) and diameter of 109,000 miles (17,500 kilometers). To make a more accurate determination, Eratosthenes, a mathematician, astronomer, and the third chief librarian of Alexandria, devised a simple experiment that was akin to treating the Earth as a giant sundial. Eratosthenes had learned that at noon on the summer solstice in the southern Egyptian city of Syene, the Sun was almost exactly overhead (at the zenith), so posts in the ground did not cast any shadows. He also knew that in his own city of Alexandria in the north of Egypt, posts in the ground did cast (small) shadows at noon on the summer solstice. He made some measurements and determined that the Sun was a little over 7 degrees south of the zenith in Alexandria. This corresponds to about 1⁄50th of the circumference of a circle, so he surmised that the circumference of the Earth was about 50 times the distance between Alexandria and Syene. With a distance of about 5,000 stadia (the stadium was an ancient Egyptian and Greek unit of measure) between Alexandria and Syene, he estimated the circumference of the Earth at about 250,000 stadia, or 25,000 miles (40,000 kilometers). Assuming that 1 stadium was about 175 yards (160 meters) to Eratosthenes, this yields a circumference of about 25,000 miles (40,000 kilometers), which, given the various uncertainties and assumptions involved in the measurements, is essentially the correct answer. Eratosthenes is widely regarded as the father of geography—indeed, he coined the word. It seems appropriate, then, that he was the first to accurately determine the size of the Earth. His method is also a fabulous example of the power of a simple, well-timed experiment. Archimedes had once quipped about levers, “Give me a place to stand, and I will move the Earth.” Eratosthenes could easily have retorted, “Give me a few sticks and some shadows, and I will measure the Earth.” SEE ALSO Earth Is Round! (c. 500 BCE), Greek Geocentrism (c. 400 BCE), Sun-Centered Cosmos (c. 280 BCE). [image: ] Cartoon illustrating Eratosthenes’s simple method for measuring the circumference of the Earth. A vertical post in Syene (lower inset) casts no shadow at noon on the summer solstice. The same post in Alexandria (upper inset), however, casts a shadow that indicates it is 1⁄50th of a circle away from Syene. 1977 Uranian Rings Discovered James L. Elliot (1943–2011), Edward W. Dunham (b. 1952), Douglas J. Mink (b. 1951) In 1659, the Dutch astronomer Christiaan Huygens observed and explained the rings of Saturn as a thin disk of material circling the sixth planet. Saturn appeared to be the only planet with a ring system until 1789, when, shortly after discovering Uranus, the English astronomer William Herschel thought he detected a faint ring around that planet as well. But subsequent visual and photographic observations of Uranus couldn’t verify Herschel’s claim. Nearly two hundred years later, on March 10, 1977, the American team of planetary scientists James Elliot, Ted Dunham, and Douglas Mink was preparing to observe Uranus pass in front of a relatively bright star—an eclipse of sorts known as an occultation. To ensure that they were in the precise location to catch the rare occultation, the team observed the event from the Kuiper Airborne Observatory, a telescope mounted in a NASA C-141A jet that flew missions in the stratosphere, above most of our atmosphere’s clouds and water vapor. The occultation yielded an exciting surprise. Just before the star was eclipsed by Uranus, its brightness briefly dropped dramatically, five separate times. Then, just after the star reemerged from the eclipse, it happened five times again. Analysis by Elliot and his team revealed that the drops in starlight occurred at the same radial distance from Uranus on either side of the planet. They had discovered a faint set of narrow rings—meaning that the seventh planet had become the second known planet with rings. Follow-up observations revealed four more narrow rings around Uranus, bringing the total to nine. Two more rings were discovered during Voyager 2’s 1986 flyby—which revealed that the rings are extremely dark and are likely made of centimeter- to meter-size icy blocks that have been darkened by interactions between Uranus’s magnetic field and icy, organic molecules—and two more were discovered in the early twenty-first century by astronomers using the Hubble Space Telescope, bringing the total to 13. Faint rings were later also discovered around Jupiter and Neptune (also in Voyager data), meaning that all the giant planets in our solar system—not just Saturn—are ring worlds. SEE ALSO Uranus (c. 4.5 Billion BCE), Saturn Has Rings (1659), Jovian Rings (1979), Rings Around Neptune (1982). [image: ] Voyager 2 image of the rings of Uranus, acquired during the probe’s flyby in January 1986. Background stars appear as streaks in this long-exposure photograph. The blinking on and off of blocked starlight passing through the rings is what earlier had led to their discovery in 1977. 1969 First on the Moon Neil A. Armstrong (1930–2012), Edwin G. “Buzz” Aldrin (b. 1930), Michael A. Collins (b. 1930) After Yuri Gagarin became the first human in space, the race between the United States and the Soviet Union quickly became focused on the next big milestone: landing astronauts on the Moon and bringing them safely back to the Earth. The Soviet Vostok program was reoriented toward the larger rockets and landing systems needed for a lunar landing and return. In America, the challenge was to beat the Russians and to meet slain president John F. Kennedy’s 1961 goal of doing it “before this decade is out.” A series of incrementally more advanced US missions were conducted between 1961 and 1969, starting with the Mercury single-astronaut flights, continuing with the Gemini two-person Earth orbital docking and rendezvous flights, and culminating in the three-person Apollo missions to the Moon. Apollo 8 achieved an important first in 1968 by sending the first humans to orbit the Moon and view the whole Earth and the lunar far side firsthand; the feat was repeated in early 1969 with the flight of Apollo 10, a full dress rehearsal of a lunar landing that sent astronauts to within 10 miles (16 kilometers) of the lunar surface before returning home. Meanwhile, the Russians continued to make progress in their own secret lunar cosmonaut program. Several catastrophic unmanned launch failures in 1969 set their program back significantly, however, opening the door for an American victory. [image: ] Buzz Aldrin’s bootprint in the fine, powdery lunar soil. That victory came on July 20, 1969, with the entire world watching as astronauts Neil A. Armstrong and Edwin G. “Buzz” Aldrin became the first humans to land, walk, and work on the moon. Armstrong and Aldrin landed on the ancient volcanic lava flows of the Mare Tranquillitatis (Sea of Tranquillity) impact basin (age dating of the samples showed them to be 3.6–3.9 billion years old), and spent about two and a half hours collecting samples and exploring the terrain. After less than a day, they took off and rejoined command module pilot Michael A. Collins back in lunar orbit for the three-day trip home—as world heroes. SEE ALSO Birth of the Moon (c. 4.5 Billion BCE), Liquid-Fueled Rocketry (1926), First Humans in Space (1961). [image: ] Apollo 11 astronaut Aldrin unloads scientific equipment from the lunar module Eagle at the landing site in Mare Tranquillitatis (photo taken by Neil Armstrong). c. 964 Andromeda