While Mercury may appear as a battered surface, a NASA spacecraft gliding over its surface for the second time this year has revealed more previously unseen real estate on the innermost planet. The probe also has produced several science firsts and is returning hundreds of new photos and measurements of the planet's surface, atmosphere and magnetic field. Image of Mercury captured by MESSENGER on the probe's second approach. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

"Now that MESSENGER's cameras have imaged more than 80 percent of Mercury, it is clear that, unlike the moon and Mars, Mercury's surface is more homogeneously ancient and heavily cratered, with large extents of younger volcanic plains lying within and between giant impact basins," said co-investigator Mark Robinson of Arizona State University in Tempe. Spacecraft images also are revealing for the first time vast geologic differences on the surface.

The MErcury Surface, Space ENvironment, GEochemistry, and Ranging, or MESSENGER, spacecraft flew by Mercury shortly after 4:40 a.m. EDT, on October 6. It completed a critical gravity assist to keep it on course to orbit Mercury in 2011 and unveiled 30 percent of Mercury's surface never before seen by a spacecraft.

This is no small task. Point your spacecraft in the right direction to pass by the planet as it orbits the Sun, and get whatever information you can during the high-speed rendezvous. That was NASA's strategy during the early days of Solar System exploration, and it provided them with an incredible amount of insight into the universe. The difficulty level gets turned up a notch when talking about "orbital insertion". You're not just flying by the planet; you're getting there and then slowing down to the point where the world that you want to explore pulls your spacecraft into orbit around it. Too slow and your probe will drop into the planet's atmosphere and burn up. Too fast and you'll fly off into space, stuck helpless in a solar orbit for billions of years.

NASA's MESSENGER will head into space with all the energy and speed it had when it left the Earth, and then have to slow down to meet up with Mercury. This means MESSENGER has to burn off a lot of energy during its journey. One way is to carry a large amount of fuel and perform braking maneuvers during the trip. But fuel already accounts for more than half of MESSENGER's total launch weight; adding more would weigh down the spacecraft to the point where it wouldn't be practical to launch at all. Image to left: Workers at Pad 17-B prepare the Delta II payload fairing for installation around MESSENGER. On August 2, 2004, as the spacecraft prepared for a 7-year trek to Mercury. Credit: NASA.

"The region of Mercury's surface that we viewed at close range for the first time this month is bigger than the land area of South America," said Sean Solomon, principal investigator and director of the Department of Terrestrial Magnetism at the Carnegie Institution of Washington. "When combined with data from our first flyby and from Mariner 10, our latest coverage means that we have now seen about 95 percent of the planet."

The spacecraft's science instruments operated throughout the flyby. Cameras snapped more than 1,200 pictures of the surface, while topography beneath the spacecraft was profiled with a laser altimeter. The comparison of magnetosphere observations from the spacecraft's first flyby in January with data from the probe's second pass has provided key new insight into the nature of Mercury's internal magnetic field and revealed new features of its magnetosphere. The magnetosphere is the volume surrounding Mercury that is controlled by the planet's magnetic field.

"The previous flybys by MESSENGER and Mariner 10 provided data only about Mercury's eastern hemisphere," explains Brian Anderson of the Johns Hopkins University Applied Physics Laboratory, known as APL, in Laurel, Md. "The most recent flyby gave us our first measurements on Mercury's western hemisphere, and with them we discovered that the planet's magnetic field is highly symmetric."

The Mercury Atmospheric and Surface Composition Spectrometer observed Mercury's thin atmosphere, known as an exosphere. The instrument searched for emissions from sodium, calcium, magnesium, and hydrogen atoms. Observations of magnesium are the first detection of this chemical in Mercury's exosphere. Preliminary analysis suggests that the spatial distributions of sodium, calcium, and magnesium are different. Simultaneous observations of these spatial distributions, also a first for the spacecraft, have opened an unprecedented window into the interaction of Mercury's surface and exosphere.

The project is the seventh in NASA's Discovery Program of lower-cost, scientifically focused missions. APL designed, built and operates the spacecraft and manages the mission for NASA's Science Mission Directorate in Washington. Science instruments were built by APL; NASA's Goddard Space Flight Center in Greenbelt, Maryland; the University of Michigan, Ann Arbor; and the University of Colorado, Boulder. GenCorp Aerojet of Sacramento, California, and Composite Optics Inc. of San Diego, provided the propulsion system and composite structure.