Spacecraft Set to Swing by Mercury
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

A space probe is headed for a second swing past Mercury to pick up a gravitational boost and eventually become the first spacecraft to orbit the closest planet to our sun.

Scientists expect to get more than 1,200 pictures when NASA's MESSENGER spacecraft zips past Mercury early Monday, which would help reveal most of planet?s remaining unmapped terrain. The flyby should also provide a gravity assist that will prepare MESSENGER to enter orbit around Mercury in March 2011.

"For needles with smaller and smaller eyes, this team is getting better and better," said Sean Solomon, MESSENGER's principal investigator at the Carnegie Institution of Washington, during a Wednesday teleconference. He described the maneuvers as a "threading exercise" requiring the highest precision.

MESSENGER, short for the bulky name MErcury Surface, Space ENvironment, GEochemistry, and Ranging, should pass 125 miles (200 km) above Mercury, or roughly the same as the separation distance during a first flyby on Jan. 14. Its cameras and instruments will cover 30 percent of the planet surface, including never-before-seen areas on the western side of the planet opposite to the first flyby's coverage.

No spacecraft has caught such close looks at the planet since NASA's Mariner 10 probe, which zipped by Mercury three times in 1974 and 1975.

The earlier Mariner probe managed to map just 45 percent of the planet's surface during its three flybys, while MESSENGER scoped out half of the planet's uncharted surface during its first flyby.

However, MESSENGER still needs to perform an intricate dance with Mercury before it can turn full-time photographer. Each flyby requires precise earlier adjustments to the spacecraft's course that normally use up precious onboard propellant.

Mission planners skirted this issue and saved up more propellant for the spacecraft's later mission by taking advantage of the solar wind.

"MESSENGER is first interplanetary mission to use solar sailing as a means to control its trajectory," said Daniel O'Shaughnessy, the lead MESSENGER navigator at Johns Hopkins University Applied Physics Laboratory in Laurel, Md. He added that they achieved accuracy within a third of a mile (1 km) using "only the subtle push of sunlight and without a single drop of propellant in over six months."

The spacecraft will begin its 15,000 mph (24,140 kph) flyby in the early morning hours of U.S. EDT on Monday, Oct. 6. Its team will occasionally lose contact as MESSENGER turns this way and that to take pictures and compile seven large image mosaics of the planet surface. The closest approach during the roughly 30-hour encounter is set for about 4:45 a.m. EDT (0845 GMT).

A 17-minute power outage will occur as MESSENGER passes into Mercury's shadow, requiring the spacecraft to rely on internal batteries instead of its solar panels.

The first MESSENGER flyby found evidence that volcanoes and not impacts had created Mercury's flat, smooth plains. It also showed that Mercury's magnetic field is elongated like a tear drop, with the solar wind pushing against the side closest to the sun and pressing it close to the surface.

MESSENGER approaches its upcoming flyby as an experienced planetary hopper, having revisited Earth once and swung by Venus twice since its August 2004 launch. A third flyby of Mercury is also scheduled for Sept. 2009, before the spacecraft enters orbit in 2011.