Designed and built at the Applied Physics Laboratory, the solar-powered MESSENGER measures 4.7 feet tall, 6.1 feet wide and 4.2 feet deep. Its two square side-mounted solar panels extend 20 feet tip to tip. The spacecraft is equipped with seven miniaturized scientific instruments, a computer system, maneuvering thrusters, a main engine, communications equipment, navigation sensors and a large sunshade, all crammed into a half-ton dry-weight package.

Artist's impression show MESSENGER at Mercury. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

The sun shines 11 times brighter at Mercury than at Earth and MESSENGER will experience temperatures as high as 700 degrees when the planet is closest to the sun. While getting to Mercury posed one major challenge, engineers also had to come up with a way to protect the compact spacecraft from the extreme heating it will experience in Mercury orbit.

The solution was an 8-by-6 foot sunshade, made of Nextel ceramic cloth surrounding multiple layers of Kapton insulation, mounted on a titanium frame attached to the side of the spacecraft that will always face the sun.

"The front of the sunshade will get up to about as hot as a pizza oven while the rest of the spacecraft will be at nearly room temperature," said James Leary, mission systems engineer at APL.

Radiators and heat pipes also are in place to carry heat away from the body of the spacecraft, which is protected by multilayer insulation. As an added precaution, MESSENGER's orbit was designed to minimize the effects of heat radiating away from Mercury's surface.

As a result of all that thermal protection, MESSENGER's subsystems and science instruments did not require expensive high-temperature electronics. But the spacecraft still requires an unusual level of complexity.

MESSENGER's twin solar panels measure 5-by-5.5 feet each and generate a combined 640 watts in Mercury orbit. They could produce up to 2,000 watts of power, but output is limited to what the probe's electrical system actually needs to minimize stress.

Two-thirds of each sun-facing panel is made up of rows of mirrors, with two rows between each row of solar cells. With the arrays tilted to reduce heating, only 28 percent of the sunlight hitting the panels will be converted into electricity.

MESSENGER is equipped with a main engine capable of producing 150 pounds of push, along with four 5-pound maneuvering thrusters and a dozen 1-pound jets for small maneuvers. To beam back its scientific data, the spacecraft will use two phased array high gain antennas, one on each side of the craft. Rather than using more common steerable antennas that must be mechanically aimed at Earth, MESSENGER's stationary antennas are electronically steered, the first such antennas ever used on a NASA deep space mission.

The craft's electronic brain is made up of redundant Integrated Electronics Modules, each one built around a radiation-hardened RAD-6000 PowerPC processor. Two solid state data recorders, each capable of storing a gigabyte of information, are on board to store images and other science data until they can be transmitted to Earth.

Once in orbit around Mercury, MESSENGER will beam back eight hours of data per day as it passes through the high point of its orbit. But the distance between Earth and Mercury varies from 54 million to 132 million miles. When the planets are at their closest, up to 104,000 bits of data can be transmitted to Earth each second. When they're at their farthest, the data rate drops to 9.9 bits per second. As a result, data will be prioritized and only the most significant imagery and engineering data will be transmitted when Mercury is on the far side of the sun.

"It's a wonderful mission, it's one that's addressing a range of scientific questions at the scale of an entire planet," Solomon said. "One must ask why it hasn't been done for 30 years. And that has to do with the engineering challenges of first building a spacecraft that can withstand the environment at Mercury and then designing a very clever mission that makes use of advances in orbital mechanics."

MISSION PREVIEW
   PART 1: MESSENGER MISSION READY TO LAUNCH
   PART 2: THE LONG AND WINDING ROAD
   PART 3: THE 'HEAT' IS ON
   PART 4: A COMPLEX SCIENCE AGENDA