"The Discovery program has been a spectacular success, more than anyone bargained for," he says. "Now we're looking to expand it. In the coming year, we're going to start planning dramatic improvements in the kind of missions we can undertake."

Any hints? "No," says Stern. "We're still studying our options. This is just a teaser."

Listen carefully, however, and you might catch some clues leaking out at the Discovery@15 conference held Sept. 19-20 at the Marshall Space Flight Center.

"We're getting together to commemorate the 15th anniversary of the Discovery program," explains program scientist Michael New of NASA Headquarters. "Dozens of scientists, engineers, professors and students will converge on Huntsville, Alabama, to review past successes, share lessons learned and to plan the next wave of missions." New and Stern are keynote speakers at the event.

"When we started the Discovery program 15 years ago, we weren't sure it would work," says New. "Now we know: it's a great way to explore the solar system."

What's different about Discovery?

Consider the traditional way of doing business: "Before Discovery, NASA would announce a 'Big Mission'-sending the Viking Landers to Mars, for instance, or the Voyagers to tour the outer planets. NASA would figure out what to do and how to do it, and invite the scientific community to participate." Traditional missions tend to be expensive and sweeping in scope, able to investigate a wide-range of scientific questions. The Cassini mission, for instance, dropped a lander on Titan, studies the dynamics of Saturn's rings, monitors Saturnian weather and much more. "Some great science has been done this way," New says.

But Discovery takes a different tack. "Instead of announcing a Big Mission and expecting everyone to fall in line, we now ask researchers what do you want to do? They submit proposals for relatively inexpensive ($425 million cost cap), fast turn-around missions (launches happen every 24 months) to destinations of their choice." Typical competitions pit a few dozen proposals against one another, and only the most highly-ranked are approved. "After a proposal is accepted, it is the scientist who leads the mission while NASA offers support-a major role reversal."

"This approach unleashes a tidal wave of ingenuity and curiosity," says New.

When asked to name the top five accomplishments of the Discovery Program so far, New and Stern came up with nine different answers. "There's so much to choose from," says New.

Both agreed on one: The discovery of "refractory inclusions" in dust from Comet Wild 2. In plain language, samples of Wild 2 returned to Earth by the Stardust spacecraft harbor chemical evidence that the cold, icy comet may have formed surprisingly close to the Sun--or alternately that material close to the Sun mysteriously traveled to the cold outer reaches of the solar system and peppered young Comet Wild 2 with Earth-like minerals. Either way, this upends traditional ideas of how the solar system was born.

Both also agreed that Lunar Prospector was a top five accomplishment-but for different reasons: Stern credits Lunar Prospector with the discovery of hydrogen-rich material--think H2O--at the Moon's poles. "There may be water there for future explorers," Stern says. On the other hand, New argues that "Lunar Prospector's most important discovery is the Moon's chemical asymmetry." About 30% of the Moon's nearside is covered with "KREEP"-a mixture of Potassium (K), rare earth elements (REE) and phosphorus (P). The other ~70% of the surface is not. "This challenges a long-held idea: When the Moon was very young, it was supposed to have been covered by a global ocean of magma, well mixed and uniform all around. Eventually, the magma cooled to form the Moon's solid surface-or so the theory goes." But why would a well-mixed ocean deposit KREEP in some places and not in others? Could our ideas of lunar genesis be so wrong? "It's a real puzzle."

New rounded out his top-5 choices with Mars Pathfinder ("a new paradigm for landing on Mars"), Deep Impact ("the first mission to blow a hole in a comet") and NEAR's exploration of 433 Eros ("we orbited and landed on a near-Earth asteroid").

Stern notes that before reaching 433 Eros, NEAR flew by asteroid Mathilda and gathered surprising evidence of "macro-porosity"-in other words, Mathilda is full of big holes. How did it get that way? "We don't know," says Stern, "but this is the kind of information we need in case we ever have to deflect or destroy an incoming space rock."

Stern's top five ends with comets: When Stardust and Deep Impact flew past their respective targets, cameras recorded surprising new kinds of comet-terrain including craggy cliffs, icy spires, strange "sedimentary" layers and powdery flows. "It's nothing short of mind-boggling."

And this program is about to get better? Fasten your seat belts, indeed.

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