Spacecraft smashes into Mercury
An American spacecraft's pioneering mission to Mercury has come to a dramatic end with the probe smashing into the planet at more than 8,750 mph, mission controllers have confirmed.
Messenger, which has been orbiting the sun's closest planetary neighbour for the last four years, hit the surface with the explosive force of a ton of TNT.
The impact left a new crater 50ft wide on Mercury's scorched and pock-marked surface.
Messenger's destruction had been planned after the 1130lbs craft ran out of fuel.
Scientists have paid tribute to the highly successful mission, which has transformed our understanding of the solar system's smallest planet.
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 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.
The probe's principal investigator Dr Sean Solomon, director of Columbia University's Lamont-Doherty Earth Observatory, said: "Today we bid a fond farewell to one of the most resilient and accomplished spacecraft ever to have explored our neighbouring planets.
"Our craft set a record for planetary flybys, spent more than four years in orbit about the planet closest to the sun, and survived both punishing heat and extreme doses of radiation.
"Among its other achievements, Messenger determined Mercury's surface composition, revealed its geological history, discovered that its internal magnetic field is offset from the planet's centre, taught us about Mercury's unusual internal structure, followed the chemical inventory of its exosphere with season and time of day, discovered novel aspects of its extraordinarily active magnetosphere, and verified that its polar deposits are dominantly water ice.
"A resourceful and committed team of engineers, mission operators, scientists, and managers can be extremely proud that the Messenger mission has surpassed all expectations and delivered a stunningly long list of discoveries that have changed our views not only of one of Earth's sibling planets but of the entire inner solar system."
Dr John Grunsfeld, associate administrator for Nasa's Science Mission Directorate in Washington DC, said: "Going out with a bang as it impacts the surface of Mercury, we are celebrating Messenger as more than a successful mission.
"The Messenger mission will continue to provide scientists with a bonanza of new results as we begin the next phase of this mission - analysing the exciting data already in the archives, and unravelling the mysteries of Mercury."
The crash could not be observed from Earth because it took place on Mercury's far side. Controllers were able to confirm the end of Messenger when the spacecraft failed to emerge from behind the planet and signal that it was still orbiting.
Experts predicted the spacecraft would pass several miles over Mercury's lava-filled Shakespeare impact basin before striking an unnamed ridge around 54.5 degrees North latitude and 210.1 degrees East longitude.
One of Messenger's most important discoveries is that Mercury appears to harbour 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.
British 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 yards 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, brought it to within 10 miles of Mercury and swung it thousands of miles away from the planet.
In total, the probe 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 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.