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.

Even though it doesn't have tectonic plates, MESSENGER also found signs of extensive volcanism and other tectonic features, possibly resulting from this contraction. The probe was also able to generate data that allowed researchers to make inferences about the planet's interior. It appears that, like Earth, Mercury has a solid crust on top of a mantle of semi-molten rock. Unlike the Earth—or any other planet we've studied—Mercury seems to have a solid layer between the mantle and its molten core.

MESSENGER also had to operate for an extended period in an unusually harsh environment. Not only was it subjected to much more intense sunlight, but it was also bombarded by far more energetic particles produced by the Sun. The probe had to be wrapped in a highly reflective sunshade, one that heated up to over 350°C, but kept the contents of MESSENGER in the area of room temperature. Mercury's hot surface also posed problems during low orbits, so additional radiators and heat pipes were used to get rid of this excess heat.

The sunlight it receives is so intense that the solar panels are nearly 70 percent mirror, reflecting away enough light to keep the remaining photovoltaic hardware from overheating.

But MESSENGER can't convert sunlight to propellant, and eventually it ran out. Careful conservation of on-board resources kept it going for several years beyond its expected one-year lifetime in orbit, and it's currently on a one-month mission extension that's using its gradual drop towards the planet's surface to achieve some low-altitude photography. Somewhat ironically, it's now close enough to resolve craters similar in size to the one it will create.

Everything seems to indicate that it will smash into Mercury's surface tomorrow, though the exact timing depends on the topography of the planet that it's done so much to reveal. Current estimates are that it will occur at 19:26:02 UTC.

If you'd like first-hand experience with some of MESSENGER's findings, you can open a map of the planet in Google Earth.