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 on April 30.

Nasa officials gave tribute in a briefing last Thursday to the spacecraft, the first to orbit Mercury and which they say has fundamentally altered our understanding of this scorched little world.

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

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: 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 7km 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, sulphur, sodium and chlorine.

Scientists had not expected it, as these elements are thought to be among the first to escape a planet, particularly when it’s so close to the Sun.

The most interesting, he said, was probably the second on his list – the presence of polar water ice in permanently shadowed regions at the bottoms of craters.

“Those polar regions, I think, are calling out to people like Jim Green and saying, ‘Send us another spacecraft, we have more stories to tell’,” Solomon said, eliciting laughter from Green and others.

“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 14,000kph, she added.

On impact, the spacecraft will make a roughly 16m-wide crater, a planetary scar with characteristics that may provide useful study by future spacecraft.

And there will be more: the joint Europe/Japan space agencies mission BepiColombo is set to arrive in 2024. — Los Angeles Times/Tribune News Service