Probe to hit Mercury in mission end
A spacecraft that has been orbiting Mercury for the last four years is set to bring its mission to an explosive end tonight by crashing into the planet.
Messenger is due to make its dramatic exit at around 8.26pm UK time, smashing onto the planet at more than 8,750mph after running out of fuel.
The blast of the impact will be equivalent to a ton of TNT exploding and leave a new 50 foot-wide crater on Mercury's pock-marked surface.
Scientists have paid tribute to the highly successful mission, which has transformed our understanding of the smallest and closest planet to the sun.
The American space agency Nasa launched Messenger, short for MErcury Surface, Space ENvironment, GEochemistry, and Ranging, on August 3 2004.
After a seven year space odyssey that included 15 trips around the sun and gravity-boosting "slingshot" fly-bys of the Earth, Venus and Mercury itself, the probe went into orbit on March 8 2011.
The 513 kilogram spacecraft was the first to circle airless, enigmatic Mercury.
Mariner 10 made three passing visits in the 1970s and sent back detailed images, but left much of the planet unmapped.
Thanks to the thousands of detailed images sent back by Messenger, the surface of Mercury can now be viewed on Google Earth.
Messenger was originally only meant to study Mercury for one year, but Nasa extended the mission for as long as possible.
It had to come to an end when the probe's supply of fuel, which made up nearly 55% of its launch weight, eventually ran out.
Although the probe's destruction will be spectacular, it will occur out of sight of observers on Earth on Mercury's far-side near the planet's north pole.
Mission controllers will only be able to confirm that the impact has taken place when the craft fails to appear from behind the planet and make contact hours later.
John Grunsfeld, associate administrator for the science mission directorate at Nasa headquarters in Washington, said: "While spacecraft operations will end, we are celebrating Messenger as more than a successful mission.
"It's the beginning of a longer journey to analyse the data that reveals all the scientific mysteries of Mercury."
One key discovery that has already emerged suggests that Mercury harbours abundant frozen water in its permanently shadowed polar craters.
Data received in 2012 indicated enough ice in the planet's polar regions to create a layer two miles thick if spread over an area the size of Washington DC.
Dark deposits on the ice are believed to be composed of organic compounds.
The find supports the theory that both water and the building blocks of life were delivered from the outer solar system to the inner planets, including the Earth.
Planetary geoscientist Professor David Rothery, from The Open University, said: "Messenger has been an amazing mission.
"The only previous Mercury mission, Mariner 10, flew past the planet three times in 1974/75, giving us only an incomplete view.
"Messenger revealed the whole globe in detail, especially its northern hemisphere to which its deliberately eccentric orbit took it closest before soaring upwards to escape the furnace-like conditions near the 400C (752F) surface.
"It is now apparent that Mercury is a misfit planet that seems not to belong where we now find it.
"It is dense even for a rocky planet, showing that its iron-rich core occupies more than 80% of Mercury's radius.
"The outer part of the core must still be molten, because this is where Mercury's magnetic field is generated - a characteristic shared with the Earth, but not with Venus, Mars or the Moon."
Messenger had also revealed evidence of recent explosive volcanic eruptions punching through Mercury's vast ancient lava fields, he said.
Another major discovery was Mercury's mysterious "hollows" - steep, flat-bottomed depressions where the top 10 metres or so of the planet's surface had "simply vanished".
Prof Rothery added: "Airless Mercury has no wind to blow it away, and there are no signs of collapse into underground cavities, so we are forced to conclude that something in the ground has been turned to vapour and lost to space.
"What this stuff is, and whether it turns from solid to vapour through heating or because chemical bonds are broken by radiation of some sort, we don't know - but it is another piece in the jigsaw puzzle that must one day be fitted together if we are to understand where and how the sun's smallest and closest planet formed."
Messenger's highly eccentric egg-shaped orbit, which has varied during its mission, has brought it to within 10 miles of Mercury and swung it thousands of miles away from the planet.
In total, the probe has circled the planet more than 4,000 times.
A series of last minute manoeuvres allowed controllers to hold off the crash, keeping the craft skimming the surface at an altitude of five to 39km (three to 24 miles) at its closest approach while continuing to make low-level observations.
After the death of Messenger, Mercury will remain unvisited until the European Space Agency's BepiColombo mission, which is due to place two spacecraft in orbit round the planet in 2024.