Huygens began its two-hour descent to Titan's surface at about 2 a.m. PST (5 a.m. EST), aided by a parachute, and began transmitting a signal to scientists monitoring its progress at the European Space Agency's Space Operations Center in Darmstadt, Germany and Nasa's Jet Propulsion Laboratory in Pasadena.

The team of ESA and Nasa scientists in Darmstadt cheered with relief on receiving confirmation of the carrier signal from the JPL at about 2:30 a.m. PST (5:30 a.m. EST), and gathered around a monitor at the Space Operations Center.

"There is a signal coming back from Titan," ESA scientist John Dodsworth said. "This is a tremendously exciting moment."

Dodsworth said the probe at that point was "busy taking data" about 37.3 miles above the moon's surface.

"It looks like we heard the baby crying," mission manager Jean-Pierre LeBreton said. "It tells us that the probe is alive. The entry is successful and we are on the parachute."

Scientists believe the organic chemical reactions taking place on Titan resemble the processes that gave rise to life on Earth 4 billion years ago. Its atmosphere is mostly nitrogen, like Earth's, but its surface temperatures of about minus 292 F make it inhospitable to life.

The saucer-shaped probe was designed to rotate on its way down, snapping the first-ever high-definition, panoramic images of Titan's thick, smoggy atmosphere and its landscape.

Along with its six scientific instruments that measure the components of Titan's atmosphere, Huygens carries a sound recorder and a lamp to look for signs of surface liquid.

Huygens was expected to reach Titan's surface at 4:30 a.m. PST (7:30 a.m. EST). The first data was expected to reach Earth about 7:30 a.m. PST (10:30 a.m. EST).

Titan, believed to be the only moon in the solar system with an atmosphere, is larger than the planets Mercury and Pluto.

It is believed to have liquid methane and ethane on its surface, but the moon's heavy fog blanket makes it unclear what Huygens will encounter when it reaches its landing site.

The $3 billion Cassini-Huygens mission, a joint project of Nasa and the European and Italian space agencies, was launched in 1997 from Cape Canaveral, Florida, to study Saturn, its rings, its moons and its magnetosphere.

In December, Cassini dropped off Huygens on a three-week journey toward Titan, now culminating in the probe's parachute-assisted plunge to the moon's surface.

The 705-pound probe was supposed to immediately begin transmitting data gathered by its onboard instruments and camera to Cassini.

Scientists will not know whether the instruments worked and the data was sent until Cassini turns toward Earth and begins sending the information back home.

Some science team members monitoring the flight have waited decades to see the first of 750 planned images and other scientific readings from the yellow-skied moon.

Huygens was named for the Dutch scientist who discovered Titan in 1655, Nasa officials said.