Just as the planet Venus passed close to the Pleiades last week, the smaller and more elusive planet Mercury will also skim this star cluster on the evening of April 30. The planet and cluster will be low in the northwestern sky after sunset, about 10º above the horizon in the northern hemisphere. This is a golden opportunity to spot the little planet before it heads back towards the Sun. And more remarkably, on the same day, NASA’s MESSENGER spacecraft, which has been orbiting Mercury for four years, will crash into the planet at more than 8,000 mph and end its long mission.
MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) launched from Cape Canaveral on August 3, 2004. A direct flight to Mercury was too difficult because the craft would gain too much speed as it approached the Sun, and it was impractical to carry enough fuel for braking. So to manage its speed, MESSENGER took a roundabout route to Mercury with a flyby of Earth, two flybys of Venus, and three flybys of Mercury itself. MESSENGER’s flybys of Mercury were the first by any craft since the Mariner 10 mission in 1975.
MESSENGER settled into a highly elliptical orbit around Mercury on March 18, 2011. During its more than 4,000 orbits of the planet over the past four years, the spacecraft mapped and imaged the inner planet, probed its magnetic field and gravity, and discovered a surprising amount of water in Mercury’s exosphere. It also found evidence for past volcanic activity and the possibility that Mercury has a liquid-iron core. Late in 2014, scientists also suggested MESSENGER discovered signs of an annual meteor shower on Mercury.
Because the craft operates so close to the Sun, its orbit slowly degrades as the Sun pulls it closer to the surface of the planet. Early in April, MESSENGER’s orbit will quickly decay as the craft runs out of hydrazine fuel. Engineers will maintain orbit as long as possible using the helium pressurant gas that remains in the fuel tanks. The craft is now orbiting between 6 km and 39 km above the surface of the planet and is collecting more scientific data on Mercury’s exosphere and gravitational field.
As the last bits of pressurized helium run out, the half-ton spacecraft will finally impact the planet at 8,720 mph on April 30. NASA engineers expect MESSENGER will impact Mercury between 20:25 and 20:30 UT, although it may complete one more 8-hour orbit of Mercury before impact. The impact site will not be visible from Earth.
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