Shoot the Moon

Two days before the big event, mission control at NASA Ames Research Center in California will send the command, radioing a signal 240,000 miles to the Lunar Prospector orbiting the moon. The signal will fire engines to boost the polar-bear-size craft into an orbit that swings 140 miles out, the better to get a long running start for what's to come. Another signal will goose the craft's spin from 12 to 23 rpm, so that the last dregs of its fuel slosh to the outside of its three tanks, the better to keep the craft stable for its big day. The $70 million Prospector has been sweeping around the moon since 1998, mapping its gravity and scoping out its composition, but now the little craft's battery is dying, its fuel is spent and the money to operate it has been zeroed out by NASA. Fans of the spunky craft have decided to send it out in a blaze of glory. On July 31, as Prospector swoops over the moon's equator from the dark side, its engine will brake, and "Prospector will intersect the moon's surface with a bang," says aerospace engineer David Goldstein of the University of Texas at Austin. Crashing into a shallow crater at the south pole at 3,800 miles per hour, Prospector should throw up a pillar of debris 300 miles high that telescopes will scrutinize for water vapor: the moon's eternally cold and dark craters might be carpeted with ice, which Prospector's impact would vaporize and hurl skyward, telling Earthlings that the lifeless chunk of rock might still harbor some secrets.

In short, we aren't finished with the moon yet.

Thirty years ago this week, Neil Armstrong and Buzz Aldrin galumphed from the lunar module onto the Sea of Tranquillity, becoming the first humans to touch another world. Five more moon landings followed Apollo 11, but by then an enterprise that had, for a shining moment, seemed like a glorious quest instead looked like a pointless waste of money. But that was then. Although the moon program was conceived in cold-war politics, 30 more years of space discoveries suggest that we were on the right track after all. If Prospector confirms previous hints of ice on the moon, then the lifeless lunar desert that Apollo showed us will be transformed into a potential oasis. Already, dreamers envision the moon as a steppingstone to the stars, or at least as a base from which to mine fuel for satellites in geosynchronous orbit. Both Japan and Europe are planning unmanned lunar missions. And our moon isn't the only moon in play: there are 65 others. Apollo may have inspired a vision of Earth as a lonely drop of life in a lifeless solar system--that famous Apollo 8 photo of a blue Earth rising over the moon--but the message of the 30 years since is the opposite: there's a lot of cool real estate in the solar system, and moons may be the coolest. "Moons of the outer solar system have environments that are like natural labs for studying the conditions that might have led to life on Earth," says geologist James Head of Brown University.

That's why moons have become some of the hottest destinations in the solar system. Probes are or will soon be en route to five moons that might be starting to cook up primitive life, and to comets that might have seeded distant worlds with the building blocks of life. The missions will look for signs of water, heat, atmospheres and organic molecules. If they find such preconditions for life, it will be one more hint that life might be easier to start than we thought.

One destination is Titan, Saturn's icy satellite. It has an atmosphere denser than Earth's and, like Earth's, is rich in nitrogen. Even more intriguing, Titan's atmosphere seems to contain ethane and methane. Those are organic compounds; that is, they contain the carbon that is the basis for life. Many astronomers think the best place to look for hints of the chemistry of life be-yond Earth is on Titan. In 2004 the little Huygens probe, launched with the Cassini mission to Saturn in 1997, will arrive, dropping onto Titan with the meter running: it has four hours of battery life to measure wind, weather, topography and soil and air chemistry. No one is expecting to find Saturnians, but Titan may show how chemistry becomes biology.

There is no life without liquid water because it lets molecules bump into one another easily and undergo chemical reactions. But is there water without life? Jupiter's moon Europa is a good place to find out. Sixty miles of water cover it, and although by rights the water should all be frozen--it's cold out by Jupiter--new discoveries hint otherwise. Both Jupiter and neighboring moons tug and pull the little satellite. Those tidal forces deform Europa's crust and may create enough heat to keep the water liquid. And last year geophysicist Krishan Khu-rana of UCLA found more evidence for a Europan sea. Something, measurements showed, was distorting Jupiter's magnetic field. One likely suspect, says Khurana, is a salty sea tens of miles thick.

