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Published: 2015-04-30 06:28:00
Updated: 2015-04-30 08:18:34
Posted April 30, 2015
By Tony Rice
On Thursday, April 30 at 3:26:02 p.m. EDT, NASA’s MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft is expected to crash into the surface of Mercury at more than 8,750 miles per hour.
After a 10+ year mission which included two extensions, the spacecraft has run out of fuel needed to overcome solar gravity and keep it in orbit.
End of mission was again delayed beyond the capabilities of the liquid fuel onboard when the team pressurized propellant tanks with remaining gaseous helium. Like Letting air out of balloon, that gas was strategically released adding an additional four weeks of close range observations of the planet closest to the sun.
The mission will end, in a process the team light heartedly calls “lithobraking,” on a ridge slightly northeast of the 250-mile-wide Shakespeare impact basin, out of Earth view. Mercury’s craters are named for artists, musicians, painters, and authors who have made fundamental contributions to their craft. While the main spacecraft body is just a bit larger than a loveseat (52” x 73” x 50”) it is expected to create a new crater 52 feet in diameter, wide enough to fit a tractor trailer.
MESSENGER spent seven years reaching Mercury. A series of five deep-space maneuvers brought the spacecraft past Earth once, Venus twice and Mercury four times before orbit insertion in March 2011. Fuel saved by that circuitous route enabled a suite of eight instruments studying gamma-rays, x-rays, magnetic fields, and energetic particles. Additional instruments mapped the surface and topography of the planet with cameras and a LASER altimeter.
Instruments were created to study the surface chemical composition, geologic history, exosphere, core and magnetic field. Mercury does have a very thin atmosphere, more accurately called an exosphere it is so thin, 2000 times thinner than our moon’s. One big surprise that came from the mission was discovery of water vapor in that exosphere.
MESSENGER also observed a bright orange glow coming from sodium scattering sunlight in the exosphere. An observer standing on the night side of the planet would notice a glow similar to light pollution created by older sodium vapor street lights. The glow varies by time of year as Mercury’s orbit varies its distance from the sun.
Days and years are quite different on Mercury. The planet completes more than 4 revolutions around the sun in an Earth year. Each Mercury year isn’t even two Mercury days long, though. The planet takes 58.6 Earth days to rotate just once on its axis.
MESSENGER's biggest legacy is how it changed how we look at the formation of our solar system. The mission discovered a surprising amount of volatile compounds like sulfur, potassium and sodium. This eliminated theories about the planet’s formation involving high temperatures for long periods of time. This caused scientists to rethink how this, other planets and even our moon formed.
Studies of Mercury will be continued by BepiColombo, a joint mission between the European Space Agency and the Japan Aerospace Exploration Agency (JAXA). BepiColombo is scheduled for launch in 2017 and will focus even more on the planet’s magnetosphere.
Tony Rice is a volunteer in the NASA/JPL Solar System Ambassador program and software engineer at Cisco Systems. You can follow him on twitter @rtphokie.