Company for Mercury

Tiny Mercury, the closest planet to the sun has a guest.  NASA’s MESSENGER spacecraft inserted itself into orbit around the planet last Thursday (St. Paddy’s Day) and will soon begin a one year mission of data collection to enhance our understanding of the little planet.  The MESSENGER mission, managed for NASA by the Johns Hopkins University Applied Physics Lab (APL) in Laurel, MD, began with a launch in August, 2004.  Six and a half years of traversing the inner solar system followed before MESSENGER became the first spacecraft in history to enter Mercury’s orbit.  The APL team made it look easy with the satellite’s trajectory perfectly matching the ideal path.  Remotely commanding a spacecraft to execute the proper sequence of thruster burns to allow it to be orbitally captured by a planet’s gravity is no simple matter.  The Japanese Space Agency learned this last December when their AKATSUKI spacecraft failed to enter orbit around the planet Venus and skipped off into space due to a propulsion system problem.  They’ll have another opportunity in six years. 

The orbit of MESSENGER is highly elliptic with the spacecraft coming within 200 km of the planet at its closest point, and 15,000 km at its farthest point.  This is a polar orbit so that as Mercury rotates on its axis below it, MESSENGER will get a view of the entire planet during its mission.  MESSENGER’s thermal shield will face the sun throughout the mission, protecting its instruments from the radiative heat and keeping them near room temperature.

Although a scant 36 million miles from the sun, the shaded side of Mercury is very cold and its poles may even contain ice.  MESSENGER’s instruments will map the surface of the planet, provide data on the composition of the planet’s core, study the magnetic field, and examine the constituents in the thin Mercury atmosphere.  The instruments will be activated in early April and planetary scientists are anxiously waiting for what they will learn about our hot little neighbor.