Messenger Nears Third Flyby of Mercury

Blogger update, 7:30 p.m.: A NASA update reports the Messenger spacecraft is performing well following its close approach. The first images are expected to be released by 10 a.m. Wednesday.

A NASA spacecraft that launched from Cape Canaveral in 2004 is hours from a final close approach of Mercury before its planned 2011 entry into orbit around the solar system's innermost planet.

The Messenger spacecraft will swing within 142 miles of the planet's surface at 5:55 p.m. during a flyby that will take more than 1,500 images and make targeted observations with seven science instruments.

In addition to those observations, the flyby will use Mercury's gravity to slow the spacecraft by 6,000 mph, adjusting its trajectory as it prepares to drop into orbit in 18 months.

"The maneuver itself is absolutely critical to the rest of our mission," said Sean Solomon, the mission's principal investigator from the Carnegie Institution of Washington, during a media briefing last week. "We must fly by Mercury hitting the right aim point."

Check out real-time tracking of the flyby here.

The $446-million Messenger mission launched Aug. 3, 2004, from Cape Canaveral Air Force Station on a Delta II rocket.

During two previous flybys in January and October of last year, the 2,400-pound spacecraft has imaged 90 percent of Mercury, much of it areas that had never before been seen. Mariner 10 imaged only about half the planet in the early 70s.

The third flyby today will add another 5 percent.

"We now have a nearly global view," said Solomon. "We'll be missing only some polar areas that we'll catch in the orbital phase."

The flyby will provide more data about the comet-like "tail" of atoms streaming from Mercury's tenuous atmosphere, pushed away by solar radiation, and more detail about the planet's composition.

Messenger will be flying 30 million miles from the sun, about two-thirds closer than the Earth. Protected by a sun shade, its instruments will run at room temperature, swiveling back and forth to measure targeted areas, while the temperature on the shade exceeds 600 degrees Fahrenheit.

The spacecraft will whiz by at a relative velocity of 12,000 mph. After the gravity assist, Messenger's orbit around the sun will decrease by 13 days, close to Mercury's 88-day period.

Messenger - an acronym for the Mercury Surface, Space Environment, Geochemistry and Ranging spacecraft - is expected to begin returning data from the flyby to Earth close to midnight tonight.

IMAGE NOTE: The Messenger spacecraft took the image above of Mercury 55 hours prior to today's closest approach, which will occur at 5:55 pm EDT. The image shows previously unseen terrain but at a much lower resolution than will be obtained when the spacecraft is closer to the planet. Credit: NASA. Below, an artist's concept of Messenger at Mercury. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.