By Edward S. Goldstein
Think about how you first learned the names of the planets closest to Earth and the phrase, “My very educated mother just served us nine pizzas,” may come to mind. What you learned, though, may not be correct – because new discoveries are reshaping ideas about our solar system.
Planetary family photo - A montage of images taken by NASA spacecraft of the planets. From top to bottom: Mercury (Mariner 10), Venus (Magellan), Earth (Galileo) (and moon), Mars (Viking), and Jupiter, Saturn, Uranus and Neptune (Voyager).
To appreciate this point, some mental time traveling is in order. “If you lived in the 1850s and you pulled out a library book on the solar system it told you about 23 planets, because they were first discovering objects in the asteroid belt that were initially labeled as planets,” noted James Green, NASA’s Planetary Science program director. Later, astronomers recognized that many of these bodies were too small to be called planets, leaving the solar system with just the nine planets with which we are so familiar. But now, said Alan Stern, NASA’s associate administrator for Space Science, if you add the planet-sized large asteroids and the recently discovered planet-sized Kuiper Belt objects, and predicted numbers of planet-sized objects in the still more distant Oort Cloud, the number of bodies called "planets" may total more like 900 than nine.
Whatever number of objects truly qualify as planets, this much is certain: What humanity knows today about the fascinating bodies in our corner of the universe is largely due to ingenious robotic explorers – orbiters, probes and rovers – ably guided by NASA scientists and engineers. NASA’s drive to explore the planets has led to moments of high drama – recall the anxious moments before the Mars Exploration Rover Spirit safely bounced to a successful landing on the Red Planet’s surface in January 2004 – and of sublime moments of awe every time mysterious, dynamic worlds were seen up close for the first time.
“Our level of knowledge of the solar system in 1957 was so primitive by today’s standards that it is almost incomprehensible how far it has come,” said Stern. “It’s more than just night and day. It is as if you grew up on a desert island and somebody took you on a world tour.” Added Stern, “We did not know, for example, what the appearances of most of the planets and their satellites were. We didn’t know that worlds in the outer solar system would be diverse. We didn’t realize that there are numerous oceans in the solar system with most of them on the inside of planets. We didn’t know that the third zone of the solar system, the Kuiper Belts, exists. We didn’t know what comets were. We didn’t know that ring systems are ubiquitous and that they come and go with time. We did not know that Venus is a toxic wasteland, 700 degrees Kelvin. We didn’t know the importance of giant impacts. And we didn’t understand that most of the solar system’s planets are in fact dwarf planets – like Pluto. We basically didn’t know anything.” Louis Friedman, executive director of the Planetary Society, a former NASA Jet Propulsion Laboratory (JPL) project manager, observed, “It’s very much like it must have been like to live in Europe in the 16th and 17th centuries as people began to get an understanding of the whole Earth and the different exotic countries that were beyond Europe’s borders around the globe. We very much are in the same sort of time period in which our simplistic views now of an Earth that existed alone in the universe as the only world that we knew of, and the only place that could have been imagined as habitable, a special little zone has come into question. Now we see a solar system that is chock full of worlds, with a liquid ocean on Jupiter’s moon Europa, a hydrocarbon atmosphere surrounding Saturn’s moon Titan…” Added Friedman, “The discoveries of Venus and Mars particularly, but other planets as well – have provided the single biggest benefit of the space program. They have given us a new understanding of our home planet. We never heard of a runaway greenhouse [effect] before we went to Venus. Now we know it is one of the greatest threats to our planet. We never knew of the effect of climate change from such things as aerosols and dust particles in the atmosphere until we looked at other planets. Those are incredibly important problems to us now.”
Sister planet - Venus, blanketed by a thick veil of clouds made up of sulfuric acid and water vapor, imaged by Mariner 10 in 1974.