Sighted ‘Abd al-Rahmān al-Sūfī (903–986) Another important early astronomer from the Arab world was ‘Abd al-Rahmān al-Sūfī of Persia (modern-day Iran). Like most other astronomers of the Middle Ages, al-Sūfī was aware of the major aspects of classical Greek astronomy and cosmology, including Ptolemy’s Almagest, which he translated into Arabic. He and others sought to expand on Ptolemy’s ideas, and to synthesize them with new observations and theories from early Arabic astronomy. His results were published around the year 964 in a landmark work called The Book of Fixed Stars. Al-Sūfī’s book was essentially a detailed map of the stars in the 48 then-known classical Greek constellations, using star data based on Ptolemy’s older catalog in the Almagest and Hipparchus’s Stellar Magnitude system, but refined or corrected using his and other newer observations of stellar brightnesses and colors. The Book of Fixed Stars uses the Arabic names for the bright stars in each constellation; we still use many of these star names—including Altair, Betelegeuse, Deneb, Rigel, and Vega—today. [image: ] The inset above shows a drawing of part of the constellations Andromeda and Pisces from al-Sūfī’s 964 Book of Fixed Stars. In the section of al-Sūfī’s book devoted to the constellations Andromeda and Pisces, he notes a “little cloud” detected among the major stars. Although he could never have known it, al-Sūfī is widely believed to have made the first recorded observation of the Andromeda galaxy, the nearest (about 2 million light-years distant) spiral galaxy to the Milky Way. Also known as Messier object 31, Andromeda is nearly eight times larger than the full Moon, but it is extremely faint and thus takes excellent eyesight and patience to detect. Al-Sūfī was also the first to detect other faint star clusters, nebulae, and “clouds,” including one of the Milky Way’s faint elliptical companion galaxies in the southern skies, an object that, 550 years later, was named the Large Magellanic Cloud because it was prominently noted and popularized in Europe after Ferdinand Magellan’s voyage around the world in 1519. SEE ALSO Ptolemy’s Almagest (c. 150), Early Arabic Astronomy (c. 825), Messier Catalog (1771). [image: ] A modern digital astronomical photograph of the Andromeda galaxy, viewed in ultraviolet light from the NASA Galaxy Evolution Explorer satellite. 1684 Zodiacal Light Giovanni Domenico Cassini (1625–1712), Nicolas Fatio de Duillier (1664–1753) Seasoned and amateur sky watchers alike know that the best way to view the full splendor of the night sky is to go out on a clear, moonless night to a dark, isolated observing site far from city lights. Even then, however, the sky is not completely dark. Besides the thousands of visible stars and the diffuse glow of the Milky Way galaxy, another faint glow can often be observed, especially in the west after sunset or the east before sunrise. This glow is called the zodiacal light because it appears as a whitish band or faint spire from the horizon away from the Sun, roughly along a line that follows the path of the constellations of the zodiac. This line is also called the ecliptic, the plane of the Sun’s equator and the orbits of most of the planets in the solar system. Islamic astronomers referred to the zodiacal light as the “false dawn,” and understanding its behavior and influence on the determination of true sunrise or sunset was an important part of determining the timing of daily prayers. Many Renaissance astronomers, including Giovanni Domenico Cassini, who observed the “luminous streak” in 1683, believed it was just an extension of the Sun’s atmosphere. It remained a puzzle why it was confined to being bright only along the Sun’s equatorial plane. The first to propose the explanation that turned out to be right was the Swiss mathematician Nicolas Fatio de Duillier, who worked under Cassini at the Paris Observatory and who proposed that the glow was caused by particles reflecting the Sun’s light. Fatio’s hypothesis was shown to be correct by modern spectroscopy and space probes that showed that the zodiacal light is the result of sunlight reflecting off of interplanetary dust grains that measure from a few hundredths to a few hundred microns in size (the width of a human hair is about 100 microns). Because these grains slowly spiral in to the Sun because of the absorption of sunlight (an effect called Poynting-Robertson drag, discovered by early-twentieth-century physicists), there must be a continuous source to resupply them. Astronomers believe that dust from comets and occasional collisions between asteroids—most of which travel in or near the ecliptic—provide that continuous source of cosmic dust. SEE ALSO Milky Way (c. 13.3 Billion BCE), Main Asteroid Belt (c. 4.5 Billion BCE), Halley’s Comet (1682). [image: ] A December 2009 photo of the white glow of the zodiacal light seen from Cerro Paranal, Chile, near the site of the European Southern Observatory’s Very Large Telescope (VLT). 2019 James Webb Space Telescope James E. Webb (1906–1992) The power and beauty of space-based astronomy has been dramatically demonstrated by a variety of small, medium, and large space telescopes, culminating in the successful missions of NASA’s four Great Observatories: the Hubble Space Telescope (HST), Compton Gamma-Ray Observatory, the Spitzer Space Telescope, and the Chandra X-ray Observatory. Like all complex spacecraft, however, these missions have finite lifetimes. Only HST has been capable of being repaired or upgraded by astronauts—but the retirement of the space shuttle in 2011 means the end of service calls to HST. NASA has thus been pondering its replacement for some time. NASA’s planned next-generation space telescope is known as the James Webb Space Telescope (JWST), after NASA’s second administrator, James E. Webb, who oversaw the space agency during the Mercury, Gemini, and early Apollo astronaut programs. Planning for JWST actually began in 1989, the year before HST was launched. Over the course of more than 20 years the design has been revised numerous times. Currently the telescope is in final development, with a scheduled launch in 2019. JWST will combine some capabilities from the Hubble Space Telescope (such as high-resolution imaging), the Keck Observatory (a precisely controlled segmented mirror design), and the Spitzer Space Telescope (sensitivity in the infrared) to become the scientific workhorse for space-based astronomy for at least a decade. Its 21-foot (6.5-meter) segmented primary mirror has six times the light-collecting area of HST, and the telescope is cooled to only 40 degrees above absolute zero to make it highly sensitive to faint, distant objects in the cosmos. Astronomers have ambitious plans for JWST science across the entire range of visible through infrared astronomy and astrophysics research areas. Major science themes include studying the first stars and galaxies, formed after the early dark ages of the universe; studies of dark matter; new stars and their associated protoplanetary disks of gas and dust; the formation of planets; and the search for extrasolar planets and other cosmic environments conducive to life. JWST should be a spectacular astronomical discovery machine! SEE ALSO First Astronomical Telescopes (1608), Hubble Space Telescope (1990), Giant Telescopes (1993), Chandra X-ray Observatory (1999), Spitzer Space Telescope (2003), Dawn at Ceres (2015). [image: ] Artist’s conception of the James Webb Space