NASA to send Messenger

NASA’s Messenger spacecraft is scheduled to launch Sunday on a 5 billion-mile trip to Mercury – the first visit in almost 30 years to the most extreme and least studied of the inner planets.

It will take seven years for the $427 million probe to reach Mercury, the closest planet to the sun. The probe will orbit the planet for one year, taking color pictures of the entire surface, as well as gathering data on the composition and structure of the crust, the shape and strength of the magnetic field, the makeup of the core and the nature of the material in the polar craters.

Mercury is surprisingly different from the other rocky planets – Venus, Earth and Mars. The planet is made mostly of iron, making it the densest for its size in the solar system. No one knows why.

It has a magnetic field, even though scientists say such a diminutive planet – only Pluto is smaller – should not be able to maintain one. And it may have some water ice hiding in the perpetual shadows of the polar craters – despite temperatures as high as 850 degrees on the sun.

“It’s a pretty bizarre place,” said Sean Solomon, Messenger’s principal investigator from the Carnegie Institution in Washington, D.C.

Scientists are hoping that the planet’s strangeness may illuminate fundamental questions about the formation of the early solar system.

“What we’re really chasing is the processes that led to the formation of Venus and Mars and the Earth, and why they produced such different outcomes,” he said.

Mercury was last visited by Mariner 10, which swung by three times in 1974 and 1975 and glimpsed barely half of the planet’s wrinkled surface.

Messenger, managed by the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., will launch from Cape Canaveral Air Force Station in Florida.

For protection from the sun, which will be shining up to 11 times brighter than on Earth, the 1.2-ton spacecraft is constructed out of heat-resistant carbon-fiber material and shielded by a special ceramic cloth.

The cloth is such a good insulator that the front can be 700 degrees and the back will be room temperature, said Robert Gold, the mission’s payload manager.

With all the solar energy bombarding the craft, one might think that powering it would be simple. But too much power is a problem, Gold said. The solar panels must tilt so they are almost edge-on to the sun to avoid frying in the radiation; even then they are mirrored to help reflect excess energy.

A Mercury orbiter became feasible in the 1980s when Chen-Wan Yen of the Jet Propulsion Laboratory in Pasadena, Calif., discovered a complex flight path that uses gravity from three planets to slow the spacecraft down, passing by Earth once, Venus twice, and Mercury itself three times.

Even so, more than half of Messenger’s weight is fuel – almost all for the final entry into orbit, when the craft must slow down by more than 5,000 mph. It will settle into an elongated oval-shaped orbit, passing as close as 124 miles to the surface, and as far away as 9,400 miles.