After traveling 7.8 billion kilometers (4.9 billion miles) in over a decade, NASA’s Messenger spacecraft will plunge into Mercury at more than 3.91 kilometers per second (8,750 miles an hour) around April 30 when it runs out of propellant.
Launched on August 3, 2004, Messenger (for Mercury Surface, Space Environment, Geochemistry, and Ranging) traveled more than six and a half years before it began orbiting around that first rock from the sun. Its main mission was to orbit Mercury and collect data for one (Earth) year. "We're at the end of a really successful mission, and we can't do anything anymore to stop it from doing what it naturally wants to do,” University of Michigan’s Thomas Zurbuchen explains in a news release. “The sun is pulling on it. The planet is pulling on it. It's just physics. It has to crash."
In 2012, the spacecraft provided evidence to support the presence of abundant frozen water and volatile materials in Mercury’s polar craters. Ice at the planet’s poles would be more than 3 kilometers thick if spread over an area the size of Washington. And a dark layer covering the water ice deposits suggests that organic compounds were delivered to the inner planets (including Earth) from the outer solar system. “The water now stored in ice deposits in the permanently shadowed floors of impact craters at Mercury’s poles most likely was delivered to the innermost planet by the impacts of comets and volatile-rich asteroids,” Columbia University’s Sean Solomon says in a NASA release. “Those same impacts also likely delivered the dark organic material.”
According to Solomon, the mission’s principle investigator, some of Messenger’s other top discoveries about Mercury include: an exceedingly thin atmosphere that changes with the seasons, that the planet shrank by nearly 14.5 kilometers (9 miles) in diameter, and how volcanism helped shape the planet’s surface, Los Angeles Times reports.
Messenger will also be remembered for a few technological firsts. Its heat-resistant, highly reflective ceramic cloth sunshade, for example, isolated instruments and electronics from direct solar radiation -- which is especially important that close to the sun. “The front side of the sunshade routinely experienced temperatures in excess of 300 degrees Celsius (570 degrees Fahrenheit), whereas the majority of components in its shadow routinely operated near room temperature (20 degrees C or 68 degrees F),” Helene Winters of Johns Hopkins University Applied Physics Laboratory (APL) explains in a university statement.
Earlier this week, researchers at mission control at APL completed the fourth in a series of orbit correction maneuvers that were designed to delay the impact. The last of these maneuvers is scheduled for April 24. "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,” APL’s Daniel O’Shaughnessy says. “At that point, the spacecraft will no longer be capable of fighting the downward push of the sun's gravity.”
Messenger is expected to barrel down into the side of the planet facing away from Earth, and that means we won’t be able to view it in real time. Farewell Messenger, and thanks.
Images: NASA (top), NASA/JHU APL/Carnegie Institution of Washington (bottom)