Human beings have a habit of making traffic wherever they go. Give us a new means of transportation, and pretty soon highways, sea-lanes and airline routes are filled with vehicles. Now add to that deep space.
For all the attention that the shuttle, the space station and other manned spacecraft get, the real foot soldiers of space exploration have always been the unmanned ships--and right now they're enjoying something of a golden age. The U.S. currently has no fewer than 11 interplanetary probes scattered about the solar system; five are orbiting, roving or approaching Mars alone, and the others are targeting Mercury, the sun, Saturn and numerous comets or asteroids. One probe is heading for a never before rendezvous with Pluto, a destination it won't reach until 2015.
This spring three of the rugged ships stand out from the rest. Near Saturn, the Cassini orbiter, launched by the Jet Propulsion Laboratory, just executed a dramatic dive through an icy geyser that reaches 950 miles (1,530 km) into space from the Saturnian moon Enceladus, and there are plans to follow that up with even higher-risk maneuvers. In May NASA's Phoenix Lander will set down in Mars' arctic region in search of water ice. And later this month NASA and the European Space Agency will retire their Ulysses solar surveyor after a 17-year mission that has reframed our understanding of the sun.
All three missions have thrilled and surprised scientists--who pride themselves on knowing more or less what to expect. "I sit back with my mouth open, watching paradigms shift," says Linda Spilker, Cassini's deputy project scientist.
The orbiter's plume dive was responsible for some of that shifting. Passing just 120 miles (190 km) above the surface of Enceladus, Cassini sampled an icy exhaust that researchers didn't even know existed until the spacecraft spotted it three years ago. NASA expects to release detailed composition information soon, but the ice hints at subsurface water and the attendant possibility of life. Seven more close-brush flybys are in the offing, including one high-wire plunge that will drop the spacecraft a scant 15 miles (24 km) above Enceladus' surface. Says JPL's Spilker: "We're going to taste and sniff everything."
Before the orbiter attempts that maneuver, it will execute two flybys of the moon Titan, whose opaque orange atmosphere has been increasingly pierced by the spacecraft's radar. And this summer Cassini will make an unusually high orbit above Saturn's massive B ring, promising unique images of the ring, spread like an immense halo around the planet. The ship will also have the rare opportunity to observe the sun cross the plane of the ring from south to north, literally shedding light on the B ring's complex particle structure. "We want to know what a particle would look like if you could pick one up and hold it in your hand," Spilker says, "and we can do that by studying how they heat and cool."
Don't mention cooling to the researchers behind the Phoenix Mars Lander. Their ship will have just six months to sample and study the water ice at the Martian north pole before -200°F (-130°C) winter temperatures hit the region. "We last until the sun goes down. Then we freeze to death," says principal investigator Peter Smith, a planetary scientist at the University of Arizona, Tucson. Before it does, Phoenix Lander will probably offer a first look at actual Martian water ice rather than the dry water scars of millenniums past. To do that, the lander will use a digging arm and a suite of mineralogy instruments to hunt for salts, clays and other signs that liquid water is manipulating the soil. If Phoenix Lander hits its targets, this will be a big step toward later missions that will search for microscopic organic life. "Pay attention," Smith says, "because it's the polar region. No one's ever been there, and it's going to be fun."
Less glamorous but more sweeping than the half-year Phoenix mission was the long-running Ulysses mission, which took the first full measure of the sun's polar regions. If it swirls, floats or emanates near the sun, Ulysses studied it. The spacecraft discovered that the sun's magnetic field determines the regions that produce the solar wind, and ruffled more than a few scientists' feathers when it showed that a hot corona produces the fastest solar winds--exactly the opposite of prevailing theories.
Ulysses also tracked interstellar dust particles all the way from the sun to Earth, and in so doing helped map the planet's magnetic fields. The big surprise came when Ulysses stumbled on the tails of two comets and found that those feathery streams were more than 93 million miles (150 million km) in length. That's about the distance from the sun to Earth. "Totally unexpected," JPL project scientist Ed Smith says simply.
A diminishing power supply means the Ulysses mission ends on March 30, but the textbook rewrites will go on as fresh ships continue to take the place of old ones. The Lunar Reconnaissance Orbiter (LRO), which will launch later this year, will conduct the most comprehensive surveys of the moon the U.S. has ever attempted, using cameras that can spot an object as small as a football. The mission will help scout for landing sites, as NASA is holding fast to its plans to return astronauts to the moon by 2020. LRO will also hunt for signs of water ice on the moon, as well as help study the irregular lunar gravity field, caused by dense concentrations of mass beneath the surface--the geological equivalent of lumps in oatmeal. Most dramatically, it will collect detailed images of all six Apollo landing sites, which have stood unseen for close to 40 years. "LRO's job is to open up the lunar frontier," says Jim Garvin, chief scientist at Goddard's Space Flight Center, where the craft is being assembled. "Right now we have a view from the 1970s, and here we are in the 21st century."
More missions to Mars are anticipated, including one that would return soil samples, possibly shedding fresh light on Martian life and allowing NASA to rehearse the round-trip skills that would be necessary for a manned mission. And even as the new ships are readied, some of the great historic ones are still in flight. Voyagers 1 and 2, launched in 1977 on a grand tour of the outer planets, are now on their way out of the solar system, with the last breaths of solar wind at their backs. Remarkably, NASA may be able to stay in touch with them for up to 30 more years--meaning the granddaddy ships could remain online long after some of the newest ones have winked out. As traffic jams go, that's not bad.