There's nothing like actually going there to turn inferences into facts, though. NASA will launch Europa Orbiter in 2003, and sometime after that the agency hopes to send a dwarf submarine. The "hydrobot" would pierce Europa's icecap and check out what's beneath. Says geologist Michael Carr of the U.S. Geological Survey, "When you look beyond Earth, there aren't many places where water is liquid. Europa is the best bet." The question is whether life can live under a roof of ice that keeps out sunlight.

Two other Jovian moons, Ganymede and Callisto, show signs of having liquid water now or in the past. "On Ganymede, you've got all these amazingly fractured surfaces that have been filled in by ice-water flows," says Jim Head. Although Ganymede shows no sign of liquid water today, what it does show are recent micrometeoroid impacts, scientists announced last month. Meteors are suspected of seeding Earth with organic molecules that evolved into this thing called life. On Callisto, Khurana's magnetic-field measurements hint that it, like Europa, has an ocean under a crust of ice. And Callisto, too, has been bombarded by meteors.

The reason meteors bring organic molecules is that interstellar space is awash in the things, which stick to bits of water ice like lint to a hard candy in your pocket. Seventy amino acids have been found in meteorites, for instance; of the 70, eight are the kind that form proteins in living cells on Earth. It turns out that if you freeze organic molecules called PAHs down to the temperatures typical of interstellar space, then hit them with ultraviolet light (which space is full of) and reheat them, you've performed alchemy: complex organic molecules called alcohols, esters and quinones form. One molecule resembles the active ingredient in St. John's wort, and one looks like a constituent of nutshells. "You have the synthesis of biologically interesting molecules," says Louis Allamandola of NASA Ames Research Center. "Maybe these were the molecules at hand when life started."

Comets could have delivered the organics to a hospitable home. Observations of Halley, Hale-Bopp and Hyakutake found that these celestial streakers are full of organics. As the comets orbit the sun, they trail particles like a toddler wandering around with a cookie. Planets and moons pass through these comet tails. If a molecular precursor of life "literally dropped from the sky," says Ames's Scott Sandford, and landed in an environment friendly to life, "it could have jump-started a simple chemical reaction [that was] key to early life." Five missions are headed to comets to investigate whether they truly carry the stardust of which all life is made.

Our own moon got bombed too. Many astronomers now suspect that, over billions of years, comets and meteorites delivered billions of tons of ice to the lunar surface. Most of it evaporated back to space. But some of it--enough to fill Utah's Great Salt Lake--was trapped in craters at the poles, calculates Stewart Nozette of the Naval Research Lab. That's why he and colleagues concluded in a recent paper that "living on and using the moon might be easier than we thought." For now, there is no political will for the "living" part. But moon-boosters have another offer. Water can be dissociated into its component hydrogen and oxygen. The cryogenic forms of both are rocket juice. "We could go back to the moon with a robotic mission and mine the ice," says geologist Paul Spudis of the Lunar and Planetary Institute. "It could be boosted to a refueling station in low Earth orbit." Satellites would launch from Earth with just enough fuel to reach the great Exxon in the sky, where they would fill the tanks of their third-stage rocket in space, then blast to geosynchronous orbit.

As Apollo 11 approached the moon in July 1969, NASA reminded President Nixon's aides that it might all end in disaster. They asked speechwriter William Safire, now a New York Times columnist, to pen something Nixon could tell the nation if, in the most horrific scenario, Armstrong and Aldrin could not blast off from the moon to rejoin Michael Collins in the orbiting Apollo 11. "Fate has ordained that the men who went to the moon to explore in peace will stay to rest in peace," Nixon would have said. Today Armstrong believes that "the important achievement of Apollo was a demonstration that humanity is not forever chained to this planet, and our visions go rather further than that." With the voyages of the new millennium, those visions are becoming reality as we search for signs that we are not alone, and thus free the human imagination from the bonds that tie it to its natal planet.