Engineers and scientists at the Johns Hopkins University Applied Physics Laboratory in Laurel, Madison, had to design and build a special spacecraft to survive the rigors of the space around Mercury. The planet orbits at an average distance of 58km million from the sun during its 88-day year, about 80km million closer than the Earth, and is subjected to sunlight 11 times as intense.
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, powered by two electricity-generating solar panels, weighs 1,099kg at launching, half of that rocket fuel for its maneuvering engine and control thrusters.
The most distinctive feature of the spacecraft is a large, highly reflective, heat-resistant sunshade attached to the front on a titanium frame. Measuring 2.4m-tall and 1.8m-across, the 0.6cm-thick shield is made of front and back layers of Nextel ceramic cloth surrounding inner layers of Kapton plastic insulation.
Temperatures on the front of the white shield could reach 371?C when Mercury is closest to the sun, engineers said, but the spacecraft on the shady side should operate at a of 20?C. Messenger is programmed to keep the shade between itself and the sun at all times.
Messenger is to circle Mercury for one Earth year in an elliptical orbit that takes it 200km above the surface at its perigee, the nearest point, and 15,160km at its apogee, the farthest. Cameras will transmit images many times sharper than Mariner's, showing features as small as 18m across; 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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