NASA bids farewell to MESSENGER, its Mercury orbiter

A bit over 3,900 days ago, NASA sent a probe on a meandering course through the inner Solar System. After seven years of gravitational maneuvering, MESSENGER finally arrived at its destination: Mercury, which it became the first spacecraft to visit since 1975's Mariner 10 flyby. Since then, it's been gathering data without much fuss, and without the attention-grabbing surroundings that Cassini benefits from while orbiting Saturn. But its time there has come to an end; out of fuel, the probe is expected to crash into the planet's surface tomorrow.

Mercury's lack of glamor hasn't meant for lack of interesting science. MESSENGER stands for MErcury Surface, Space ENvironment, GEochemistry, and Ranging, which gives you some sense of the extensive suite of instruments it carries.

We've covered a number of its findings. Despite its proximity to the Sun, MESSENGER found hints of water and other volatile chemicals in the shadows of crater rims at the planet's poles. Researchers had also suggested the planet had shrunk as it cooled off after its formation, but MESSENGER was able to put some numbers on the shrinkage: its radius went down by somewhere between five and seven kilometers.

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