On March 18, 2011, NASA's MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft finally inserted itself into orbit around Mercury after six and a half years of traveling across our solar system. Now the 11-year-old spacecraft is finally retiring, with plans to end its career by leaving its mark on the planet it observed for so long.

Earlier this week, mission operators at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md. were working hard to correct the decay of the spacecraft's orbit, as it slowly sinks towards Mercury's surface. The last maneuver MESSENGER will ever make is slated for next Friday (April 24), buying the orbiter just a bit more time.

"Following this last maneuver, we will finally declare the spacecraft out of propellant, as this maneuver will deplete nearly all of our remaining helium gas," Daniel O'Shaughnessy, mission systems engineer at the APL, said in a statement. "At that point, the spacecraft will no longer be capable of fighting the downward push of the Sun's gravity."

Since Mercury doesn't have an incredibly thick atmosphere - one made up of very small amounts of hydrogen, helium, and oxygen - MESSENGER will not burn up as it draws closer. This means that the spacecraft can end its career kissing the surface of the planet it has been eyeing for so long - leaving a small impact crater as if to say "MESSENGER was here." (Scroll to read on...)

And while the crater may be the spacecraft's physical legacy, its impact on science has been far more impressive.

"For the first time in history we now have real knowledge about the planet Mercury that shows it to be a fascinating world as part of our diverse solar system," said John Grunsfeld, associate administrator for the Science Mission Directorate at NASA Headquarters in Washington, DC.

The spacecraft was the first to confirm that Mercury - one of Earth's nearest planetary neighbors - harbors a surprising amount of water frozen in its permanently shadowed craters.

This was key evidence for growing theories concerning how our solar system received its abundance of water and organic matter, explained Sean Solomon, the mission's principal investigator.

"The water now stored in ice deposits ... was delivered to the innermost planet by the impacts of comets and volatile-rich asteroids," he said.

That evidence is helping back other investigations, and untapped data from the MESSENGER mission could hold even more secrets.

"While spacecraft operations will end, we are celebrating MESSENGER as more than a successful mission,  added Grunsfeld. "It's the beginning of a longer journey to analyze the data that reveals all the scientific mysteries of Mercury." 

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