MELISSA BLOCK, HOST:
Today a journey in deep space came to an end. As expected, NASA's Messenger spacecraft, which has been orbiting Mercury for four years, plunged into the surface of the planet. The spacecraft had run out of fuel earlier this month and its demise was only a matter of time. NPR's Geoff Brumfiel talked with some of the scientists behind the mission and has this remembrance.
GEOFF BRUMFIEL, BYLINE: The planet Mercury is so close to the sun that the Messenger spacecraft needed a hat to keep cool, a glass shield wrapped around one side of its boxy frame. In science-speak it's called a sunshade.
RALPH MCNUTT: The front of the sunshade goes up to something like about 390 degrees Celsius. And it's room temperature behind it.
BRUMFIEL: Ralph McNutt was the project's scientist on the Messenger mission. In addition to enduring brutal temperatures, the spacecraft was sometimes bombarded by solar radiation. McNutt and fellow researchers sent Messenger to this harsh environment because Mercury is a strange world. It's a rocky planet like the Earth, Venus and Mars. But it's cratered, barren. It doesn't have an atmosphere. And Mercury is super dense.
MCNUTT: The surface gravity is about the same as the surface gravity at Mars, even though Mars is much bigger.
BRUMFIEL: So what scientists wanted to figure out was whether Mercury was truly a sibling to the other rocky planets or something else entirely. And the result?
MCNUTT: It's a lot less the odd-man-out than we thought that it was going to be.
BRUMFIEL: The planet has a huge iron core that gives it a magnetic field like the earth. It even has ice hidden in cold shadows near its north and south poles. So Messenger answered that question - Mercury is like the other inner planets. But then it raised another one about how Mercury formed. Scientists thought maybe it used to be a bigger planet that had its surface burned away by the sun, but chemicals seen by Messenger show that's not the case.
MCNUTT: Those kinds of formation scenarios where that you basically had Mercury as the marshmallow too close to the fire, those just simply seem to not be applicable.
BRUMFIEL: Messenger's discoveries are now done. The sun's gravity was pulling it ever closer to Mercury. This afternoon it disappeared behind the planet and never reemerged.
SEAN SOLOMON: It's rather a quiet and solitary end to our mission.
BRUMFIEL: Sean Solomon is a planetary scientist at Columbia University. He's spent two decades working on Messenger. To him, it's a family member.
SOLOMON: Messenger's a tough little probe. So it's hard not to be personally attached.
BRUMFIEL: The mission was a success, he says, but saying goodbye still isn't easy. Geoff Brumfiel, NPR News. Transcript provided by NPR, Copyright NPR.