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Huygens probe set for final descent

European spaceflight controllers are now making their final communications with the Huygens probe, soon to be deployed on the last leg of its journey to Titan.
This image of the Saturn moon Titan was taken by the Cassini spacecraft earlier this month during a flyby.
This image of the Saturn moon Titan was taken by the Cassini spacecraft earlier this month during a flyby. NASA / JPL / SSI

European spaceflight controllers are now making their final communications with the Huygens probe, soon to be deployed on the last leg of its journey to Titan. Once it separates from the Cassini mother ship at 10:08 PM ET on Dec. 24, they will never be able to speak to it again. And even when (or if) they again hear its voice, it will only be an echo after it has already died.

Seven years of space cruising, and before that a decade or more of design and assembly work, now comes down to a four-hour window on Jan. 14. That is when, as the probe hurtles the last few miles to the surface of Titan, Huygens is supposed to wake up, measure the Saturn moon's atmosphere, snap images of the approaching surface, and then -- it is hoped -- survive the landing for at least a few minutes and perhaps as long as two hours.

What happens after that won't matter, because the probe will have no way to communicate with the Cassini relay ship after it "sets" in the sky of Titan. Cassini’s orbit around Saturn and Titan’s rotation on its axis will take the Huygens landing site over the horizon and will sever the line-of-sight link.

Only then will Cassini turn its large antennas back toward Earth and beginning playing back what – if anything – it heard from Huygens. One potentially fatal flaw already has been diagnosed in the radio system, and engineers hope both that workaround plans will dodge that space bullet and that there aren’t any more such bullets out there in deep space.

A separate team of radio astronomers are trying to patch together a worldwide net of coordinated radio telescopes that may actually be able to hear the original weak signals from the Huygens probe.

Playbook for a space playback
Huygens is expected to feel the first wisps of Titan's atmosphere at 790 miles (1270 km) above the moon's surface, a destination it is set to reach at 4:07 AM ET on Jan. 14. Fifteen minutes earlier, a timer is supposed to wake up the probe so that it can begin sending data back to Cassini.

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The probe is expected to take about 135 minutes, give or take 15, to make the descent to the surface, all the while streaming its observations (including more than a thousand images) skyward. Two separate channels are used for redundancy. One data stream lags the other by six seconds, so that a brief communications hiccup wouldn't result in the loss of any information.

If Huygens survives its landing on Titan, it could continue to send data from the surface for some time, but the batteries are charged only to last 156 minutes, with possibly about half an hour more power.

Only after the last possible radio receptions are received from the probe will Cassini begin transmitting the precious signals to Earth. Multiple copies of the data will be stashed in different locations in the spacecraft's computer memory, so that any circuit failure won’t lose it, and the data will be sent back to Earth more than a dozen times during the next day. Racing at the speed of light, the signals will take more than an hour to cross the chilling distance of space from Saturn to Earth.

The first full copy of the data set should begin to arrive on Earth at 10:24 AM ET on Jan. 14, and be completed within half an hour. Only then will the world know the fate of the Huygens probe.

The drama of the Titan landing has been heightened by concerns over whether the communications equipment on Huygens will even work. Already, engineers on Earth discovered one astounding fatal design flaw in the hardware. Fortunately, they had enough time to develop a workable backup procedure.

Because the probe will slow down dramatically when it hits the moon's atmosphere, the radio signals it sends to Cassini will undergo a sudden severe Doppler shift. The European designers of Huygens knew this and built their equipment to accommodate it.

What wasn't correctly implemented was a feature the receiver hardware on Cassini needs to correctly break up the data stream into the intended words. Each word in the stream is punctuated by a special "synchronization pulse" to indicate its beginning -- and that pulse is also affected by the Doppler shift, turned into a shorter burst.

The receiver's own clock was not flexible enough to respond to this large a change in duration, however, and as a result, the incoming data would have been interpreted as gibberish. The science and imaging information would have been unreadable and probably unrecoverable. A series of design review boards studied the plans, and all missed this flaw.

However, communications engineers still worried about this feature of the design, and after Cassini was launched they conducted a series of long-range radio communications tests that uncovered the flaw. NASA trajectory designers then reworked Cassini’s flight path so that it was offset from the path of the descending Huygens. This lowered the amount of Doppler shift into a range that the existing hardware could handle.

Intense design reviews showed no further flaws, but now it is up to Titan itself to put the designs to the ultimate test.

Peeking at the data stream
Radio astronomers have recently designed an experiment to allow them to precisely track the probe. This will require several widely-spaced radio telescopes to each detect the weak signals from Huygens during its descent, thus providing advance knowledge of its success or failure prior to the replay of data from Cassini.

The JIVE radio telescope in Dwingeloo, The Netherlands will coordinate a worldwide network of about twenty radio telescopes in an attempt to combine the receptions into one precise path.

“They intend to use a technique called Very Long Baseline Interferometry (VLBI) to pick-up the probe’s faint radio signal here on Earth and use it for reconstruction of the trajectory of descent of the probe in the atmosphere of Titan,” the European Space Agency said earlier this month. The scientists expect to be able to pinpoint the probe’s location to within about half a mile at a range of 900 million miles.

To practice, the network has already tuned in to the beacon on Huygens twice in the last four months, while it was orbiting Saturn still attached to Cassini.

"The Huygens tracking experiment pushes the applications of VLBI technologies to their extremes,” ESA admitted, but hailed it as "an example of the synergy between experimental techniques developed in fundamental astrophysics and based on radio physics, quantum electronics and digital signal processing, with planetary science and exploration of the Solar System by deep space probes."

Large radio telescopes in the United States, such as Green Bank in West Virginia, will also attempt to detect the signal. NASA spokeswoman Caroline Martinez said that all real-time tracking announcements will be coming out of the ESA center in Germany, but that the Jet Propulsion Lab was "sending a few people to Green Bank to help with this experiment.”

Martinez also noted that the hoped-for tracking precision would take several weeks of computer processing, as opposed to the relatively simple task of simply picking up the signal at any one location.

If Huygens does make it successfully to Titan it will mark the the sixth alien world to see a human-built robot.