NASA lost contact with the Venus-orbiting Pioneer 12 spaceship Thursday as the craft began a slow, fiery death plunge into the planet's atmosphere.
The radio signal loss came 14 years after starting a mission meant to last 243 days.
"Initial indications are that it is gone or damaged," said Jack Dyer, deputy chief of space projects at NASA's Ames Research Center in Mountain View.
The signal was lost, possibly forever, after the spacecraft made its closest approach to Venus after 3 p.m. EDT, Dyer said.
Before the spacecraft stopped transmitting, it collected valuable measurements of unexplored regions of Venus' upper atmosphere, NASA said.
The Pioneer Venus Orbiter was launched from Cape Canaveral on May 20, 1978. On Dec. 4 of that year, it became the first U.S. spacecraft to go into orbit around the second planet from the sun.
The 810-pound spacecraft, built by Hughes Aircraft Co. in El Segundo, was designed to study Venus for 243 days, and most engineers thought it would operate only two or three years, said Dyer.
"We never dreamed it would last this long," he said. "We're extremely pleased with how long it lasted."
Pioneer was plunging lower into Venus' atmosphere each time it completed another long, elliptical orbit, Dyer said. Friction from the spacecraft's hypersonic entry into the atmosphere had been expected to melt its radio equipment, he said.
Even if contact is briefly restored, much of the spacecraft will burn up during the next few days as its hulk circles the planet and continues its plunge, Dyer said. It is possible some titanium metal parts might crash to the planet's surface, he said.
The orbiter was part of a joint mission with Pioneer 13, which was launched in August 1978 and sent four measurement-making probes into the Venusian atmosphere before it was incinerated.
Pioneer 12 was the first spaceship to use radar to peer through Venus' thick clouds and make a crude topographic map of 93 percent of the planet's surface. The radar pictures revealed canyons, plateaus, plains and mountains, including 7-mile-high Maxwell Montes, Venus' tallest peak.
Since early September, engineers ordered Pioneer 12 to fire its main thrusters every five days to prevent the spacecraft from plunging into the atmosphere. It ran out of thruster fuel Saturday.
Pioneer 12 was the 11th spacecraft to reach Venus. To date, 22 Soviet and American spaceships have visited the planet.