So what is (soon to be was) the Messenger spacecraft? It's a probe that was launched in 2004 with the mission of mapping the entire surface of Mercury. Its name is short for the name is short for MErcury Surface, Space Environment Geochemistry, and Ranging, meaning that it had instruments that collected data on each of those elements. Messenger has a solar panel (as opposed to wing) span of 10 feet (3 meters), and it weighs about two and a half tons. The entire mission cost about 4 and a half million dollars.
Why did we build Messenger and send it to Mercury? Well, before Messenger, Mercury was, by far, the least studied of the inner planets in our solar system. The only other spacecraft that has ever been there was NASA's Mariner 10 in 1974, and all it did was a few flybys. Messenger, in contrast, during its ten year mission, has orbited Mercury 3,308 times, sent back 255,858 images, done six flybys of the inner planets and sent 10 terabytes of publicly released information back to us here on Earth. It could certainly be argued that we have gotten our money's worth.
Why has there been so little study of Mercury? One reason is that many scientists argued that it was impossible to orbit Mercury, as it is so close to the Sun, and therefore within its gravity well, without an extremely massive, extremely expensive spacecraft. But a team at Johns Hopkins worked out an ingenious plan that used planetary flybys to slow Messenger down to a speed at which it could achieve orbit. Even with this plan, Messenger did use a lot of fuel to maintain its orbit. Almost half of the craft's weight at launch was propellant.
So what do we know now that we didn't before Messenger's mission? We learned that in its polar regions Mercury has areas in some of its craters that are constantly in shadow, and therefore constantly very cold, cold enough to harbor frozen water, which Messenger found some evidence of. We learned that Mercury is actually shrinking. Due to the fact that it is cooling, since its formation, Mercury has shrunk by about 8.5 miles. We've learned that Mercury has a very violent, very volcanically active past. Part of Messenger's mapping of Mercury included analysis of its chemical makeup, allowing us to discover that there is an abundance of volatile elements, like potassium and sulfur, and probably helium3. And that's just the stuff that we learned right off the bat. Messenger sent back so much information that, according to principal investigator Sean Solomon, director of Columbia University's Lamont-Doherty Earth Observatory, Planetary scientists will be looking at Messenger's huge stash of data "for years, probably for decades, as we try to understand the origin and evolution of Mercury."
But now all of Messenger's fuel is all used up and, as planned, Messenger will soon meet its fiery fate, a fate perhaps best summed up by at tweet from the Messenger team earlier this week: "I guess the end is coming. After 10 years, spacecraft will end life as just another crater on Mercury's surface."
(Sources: phys.org, NASA, Wikipedia)