Spacecraft that altered understanding of Mercury to crash into planet

After more than four years of orbiting Mercury, NASA’s Messenger spacecraft is about to end its mission with a bang. After more than 4,100 orbits around the closest planet to the sun, the satellite will crash into Mercury’s crater-pocked surface April 30.

NASA officials gave tribute Thursday to the Messenger spacecraft, which was the first to orbit Mercury and which they say has fundamentally altered understanding of this scorched world.

“The spacecraft and the instruments have worked virtually flawlessly over those four years,” said James Green, director of NASA’s Planetary Science Division.

Launched in August 2004, the spacecraft has revealed many unexpected insights about that “first rock from the sun”: that, even within scorching distance, it has reserves of polar ice holding frozen water; that organic matter also coats protected areas near the poles; and that the tiny planet has a strong but lopsided magnetic field.

Mercury is among the least-studied planets in our solar system. Messenger was the first mission since the Mariner 10’s final flyby in 1975 to study it up close.

With so little previously known about Mercury, Messenger has opened up a trove of new information – and several surprises – in its three flybys and four years of orbiting the planet, said Sean Solomon, the mission’s principal investigator and director of Columbia University’s Lamont-Doherty Earth Observatory in New York.

Solomon put together a top-10 list of greatest discoveries made possible by Messenger. Among them: that the planet has an exceedingly thin atmosphere that changes with the seasons – and that sometimes trails behind the planet in a cometlike tail; that Mercury has shrunk by as much as 7 kilometers in radius; and that volcanism played a major role in shaping the planet’s surface. There are even different types of volcanic material on the surface that probably came from different reservoirs in the planet, he added.

“We have a record, if only we could read it – and we’re working on that now,” Solomon said.

The most important of the discoveries, he said, tops his list: that Mercury was surprisingly high in volatile elements, including potassium, sulfur, sodium and chlorine. Scientists had not expected this planet to be so high in these elements, which are thought to be among the first to escape a planet, particularly when it’s so close to the sun.

“It allows us to reject most of the ideas for how Mercury was assembled as a planet at the beginning of the history of the solar system,” Solomon said.

“At the end of this month – we will lose our battle with solar gravity,” said Messenger’s project manager Helene Winters of the Johns Hopkins University Applied Physics Laboratory in Maryland. It will crash into the surface at about 8,700 mph, she added.