PASADENA, CALIF., SEPT. 24 -- When the scientists exploring Venus with the robot Magellan spacecraft Sunday got their first good look at a cluster of craters on Earth's sister planet, they whooped with delight at the astonishing detail captured by the spacecraft's radar eyes.

After a rough start last month, in which Magellan kept switching itself off, flight contollers here at the Jet Propulsion Laboratory (JPL) have tamed the spacecraft, which now appears to be working flawlessly, and started its epochal, 243-day mission to map the surface of one of the most elusive planets in the solar system.

Perpetually shrouded in clouds of acid vapor, almost none of the Venusian surface has been seen by human eyes. Magellan's radar, however, beams through the clouds and records a reflected radar image as clear and sharp as a photograph.

The image before the dozen planetary geologists, vulcanologists and radar specialists who quickly gathered to peer and compare ideas showed that vast, complex forces have dramatically carved, scarred, fractured and twisted the surface of Venus.

"Some of the things we're seeing are very exciting," said Duane Bindschadler, a geophysicist, as he sat at a video screen and stared at one image.

He scanned around the picture, describing the various forms -- meandering channels, shear points where the Venusian crust had been wrenched apart, a wide valley that looked like the rock walls had collapsed into it.

"Here, there have been numerous flows. It's complex, radar-bright, linear, either compressional or extensional," Bindschadler said. "Yeah, this is great. Here is a smooth, radar-bland plain, and set in that are these troughs. That suggests lava flow, but it's bendy, trending east and west. From that, I infer that they're controlled by structural trends. Maybe it's a collapsed lava flow.

"The surface originally was faulted. There was extensive vulcanism . . . and this area here looks really interesting. We see these upwellings on Earth, but it's important to try to understand how similar processes occur on Venus."

Bindschadler described a sense of wonderment that comes with discovering a new world: "It's the element of exploration -- the discovery of new things. I grew up on 'Star Trek' -- 'where no man has ever gone.' And now, with Magellan -- before this image was taken, no one had ever seen this before!"

With every orbit Magellan's radar mapper scans a strip of the planet's surface about 16 miles wide and thousands of miles long. "Like looking through a crack in a fence," said one researcher. But now, imaging specialists at JPL have electronically assembled adjacent strips, revealing a surface area hundreds of miles wide -- the equivalent of a knothole in the fence.

The cluster of craters, apparently caused by huge meteorites that crashed randomly into Venus millions of years ago, stands in an expanse of surfaces webbed by cracks, meandering troughs or depressions, scars where the crust has pulled apart and mounds of material ejected from the meteorite impacts and strewn like irregular breastworks across the Venusian landscape.

Although the dim outlines of the cluster, called The Crater Farm by Magellan scientists, had been detected in earlier Soviet and U.S. efforts to map Venus, Magellan's image of the formation stunned researchers with its detail.

The excitement mounted as they milled around, peering over each other's shoulders at the transparency, roughly 8 by 10 inches in size, that lay on the table.

"They look incredibly wonderful," said Ellen Stofan, a post-doctoral geologist from Brown University. "These fractures are amazing."

Project scientist R. Stephen Saunders said that after just a few days of mapping, Magellan's images suggest startling new avenues of investigation and thought about Venus. "We're going to have to work harder and think harder," he said.

It has taken years of work, heart-wrenching delay and more than $700 million to get to this moment. But scientists say they hope Magellan will yield clues about why Venus, long thought to be geologically like Earth, may be a harbinger of Earth's fate if global warming theories prove correct.

Because the atmosphere of Venus is thick with heat-trapping carbon dioxide, is thought to be a planet in the throes of a runaway greenhouse effect. Its surface temperature is about 900 degrees Fahrenheit. One hope is that the images will suggest how a geologically Earth-like planet could get in such trouble.

Day by day, as Venus rotates under Magellan's orbit, the spacecraft will pass over a new ground-track, scanning a new swath of surface, or "noodle" as some here call it.

For quick scanning by the investigators, noodles are produced in long strips, like contact prints from movie film, that are tacked up on the lab walls. The images are also stored in digital form on compact discs, with the idea that eventually a researcher would be able to simply type in a set of coordinates and pull up a display on a video screen.

Magellan researchers are grouped into teams headed by senior professors from U.S. and foreign universities and by scientists with the U.S. Geological Survey, NASA, JPL and private corporations. Staffed with graduate students and "post-docs," who already have their PhDs, the groups specialize in specific features such as craters or large volcanoes or small volcanoes. There is even one group focusing on surface-modification processes -- the "dirt group."

John A. Wood, a geologist with the Harvard-Smithsonian Astrophysical Observatory and leader of the dirt group, is seeking evidence of erosion -- a phenomenon difficult to imagine on a planet without surface water and where the surface winds are generally thought to waft at a zephyr-like 2 mph to 4 mph. But on Venus it isn't that simple. The pressure of the Venusian atmosphere is 90 times the sea-level pressure on Earth -- about the same as pressure 2,700 feet beneath the ocean.

"You'd think two to four miles per hour wouldn't blow things around much, but the density of Venus's atmosphere is far greater than ours," said Wood. "It's about halfway between water and air."

The Venusian atmosphere is distinctive in another way. "The {amount of} carbon in both planets {Earth and Venus} is about the same, but on Earth, almost all the carbon is fixed in fossil fuels, in limestone formations in the rock," Wood said. "On Venus most of the carbon is in the atmosphere -- as if we had burned all the oil, coal, timber, limestone, and so forth, on Earth. Which of course, is exactly what's happening."