Telescope, with its gold-coated primary mirror (21 feet [6.5 meters] in diameter) and deployed radiation shields, designed to protect the telescope from the contaminating light and heat of the Sun, Earth, and Moon. 1995 Galileo Orbits Jupiter The Pioneer 10 and 11 and Voyager 1 and 2 flybys of Jupiter revealed a beautiful, complex, and enigmatic mini solar system at the giant planet, including a dynamic atmosphere of zones, belts, and long-lived cyclonic storm systems, such as the Great Red Spot; a veritable zoo of icy and rocky satellites, including four that are essentially planets in their own right (Io, Europa, Ganymede, and Callisto); a dusty system of narrow rings; and an enormous magnetosphere that bathes much of the system in high-energy radiation. The discoveries during those brief flybys convinced planetary scientists that the next logical step would be to send a spacecraft to orbit Jupiter and stay for a while. With congressional and international support, the Jupiter Orbiter and Probe mission, renamed Galileo in honor of the astronomer who first studied Jupiter and its moons through a telescope, was approved for funding in late 1977. Overcoming a number of technical and financial hurdles, Galileo was eventually launched on the space shuttle Atlantis in late 1989, and used gravity-assist flybys of Venus and Earth to slingshot out to Jupiter, where it arrived and entered orbit in December 1995. Along the way, Galileo performed the first close asteroid flybys (of 952 Gaspra and 243 Ida) while passing through the main asteroid belt. [image: ] The mission was handicapped by the failure of the main communications antenna to open properly, but mission engineers and scientists designed a new mission using the backup low-data-rate antenna. Using the spacecraft’s cameras, spectrometers, and fields and particles instruments, the Galileo team directed the spacecraft through 34 elliptical orbits of Jupiter over the course of nearly eight years, flying past large and small moons to determine their composition and internal structure, studying the rings and magnetic field in detail, and releasing a probe into the atmosphere that relayed direct measurements of composition, temperature, and pressure. The Galileo spacecraft is gone, crushed and vaporized by Jupiter’s deep atmosphere at the end of its mission, but its scientific legacy will be long-lasting—living up to the larger legacy of the name Galileo. SEE ALSO Jupiter (c. 4.5 Billion BCE), Great Red Spot (1665), Io (1610), Europa (1610), Ganymede (1610), Callisto (1610), Himalia (1904), Jupiter’s Magnetic Field (1955), Pioneer 10 at Jupiter (1973), Active Volcanoes on Io (1979), Jovian Rings (1979), An Ocean on Europa? (1979). [image: ] A montage of Jupiter’s Great Red Spot and the four Galilean satellites (Io, Europa, Ganymede, and Callisto), all studied in detail from 1995 to 2003 by the NASA Galileo orbiter (left). 2010 Rosetta Flies by 21 Lutetia Spectroscopy and color measurements from telescopic observations during the second half of the twentieth century revealed that main belt and near-Earth asteroids can be grouped into a small alphabet soup of compositional classes. For example, asteroids that show colors and spectra indicating the presence of typical planet-forming volcanic minerals, like those found in stony meteorites, are known as S-type asteroids; darker objects with grayer colors and spectra that are more like those of carbonaceous (carbon-bearing) meteorites are known as C-type asteroids; objects with spectra similar to metallic meteorites are known as M-types, and so on. Dozens of different asteroid types have been proposed, depending on the classification scheme and research group. Prior to 2010, spacecraft had encountered both S-type (for example, Eros, Gaspra, Ida, and Itokawa) and C-type (253 Mathilde) asteroids, but no others. Thus it was especially exciting when the European Space Agency’s Rosetta spacecraft made a close flyby past the M-type asteroid 21 Lutetia on July 10, 2010. Rosetta is a comet rendezvous mission that was launched in 2004 and will rendezvous and deploy a lander onto the surface of periodic comet Churyumov-Gerasimenko in 2014. In addition, like many other space mission teams, the Rosetta team has been able to do some excellent “bonus” science by flying past other objects on the way. The Rosetta images of Lutetia revealed it to be the largest asteroid then encountered by spacecraft (at 82 × 63 × 48 miles [132 × 101 × 76 kilometers] in size). It is also one of the densest (at 3.4 grams per cubic centimeter), suggesting a possible rocky, metallic composition consistent with its M-type classification. In terms of its visual appearance and geology, however, Lutetia shares many similarities with the other asteroids that have been photographed up close: a lumpy, irregular surface heavily covered by both relatively fresh and relatively degraded impact craters in a variety of sizes. Lutetia also shows evidence of a surface layer of fine-grained, mobile, impact-generated debris—what planetary scientists call a regolith. Why Lutetia’s spectrum appears similar to metallic meteorites, and how such small objects with such low gravity (less than 0.3 percent of Earth’s) can retain fine-grained regolith materials, are active areas of research and debate motivated by Rosetta’s flyby measurements. SEE ALSO Ceres (1801), Vesta (1807), Asteroids Can Have Moons (1992), 253 Mathilde (1997), NEAR at Eros (2000), Hayabusa at Itokawa (2005). [image: ] Photo of the asteroid Lutetia, taken by the European Space Agency’s Rosetta spacecraft during its July 2010 flyby. Lutetia is a heavily cratered Main Belt asteroid and was the largest asteroid yet encountered, prior to the Dawn mission’s exploration of Vesta and Ceres. 1895 Milky Way Dark Lanes Edward Emerson Barnard (1857–1923), Max Wolf (1863–1932) People fortunate enough to live in or at least occasionally visit truly dark, non-light-polluted night skies on moonless nights are treated to a stunning view: the grand Milky Way sweeps from horizon to horizon, with bright starry bands and black inky lanes stretching out like a celestial Jackson Pollock painting splashed across a grand cosmic canvas. On such wonderful nights it’s easy to understand our ancestors’ reverence for the night sky, as well as their need to try to make sense of what they were seeing. In the late nineteenth century, many of the world’s major cities could still be considered “dark sky” observing sites; the electrification of the night sky didn’t become truly ubiquitous until sometime after World War II. Thus, the American astronomer E. E. Barnard jumped at the opportunity to move to the University of Chicago in 1895 to gain access to what had just become the world’s largest refracting telescope, the giant 40-inch (102-centimeter) lens at Yerkes Observatory. Armed with a great telescope and his newfound interest in the nascent field of astrophotography, Barnard began taking the best data ever acquired of bright star fields and dark, seemingly empty gaps across the Milky Way’s grand sweep. An important collaborator on Barnard’s Milky Way studies was the German astronomer and astrophotographer Max Wolf. Wolf was aware that many astronomers were puzzled by the Milky Way’s dark lanes—what the English astronomer William Herschel had called holes in the sky. Barnard’s photos and Wolf’s analysis revealed that these “holes” weren’t really holes at all—careful observations could reveal faint embedded stars, or even background stars, that could be used to derive the properties of the dark lanes. Wolf made a convincing and ultimately accurate argument that the dark areas in the Milky Way are enormous clouds of relatively opaque dust that obscure the otherwise blazing light of the background stars and prevent them from shining through. He noticed that the dark lanes were often associated with pockets of bright nebulosity, potentially from newly formed stars, and deduced that the dark regions might be cosmic cocoons, places where dust and gas are being compressed and thickened and are “about to form new suns.” Wolf and Barnard’s early speculations about the origin of the dark lanes have turned out to be spot-on. SEE ALSO Milky Way (c. 13.3 Billion BCE), Solar Nebula (c. 5 Billion BCE), First Astrophotographs (1839). [image: ] The glorious spectacle of the Milky Way and its dark, dusty lanes rises over Long’s Peak (14,259 feet [4,346 meters] high), in the Rocky Mountain National Park in Colorado. 