6:05 a.m. EST:

Space News Staff Writer Peter de Selding reports live from Huygens mission control:

DARMSTADT, Germany-- A network of powerful ground telescopes has picked up the signal of Europe's Huygens descent probe 1.2 billion kilometers away, confirming that the probe is alive as it begins its descent into the thick atmosphere of Saturn's largest moon, Titan.

European Space Agency officials at Huygens mission control here said the signal -- no more than the equivalent of a telephone dial tone -- was detected by a network of 18 telescopes deployed to listen for a signal coming directly from Huygens.

The biggest of these antennas is the 100-meter-diameter Robert C.

Byrd Green Bank Telescope, operated by the National Radio Astronomy Observatory in Green Bank, West Virginia.

The signal did not confirm anything beyond the fact that Huygens is alive. But it was enough to cause a burst of applause here when announced at 5:35 a.m. EST (10:35 a.m. GMT)  today. 'There is a lot of emotion in this room," said Jean-Pierre Lebreton, Huygens program manager at ESA. "It's great news."

Leonid Gurvitz, mission manager for Huygens' communications with the ground telescope network, said 18 telescopes including Green Bank had been trained to pick up a signal and that it is the network itself, more than any single telescope, that received the Huygens signal.

NASA's Cassini satellite, which carried Huygens to Saturn orbit, has been moved into position to receive Huygens mission data during the probe's 2.5-hour descent into Huygens' thick atmosphere.

A more-complete assessment of whether Huygens' parachutes have deployed and its heat shield jettisoned to permit the start of observations is expected to be received by science teams from Cassini around 11:20 a.m. EST (1620 GMT) today.

 6:00 a.m. EST: If it switched on as planned, a microphone instrument aboard Huygens may allow researchers to recreate the sound of the probe's descent as it plunged through Titan's atmosphere, ESA mission scientists said. The instrument may also record thunder, and Huygens scientists hope to have at least initial data to present within 24 hours.

5:50 a.m. EST: Cautious ESA commentators stress the Huygens signal is just a carrier tone. There is no confirmation that the six science instruments aboard the probe are working as planned.

"It looks like we heard the baby crying," said Huygens mission manager Jean-Pierre Lebreton from the floor of ESOC mission control. "But clearly it tells us the probe is alive, the entry has been successful and we are under parachutes."

5:35 a.m. EST: Huygens speaks from Titan! ESA has confirmed that the Green Bank Telescope successfully detected a Huygens signal tone. The signal, a confirmation that Huygen's transmitter is at least functioning, and activated on time at about 5:18 a.m. EST.

About 600 people are at ESOC mission control for Huygens Titan descent and some engineers crowded around computer monitors when the signal confirmation was announced.

"It's a tremendously exciting moment," said John Dodsworth, Huygens ground manager at ESOC.

5:30 a.m. EST: If everything is going well, researchers may be able to assemble Huygens first pictures of Titan within 24 hours, U.S. astronomer Martin Tomakso, the international lead of the probe's only optical instrument, has said.

"The first images may be pretty murky," Tomasko said from Darmstadt, adding that Titan's nitrogen-rich atmosphere is quite hazy. "But we think the haze has a bottom, and that once we get through it we'll have a clear view of the surface."

Tomasko said that during its slow descent, Huygens will most likely be swinging and rocking while taking numerous images that will later have to be assembled into mosaics. The Sun should appear 10 times smaller from Titan than it does from Earth, so researchers expect a sort of twilight environment, he added.

5:15 a.m. EST: According to its timeline, Huygens should now be transmitting data to its Cassini mothership, after deploying a series of parachutes to slow its descent. The first few measurements could already be in Cassini's data file, researchers say. But they will not know for sure whether Huygens transmitter is working unless Green Bank is successful in picking up the tone, or from Cassini once it turns back toward Earth.

5:00 a.m. EST: ESA's Huygens probe is scheduled to reach Titan in the next few minutes. While all Huygens data will be recorded by Cassini for later playback to Earth, astronomers are hoping that the powerful 100 by 110-meter Green Bank radio telescope in West Virginia will pick up a simple tone from the probe.

4:15 a.m. EST: Al Diaz, NASA's associate administrator of the Science Mission Directorate, is monitoring the joint Cassini-Huygens operations at ESA's ESOC mission control center in Darmstadt.

"It's a mission unlike anything we've tried before," he said today of Cassini-Huygens.

4:00 a.m. EST: ESA is providing a real time tracking of Cassini and Huygens. You can see find it here. At last report, Huygens was set to reach Titan at about 5:05 a.m. EST, then deploy parachutes a few minutes later. The Cassini orbiter was last reported about 72,000 kilometers from Titan. It has turned away from Earth to record any data Huygens is able to broadcast.

3:30 a.m. EST: ESA mission controllers are playing a waiting game now with Huygens' mothership Cassini. The NASA orbiter has apparently turned away from Earth and is oriented toward Titan to receive data fromthe Huygens probe.

European commentators have their fingers crossed that Huygens' descent goes better than a recent - but fictional - probe to Titan in the BBC science fiction series Space Odyssey. In that series, which follows a crew of astronauts exploring the solar system, an automated Titan probe fails after deployement.

3:25 a.m. EST:  Huygens mission controllers report the probe is hurtling toward Titan at about 22,000 kilometers an hour.

3:20 a.m. EST:  John Dodsworth, of ESA's ESOC mission control center at Darmstadt, said Huygens is about two hours from reaching Titan interface - about 1,270 kilometers above the surface. The probe is right on target and will land well within its target, he added. 

3 a.m. EST: ESA officials are counting down to the Huygens probe's arrival at Titan. The probe is still a few hours off from encountering the moon's atmosphere.

"I certainly am jumping all over in anticipation," said Claudio Solazzo, ESA Huygens Mission operations manager, from Darmstadt, Germany. "Today is a great day."

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The Cassini Quest Learn more about the Cassini-Huygens project with these video journey behind the mission.