Laurel, Maryland—On the appointed afternoon last March 17, as the Messenger spacecraft prepared to insert itself into orbit around Mercury, the Johns Hopkins University Applied Physics Laboratory braced for hundreds of visitors intent on sharing the historic moment. The lab—called by its acronym APL (rhymes with JPL, as in the NASA facility on the West Coast)—lies on a bucolic campus halfway between Baltimore and Washington, D.C. Most of the facility’s 400 acres are restricted, including the areas dedicated to the construction of marvels such as Messenger and the New Horizons spacecraft now heading for a 2015 rendezvous with Pluto. Aside from anxiously anticipating the evening’s crucial maneuver, Messenger project manager Peter Bedini was concerned about granting his official guests the prime viewing positions they coveted inside the tense, cramped quarters of the Mission Operations Center, APL’s version of Mission Control.
The difficulty of orbiting Mercury, the solar system’s innermost planet, has discouraged attempts until now. To catch Mercury in its tight orbit around the sun, a spacecraft must change speed by more than 60,000 miles an hour after leaving Earth, difficult to do with rocket propulsion alone.
To meet the planet at the proper speed, Messenger had to ply a looping, roundabout route. After its August 2004 launch from Cape Canaveral, the spacecraft began circling the sun on a constantly shifting course, flying by Earth once (in 2005) and then Venus twice (2006 and 2007), in order to narrow its orbit and lose a little momentum at each planetary encounter. By the time it reached Mercury’s vicinity (in January 2008), it was still traveling much too fast to be pulled into orbit around the little world. So Messenger continued to circle the sun, brushing past Mercury two more times (in October 2008 and September 2009) to further brake its speed.
