Back when the first issue of discover hit the newsstands, the solar system was a sleepy place. We had nine planets and dozens of moons, but they seemed like inert, dead places—to the extent that we knew them at all. No action, no change.
Today the picture couldn’t be more different. NASA probes have found evidence of geological activity on at least three planetary bodies: Jupiter’s hellish moon Io, Saturn’s Enceladus, and Neptune’s Triton. The Mars Reconnaissance Orbiter has documented landslides and dust devils on the Martian surface. Observers around the world saw Jupiter whacked by impacts on three occasions, including the dramatic multiple beating it took in 1994 by the comet Shoemaker-Levy 9.
And, perhaps most amazingly, scientists are seriously considering the presence of life elsewhere in the solar system. Mars still ranks high on the list of places to look. But another of Jupiter’s satellites, icy Europa, is a contender, as are Enceladus and the largest of Saturn’s moons, Titan. The last of these even has liquid methane lakes that expand and shrink with the seasons.
Many people think that Apollo represented the glory days of America’s space program, but you’d have to qualify that with the adjective manned. The unmanned program is having its heyday right now. NASA has spacecraft orbiting Saturn, Mars, the moon, and the sun and will soon have one around Mercury; the European Space Agency (ESA) has probes around the sun and Venus. ESA has a comet in its sights as well: The Rosetta mission will touch down on one in 2014. In counterpoint, NASA’s Dawn spacecraft will orbit the asteroids Ceres and Vesta later this decade. All eight planets have now been seen up close. If you’re a Pluto fan, you’ll have to wait just a few more years for New Horizons’ 2015 flyby.
Whole libraries could be filled with the wonders gleaned from the robotic exploration of the solar system, and we’ve only scratched the surface. When humans set foot once again on the moon—or for the first time on Mars—what they find will surely be enriched by all that our robotic messengers have already told us.
Phil Plait worked on the Hubble Space Telescope for 10 years and writes DISCOVER’s Bad Astronomy blog.