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For the past year, a small spacecraft called Messenger has been orbiting the planet Mercury. Meantime, back on Earth, astronomers are meeting in Texas to talk about Messenger.

And today, as NPR's Joe Palca reports, they revealed what they've learned so far about our solar system's innermost planet, including some surprises.

JOE PALCA, BYLINE: Messenger's orbit takes it to within 125 miles of the planet, and from there you can get a pretty good view. But don't expect any dazzling color pictures.

DR. MARIA ZUBER: Mercury is very gray and it's pretty dark gray; even less color variations on Mercury then we see on the Moon.

PALCA: Maria Zuber is an astronomer at the Massachusetts Institute of Technology and one of the lead scientists on the Messenger mission.

ZUBER: So, Mercury is not a very color for planet.

PALCA: But interesting.

ZUBER: But fascinating.

PALCA: That's why Zuber has been willing to hang around for the data she knew Messenger would be providing. And she's been waiting for that data for quite a while.

ZUBER: The launch date was August 3rd, 2004.

PALCA: Now, you may be thinking, wait a minute, Mercury isn't all that far from the Earth, spitting distance in astronomical terms. Why does it take nearly eight years to get there? Zuber says the answer is gravity. Mercury's orbit is close to the Sun and if you launch a rocket towards the Sun, which is massive and has a much stronger gravity field than tiny Mercury, it's going to make the spacecraft speed up and head directly for the Sun.

ZUBER: And in order to get into orbit around the planet, the spacecraft has to slow down enough for the planet's gravity field to capture it.

PALCA: If you have a giant rocket engine, you can use that to slow you down. But Messenger didn't have one. So instead, Mission managers chose a route worthy of Rube Goldberg. Messenger did a loop around Earth and two close encounters with Venus, using those gravity fields as a kind of brake, counteracting the Sun's pull. When it arrived at Mercury in 2008, it was still going too fast to go into orbit, so it flew by Mercury three times, slowing down a little more each time.

ZUBER: And the fourth time it came by Mercury it was slowed down enough that when we fired the spacecraft's main engine Mercury's gravity field was able to capture it.

PALCA: Messenger has an orbiting Mercury since March 2011. As Zuber and her colleagues reported today at the Lunar and Planetary Science Conference in Houston - and are publishing this week in the journal Science - Mercury has some puzzling features.

For example, Zuber used Messenger's instruments to study a large crater 900 miles across called Caloris.

ZUBER: The northern half of Caloris' floor has actually been uplifted so that it's actually higher than its rim. And, of course, craters are holes in the ground. But this hole in the ground was uplifted so high that the base of the crater is above its rim.

PALCA: Another surprise is how big Mercury's core is. It's big. Zuber says to get an idea, if Mercury were an orange the core would be the size of the fruit, while the outer crust and mantle would only be as thick as the orange peel. That's a much bigger core proportionally than the Earth has.

Sean Solomon, of the Carnegie Institution for Science, is another one of Messenger's principal scientists. He says the spacecraft's original mission was supposed to end after one year, but it was working so well they convinced NASA to keep it going.

DR. SEAN SOLOMON: Just on Sunday we began the Messenger extended mission.

PALCA: Which will last another year, so astronomers expect they'll have more to say at next year's meeting.

Joe Palca, NPR News, Washington.

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