This bounty of knowledge was non-existent five years into the Space Age, when a Newsweek cover story (Oct. 8, 1962) featured the known planets and their orbits around the sun. At that time none of them had yet to be seen up close by robotic emissaries from Earth. The NASA JPL team, led by New Zealand-born JPL director William Pickering, under orders since 1959 from NASA Headquarters to give priority to lunar impacters (Project Ranger), was about to change this state of affairs. On Dec. 14, 1962, the Mariner 2 spacecraft flew by Venus. “The United States had at least [last] beaten the Soviet Union in scoring a spectacular and impressive first in the space race,” wrote The New York Times. To honor the achievement, Pickering was named Grand Marshall of the 1963 Tournament of Roses parade. He then set his sights on Mars, which Mariner 4 flew by on July 14, 1964. Pickering’s long-term goal was to mount a mission involving an “interplanetary vehicle, which could give an answer to the question of life on Mars.” Following these early triumphs NASA has led the way in advancing knowledge about every planet from Mercury (recently explored by the MESSENGER spacecraft on Jan. 14, 2008) to Neptune and multiple objects in between. Now with the New Horizons robotic spacecraft on the way to a 2015 close flyby of Pluto and its system of moons, we will soon learn about completely new kinds of bodies in the Kuiper Belt.
While far away, the Kuiper Belt objects may hold vital clues to how life first formed on Earth, and therefore may be one of the most enticing targets for future exploration. James Green pointed out that some scientists believe and some computer models suggest that in Earth’s infancy, our home planet was bombarded by material from the Kuiper Belt, composed of frozen volatiles such as methane, ammonia and water, which may have “rained in our upper atmosphere, bringing a significant amount of water … maybe enough to populate the oceans. These objects may have been fundamental to bringing water to Earth at a stage where life could evolve.” While this issue is still a subject of debate in the scientific community, some scientists believe that discovering the oldest ices in our solar system may be the key to understanding the birth of our own oceans, where life as we know it may have been initiated nearly 4 billion years ago.
As a result of up close planetary exploration, and building on the work of Earth-based and orbiting telescopes, we now know that Venus, Mars, Jupiter, Saturn, Uranus and Neptune have significant and dynamic atmospheres. We have found evidence of the existence of a “frozen in stone” or “fossil” river delta 25 degrees south of the Martian equator dating from a time when liquid water persisted on the Martian surface, as well as evidence for shallow salty seas and lakes in the Red Planet’s history. NASA spacecraft have captured stunning pictures of volcanic eruptions on Jupiter’s moon Io, a geyser spewing water ice from the south pole of Saturn’s moon Enceladus, and cryovolcanic geysers on Neptune’s moon Triton. Also proving to be inviting targets for future missions to search for evidence of biology beyond Earth are Jupiter’s moon Europa, with a smooth, bright, shiny surface that might well be a water ice mixture covering a potentially liquid ocean, and Saturn’s moon Titan, which may have hydrocarbon seas or lakes, similar to those that may have existed on the early Earth. “There is a revolution going on in planetary science that is aided tremendously by NASA’s missions and discoveries,” said Green. “We’re following up on these discoveries and we’re making gratifying progress.”
Reaching Out to Venus and Mars
Under salmon skies - The Viking 2 spacecraft’s view in 1976 of the boulder-strewn field of red rocks on Mars’ Utopian Plain.
Solar system exploration did not take place by magic. It required visionary leadership, strong-willed management, and persevering execution. The use of a spacecraft to explore a planet actually predated NASA. The first American satellite, Explorer 1 (Jan. 31, 1958) contained James Van Allen’s micrometeorite detector and cosmic ray experiment which discovered the region of charged particle radiation trapped by Earth’s magnetic field.
With the help of Van Allen (who consistently advocated that NASA place greater emphasis on robotic exploration instead of making human space flight its top priority) and other scientists, the foundation for using spacecraft-borne instruments to explore other planets was laid in the 1960s. In addition to JPL, NASA’s Goddard Space Flight Center, Ames and Langley Research Centers have built and managed sophisticated planetary missions. Over the years, NASA’s planetary teams have developed ingenious methods to use the gravitational influence of planets to slingshot spacecraft to more distant targets, and to obtain reams of data across millions of miles once those targets are reached, often within seconds of their predicted arrival time.
Venus was the prize in the first interplanetary race between the United States and the Soviet Union. Outside of the prestige implications of getting there first, scientists in both countries saw the attraction of studying a planet shrouded in a mysterious cloak of clouds permanently hiding the surface from view. On Dec. 14, 1962, the Mariner 2 spacecraft skirted by the planet at a distance of 21,641 miles. It probed the clouds, estimated planetary temperatures, measured the charged particle environment, and looked for a magnetic field similar to Earth’s magnetosphere (but found none). In 1967, Mariner 5 began investigating Venus’ massive atmosphere. The flight demonstrated that Venus was a very inhospitable place for life to exist, determining that the entire planet’s surface was a fairly uniform 800 degrees Fahrenheit, thus ending the probability that life – at least as humans understood it – existed on Venus. During this heady time a young astronomer named Carl Sagan correctly speculated that Venus’ high temperatures could be explained by its thick carbon dioxide-filled atmosphere causing a runaway “greenhouse effect.”