1932 The Öpik-Oort Cloud Ernst Öpik (1893–1985), Jan Oort (1900–1992) By the early twentieth century, hundreds of years of careful observations had enabled accurate orbits to be calculated for dozens of bright comets. They appeared to come in two varieties: periodic short-period comets, with orbits that bring them back into the inner solar system roughly every 20 to 200 years; and periodic long-period comets, with long, highly eccentric orbits that could take hundreds to thousands of years or more to complete (or not to repeat at all, for the related nonperiodic single-apparition comets). The trajectories of the longest-period comets take them extremely far from the Sun, with some traveling as far out as 50,000 to 100,000 astronomical units (almost a third of the way to the nearest star) when near their aphelion, or farthest point in their orbit. Several researchers independently noted that the cluster of aphelion distances at those extreme ranges probably meant that there was a supply or reservoir of comets that originated at those extreme distances. The fact that long-period comets come from all directions in the sky (not just along the plane of the ecliptic, like the planets and most short-period comets) also meant that this reservoir was likely spherical, like a huge cloud surrounding the solar system. In 1932 the Estonian astrophysicist Ernst Öpik was the first to postulate the existence of this vast reservoir of comets in a paper describing the role that passing stars might have in gently nudging comets from this distant cloud into new orbits that would take them in toward the Sun. Independently, in 1950, the Dutch astronomer Jan Oort also came up with a similar idea, but expanded to include the role of Jupiter and the other giant planets in flinging inner solar system comets out into this enormous distant cloud. Subsequent studies of new long-period comets (about one new one is discovered every year) confirm the idea: even though it’s never been directly seen, a vast cloud of distant comets appears to surround the Sun. Astronomers now call this the Oort cloud (or Öpik-Oort cloud). By some estimates there could be a few Earth masses in a trillion or more kilometer-size comet nuclei out there, some formed closer in to the Sun but ejected into a perennial deep freeze, others formed at the edge of the Sun’s gravity and waiting for their first gentle stellar nudge into the warmth. SEE ALSO Halley’s Comet (1682), Kuiper Belt Objects (1992), “Great Comet” Hale-Bopp (1997). [image: ] The inner solar system (top) extends out to the main asteroid belt, and the outer solar system (middle) out to the Kuiper Belt. The Oort cloud (bottom) is hypothesized to extend out much farther, perhaps a third of the way or more to the nearest star. 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Hurt (SSC): 393; JPL-Caltech/Space Science: 45, 441; JPL-Caltech/SwRI/MSSS/Gerald Eichstadt/Sean Doran: 479; JPL-Caltech/UCLA/MPS/DLR/IDA: 189, 474, 475; JPL-Caltech/University of Arizona: 225, 227, 445, 461; JPL-Caltech/University of Maryland: 445, 461; JPL/Cornell University: 226, 358; JPL/Cornell/USGS: 438, 439; JPL/DLR: 407; JPL/IMP Team: 415; JPL/JHUAPL: 413; JPL/LASP: 359; JPL/MSSS: 417; JPL/SSI: 145, 152, 153(x2), 158, 159 180, 181, 183, 207, 239; JPL/SSIitute: 43; JPL/USGS: 357, 382, 389; JSC: 3, 284, 285, 335, 409; S. Kulkarni (Clatech)/D. Golimowski (JHU): 403; Emily Lakdawalla (The Planetary Society): 459; S. Lee (University of Colorado)/J. Bell (Cornell University/M. Wolff (SSI): 39; J. Morse/STScI: 29; Mount Stromlo and Siding Springs Observatory: 400; MSFC: 424; National Radio Astronomy Observatories: 273; NEAR Project/JHUAPL: 429; NSSDC Photo Gallery: 330, 336, 337; Tim Pyle: 454; Pat Rawlings/SAIC: 503; T.A. Rector (University of Alaska-Anchorage, NRAO/AUI/NSF)/B.A. Wolpa (NOAO/AURA/NSF): 303; H. Richer (University of British Columbia): 515; Jim Ross: 457; SAO/CSC: 219; SDO: 263; N.A. Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF: 193; STS-119 Shuttle Crew: 419; STScI: 85, 387, 491; Swift/Mary Pat Hrybyk-Keith and John Jones: 345; USAF/J. Strang: 295; Corby Waste: 416; Fred Walter (SUNY Stony Brook): 277; WMAP Science Team: 21, 175, 433 NOAA: 168 © Tyler Nordgren: 233 NYU Archives: 197 Orbis World Globes/www.earthball.com: 75 © J.M. Pasachoff, W.G. Wagner, and H. Druckmülerová: 223 The Planetary Society: 473 Private Collection: 77, 79, 81, 84, 91, 100, 113, 115, 117, 146 © Massimo Ramella: 384 © Pedro Ré: 74 © Casey Reed/Courtesy of Penn State University: 517 Kristian Resset: 69 © Graham Rigg of South Shields Daily Photo (http://southshieldsdailyphoto.wordpress.com): 97 Royal Swedish Academy of Sciences: 431 Science Source: © Julian Baum/New Scientist: 213; © Mark Garlick: 275 © Scott S. Sheppard/Carnegie Institution: 245 Shutterstock: © Bertold Werkmann: 235 © courtesy of A. Siddiqi/Fordham University: 305 © John Spencer: 289 © Lawrence Sromovsky/University Wisconsin-Madison/W.M. Keck Observatory: 47 © STFC: 291 © Barry Sutton: 57 © R. Svoboda (UC Davis)/K. Gordan (LSU): 290 © Joe Tucciarone: 53 courtesy of University of Cambridge, Institute of Astronomy: 194 © Massimo Mogi Vicentini: 87 © David Wall/Paul Hewitt: 229 © M. Weiss (CXC): 121 courtesy of Wikimedia Foundation: 89, 99, 109, 114, 125, 126, 127, 128, 130, 132, 134, 138, 139, 154, 169, 182, 186, 221, 252, 472; Adamt: 105; Brian Brondel: 83; CXC/SAO, Infrared: NASA/JPL-Caltech; Optical: MPIS, Calar Alto, O. Krause et al.: 116; EugeneZelenko: 317; Adam Evans: 511; Goodvint: 333; HTO: 63; Jastrow: 123; E. Kolmhofer/ H. Raab/ Johannes-Kepler-Observatory, Linz Austria: 411; KuniyoshiProject.com: 73; Ricardo Liberato: 71; Marco Polo: 500, 501; Marsyas: 86; Mdf/NASA/JPL/IAU: 41; Meteorite Recon: 185; Mukerjee: 95; orci: 240; Alex Ostrovski: 111; Pfalstad: 205; Prolineserver/Holgar Motzkau: 229; Reyk YO!: 217; Daniel Sancho: 209; Ferdinand Schmutzer: 247; Silvercat: 298; skatebiker/Chris Heilman: 240; John Sullivan: 321; taking.kwong: 110; Mike Young/Ed Grafton: 332* © Robert Wilson: 163 © Yale University: 280 © Michael Zeiler/elipse-maps.com: 484 © Mila Zinkova: 25 1967 Study of Extremophiles Thomas Brock (b. 1926) Astrobiology is the study of the origin, evolution, and distribution of life and habitable environments in the universe. It is perhaps a unique discipline in that it has only one data point with which to conclusively justify its existence. So far, we know of only one example of life in the universe—that is, life on earth—all of