NASA spacecraft to visit Mercury

January 14, 2008 7:47:49 AM PST
A NASA spacecraft will fly by the planet Mercury on Monday, the first visit to the sun's closest neighbor since the 1970s. The space probe Messenger will skim 124 miles above the planet's surface, the first of three passes before it settles into orbit three years from now.

The flyby will provide up-close views and, in a few weeks, pictures.

"We're expecting some pretty major surprises out of this," said Faith Vilas, a Messenger scientist and director of the MMT Observatory at Mount Hopkins, Ariz.

Scientists are hoping that what they learn next week will help them begin to answer lingering questions about the planet's origin, magnetic field, atmosphere and what that means about our own planet.

Mercury is a "real oddball," said lead researcher Sean Solomon of the Carnegie Institution of Washington. The planet is so close to the sun that temperatures vary at the equator between day and night by 1,100 degrees. At the same time, there is also evidence of ice at the poles.

It's the smallest planet in the solar system. For comparison, if Earth were the size of a baseball, Mercury would be a golf ball.

"We really need better information on Mercury to make sure that our ideas on how the Earth and sister planets formed" are accurate, Solomon said.

Among the mysteries is Mercury's magnetic field, discovered by NASA's Mariner 10 in the 1970s.

Scientists had thought that because of its small size, the planet's core had long ago solidified and would not have a molten interior, the most common explanation for a magnetic field. Last year, scientists using powerful radar signals to examine Mercury's rotation found evidence of wobbling that they say shows the planet has a molten outer core. Messenger's magnetometer is expected to provide further clues.

Mariner 10's three passes by Mercury provided information on less than half of the planet.

"Probably the first thing that most of us want to see is what the other 55 percent of Mercury looks like," said Vilas.

William McClintock, who led the development of a Messenger spectrometer at the University of Colorado at Boulder's Laboratory for Atmospheric and Space Physics, said scientists have only a "vague idea" about the composition Mercury's surface. Readings taken during Monday's pass should provide much more information on the planet's composition, formation and evolution, he said.

In addition to more than 1,200 images of the planet, Messenger will collect other data that are expected to begin streaming back to Earth when Messenger emerges from behind Mercury on Tuesday. The data are expected to take about a week to transmit, said Eric Finnegan, Messenger mission systems engineer.

NASA plans to release images and results from the flyby on Jan. 30.

Messenger, which began its journey in 2004, is the seventh in NASA's Discovery program of lower-cost, scientifically focused space missions. The Johns Hopkins University Applied Physics Laboratory, which built and operates the probe, manages the mission for NASA.