For ages the object of scientific inquiry and literary speculation (e.g. H.G. Wells’ War of the Worlds), Mars was the next target for NASA’s attention. In July 1965 Mariner 4 flew by Mars, taking 21 close-up pictures with its advanced digital vision camera system, revealing a Mars (in the southern highlands) that was lunar-like, rather than the Earth-like world that had been imagined because of Earth-based telescopic observations. Mariners 6 and 7, launched in February and March 1969, each passed Mars in August 1969, studying its atmosphere and surface to lay the groundwork for an eventual landing on the planet. Their pictures further verified the moon-like appearance of Mars and gave no hint that the Martian landscape was hospitable to life. Mariners 6 and 7 also showed there were linear features on its surface resembling tectonic cracks, but no canals, as had been the subject of previous speculation.
Close encounter with Io - During its closest approach (81,000 miles) to Jupiter’s moon Io on July 3, 1999, the Galileo spacecraft acquired this high resolution image.
Mariner 9, NASA’s first planetary orbiting mission, started circling the Red Planet in the Martian summer of 1971 only to find a rapidly spreading a near global dust storm which eventually obscured the entire planet’s surface. Mariner 9’s first pictures showed a featureless disk, marred only by a group of black spots in a region known as Nix Olympia (Snows of Olympus). After the dust storm finally subsided months later, the four spots emerged out of the dust “veil” to become the topographic expression of giant extinct volcanoes dwarfing anything known on the Earth. Olympus Mons, the largest of the four, was 400+ miles across at the base with a summit crater 45 miles wide, similar to smaller “calderas” on large terrestrial volcanoes such as Hawaii’s Mauna Loa. Rising 20 miles from the surrounding plains, Olympus Mons was found to be three times the height of Mount Everest. Later pictures from Mariner 9’s digital camera showed a canyon, Valles Marineris, 2,500 miles long and approximately 3.5 miles deep (on average). As the dust further settled, meandering “river” channels were spotted indicating that, at some distant time, fluid had flowed on Mars’ surface. Mariner 9 further found evidence of wind and water erosion and deposition, weather fronts, and fogs. Suddenly, Mars was once again an object of fascination. Mariner 9 also documented the seasonal carbon dioxide “frost” that forms in the Martian high-latitudes, and many other enigmatic features, providing strong impetus for future robotic landings.
In America’s bicentennial year, NASA’s first Mars landers, Viking I and II, provided important scientific data about Mars’ surface and atmospheric composition. Significantly, the Viking landers conducted biological experiments, designed by teams led by project scientist Gerald Soffen, to detect extant or dormant life in the Martian soil if it existed. While one experiment produced results that to a few suggested the detection of life-related chemical (metabolic) processes, all other life-related experiments failed to reveal any evidence of Earth-like life, including organic molecules in the soil. Consequently, most scientists became convinced that the interesting results were likely caused by nonbiological chemical reactions resulting from highly oxidizing soil conditions that were discovered (and unanticipated). One of the great prizes from Viking was a stunning photograph of the sun setting through the planet’s salmon colored atmosphere. While the Viking landers revolutionized our understanding of the martian near-surface and its hazards, the twin Viking orbiters produced the first global, remote-sensing datasets for the Red Planet, including a spectacular digital image mosaic that kept the Mars science community moving forward for 20 years.
Although success is known to have a thousand fathers, a few individuals stand out in the Viking saga, and other triumphs in NASA’s planetary exploration during this period. Noel Hinners, serving as associate administrator for the agency’s Office of Space Science and Applications from 1974 to 1979, and from 1982 to 1987 as director of the Goddard Space Flight Center, is credited by current Goddard chief scientist James Garvin with allowing the agency to “leverage the capabilities of Apollo to enable a heyday of robotic planetary exploration. … Hinners influenced a generation of planetary exploration missions from Viking to Magellan and on to the first ever sample return mission from the solar wind [Genesis] and a comet [Stardust].” Thomas A. Mutch was the leader of the Viking imaging team, whose love of adventure led to his untimely death in a Himalayan mountain climbing accident in 1980. In his honor NASA renamed the Viking 1 Lander in Chryse Planitia the Thomas Mutch Memorial Station. The Viking 2 Lander was also renamed, after the late Gerald Soffen. Another Mars exploration hero is still-active Michael Malin, planetary exploration’s only MacArthur Fellow, whose bold discoveries about Martian landscapes beginning in the 1970s revolutionized thinking about the Red Planet, culminating in his hypothesis that liquid water has been flowing at the Martian surface in the modern era from years’ worth of orbiting camera observations.
