Computer animation depicts the NEAR spacecraft resting on the surface of Eros. Photo: JHU TV/Spaceflight Now

The five-year, 2-billion mile voyage of the NEAR Shoemaker science probe came to an abrupt and most unexpected ending on Monday, surviving a four-hour descent to touch down on the surface of the asteroid Eros and live to relay a beacon signal to its home planet.

"This was a bonus," said Robert Farquhar, mission manager of the Near Earth Asteroid Rendezvous program, which is run by the Johns Hopkins University Applied Physics Laboratory for NASA.

For the past year, NEAR has been locked in orbit around Eros, a potato-shaped 21-mile long asteroid orbiting about 196 million miles from Earth. It is the first space mission dedicated to an asteroid.

NEAR provided unprecedented close-up views of the asteroid's surface as it swooped down. Photo: NASA TV/Spaceflight Now

NEAR was not designed as a lander, but program managers figured they had nothing to lose by attempting the unprecedented touchdown. As an engineering exercise, slowing the probe so that Eros' weak gravity field would pull it to the surface was interesting to scientists, but what they were really after were close-up, detailed photos of the asteroid's rocky surface during the descent. No one was disappointed on Monday, with NEAR performing flawlessly and transmitting richly detailed pictures until its radio link to Earth was broken by the probe's landing.

"The risk was worth taking," said Farquhar. "During our year-long study of Eros, we collected 10 times more data than originally planned. And now, at the end of the mission, we had a chance to gather close-up images of Eros'surface... so we took it."

Officials in mission control watch as NEAR beams back pictures to Earth. Photo: JHU TV/Spaceflight Now

A successful engine burn at 10:31 a.m. ET nudged NEAR out of its 16-mile high orbit and four braking maneuvers later in the day slowed the spacecraft to about 4 mph, relative to Eros -- slow enough to fall to the asteroid's surface. Managers didn't expect the probe to survive touchdown and only in their most optimistic moments did they ever expect to know NEAR's ultimate fate. But a beacon signal picked up by ground tracking stations after NEAR's 3:02 p.m. ET touchdown confirmed that the probe was still alive.

The images relayed during NEAR's free-fall to Eros join a 160,000-plus library of pictures that raise as many questions as they answer about the asteroid's origin and structure. Scientists now know that Eros is not a pile of rubble, but a solid body believed to have been formed about 4.5 billion years ago when the solar system was born. Among Eros' mysteries are why many of the asteroid's rocks are crumbling and how dust is being distributed over the asteroid's surface.