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Mercury Mission to Take Long, Cool Look at a Hot Planet

Posted on: Thursday, 29 July 2004, 06:00 CDT

Mercury, the planet closest to the sun, will be getting its chance to become more scrutable starting next week when an instrument-crammed spacecraft called Messenger is to take off on a seven-year journey.

Following a circuitous trip through the inner solar system, the craft will become only the second to visit mysterious Mercury and the first to orbit it for long-term study.

Messenger is scheduled to lift off for Mercury from Cape Canaveral, Florida, on Monday atop a Boeing Delta 2 rocket, but engineers will have only a 12-second launch window that day.

Mercury, a small, rocky body slightly larger than Earth's moon, is difficult to study with spacecraft because of its proximity to the sun. The planet's size and solar orbit also make it difficult to slow a speeding spacecraft enough to be captured by its gravity for orbital studies.

Three decades ago, the National Aeronautics and Space Administration sent its Mariner 10 spacecraft to Mercury on a pioneering mission. Looping around Venus and the sun, the craft made three swift flybys of Mercury during 1974 and 1975, sending back almost 1,000 pictures that mapped only 40 percent of the surface of the heavily cratered planet. Mariner found iron-laden Mercury to be the densest planet in the solar system and the only inner planet besides Earth with a global magnetic field, but left scientists wanting to know more.

"For nearly 30 years we've had questions that couldn't be answered until technology and mission designs caught up with our desire to go back to Mercury," said Sean Solomon of the Carnegie Institution of Washington, Messenger's principal investigator. "Now we are ready."

Since Mariner 10, ground-based observations have shown that Mercury is surrounded by an immense sodium cloud, but no one knows how it is being sustained. And radar probes from Earth show highly reflective areas in the polar regions that some scientists suggest may indicate some form of ice in cold, permanently shadowed craters.

"Studying Mercury's surface, tenuous atmosphere and magnetic field" is a key to understanding the evolution of the inner solar system, including Earth, said Daniel Baker of the University of Colorado, a lead investigator for one of Messenger's seven scientific instruments.

Messenger is one of NASA's lower-cost, rapidly executed Discovery Program robot missions designed to go from planning to flight in about three years. It uses a relatively small, inexpensive launching vehicle. The $427 million mission requires getting gravitational boosts from planetary flybys to get the spacecraft to Mercury and maneuver it into orbit.

Engineers are sending Messenger on a journey that takes it past Earth once, Venus twice and Mercury three times. The craft will swing by Earth a year after launching for a boost to Venus, which it will fly by in October 2006 and June 2007 for course changes that take it past Mercury in January and October 2008, and again in September 2009.

Solomon said the Venus flybys would be used to test Messenger's instruments and, he hoped, gather new scientific information, while the Mercury flybys would be used to map the previously uncharted parts of the planet and gather other data critical to planning the orbital phase of the mission.

The importance of timing to the mission is illustrated by an 11- week delay that added almost two years to getting the spacecraft into Mercury's orbit. Messenger was originally scheduled for launching around May 11, but NASA delayed the flight to resolve software issues and spacecraft testing. The change meant scrapping a flight plan that involved no Earth flyby, three trips around Venus instead of two and two passes past Mercury instead of three that would have put the craft in orbit in July 2009 instead of March 2011.

Engineers and scientists at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, had to design and build a special spacecraft to survive the rigors of the space around Mercury.

David Grant, project manager at the laboratory, from which the mission also will be directed, said designing a spacecraft hardened against the heat, solar radiation and brightness of the nearby sun was a challenge. "We're doing something no one has ever tried before," he said.

The main body of the spacecraft, made of a lightweight, heat- tolerant graphite composite material, is covered with multilayered insulation and peppered with radiators and heat pipes to dissipate heat. The craft is powered by two electricity-generating solar panels. The most distinctive feature of the spacecraft is a large, highly reflective, heat-resistant sunshade attached to the front on a titanium frame. The shield is made of front and back layers of Nextel ceramic cloth surrounding inner layers of Kapton plastic insulation.

Messenger is to circle Mercury for one Earth year in an elliptical orbit that takes it 124 miles, or 198 kilometers, above the surface at its perigee, the nearest point, and 9,420 miles at its apogee, the farthest. Cameras will transmit images many times sharper than Mariner's; a battery of spectrometers will identify the elements and compounds on the surface; a laser altimeter will map landforms and distances; and other experiments will study variations in the planet's gravity and magnetic field.

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