The Voyagers and the Grand Tours
A month away - Thirty-six days and 12.4 million miles from reaching Saturn, the Cassini spacecraft returned this image of the planet and its rings on May 16, 2004.
The farthest traveling man-made objects from Earth are the Voyager 1 and 2 spacecraft, launched in 1977 to take advantage of a rare alignment in the orbits of Jupiter, Saturn, Uranus and Neptune. This alignment allowed both spacecraft to use the gravity of each of the four planets to boost themselves on toward the next planet to allow flyby reconnaissance of the gas giants and their enigmatic icy moons. Following the 1972 through 1974 initial reconnaissance of Jupiter by Pioneers 10 and 11, the Voyagers’ Grand Tour collected a wealth of information about all the giant planets, discovering moons, rings (including a ring around Jupiter), atmospheric compositions and magnetic fields. “I had no idea there would be so much discovery on this mission,” said Voyager project scientist and later JPL Director Edward C. Stone. “We knew there was going to be a lot of discovery, but what we learned was that nature is much more prolific than our imagination. What we discovered is a remarkable diversity of objects in the solar system.”
Now heading toward interstellar space, Voyager 1 as of Jan. 30, 2008, about 9.8 billion miles from the sun, is now in the heliosheath, the final boundary of the solar system. Voyager 1 is expected to cross into interstellar space some time around 2014. The Voyagers continue to provide data, running on radiactive generators that produce less than 300 watts of power, the amount needed to light up a bright light bulb. Both spacecraft carry golden discs (primitive DVDs) created by a team headed by Carl Sagan. They contain sounds and images portraying the diversity of life and culture on Earth. Perhaps 40,000 years from now, when the Voyager spacecraft come within the vicinity of nearby stars, these records will be discovered and played by intelligent alien beings, if such exist. During the heyday of the Voyagers, on the television program Saturday Night Live, comedian Steve Martin breathlessly announced that extraterrestrials had recovered one record and sent back the message, “Send more Chuck Berry.”
The Dawn of a New Age in Space Science
In the late 1980s a new era of planetary exploration began, leading to new discoveries and prompting new scientific debates. The Magellan mission to Venus provided significant scientific data and raised new questions about our sister world. According to James Garvin, chief scientist at Goddard Space Flight Center, Magellan’s findings “of dramatic volcanic landscapes associated with volcanoes as well as exotic features related to Venusian ‘continents,’ with their own unique deformation histories … attest to a much more geologically active planet than ever thought before, perhaps due to massive amounts of volcanism, a potential ongoing source of energy and even water (as vapor today). Magellan’s discoveries have drawn us all to the urge to visit Venus’ surface to learn about its chemical history and whether any signature of the history of water as ancient oceans can be discovered.”
The Galileo mission, launched in 1989 was the first spacecraft to orbit Jupiter. Prior to reaching its destination in 1995, Galileo had become a source of great concern to both NASA and public officials because not all of its systems were working properly (i.e., its large high-gain telecommunications antenna failed to unfurl as intended), but once it arrived at Jupiter and carried on its mission through 2003, it returned significant scientific data, including evidence of sub-crustal oceans on Europa, Jupiter’s large ice-rock moon. Among Galileo’s other successes was capturing imagery of comet Shoemaker-Levy 9’s collision with Jupiter in 1994, discovering a turbulent Jovian atmosphere, complete with lightning and thunderstorms a thousand times the size of those on Earth, and conducting close-up inspections of the Jovian moons Ganymede, Callisto and Io. While passing by the latter moon, Galileo observed eruptions of Io’s Loki volcano, the largest and most powerful currently known in the solar system. Galileo also sent a probe into Jupiter’s atmosphere, whose findings, wrote historian Michael Melzer, “made it necessary for scientists to revisit many of their beliefs about the formation and evolution of our solar system’s giant gaseous planets. Measurements of atmospheric composition, wind velocities, temperatures, cloud characteristics, electrical storms and elemental and molecular abundances painted a very different picture of Jupiter from what was expected.”
