Space
4:09 pm
Wed March 21, 2012

Spacecraft's Wild Ride To Mercury Yields Surprises

Originally published on Wed December 12, 2012 5:40 pm

There's a small spacecraft called Messenger that's been orbiting the planet Mercury for a year. Today, at the Lunar and Planetary Science Conference in Houston, astronomers revealed what they've learned about the innermost planet in our solar system, and some of the new knowledge is puzzling.

Maria Zuber, a planetary scientist at the Massachusetts Institute of Technology, studied a large crater 900 miles across called Caloris.

"The northern half of Caloris has actually been uplifted so that it's actually higher than its rim," she says. "And of course craters are holes in the ground, but this hole in the ground has been uplifted so high that the base of the crater is above its rim."

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. For comparison, if the Earth were an orange, the peel and fruit would be about the same width.

Astronomers like Zuber have been waiting for a long time for Messenger's data. The spacecraft launched nearly eight years ago, on Aug. 3, 2004.

Now you may be thinking, "Wait a minute. Mercury isn't all that far from the Earth. That's spitting distance in astronomical terms. Heck, it's less than 50 million miles at its closest approach. Why does it take so long to get there?"

Zuber says the answer is gravity. Mercury's orbit is closer to the sun than the Earth's, and if you launch a rocket toward the sun, the sun's gravity is going to cause your spacecraft to speed up.

"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," she says.

If you have a giant rocket engine, you can use that like a retrorocket to counteract the sun's pull, just like Scotty does when the Starship Enterprise is about to fall into a black hole. But Messenger didn't have a giant engine.

So instead, it used the gravity of other planets to slow it down with respect to the sun. In a trajectory worthy of Rube Goldberg, Messenger looped once around the Earth, then made two close encounters with Venus. When it arrived at Mercury in 2008, it was still going too fast, so it flew by Mercury three times, slowing down a little more each time. "The fourth time it came by Mercury it was slowed down enough that when we fired the main engine, Mercury's gravity field was able to capture it," says Zuber. Messenger has been orbiting Mercury since March 2011.

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 things were going so well they persuaded NASA to extend the mission. "Just on Sunday we began the Messenger extended mission," he says.

The extended mission is scheduled to last another year, so astronomers expect they'll have more to say at next year's meeting.

The reports presented at today's conference will also appear in this week's edition of the journal Science.

Copyright 2013 NPR. To see more, visit http://www.npr.org/.

Transcript

AUDIE CORNISH, HOST:

From NPR News, this is ALL THINGS CONSIDERED. I'm Audie Cornish.

ROBERT SIEGEL, HOST:

And I'm Robert Siegel.

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. Transcript provided by NPR, Copyright NPR.