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January 31, 2008—A color image taken by the MESENGER spacecraft shows the side of Mercury previously unseen by human eyes.

The shot, released yesterday, is a mosaic of images from the craft's 11 narrow-band color filters.

Subtle patterns revealed by the filters—which can capture light in wavelengths invisible to the naked eye—will help astronomers determine the mineral composition of the planet's surface.

The bright spots with a bluish tinge are relatively recent impact craters. Some of these have bright streaks, called “rays.” The streaks are made from crushed rock that was blasted outward during an impact.


The large, light-colored circle in the upper right of the image is the inside of the Caloris Basin. The only previous mission to Mercury, Mariner 10, viewed only the eastern (right) portion of this enormous impact crater.

MESSENGER has now shown that Caloris is filled with smooth plains that are brighter than the surrounding terrain—the opposite of the shading differences on Earth's moon.


Louise Prockter of Johns Hopkins University, a member of the MESSENGER science team, says she and her colleagues are mystified by a unique feature they're calling ''the Spider,'' seen above.

The formation lies in the middle of the large, 3.8-billion-year-old impact crater called Caloris Basin, and it consists of more than a hundred narrow troughs radiating from a complex central region.

The whole feature is raised off the floor of the basin. The scientists aren't sure whether the small crater near the center of the Spider has anything to do with its odd, raised shape.

One interpretation suggests that the troughs formed when materials in the Caloris Basin floor broke apart.


Since Pluto's demotion to ''dwarf planet'' last year, Mercury is the smallest planet in the solar system. At just 3,030 miles (4,880 kilometers) across, it's barely bigger than Earth's moon. Still, it's been a frequent target for meteoroid and asteroid impacts.

NASA's MESSENGER mission photographed impact craters all over Mercury's surface during a January 14 flyby.

Some of the craters are 100 feet (30 meters) or bigger. Smaller craters were probably made by castoffs from larger impacts. In some of the craters, volcanic material has apparently oozed out to form a smooth floor.

The density of craters on a planet's surface can indicate the relative age of different areas. That's because craters made long ago would weather with time, creating a smoother surface, while younger regions sporting recent impacts would show sharper features.

In this image alone, 763 craters have been identified and measured (shown in green) along with 189 hills (shown in yellow).


The Mariner 10 mission to Mercury in the 1970s hinted at long, high cliffs traversing the planet's surface. But that mission imaged just under half of the planet—and astronomy's cameras have improved since then.

This image from the MESSENGER flyby shows a region, previously unseen by any spacecraft, that includes a large cliff crossing vertically through the far right.

The presence of many long, high scarps suggests a history for Mercury unlike that of any other planet in the solar system. The giant cliffs probably formed when Mercury's interior cooled and the entire planet shrank as a result.

A large U.S. spy satellite has lost power and could hit Earth in late February or early March, government officials said Saturday.

The satellite, which no longer can be controlled, could contain hazardous materials, and it is unknown where on the planet it might come down, they said.

The officials spoke on condition of anonymity because the information is classified. It was not clear how long ago the satellite lost power or under what circumstances.

"Appropriate government agencies are monitoring the situation," said Gordon Johndroe, a spokesman for the National Security Council, when asked about the situation after it was disclosed by other officials.

"Numerous satellites over the years have come out of orbit and fallen harmlessly. We are looking at potential options to mitigate any possible damage this satellite may cause."

He would not comment on whether it is possible for the satellite to perhaps be shot down by a missile, saying it would be inappropriate to discuss any specifics at this time.

A senior government official said that lawmakers and other nations are being kept apprised of the situation.

The spacecraft contains hydrazine, a rocket fuel, according to a government official who spoke on condition of anonymity. A colorless liquid with an ammonia-like odor, hydrazine is a toxic chemical that can cause harm to anyone who contacts it.

Uncontrolled reentry of the probe could risk exposure of U.S. secrets, said John Pike, a defense and intelligence expert. Spy satellites typically are disposed of through a controlled reentry into the ocean to render the spacecraft inaccessible, he said.

Pike also said it's not likely the threat from the satellite could be eliminated by shooting it down with a missile, because that would create debris that would then reenter the atmosphere and burn up or hit the ground.

Pike, who is director of the defense research group GlobalSecurity.org, estimated that the spacecraft weighs about 10 tons and is the size of a small bus.

effrey Richelson, a senior fellow with the National Security Archive, said the spacecraft likely is a photo reconnaissance satellite.

Such "eyes in the sky" are used to gather visual information from space about the activities of adversarial governments and terror groups, including construction at suspected nuclear sites or militant training camps.

The satellites also can be used to survey damage from hurricanes, fires, and other natural disasters.

The largest uncontrolled reentry by a NASA spacecraft was Skylab, the 78-ton abandoned space station that fell from orbit in 1979. Its debris dropped harmlessly into the Indian Ocean and across a remote section of western Australia.

In 2000 NASA engineers successfully directed a safe de-orbit of the 17-ton Compton Gamma Ray Observatory, using rockets aboard the satellite to bring it down in a remote part of the Pacific Ocean.

In 2002 officials believe debris from a 7,000-pound (3,175-kilogram) science satellite smacked into Earth's atmosphere and rained down over the Persian Gulf, a few thousand miles from where they first predicted it would plummet.

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