Associated Press
A MESSENGER space probe image provided by NASA shows the planet Mercury on Monday.

CHICAGO (MCT) — After 32 years and two space probe missions, mankind knows what the back side of the planet Mercury looks like.

About the same as the front.

But pictures of the planet, pockmarked by craters, baked by the sun — gratefully received by scientists on Earth overnight Tuesday — have finally filled massive holes in the map of Mercury that persisted since the last space probe visited the sun-blasted planet in the early 1970s.

"We're looking at the last terra incognita in the inner solar system," said Ralph L. McNutt, Jr. of the Johns Hopkins University Applied Physics Laboratory, the project scientist for the mission. "And it is beautiful."

The first picture — of a gray, stark, never-before-seen landscape emerging from shadow — reached Earth Tuesday evening. It includes a good view of the 800-mile-wide Caloris basin that had been only partially visible in the past. It was formed by the impact of a large asteroid or comet and is one of the biggest and possibly youngest such basins known to exist in the solar system.

More images and data followed throughout the night. Further images, as well as some analysis, may be available later Wednesday, McNutt said.

"Everybody is drooling over it," he said Wednesday morning.

Scientists from NASA and physics laboratory were gathered in meetings to consider their astronomical windfall. The images came from a close pass of Mercury; the real meat of the mission is not scheduled until the probe actually orbits the planet in 2011.

To the untrained eye, the planet looks like Earth's moon — heavily cratered, compact, and without significant atmosphere. But among the goals of the MESSENGER space probe is to determine what its unmapped surface is made of, why it has such a comparatively plump iron core, why it has a magnetic field when Mars and Venus don't, and what lurks in its shaded polar regions.

(The probe's name stands for MErcury Surface, Space ENvironment, GEochemistry and Ranging, an awkward acronym that lets it play off the mythical Mercury's role as messenger to the Roman gods.)

Disregarding Pluto, Mercury is the smallest planet in the solar system and orbits so near the Sun that it completes a circuit once every 88 days. That's so close that it can be seen from Earth only at dawn and dusk, a brilliant but brief show. The rest of the time, the Sun's light gets in the way.

With Earth, Mars and Venus, Mercury also rounds out the solar system's inner, rocky planets and is a place of extremes. It is the solar system's densest planet but also the one with the oldest surface and the one that is the least explored. Though it gets cold sometimes, it is mostly, as one would expect, very, very hot. (Temperatures during the noon hour: 800 degrees Fahrenheit. And because Mercury rotates on its axis so slowly, that "noon hour" lasts seven Earth days.)

Knowing its structure and life cycle is considered key to full understanding of how the solar system formed.

Despite interest in the tiny planet 29 million miles from the Sun, little has been known about Mercury, even after the Mariner 10 mission swung by three times during the Nixon and Ford administrations. After going all that way, Mariner only managed to map 45 percent of its surface. Earthbound telescopes have shed little more light on the planet since.

Astronomers think much of that gap in human knowledge was captured in 1,200 photographs during a 16,000 mph, 125-mile-high flyby on Monday.

The information was sent back to Earth in coded signals on Tuesday, as the probe moves onward in its 4.5-billion-mile journey to stable orbit around Mercury — a bit more than three years and another 2.6 billion miles from now.

Thanks to the need for gravity slingshots from other planets, MESSENGER already has taken a long and photo-happy trip through the inner solar system.

It thundered away from Earth atop a Boeing Delta II rocket in August 2004, and during a planetary flyby a year later took a haunting farewell video of Earth's spinning clouds and sun-glittered oceans. It swung past Venus in October 2006, and shot video of the retreating planet's yellow crescent on a second flyby last June.

The mission is being tracked and controlled by the Johns Hopkins University Applied Physics Laboratory in Laurel, Md.

Monday's swift transit of Mercury's darkened equator will be only the first of three passes by the planet, NASA says, as the probe slows itself for a dainty orbit insertion three years from now.

Still, hints about some of Mercury's most vexing questions may already have been revealed, scientists say.

"There's some very interesting things in the geology — or in the topography," said McNutt. "Everything is spectacular."