MESSENGER probe faces imminent impact

The spacecraft will crash into Mercury on April 30.
By Andrew McDonald | Apr 28, 2015
According to a statement from the Johns Hopkins University Applied Physics Laboratory (APL), the MErcury Surface, Space ENvironment,GEochemistry, and Ranging (MESSENGER) spacecraft in orbit of Mercury is nearing the denouement of its mission: impact on Mercury's surface. On April 24, mission controllers at the APL instructed the probe to carry out the final in a series of six maneuvers that have maintained its orbit.

Mission managers used MESSENGER's deteriorating orbit as an opportunity, bringing the spacecraft closer to the surface of a planet than any have achieved before. MESSENGER had already accomplished its main scientific goals, making the low-altitude orbits and unique observational opportunities worth the risk.

The fifth in the series of maneuvers took place on April 14 and increased MESSENGER's minimum altitude from four miles to 8.3 miles. Nevertheless, the probe's altitude continued to fall. When the sixth and final maneuver began on April 24, MESSENGER's orbit brought it to 5.1 miles from the surface. The probe's four most powerful monopropellant thrusters emitted helium gas. At 3.43 miles per hour, this gradually took the probe into a higher orbit with a closest approach of 11.3 miles.

With its fuel having been depleted, MESSENGER's gaseous helium, originally used to pressurize the fuel, was the last-ditch effort to affect the probe's orbit. Now that there is no way to increase its altitude, MESSENGER will be unable to resist the Sun's gravity and will slam into the surface of Mercury on April 30. The probe will strike the surface at around 8,750 miles per hour and gouge out a new crater up to 52 feet wide.

"Operating a spacecraft in orbit about Mercury, where the probe is exposed to punishing heat from the Sun and the planet's dayside surface as well as the harsh radiation environment of the inner heliosphere, would be challenge enough," said. MESSENGER principal investigator Sean Solomon of Columbia University's Lamont-Doherty Earth Observatory. "But MESSENGER's mission design, navigation, engineering, and spacecraft operations teams have done much more. They've fought off the relentless action of solar gravity, made the most of every usable gram of propellant, and devised novel ways to modify the spacecraft trajectory never before accomplished in deep space. They've extended the duration of MESSENGER's orbital observations by more than a factor of four over the original plan, and an amazing set of scientific discoveries has been enabled by their creative efforts. This latest maneuver is icing on a multi-tiered cake of spectacular accomplishment. The MESSENGER mission will soon end, but its legacy of scientific knowledge and technical innovation will endure for as long as we study the planets and explore the Solar System."

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