Before something suddenly halted the heartbeat of Magellan Thursday night, the robot spacecraft "opened a crack in the door" to the mysterious planet Venus with images of a violently torn-up terrain similar to parts of the American West, Hawaii or Africa, scientists said yesterday.

Magellan's first pictures of Venus's cloud-cloaked surface are strips showing up to 90 miles each of a volcanic highland known as Beta Regio. They feature ridges and valley floors where volcanic flows have hardened, networks of fractures and small craters, said project scientist R. Stephen Saunders, who released the pictures at a briefing.

The new images are about 10 times as detailed as any previously obtained of Venus, Earth's sister planet, and "they tell us that Venus is intensely fractured and faulted down to . . . about 400 or 500 feet across," he said, which is the smallest scale Magellan can see. "I'm somewhat surprised to see the degree of faulting."

Aspects of the terrain seem similar to the Rift Valley system of East Africa, the inner-mountain American West and the slopes of Mauna Loa crater in Hawaii, he said.

The unexpectedly revealing slices of Venusian landscape, obtained during the craft's first test of its mapping radar sensor, were all the team got before Magellan's computers put it into a "safe mode" Thursday.

The computers reacted when, just as the spacecraft was using guide stars to check its position, "the heartbeat stopped," said John P. Slonski, spacecraft systems engineer.

The so-called "heartbeat monitor" is designed to tell an onboard computer about the health of the spacecraft's attitude control system with 90 signals, or "beats," every minute, he said. Its stoppage triggered a series of pre-programmed computer reactions that kept the spacecraft safe but also put it out of touch with Earth for more than 14 hours, sending shivers through its handlers at the Jet Propulsion Laboratory in Pasadena, which manages the project for NASA.

Magellan officials said they hope to restore the spacecraft, which is in apparent good health but still partially in suspended animation, so that it can resume its intended mission only a few days late -- possibly the first week of September -- with no loss of scientific data.

Magellan's $750 million mission is to provide the most detailed and complete map ever of the planet nearest Earth.

Teams of investigators reading through data relayed from the spacecraft have pieced together what happened after it went into the safe mode, but they haven't found out why.

"We don't know what triggered the initial safing event," said A.J. "Tony" Spear, Magellan project manager.

He and others speculated that the culprit was a cosmic ray that hit Magellan and altered a single digit in its computer program. But he said engineers are considering other possibilities, including unusual electrical charges around Venus and a fault in the spacecraft itself.

Another data dump from the spacecraft expected Aug. 29 should help engineers zero in on the answer, Slonski said.

The antenna that transmits at the high rate required for the scientific data shut down in mid-transmission when the problem occurred. It had sent back three-fourths of its photographic data; the rest is still on board waiting to be retrieved.

Last week, the scientists labored over their computers to translate those long-awaited first returns into meaningful images, even as the crisis progressed around them, and at 4 a.m. Friday, "the images snapped into focus," said radar system chief engineer William Johnson.

The scientists were "elated" at the images but because they didn't know at that point whether the spacecraft was still functional, he said, there was a brief fear that these would be the last, that "we were holding $750 million worth of data in our hands."