Distant sunset - The Mars Exploration Rover Spirit’s view of the sun (appearing 2/3 the size we see on Earth) setting below the rim of Gusev Crater on May 19, 2005, Spirit’s 489th Martian day, or sol.
Following a 21 year hiatus, the surface of Mars was visited again on the 4th of July, 1997, with the arrival of the Mars Pathfinder. Its small, 23-pound robotic rover, named Sojourner, departed the main lander with a remote imaging and chemical analysis system, while the Pathfinder lander continued to record weather patterns, atmospheric opacity, and the chemical composition of rocks washed down into the Ares Vallis flood plain, an ancient outflow channel in Mars’ northern hemisphere. After 30 days of operations on the surface, Pathfinder had captured far more data on the atmosphere, weather, and geology of Mars than scientists had expected. In all, the Pathfinder mission returned more than 1.2 gigabits (1.2 billion bits) of data and over 10,000 tantalizing pictures of the Martian landscape. Pathfinder also demonstrated that a new approach to landing payloads on Mars, via terminal-descent airbags, was cost-effective, even for sensitive landed payloads. Finally, Pathfinder proved to be one of the most followed events on the popular new communications medium known as the Internet.
A new portrait of the Martian environment began to emerge in the years since Pathfinder as a consequence of a succession of spacecraft and their new data relating to the dynamic geology, weather patterns, ice caps, and the chemical composition of the Martian surface. Indeed, Mars Global Surveyor, which began orbiting and mapping the planet’s surface in March 1998, imaged gullies on cliffs and crater walls and suggested that liquid water has seeped onto the surface in the geologically recent past to form these now ubiquitous features. Such gullies were unknown prior to the imaging observations of the Global Surveyor, and they reawakened interest in the biological potential of Mars. This was amplified by Mars Odyssey 2001, another NASA orbiter with a different set of sensitive instruments, which found that hydrogen-rich regions are located in high-latitude areas (nearly 25 percent of the planet), where water ice should be stable. The relationship between high hydrogen content with regions of predicted ice stability led scientists to conclude that the hydrogen is, in fact, in the form of water ice as a dominant constituent in the upper-most surface. Only time and more research will tell if these findings demonstrate that Mars harbors a massive water ice reservoir, partially hidden beneath the dust and soils of the high-latitude regions on the planet, as a potential refuge for modern era life.
More recently, the twin Mars Exploration Rovers Spirit and Opportunity that landed in January 2004 have greatly enhanced knowledge of the Red Planet with surface missions that have vastly exceeded their 90 day expected lifetime. Among the rovers’ most significant findings were the photographs and data obtained by Opportunity in February and March 2004 of a stratified pattern with cross bedding in the rocks of an outcrop inside a crater in Meridiani Planum, suggesting that water previously flowed there, perhaps at the margin of a lake or sea. The uneven distribution of the chemical elements chlorine and bromine discovered by Opportunity also suggests that the site was once the shoreline of an ancient salty sea, which subsequently evaporated or sublimed. With Opportunity, which is the first rover to traverse more than seven miles on the Martian surface, now having ventured into the half-mile diameter Victoria Crater, scientists are hopeful for new discoveries before this historic mission ends. The Mars Exploration Rovers also provided dramatic evidence of how advanced surface exploration prior to the arrival of human explorers can cost-effectively provide the means for opening the exciting new Martian frontier, now being further unveiled by means of the unprecedented orbital reconnaissance of the Mars Reconnaissance Orbiter.
Representing the international character of many NASA planetary missions, Cassini-Huygens, a joint effort of NASA, the European Space Agency and the Italian Space Agency, has also proved to be an incredible success. Cassini is the first spacecraft to orbit Saturn, beginning July 1, 2004, and to send a probe (Huygens) to the surface of Saturn’s moon Titan on Jan. 14, 2005. But even before its Saturnian encounter, the Cassini mission advanced science by finding individual storm cells of upwelling bright-white clouds in dark “belts” in Jupiter’s atmosphere, and by conducting a radio signal experiment on Oct. 10, 2003, that supported Einstein’s theory of general relativity. At Saturn in 2005, Cassini discovered three new moons (Methone, Pallene and Polydeuces), observed water ice geysers erupting from the south pole of the icy moon Enceladus, obtained images appearing to show lakes of liquid hydrocarbon (such as methane and ethane) in Titan’s northern latitudes, and discovered a storm at the south pole of Saturn with a distinct eye wall. Cassini, like Galileo at Jupiter, has demonstrated that icy moons orbiting gas giant planets are potential refuges of life, and attractive destinations for a new era of robotic planetary exploration. Likewise, the Mars Reconnaissance Orbiter (MRO) is witnessing Mars at scales previously only accessible from more complex landed missions, thereby paving the way for a bold return to the Martian surface by the first mobile analytical laboratory in history, the Mars Science Laboratory (MSL), slated for launch in 2009. This mission will be vastly enhanced by the assistance of MRO observations, and will be guided by a suite of supporting sensors to bring us all along for the ride using high-resolution movies.
