It's summertime and for a lot of families that means road trips. Deciding which states to hit is tricky enough, but imagine having to choose which planets to visit. That might mean a little more long-term planning.

But as NPR's Geoff Brumfiel reports, scientists gathered in the nation's capital this week to do just that.

GEOFF BRUMFIEL: It is hot in Washington. But mercifully, the National Academy of Sciences is air conditioned.

Planetary researchers from around the country have gathered here to discuss where they want to go in the solar system over the next decade - well, not literally. They plan to send unmanned probes to take measurements and collect samples. On the table: Everything under the sun, or rather orbiting it.

Dr. STEVE SQUYRES (Researcher, Cornell University): The missions that we've been studying are just so damn cool.

BRUMFIEL: Steve Squyres is running the event, which is being held at the request of NASA and the National Science Foundation. Squyres is also the head of the greatest interplanetary road-trippers to date: the Mars rovers Spirit and Opportunity.

Dr. SQUYRES: I mean, we've got 28 missions out there that range from Mercury landers to things that fly by Trojan asteroids to landers that land on Venus and then take off and land again, you know, Mars sample return, just a tremendous, breathtaking set of missions.

BRUMFIEL: There are so many ideas because there are so many questions. If Mars used to have water on its surface, researchers want to learn more about it. They also suspect present-day oceans may be sloshing under the icy surface of Jupiter's moon Europa. Jupiter is interesting for other reasons, too: New theories suggest it moved billions of miles across the solar system before reaching its present-day orbit.

Studying planets, moons and even asteroids will help scientists understand how the solar system formed and how life began.

Louis Friedman heads the Planetary Society, which promotes space exploration.

Dr. LOUIS FRIEDMAN (Co-founder, Planetary Society): This is all going to lead to some definitive discoveries about life, either positive or negative.

BRUMFIEL: But choosing one mission over another isn't easy. Which would you choose: a nuclear-powered boat cruising liquid-methane lakes on Saturn's moon Titan, or a trip to an asteroid to see whether it's home to complex molecules needed for life?

Dr. JIM GREEN (Director, Planetary Science Division, National Aeronautics and Space Administration): We want the scientists to sit and argue. We want them to say this is more important than that.

BRUMFIEL: Jim Green heads NASA's Planetary Science Division. Green says the aim is to get everyone to agree on the most important missions. That makes it easier for the agency to set priorities and to get them funded.

Dr. GREEN: The whole planetary science community will get behind these things, and that will enable us to put the best foot forward.

BRUMFIEL: But getting the whole planetary science community into a room is one thing, getting them to pick planets is another. Panel member Ralph McNutt.

Dr. RALPH McNUTT (Panel Member, National Academy of Sciences): Gets a little bit intense sometimes.

BRUMFIEL: Are you ever tempted to just do a cage match and put, you know, Mars and Jupiter in a room?

Dr. McNUTT: Oh, I think some people would be tempted at times, but it's never really quite come out that way.

(Soundbite of laughter)

BRUMFIEL: Never come to blows.

Dr. McNUTT: Never come to blows, no.

BRUMFIEL: In the end, the decision isn't all about science. Issues like which missions are closest to being ready and which could be done with other nations must also be considered. And, as always, money is important, says Squyres.

Dr. SQUYRES: You could double the planetary budget, and we'd still have a tremendous amount of oversubscription. There are just so many good ideas out there for missions that can fly.

BRUMFIEL: The group hopes to come up with their final list of destinations by some time next spring.

Geoff Brumfiel, NPR News.

SIEGEL: And you can see where scientists want to go next and vote for your favorite destination at This is ALL THINGS CONSIDERED.

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