06 August 2004
06 August 2004
NASA's Messenger spacecraft launched earlier this week 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.
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-fibre 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, according to Robert Gold, the mission's payload manager.
With an abundance of solar energy permeating the craft, one might think that powering it would be simple. But too much power is also a problem, according to Gold. The solar panels must tilt so that they are almost edge-on to the sun to avoid frying in the radiation, and even then they are mirrored to help reflect excess energy.
""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.""
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.
To run Messenger's systems and charge its 23-ampere-hour nickel hydrogen battery, the panels, each about 1.5 meters (5 feet) by 1.65 meters (5.5 feet), will support between 385-485 watts of spacecraft load power during the cruise to Mercury and 640 watts during the science orbit. The panels could produce more than two kilowatts of power near Mercury, but to prevent stress on Messenger's electronics, onboard power processors take in only what the spacecraft actually needs.
The panels are 67 percent mirrors (called optical solar reflectors) and 33 percent triple-junction solar cells, which convert 28 percent of the sunlight hitting them into electricity. Each panel has two rows of mirrors for every row of cells; a total of 648 cells and 1,296 mirrors per panel. The small mirrors reflect the Sun's energy and keep the panel cooler.
The panels also rotate, so Messenger's flight computer will tilt the panels away from the Sun, positioning them to get the required power while maintaining a normal surface operating temperature of about 150 degrees Celsius, or 302 degrees Fahrenheit.
Messenger, short for MErcury Surface, Space ENvironment, Geochemistry, and Ranging, is the seventh mission in NASA's Discovery Program of lower cost, scientifically focused exploration projects. APL manages the mission for NASA’s Office of Space Science, built the spacecraft and will operate Messenger during flight. Messenger is the 61st spacecraft built at APL.
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