which is fundamentally similar, based on similar RNA, DNA, and other carbon-based organic molecules. The search for life elsewhere is more than just the search for complex life forms like us, however. It is a search for other planetary environments that could be suitable for the most dominant form of life on our own planet—bacteria and other “simple” life forms. The best place to start a search for those conditions is right here on our own planet, where significant advances in our understanding of habitability have been made in the past 50 years. In 1967, the American microbiologist Thomas Brock wrote a landmark paper describing heat-tolerant bacteria (hyperthermophiles) that flourished within hot springs at Yellowstone National Park. He challenged the prevailing wisdom that the chemistry of life requires moderate temperatures to operate. Brock’s work helped to spur the study of extremophiles—life forms that survive and even thrive in harsh environments. Hyperthermophilic bacteria have since been identified in very hot water near deep sea hydrothermal vents as well; on the opposite extreme, psychrophiles have been found that live and thrive in near- or below-freezing temperatures. Life forms have also been found that exist over extremes of salinity (halophiles), acidity (acidophiles and alkaliphiles), high pressure (piezophiles), low humidity (xerophiles), and even high levels of UV or nuclear radiation (radioresistant). The message for astrobiologists from the history of life on our planet is clear: life can thrive in an enormous range of environments. Thus, searching for evidence of past or present extremophiles or their habitable environments in extreme places such as Mars, the deep oceans of Europa and Ganymede, or the frigid, organic-rich surface of Titan is not as crazy an idea as it used to be. SEE ALSO SETI (1960), An Ocean on Europa? (1979), Life on Mars? (1996), An Ocean on Ganymede? (2000), Huygens Lands on Titan (2005). [image: ] Morning Glory Pool, a hot spring in Yellowstone National Park, Wyoming. The colors along the spring’s outer edges are from a variety of hyperthermophilic bacteria that can survive and thrive even at the high temperatures of the spring (above 176°F [80°C]). Acknowledgments I owe a debt of gratitude to the many colleagues and mentors who, knowingly and unknowingly, fueled my interest in the history of astronomy, planetary science, and space exploration. Among the most influential of these are the late Carl Sagan, Jim Pollack, and Leonard Martin, all of whom were also outstanding scientists. I am very grateful to the many friends, colleagues, new acquaintances, and anonymous Samaritans who graciously agreed to provide their beautiful photographs or artwork for this project. I extend a huge thank you to the creators of and worldwide editors/contributors to Wikipedia (to which I contribute financially) for creating an incredible research tool and jumping-off point for additional exploration of both historical and current topics. I thank Michael Bourret at Dystel & Goderich and Melanie Madden at Sterling for their never-wavering support of this seemingly never-ending project. I also thank my astronomy colleagues Rachel Bean and Margaret Geller for their reviews of a number of entries in areas relatively far from my own astronomical expertise, as well as Jahangir Mohandesi and Shuang Gao for catching some errors in the first edition of the text. Finally, I owe great thanks to Maureen Bell, who helped enormously in the photo research for this book and who has been phenomenally patient during its long gestation period. In the words of Voltaire, “How infinitesimal is the importance of anything I can do, but how infinitely important it is that I should do it.” 1988 Light Pollution To our ancestors, the night sky was a source of reverence, inspiration, and wonder. On a clear, moonless night it was possible even from the cities to see thousands of stars with the unaided eye, including the grand, sweeping arch of the Milky Way. But the advent of modern civilization, and especially the growth of major cities and urban centers and their widespread use of electricity for artificial illumination, have significantly changed our relationship with the night sky. Rather than thousands of stars, most people in industrialized countries can now usually see only hundreds of stars on a clear night; residents of major cities might be lucky to see just 10 or 20 stars and a slew of airplanes, but certainly not the Milky Way. The night sky has lost its wonder for most people, becoming instead a dull, faintly glowing, and featureless part of the background. The culprit in this nocturnal cosmic dulling is light pollution, the alteration of natural outdoor light levels by artificial light sources. Light pollution obscures fainter stars for people living in cities or suburbs, interferes with astronomical observations of faint sources, and can even have an adverse effect on the health of nocturnal ecosystems. It’s also economically inefficient—the point of lighting a home or building at night is to light the home or building, not to spend money and kilowatts lighting up the night sky. In recognition of the growing global problem of light pollution, in 1988 a group of concerned citizens formed an organization known as the International Dark-Sky Association (IDA), with the mission “to preserve and protect the nighttime environment and our heritage of dark skies through quality outdoor lighting.” IDA now has about five thousand members worldwide who work with city and local governments, businesses, and astronomers to raise awareness about the value of dark skies and to help implement lighting solutions that are more energy efficient and economical and that lead to less light pollution. Despite some notable successes establishing ordinances and building codes that are decreasing light pollution, the effect on astronomy continues to limit the utility of major observatories near large cities (such as the Mount Wilson Observatory, perched above Los Angeles). New telescopes are now usually built in remote deserts or on isolated dark mountaintops to escape the night sky’s growing glow. SEE ALSO Milky Way (c. 13.3 Billion BCE), Stellar Magnitude (c. 150 BCE), First Astronomical Telescopes (1608). [image: ] A map of artificial night sky brightness for part of the Western Hemisphere, from the US Defense Meteorological Satellite Program. The reddest places, mostly in the eastern and western United States, are where light pollution makes the night sky nearly 10 times brighter than the natural sky. 1918 Size of the Milky Way Harlow Shapley (1885–1972), Edwin Hubble (1889–1953) During the decade since Cepheid Variable stars were found in 1908 to be useful as measuring sticks for determining cosmic distances, a number of astronomers worked to determine distances to spiral nebulae, globular clusters, and other enigmatic objects in order to get a handle on whether they were inside or outside of the Milky Way. Indeed, the size of the Milky Way itself was the subject of intense debate, with many astronomers believing that it essentially was the universe, while many others believed it to be just one of many separate “island universes,” as the distant nebulae had been dubbed by the eighteenth-century philosopher Immanuel Kant. The first astronomer to make an experimental estimate of the size of our galaxy was the American astronomer Harlow Shapley, who studied the distribution of globular clusters in the