October 28, 2006
MESSENGER: Mission to the Innermost Planet
Orbiting too close to the Sun for conventional telescopes to risk a glance and for ordinary spacecraft to approach, hot and rocky Mercury remains the
October 28, 2006
MESSENGER: Mission to the Innermost Planet
Orbiting too close to the Sun for conventional telescopes to risk a glance and for ordinary spacecraft to approach, hot and rocky Mercury remains the least explored of the terrestrial planet family that includes Venus, Earth, and Mars. But thanks to a pioneering APL spacecraft, Mercury's elusive reputation is about to change.
Launched on August 3, 2004, the APL-built MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft is slated to start orbiting the planet in March 2011. While NASA's missions to Venus and Mars have produced exciting data and theories about processes that formed the inner planets, scientists expect Mercury to yield its own fascinating story.
MESSENGER's mission: complete a detailed exploration of the inner solar system and help us understand the forces that shaped planets like our own.
Start of a Journey. Soon after MESSENGER's predawn liftoff from Cape Canaveral Air Force Station, Florida, computer screens in the MESSENGER Mission Operations Center at APL lit up with electronic word that the spacecraft had come out of a launch-induced tumble, deployed its two solar panels, and was operating normally.
The celebrations had to be short; the critical task of operating a first-of-its-kind planetary explorer was at hand. But the moment was a significant milestone for the most ambitious and challenging NASA mission that APL has ever led.
MESSENGER faces a long cosmic road: more than 6 years, nearly 5 billion miles, 15 loops around the Sun, and multiple flybys of Earth, Venus, and Mercury before beginning a yearlong study of the innermost planet. In a way it's the project's second journey, coming after a five-year odyssey in which APL and its determined partners set out to push the limits of engineering and technology and pull off a space science first.
Technical Innovation. Only one spacecraft has visited Mercury; NASA's Mariner 10 sailed past it three times in 1974 and 1975 and gathered detailed data on less than half the surface. An orbiter mission was the next logical step, but high technical risks and potential price tags ($1 billion in some studies) cooled NASA to the idea of sending a probe back to the hot planet.
APL began shaping concepts for a Mercury orbiter in the mid-1990s —incorporating new technologies, tougher materials, and creative mission designs that gradually made the mission more feasible. In 1999 NASA selected MESSENGER as the seventh in its Discovery Program of lower-cost, scientifically focused exploration projects, and gave APL the formal nod to build the spacecraft in 2001.
The technical challenges of working in Mercury's orbit are intense. The planet's average distance from the Sun is only 36 million miles, about two-thirds closer than Earth. MESSENGER could face the intensity of 11 suns while orbiting the dayside of Mercury, then endure a steep temperature drop while it passes over the nightside. (Mercury experiences the solar system's largest swing in surface temperatures, from highs above 840°F to lows below —350°F.)
The double threat of searing sunlight and roasting heat bouncing off the planet below made it necessary to build a tough spacecraft. But MESSENGER also had to be lightweight, since most of its mass would be fuel to fire its rockets and slow the spacecraft down enough for Mercury's gravity to capture it. And it had to be compact enough to lift off on a conventional and cost-effective rocket.
The MESSENGER team tackled each of these challenges. A ceramic-fabric sunshade and a mission design that limits time over Mercury's hottest regions will protect MESSENGER without expensive cooling systems. The spacecraft's graphite-epoxy composite structure—strong, lightweight, heat tolerant, and the first of its kind for a deep-space satellite—is integrated with a low-mass propulsion system that efficiently stores and distributes the propellant that accounted for 55% of MESSENGER's launch weight.
On the way, an innovative trajectory will swing MESSENGER past Earth, Venus, and Mercury several times, using the tug of the planets' gravity to adjust the spacecraft's path and position it to orbit Mercury. This "economy route" takes longer but requires less fuel than a straight shot to the planet, ultimately lowering costs and creating more room on the spacecraft for science instruments and systems.
Tackling Challeng