Genesis and Stardust
Priceless samples - Donald Brownlee, Stardust principal investigator with the University of Washington, flashes a victory sign after the successful arrival of Stardust material. Also pictured are JSC’s Mike Zolensky (left), curator and co-investigator for the project; Friedrich Horz, JSC, and Peter Tsou, JPL.
Two of NASA’s boldest missions of late, Genesis and Stardust, returned to Earth samples of the solar wind and the comet Wild 2 respectively. Genesis, launched Aug. 8, 2001, to capture particles of the solar wind on ultra-pure wafers of gold, sapphire, silicon and diamond, returned home to Earth to an apparent ignominious end in the Utah desert on Sept. 8, 2004. A parachute failure caused the sample return capsule to crash into the ground at 193 miles per hour. Fortunately, the Genesis sample return capsule was designed to survive such a landing, and remaining shards were sent to NASA’s Johnson Space Center for study. Scientists studying the Genesis material have already made important findings, using the solar wind to better understand the evolution of the moon. The Stardust mission to collect a cosmic dust sample was launched two years before Genesis, but took two more years to return to Earth. After its encounter with the comet Wild 2 on Jan. 2, 2004, the Stardust sample return capsule had a successful soft landing in Utah’s Great Salt Lake desert on Jan. 15, 2006. Stardust Principal Investigator Don Brownlee reported that the returned materials contain many surprises, including the finding that the comet’s rocky materials “formed in the hot inner regions of the young solar system and were then transported all the way to beyond the orbit of Neptune. … The inner solar system can be thought of as a factory producing rocky materials that were distributed outwards to all the bodies and regions of the solar system.”
Looking Toward the Future
Scanning the near horizon, NASA’s planetary program director James Green sees the potential for incredible missions that will provide us “a better understanding of the origin and evolution of our solar system and understanding of the context that has allowed life to exist here on the Earth but not that we know of on Mars and on Venus, and why life might occur on places like Europa.” Green points to a number of ongoing and upcoming missions that are “tremendously exciting, having enormous scientific potential.” The international Phoenix mission will land in Mars’ northern high latitudes on May 25, 2008, and use a robotic arm to search for a history of water or the existence of subsurface liquid water. In 2011 the MESSENGER spacecraft will orbit Mercury to characterize the planet’s chemical composition and geological history. The Dawn mission will explore the large but dissimilar main-belt asteroids Vesta (between 2011 and 2013) and Ceres (2015). Pluto, and its moons Charon, Nix, and Hydra, and potentially one or more Kuiper Belt objects will be observed for the first time by the New Horizons spacecraft in 2015. (In February 2007, New Horizons flew by Jupiter and observed the spectacular continuous eruption of the gigantic Tvashtar volcano on Jupiter’s moon Io.)
Slated to be launched in upcoming years are the Mars Science Laboratory, a mission scheduled that will land a long-duration mobile analytical lab on the Red Planet to study Martian surface chemistry (search for reduced carbon) in a region that may have been a habitat for ancient life, and pave the way for a future sample return mission; and Juno, a mission that will investigate Jupiter’s interior, aurora, and magnetosphere and search for clues about the formation of planets in the early solar system.
To NASA’s planetary scientists, there is no end of targets to explore in our fascinating corner of the universe. “We’re just beginning,” noted Stone. “We’ve landed only a few places on Mars. Can you imagine believing you understand the Earth from landing a few places on Earth?” Looking to the future, Stern said, “I fully expect that we will have a vigorous program with more and more autonomous vehicles. I hope that we will be able to put orbiters around each of the giant planets, return samples from Mars on multiple occasions, return samples from comets, have surface analysis of Mercury and Venus, and perhaps more importantly – begin the human exploration of the planets.” Such is within the realm of possibilities as NASA moves into its next 50 years of exploration.