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Messenger's final image before crashing into the surface of Mercury

The unmanned Messenger probe has completed its Mercury mission, going out with a bang.

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The last photo transmitted by Messenger before it crashed into the surface of Mercury. NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Mercury probe Messenger's mission has finally come to a close, with the spacecraft hurtling into the planet's surface at a speed of 8,750 mph (14,082 kph) at 3:26 p.m. EDT on Thursday.

As it plummeted, having used the last of its fuel to position itself into the gravitational pull of Mercury, the probe still continued to take photographs of the planet's surface. The final image transmitted back to Earth is of the floor of the 58-mile-wide (93 km) Jokai crater.

Messenger's impact would have created a small crater of its own, estimated to be about 50 feet in diameter.

The probe, which was originally planned for a one-year mission, was launched on August 3, 2004, reaching Mercury orbit in March 2011. It then went on to spend the next four years studying the Swift Planet in an unprecedented level of detail, quadrupling its expected mission length.

"Going out with a bang as it impacts the surface of Mercury, we are celebrating Messenger as more than a successful mission," said John Grunsfeld, associate administrator for NASA's Science Mission Directorate in Washington.

"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 impact occurred on the far side of Mercury, so NASA scientists were unable to observe it directly. Instead, the impact was confirmed at 3:40 p.m. EDT, at the time the probe was due to reappear from behind the planet. In addition, the NASA Deep Space Network Radio Science team independently confirmed the loss of a signal from Messenger.

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One of Messenger's last images, transmitted the day before impact. It shows a region some 115 km south of the centre of the Mansart crater. NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

The probe's important discoveries include the surface composition of Mercury; its geological history; its polar ice deposits; and that its magnetic field is offset from the planet's centre.

"Today we bid a fond farewell to one of the most resilient and accomplished spacecraft to ever explore our neighbouring planets," said Sean Solomon, Messenger's principal investigator and director of Columbia University's Lamont-Doherty Earth Observatory in Palisades, New York.

"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."