Almost 30 years after Mariner 10 first pulled back the veil on Mercury, NASA hopes to launch another spacecraft to pick up the investigation where it left off.
The craft, called MESSENGER (a contorted acronym for "Mercury Surface, Space Environment, Geochemistry and Ranging" mission), would launch in the spring of 2004, and arrive at the innermost planet of our solar system by January of 2008. Along the way, the spacecraft would conduct two flybys of neighboring Venus, in 2006 and 2007. MESSENGER would perform two flybys of Mercury in 2008 before settling into orbit in 2009.
Mission planners hope the $280-million project will open up a planet whose proximity to the sun and small size have made it difficult to explore. Mercury orbits the sun at 36 million miles (58 million kilometers) and is tiny; its diameter is just 3,031 miles (4,878 kilometers). These conditions have made Mercury difficult both to view and photograph from Earth and to investigate with remote devises, said Sean Solomon, the missions principal investigator.
"Its a hard target to study," said Solomon, who is director of the department of terrestrial magnetism at the Carnegie Institution of Washington.
Because of its proximity to the sun, it is hard to slow a spacecraft down as it approaches Mercury. Using gravity assists, however, Solomon said it could be done - just as Mariner 10 proved possible 25 years ago.
Mariner 10 flew by Venus in February 1974 before arriving at Mercury that March. It later flew by twice more in late September, imaging approximately 45 percent of the planets pockmarked surface.
The brief mission has left scientists with a suite of tantalizing questions about Mercury. Among them are:
*Why it is so dense? (Mercurys iron core is two-thirds the size of the planet),
*What has its past geologic history been? (There is still disagreement whether volcanic material has ever spilled onto its surface)
*What sort of materials lurk at its poles?
The last question is among the most perplexing for Mercury, a planet where surface temperatures can reach 365 degrees Fahrenheit (600 Celsius).
Radar imagery acquired in 1991 showed that deep craters at the poles remain in perpetual shadow, where temperatures remain hundreds of degrees below zero. Additional data revealed that the craters are highly reflective at certain wavelengths, indicating the presence of water ice.
The mission will also shed light on the evolution of all four terrestrial planets, Mercury, Venus, Earth and Mars, Solomon said.
The project spacecraft, selected earlier this year as part of the Discovery Program of low-cost, highly focused missions, will be designed and built by the Applied Physics Laboratory at Johns Hopkins University in Baltimore, Md.
The Europeans and Japanese are also in the early stages of planning missions to Mercury.
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