sky. Cepheid variable stars had been used to determine the distance to a nearby globular cluster, so Shapley assumed that they were all the same size and used the changing apparent diameter of other clusters to estimate their distances. By 1918 he had determined that the globular clusters form a sort of halo around the platelike disk of our galaxy, enabling him to estimate that the Milky Way is about 300,000 light-years across, with the Sun not centered but offset by about 50,000 light-years (so much for heliocentrism). This was astonishingly larger than many had thought the galaxy to be, and it convinced Shapley that there were no island universes: the globular clusters and spiral nebulae must all be bordering or within the Milky Way. It turned out that Shapley’s estimate of the size of the Milky Way was about three times too large, mostly because his assumption that all globular clusters are the same size is not really valid. The disk of our galaxy is actually about 100,000 light-years across and about 1,000 light-years thick (with a somewhat thicker central bulge), and the Sun is offset from the center by about 27,000 light-years. Shapley was right about the globular clusters being in and near the Milky Way in a diffuse halo, but he was wrong about the spiral nebulae. As shown by Edwin Hubble and others in subsequent decades, spirals and many other forms of “nebulae” are actually separate galaxies, some like our own, some not, but all are millions to billions of light-years away. SEE ALSO Milky Way (c. 13.3 Billion BCE), Andromeda Sighted (c. 964), Globular Clusters (1664), Cepheid Variables and Standard Candles (1908), Hubble’s Law (1929). [image: ] A Hubble Space Telescope photo of the face-on spiral galaxy NGC 1309. With bright bluish regions of new star formation in its spiral arms, and a central region of older, yellowish stars, this is probably close to what our Milky Way galaxy would look like if we could view it from far above. 1998 International Space Station Early-twentieth-century rocket pioneers such as Konstantin Tsiolkovsky and Robert Goddard were among the first to work out the technical details of orbiting stations and habitats in space. For most of the century, however, the idea of a human outpost in Earth orbit was only realized in science fiction books, magazines, TV shows, and movies. In the 1970s the Soviet Union launched the first of nine long-duration Salyut space research modules, followed up in the 1980s by the orbital assembly of their Mir space station—the first long-duration, multicrew outpost in space. NASA’s plans to launch a US space station (called Freedom) in the 1980s never materialized, due to cost overruns and technical delays. The fall of the Soviet Union in 1991, technical problems with the Mir station, and the high cost of launching and operating space vehicles in general all compelled NASA, Russia, and other space-faring nations to pool resources toward the design and operation of a joint International Space Station (ISS), begun in 1993. The first component of the new ISS was a Russian electrical power, propulsion, and storage module called Zarya, launched into low Earth orbit (about 230 miles [370 kilometers] above the surface) on a Russian Proton rocket in November 1998. The second component, a US docking, airlock, and research module called Unity, was launched and connected to Zarya a few weeks later by the crew of the space shuttle Endeavour. Fifteen more launches of shuttles and Russian Proton and Progress rockets over the next 13 years added additional solar panels, living quarters, laboratories, airlocks, and docking ports. Completed in 2011, the ISS now spans the area of a US football field, with a total mass of more than 920,000 pounds (420,000 kilograms), making it the largest artificial satellite ever constructed. In addition to the United States and Russia, the European, Japanese, and Canadian space agencies are also key partners. The ISS is primarily an international research laboratory designed to take advantage of its unique microgravity, orbital environment to enable space-related medical, engineering, and astrophysical research. But it also serves an important role as an outpost for a permanent human presence in space, a place where we can learn how to live and work there, and how best to prepare to venture further beyond low Earth orbit for deep space voyages of exploration. SEE ALSO Liquid-Fueled Rocketry (1926), Space Shuttle (1981). [image: ] The International Space Station orbits about 190 miles (305 kilometers) above Earth’s surface. Assembly of the space research outpost began in 1998; this 2009 photo taken by the crew of the space shuttle Discovery shows the station’s solar panels, trusses, and pressurized modules. ~1 Billion Earth’s Oceans Evaporate The life cycle of a main sequence star like the Sun is quite predictable. Astronomers in the early twentieth century figured out the basic evolutionary track of stars like the Sun by observing countless numbers of similar stars at different stages of development. By the middle of the century, theories had also been worked out about the insides of the stars, and the nuclear fusion processes that make them shine. And thanks to primitive meteorite researchers and radioactive dating methods, we know the approximate age of the Sun (4.65 billion years) and thus can predict the next milestones of our star’s life. Hydrogen is converted into helium at the enormous temperatures and pressures in the Sun’s core. Over time, then, the Sun’s hydrogen supply is slowly decreasing. To keep its balance of gravitational (inward) versus radiational (outward) pressure—and thus to stay on the main sequence—the Sun’s core is slowly getting hotter. This increases the rate of nuclear fusion in the core, offsetting the effect of the decreasing hydrogen supply and increasing the Sun’s brightness over time. Astronomers estimate that the Sun’s energy output is increasing by about 10 percent per billion years because of the decreasing supply of hydrogen. Such a dramatic change in the Sun’s energy output will have a correspondingly dramatic change on the Earth’s climate. In tens to hundreds of millions of years, it will become warm enough that the oceans will start to permanently evaporate, turning our planet into a steamy world. Scientists further predict that within about a billion years, the slow breakdown of all that atmospheric water by sunlight and the subsequent escape of the liberated hydrogen will turn our planet into a bone-dry, inhospitable desert world. Unfortunately, the future is looking too bright. But wait, it could be even worse: some long-term climate modelers think that our planet will become uninhabitable long before the oceans are completely dried up. As the climate gets hotter, more carbon dioxide will get trapped into carbonate rocks, leaving less for plants to use in photosynthesis. Within maybe a half billion years, then, much of the base of the food chain could collapse, making the biosphere overall unsustainable. It’s not a cheery long-term prognosis, but maybe by then our species (or whatever it has become) will have found a new beautiful blue-water world to call home. SEE ALSO Birth of the Sun (c. 4.6 Billion BCE), Mira Variables (1596), Main Sequence (1910), Nuclear Fusion (1939), End of the Sun (~5.7 Billion). [image: ] An artist’s conception of a “hot Jupiter,” among the most common kind of extrasolar planets initially discovered in the solar neighborhood. A billion years from now, as the Sun continues to mature and grow hotter, the oceans will evaporate, and our planet will become a “hot Earth.” 