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Research Scientist, NASA Ames Research Center
What do you think are the most significant events that have occurred in the past fifty years of robotic planetary exploration? Why?
1) 1967: Surveyor 1 -- Surveyor 1 was the first soft landing on any extraterrestrial object. Surveyor 1 sent back 11,350 images of the Moon and other valuable information about the mechanical properties of the surface and lunar environment that was used to design the Apollo lunar landers that later delivered astronauts to the Moon's surface. The pictures of the grainy, rocky lunar surface, and the depressions of the footpad, made the Moon a real object for the first time. Prior to Surveyor, serious scientists had asserted that fine dust covered the surface to such great depths that astronauts might just sink out of sight!
2) 1970: Soviet Sample Return and Lunar Rover -- Perhaps because of the Cold War, these achievements never made big headway with the public (nor do I remember them well). However, they were impressive. The Luna 16 spacecraft collected 105 grams of lunar soil, deposited them in a container, and launched them back to Earth in the world's first -- and still one of the few -- robotic sample returns. Just a month later, Russia's Lunokhod 1 rover became the first unmanned wheeled vehicle on another world. Lunokhod 1 explored the Moon for 322 Earth days.
3) 1976: Viking -- NASA's Viking 1 and 2 landers were the first spacecraft to touch down safely on Mars. Their images of the Martian surface, with its rocks and gravel, were the very first to tell us that Mars looked a lot like home. We knew so little about Mars that the science team presented the first lander images showing a blue sky. It was not until Jim Pollack carefully looked at the calibration that we all learned that the sky was its now-familiar shade of red.
I got up at four in the morning to go in to Ames to watch the first scans slowly appear on the screen in the auditorium, which was nearly full! The soil experiments on VL1 and 2 were profoundly sophisticated, carrying soil samples into incubation chambers. Some of the experiments suggested the presence of life, and it was not until months or even years later that it became better understood that it was the unexpected oxidation state of the Mars soil that was causing the exciting results, and not carbon of any sort. Moreover, Viking orbiter images were of unparalleled detail and clarity (once the dust storm cleared), showing new details on river valleys, layered polar caps, morning valley fog and spiral storm clouds, and other evidence for an active atmosphere and hydrosphere at one time, overturning the impression of a dead planet left by Mariner 9's limited sample.
4) 1979 - 1989: Voyager Exploration of the Outer Planets -- For a cornucopia of sheer eye-opening wonder and revelation of the unexpected, it is hard to match Voyager. From its stunning time lapse movies of Jupiter's boiling and churning atmosphere; to the volcanoes of Io and the pristine, fractured surfaces of the Galilean moons; to the stunning filigreed and (in stretched images) rainbow-colored rings of Saturn and the dense atmosphere of Titan; to the more serene ice giants Uranus and Neptune, and their never-before-seen tinsel-like ring systems, Voyager was seeing worlds for the first time that humanity had never seen as anything but points of light, seeing structure where we expected none. The abundance of the exploration -- four giant planets and their moons and rings -- and the fact that Jupiter and Saturn were both seen in two different geometries, as well as my own involvement and ability to work with many of the founding superstars of planetary science, made Voyager my most intense and unforgettable personal experience.
5) 1986: Halley's Comet Encounter -- A flotilla of European, Russian and Japanese spacecraft encountered and photographed the nucleus of Halley's iconic comet as it approached the sun, and obtained direct chemical samples of its dust tail. This flotilla connected the space age to the battle of Hastings!
Data from Giotto's camera, which used an automated targeting system, included a spectacular image of the potato shaped nucleus that measures roughly 15 km across. What surprised everyone was that the nucleus was not a snow-white ice ball, but dark as a lump of coal. Some craggy surface features and craters could be seen, and jets of gas and dust streaming into Halley's coma. This was the first-ever image of a "primitive body," and a highly active one at that. The automated targeting device was even fooled, homing in on a jet coming off the dark surface as the spacecraft flew past (rather than the surface itself, which was expected to be bright). Data obtained on the composition of single comet grains discovered something new -- grains of pure organic material or "CHON" -- and nothing else, proving that comets are largely organic material rather than snowballs!
6) 1990-present: Hubble Space Telescope -- Well, other people can surely say much more than I can about HST's countless inspiring images in unparalleled detail and color, not to mention the many discoveries carefully extracted from these images. Perhaps more than any other images I remember are the "Pillars of Creation", the protoplanetary nebulae in Orion, and the "Deep Survey" with thousands of galaxies seen in a piece of the sky that could be covered by our Moon. These were images that profoundly changed everyone's view of the Universe we live in.
7) 2004-present: Mars Exploration Rovers (MER), Spirit and Opportunity -- The plucky rover twins and their indefatigable travels across unexpectedly wide regions of Mars have brought a tangible feeling of "being there" to scientists and the public alike. The independence and freedom to go to what interests us is a precious capability, and the ability of the rovers to engineer themselves out of trouble (with some help from a dust devil or two) has been unique in robotic exploration. The science gained includes not only images, but actual in-situ chemical and mineralogical data which has broken open the story of ancient water on Mars.
8) 2004-present: Cassini-Huygens -- A return to Saturn, but in many key ways a "first-of-its-kind" eye-opener, especially regarding Titan and Enceladus: two of the most fascinating objects in the solar system. In some ways Cassini is a culmination of the unrealized potential of Galileo.
Cassini is the largest interplanetary spacecraft ever launched, and represented an international collaboration of unprecedented scope. In addition to its sophisticated Cassini orbiter, which has operated flawlessly, the Huygens probe provided entry in-situ data for an entirely new class of object -- a world with a hydrogen-poor atmosphere where organic chemistry can play out to its fullest. The probe provided fascinating isotopic and organic chemical data as well as descent and landing images -- the Galileo probe and Viking rolled into one, along with an airplane-level survey of the local surface. Meanwhile, the orbiter has returned a wealth of data on the planet and its rings, including unexpected time and seasonal variations in both.