Planetary scientists are preparing for another quick and close-up look at the planet Mercury on Tuesday evening as the Maryland-built Messenger spacecraft skims to within 142 miles of the sun's closest neighbor.
The flyby, around 6 p.m., will be the scientists' last look at Mercury until their spacecraft returns in March 2011 and, if all goes well, become the first ever to orbit the tiny planet.
"Every time we encounter this planet we are surprised," said Sean C. Solomon, of the Carnegie Institution of Washington, the $446 million mission's principal investigator. "It's very much like Christmas morning. We know there are presents under the tree, but we don't know in detail what's inside each of those boxes. We expect to be surprised, and we expect to be delighted."
Comet-like tail
Photographs taken during Messenger's two previous flybys, in January and October of last year, expanded the known portion of the planet's surface from 45 percent to 90 percent. This flyby will fill in another 5 percent.
Pictures have revealed previously unknown terrain battered by impacts, altered by volcanic activity and torn by seismic shifts in the planet's crust. Spectral imaging with other instruments identified some of the minerals exposed on Mercury's surface and the chemistry and physics of the planet's thin atmosphere and comet-like tail.
Mercury is just 30 million miles from the sun, two-thirds closer than Earth. It is currently rising just before the sun, below Venus, very low in the eastern sky. Before Messenger, the only spacecraft to visit was Mariner 10, which made three flybys in 1974.
The NASA-funded Messenger spacecraft was designed and built by scientists and engineers at the Johns Hopkins University Applied Physics Laboratory. The mission's control center is located on APL's campus near Laurel.
Launched in August 2004, Messenger has flown 4.9 billion miles. It zipped past Earth and Venus once, and Mercury twice.
On Tuesday, Messenger will speed by Mercury at 12,000 mph relative to the planet's surface, making its closest approach at 6:01 p.m. EDT. The pass will slow its speed by 6,000 mph to match the 88-day "year" of its target planet.
There will be plenty for mission members to worry about. For 18 minutes the spacecraft will be in Mercury's shadow, relying on batteries alone for power as it carries out 7,000 pre-programmed commands.