1961 First Humans in Space Yuri Gagarin (1934–1968), Alan Shepard (1923–1998) The Soviet Union’s successful launch of Sputnik 1 in 1957 marked the beginning of the Space Age, as well as the beginning of an epic geopolitical race for technological, military, and moral superiority with the United States. The Russians had launched the first animal into space—a dog named Laika onboard Sputnik 2—and the US was launching monkeys and chimpanzees, but both governments knew that the next big victory in the space race could only be claimed by launching a person into space. The Soviet human spaceflight program was called Vostok, and, like the original Sputnik effort, it was based on adapting existing intercontinental ballistic missile rockets to accommodate a small passenger capsule. About 20 Soviet Air Force pilots were secretly screened for the privilege of becoming the first cosmonauts (“space sailors” in Russian); the man chosen to be first was Senior Lieutenant Yuri Gagarin. At the same time, the US human spaceflight program, called Project Mercury, was on a parallel track, modifying the Redstone missile to accommodate its small single-passenger capsule. Seven test pilots, from the air force, navy, and marines, were ultimately selected and became instant celebrities, even before their flights. Navy test pilot Alan Shepard was chosen to fly the first Mercury mission. Both Vostok and Mercury had early (unmanned) launch failures; both teams had to demonstrate that their rockets would work with an empty capsule before government leaders would authorize a human-piloted flight. Both teams were neck and neck in the race to launch a person first in early 1961, and once again the Soviets scored an enormous international victory by successfully sending Gagarin into space first, for one orbit of Earth in Vostok 1 on April 12, 1961. Three weeks later, Shepard became the second person—and first American—launched into space with his successful suborbital flight in the Freedom 7 capsule. The Russians had again taken the lead. But America upped the ante shortly after Shepard’s flight, when president John F. Kennedy, in an address to Congress, called for NASA to land a man on the moon before the decade was out. SEE ALSO Liquid-Fueled Rocketry (1926), Sputnik 1 (1957), Earth’s Radiation Belts (1958), First on the Moon (1969). [image: ] Cosmonaut Yuri Gagarin preparing to board his Vostok 1 spacecraft on the morning of April 12, 1961. Seated behind him was his backup, cosmonaut German Titov, who eventually piloted Vostok 2 in August 1961, becoming the second person to orbit the Earth. 1977 Voyager “Grand Tour” Begins As robotic missions to the outer solar system were being contemplated in the late 1960s and early 1970s, mission planners and celestial dynamics experts realized that a fortuitous alignment of Jupiter, Saturn, Uranus, and Neptune would make it possible for a single probe to use gravity assists to potentially fly past all four giant planets in the 1970s and 1980s. This historic “grand tour” concept generated a lot of excitement among NASA researchers, and the idea was eventually implemented in a new pair of missions called Voyager. Voyager 2 was launched first, on August 20, 1977, on a trajectory that would get it to Jupiter in mid-1979, Saturn in mid-1981, Uranus in early 1986, and Neptune in mid-1989. Voyager 1’s launch was on September 5, 1977, on a faster trajectory that would get it to Jupiter in early 1979 and Saturn in late 1980. Voyager 1’s tour included just Jupiter and Saturn flybys because the desire to perform a close flyby of Saturn’s large, thick-atmosphered moon Titan required a course correction that made it impossible to subsequently encounter Uranus and Neptune. The missions of Voyager 1 and Voyager 2 are among the most exciting and successful adventures in the history of space exploration—or any other kind of exploration, for that matter. The probes enabled scientists to make new discoveries about the giant planets’ atmospheres, magnetic fields, ring systems, and their planet-size moons Io, Europa, Ganymede, Callisto, Titan, and Triton (plus many smaller moons). Voyager discoveries enabled even more discoveries at Jupiter and Saturn by the follow-on Galileo and Cassini orbiters, respectively, and provide the data needed for eventual orbital missions to Uranus and Neptune sometime in the future. [image: ] Artist’s conception of the Voyager spacecraft. The long piece extending to the lower right is the 42-foot (13-meter) boom for the magnetic field instrument. Like the Pioneers, each Voyager also has a message in a bottle of sorts—a golden record, containing encoded images, voices, and music—a cosmic time capsule bearing greetings from planet Earth to whoever finds it in the far future. SEE ALSO Jupiter (c. 4.5 Billion BCE), Saturn (c. 4.5 Billion BCE), Uranus (c. 4.5 Billion BCE), Neptune (c. 4.5 Billion BCE), Io (1610), Europa (1610), Ganymede (1610), Callisto (1610), Titan (1655), Saturn Has Rings (1659), Triton (1846), Pioneer 11 at Saturn (1979), Voyager 2 at Uranus (1986), Voyager 2 at Neptune (1989). [image: ] Montage of a Voyager Grand Tour trajectory diagram, superimposed on greatest-hits Voyager images of the giant planets. 1877 Phobos Asaph Hall (1829–1907) After the initial discovery of Deimos on August 11, 1877, the American astronomer Asaph Hall continued to scan the skies around Mars using the 26-inch (66-centimeter) Alvan Clark and Sons refractor of the US Naval Observatory in Washington, D.C. Hall’s efforts were frequently interrupted by fog or bad weather, but his persistence paid off. Not only was he able to confirm that Deimos was in orbit around Mars, but on August 17–18 he was able to peer even closer in to Mars and discover a second faint, small moon, which was eventually named Phobos. Hall and other astronomers quickly realized that Phobos orbits closer to Mars than any other known moon relative to its primary planet—so close, in fact, that with an orbital period of just over 7.5 hours it actually spins around Mars faster than Mars itself rotates on its axis. This meant that an observer on the surface of Mars would see Phobos rise in the west and set in the east, even though it orbits Mars in the same direction that Mars spins. [image: ] The silhouette of Phobos as it eclipses (transits) the Sun, photographed from the NASA Mars rover Opportunity on January 21, 2006. Modern space missions have been able to reveal more about this tiny, slightly reddish, asteroid-like world, but much still remains a mystery. Phobos is irregular in size—17 × 13.7 × 11 miles (27 × 22 × 18 kilometers)—and has a density near 1.9 grams per cubic centimeter, suggesting a porous, rocky, chondritic composition perhaps similar to Deimos’s. The surface is heavily cratered, and one large crater (Stickney, named for Hall’s wife, Angeline Stickney) is surrounded by a series of deep grooves that cover much of the surface. Is Phobos an asteroid that Mars somehow captured from the nearby main asteroid belt? Or is it perhaps a piece of Mars created from ejected fragments of a giant impact into the Red Planet? Two Soviet robotic probes were sent in 1988 to land on Phobos, but they failed—one en route and the other after attaining Mars orbit for only a few months. A new Russian robotic mission was launched in 2011, but it also failed, shortly after launch. New mission ideas are in the works, but in the meantime it seems that Phobos will continue to guard its secrets. SEE ALSO Mars (c. 4.5 Billion BCE), Meteorites Come from Space (1794), Deimos (1877). [image: ] The reddish Martian moon Phobos, 13 miles (21 kilometers) wide, photographed on March 23, 2008, by the NASA Mars Reconnaissance Orbiter. 