Messenger at Mercury. (Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)

U. COLORADO (US)—NASA’s Messenger spacecraft will zip by Mercury for the third and final time today, September 29, cruising within 142 miles of the planet’s surface at more than 100,000 mph. Messenger will take high-resolution color images of the surface terrain before making a clever gravity-assist maneuver that will steer it into orbit around the rocky planet beginning in March 2011.

The spacecraft also will be taking ultraviolet and visible light measurements of the harsh planet’s surface, its tenuous atmosphere, and a comet-tailed gas cloud 25,000 miles long that trails behind the planet.

Messenger is carrying seven instruments—a camera, a magnetometer, an altimeter, and four spectrometers—and includes University of Colorado at Boulder’s Mercury Atmospheric and Surface Composition Spectrometer, or MASCS.

Despite the spacecraft’s eye-popping speed, rapid rotation maneuvers during the flyby will allow the MASCS instrument to “stare” at a handful of selected targets such as surface craters as the spacecraft passes overhead, says CU-Boulder senior research associate William McClintock.

“We will be pointing at each individual target from several different angles during the flyby, which will allow us to collect more data,” says McClintock of CU-Boulder’s Laboratory for Atmospheric and Space Physics and a Messenger mission coinvestigator who led the development of the MASCS instrument. The MASCS team is particularly interested in unusual surface deposits spotted by the camera during Messenger’s previous flybys, McClintock says.

“One of the big questions planetary scientists have is how much iron there is on Mercury’s surface,” explains McClintock. “We hope to pinpoint the iron, determine what chemical form it is in and how it is bound up on the planet’s surface.” Iron, which dominates Mercury’s core, is responsible for maintaining the planet’s magnetic field.

The dynamic magnetic field of Mercury absorbs and stores energy from the powerful solar wind, periodically “snapping like a rubber band” and driving charged particles into the planet’s surface, says McClintock. The collisions cause atoms of sodium, potassium, and calcium—and likely iron, silicon, and aluminum—to be ejected into the planet’s wispy atmosphere, he adds.

Some of the atoms are then accelerated by solar radiation pressure into the gigantic gas cloud tail, while others drift back down to the planet’s surface, only to be lofted once again into the exosphere, where they make their way into the gaseous tail, he says.

McClintock says that after the third and final flyby, the researchers will have collected about the same amount of data as they will gather during a single orbit around Mercury. Once Messenger settles into a yearlong pattern of twice-a-day orbits around Mercury in 2011, analyzing the massive streams of images and data “will be like drinking from a fire hose,” says McClintock.

The 4.9-billion-mile journey to Mercury requires Messenger to make more than 15 loops around the sun to guide it closer to Mercury’s orbit. The craft is equipped with a large sunshade made from a heat-resistant ceramic fabric to protect it from the sun.

“During this third encounter, the Messenger camera will again image areas never before seen at close range, and we will obtain color images of other regions at resolutions superior to those of previous observations,” says Messenger principal investigator Sean Solomon of the Carnegie Institution of Washington.

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