After an absence of more than 25 years, a spacecraft from Earth will be going back to the sizzling planet Mercury before the end of the decade.
NASA gave the green light yesterday to the Messenger project, a $256 million effort to put a Maryland-built spacecraft in orbit for a year around the planet closest to the sun.
Built in Maryland
The craft will be designed, built and operated by the Johns Hopkins University Applied Physics Laboratory in Laurel.
"It's never been done, and the reason is there are two formidable challenges," said Sean C. Solomon, of the Carnegie Institution in Washington, the mission's principal investigator.
"The first is you've got to slow down the spacecraft enough to get into orbit when it's in the deep gravity pull of the sun," he said.
Messenger will solve that problem by using four Venus and Mercury flybys over five years to slow its orbit relative to the sun and Mercury.
The second threat is heat. Mercury is 36 million miles from the sun - a third of Earth's distance. At that distance, the sun is 11 times brighter than at noon on Earth.
A sunshade made from the same ceramic as the space shuttle will keep Messenger's instruments operating at room temperature, Solomon said.
But "the side facing the sun will experience temperatures as great as 400 degrees centigrade [752 degrees Fahrenheit]. That's hot."
Last visit in 1975
The last time Mercury had a visitor from Earth was in 1975, when Mariner 10 made the last of three high-speed flybys. It managed to photograph less than half the planet's surface.
The launch of Messenger (which is short for Mercury Surface, Space Environment, Geochemistry and Ranging) is scheduled for March 2004. The first Venus flyby will occur three months later.
The craft will enter Mercury orbit in April 2009.