2010 Comet Hartley-2 After completing the successful 2005 mission to crash a projectile into comet Tempel-1, engineers responsible for NASA’s Deep Impact spacecraft realized that there was enough fuel left onboard to operate the probe as a remote observatory for characterizing extrasolar planets using the transit method (which was also being used by the Kepler mission), and to potentially encounter a second comet nucleus as well. Deep Impact was thus recast as a new mission called EPOXI, for Extrasolar Planet Observation and Deep Impact Extended Investigation. After three Earth flybys to provide gravity-assist trajectory tweaks, EPOXI was targeted to fly closely past the nucleus of the Earth-approaching comet Hartley-2 in November 2010. Discovered in 1986 by Australian astronomer Malcolm Hartley, Hartley-2 is a short-period comet that travels in an orbit between about 1.1 astronomical units (AU) and 5.9 AU every six and a half years. Short-period comets are further divided into Jupiter family comets, such as Hartley-2, with periods less than about 20 years, and Halley family comets (named after their most famous member), with periods from about 20 to 200 years. Many short-period comets are suspected to have once been long-period comets that had their orbits dramatically changed by a close encounter with one of the giant planets. Hartley-2, for example, may be a relatively primitive object from the Öpik-Oort Cloud that encountered Jupiter relatively recently. The EPOXI data from the flyby support the idea of a primitive, outer-solar-system origin for Hartley-2. Powerful jets of ice, gas, and dust were seen spewing from the comet’s 1.4-mile (2.3-kilometer) peanut-shaped nucleus in the dramatic EPOXI images, and Spectroscopic measurements showed that the ice is dominated by carbon dioxide (dry ice) rather than water. Initial studies also point to the possible presence of some organic molecules, such as methanol, in Hartley-2’s jets and extended atmosphere. At the rate that the comet is currently losing mass from the jets as well as from sublimation (evaporation of ice) of the surface in general, scientists predict that it might only survive for about another 100 orbits (700 years) or so before breaking up into smaller pieces. Thus it is likely that this little lump of primitive ices from the original solar nebula is indeed a recent interloper in the inner solar system. SEE ALSO Solar Nebula (c. 5 Billion BCE), Halley’s Comet (1682), Tunguska Explosion (1908), Öpik-Oort Cloud (1932), Comet SL-9 Slams Into Jupiter (1994), “Great Comet” Hale-Bopp (1997), Stardust Encounters Wild-2 (2004), Deep Impact: Tempel-1 (2005), Kepler Mission (2009). [image: ] The nucleus of comet Hartley-2 photographed by the NASA EPOXI spacecraft during its November 4, 2010, flyby. Powerful jets of water vapor, other cometary gases, and dust are escaping from the comet’s interior. c. 1500 Early Calculus Mādhavan of Sangamagrāmam (c. 1350–c.1425), Nīlakantha Somāyaji (1444–1544) Astronomical research in India through the Middle Ages was initially based on the early findings and writings of Aryabhata and other mathematicians and astronomers; it was ultimately expanded by the creation of dedicated research and teaching groups like the Kerala school of astronomy and mathematics, founded in the fourteenth century by the mathematician Mādhavan of Sangamagrāmam. Mādhavan and subsequent Kerala mathematicians like Nīlakantha Somāyaji developed mathematical methods of estimating the motions of the planets based initially on geometry and trigonometry and later on newly developed techniques for modeling complex curves and mathematical shapes using combinations of functions. Among these shapes were parabolas, hyperbolas, and ellipses; their work on ellipses proved especially applicable to astronomy because they were able to show that Aryabhata’s earlier conjecture was correct: the paths of the planets could be described by elliptical orbits. The new mathematical methods developed at Kerala that focused on series of functions were early versions of calculus, predating the European development of calculus some 200 years later by scientists like Isaac Newton. Nīlakantha’s work Aryabhatiyabhasya (a commentary on Aryabhata’s Aryabhatiya), published around 1500, further demonstrated that a rotating Earth and a partially heliocentric solar system provided a more accurate way of fitting the planetary orbits. In his model, Mercury, Venus, Mars, Jupiter, and Saturn all orbited the Sun, but the Sun orbited Earth. A similar model was adopted by the sixteenth-century Danish astronomer Tycho Brahe, and some aspects of Nīlakantha’s model are also consistent with the fully heliocentric cosmology proposed in 1543 by Polish astronomer Nicolaus Copernicus. The contributions of the Kerala school, and perhaps of Indian mathematics and astronomy in general, may have previously been underappreciated in the West. It seems clear now that they should be counted among the “shoulders of giants” that supported the later discoveries of Copernicus, Newton, and others. SEE ALSO Earth Is Round! (c. 500 BCE), Aryabhatiya (c. 500), Copernicus’s De Revolutionibus (1543), Brahe’s “Nova Stella” (1572). [image: ] Planetary orbital calculations by mathematicians from the Kerala school in southern India, active between the fourteenth and sixteenth centuries, fit a heliocentric model for the solar system. This figure shows some examples from modern Indian physicists reconstructing the geometry used by Kerala school astronomers. 1936 Elliptical Galaxies Edwin Hubble (1889–1953) The work of the astronomers Harlow Shapley, Vesto Slipher, Edwin Hubble, and others on determining the scale of the galaxy and collecting spectroscopic data for large numbers of spiral nebulae during the first few decades of the twentieth century eventually led to the realization that they are other galaxies—other Milky Ways—each harboring hundreds of billions of stars of their own. As more galaxies were identified and studied, and as it became clear that they were not all the same, astronomers naturally sought to classify them into distinct categories, as they had done for the stars. As a leading observer of galaxies and with access to some of the best telescopic facilities in the world, Hubble was in a particularly unique position to take the lead on galaxy classification. And lead he did. In a series of papers and lectures, eventually compiled into a landmark 1936 book called The Realm of the Nebulae, Hubble outlined a scheme for the morphologic (shape, size, brightness) classification of extragalactic nebulae, now called the Hubble sequence. On one end of the Hubble sequence were the elliptical nebulae, now known as elliptical galaxies. Ellipticals are one of three main classes, the others being spiral galaxies, like our own Milky Way, and lenticular (lens-shaped) galaxies, intermediate in form between ellipticals and spirals. [image: ] Edwin Hubble’s original “tuning fork” galaxy classification scheme diagram, from The Realm of the Nebulae. Elliptical galaxies, as the name implies, are ellipsoidal to spherical in shape, and vary smoothly in brightness