Radio link may hamper a Titan probe

Parachuting through the orange haze of an alien moon, a small vehicle splashes down onto a frigid hydrocarbon sea. During the descent, its camera records the strange landscape while other detectors analyze the atmosphere’s organic brew. Together, the data provide a glimpse of what conditions may have been like on Earth billions of years ago, just before life got its toehold.

Artist’s depiction of Huygens probe descending through Titan’s atmosphere. JPL

That’s the scene scientists hope to see in 4 years, when the Huygens probe is scheduled to separate from its mother craft, Cassini, and descend through the cloud-bedecked atmosphere of Titan, Saturn’s largest moon. The mission could prove whether this moon harbors hydrocarbon seas (SN: 3/4/00, p. 156: Available to subscribers at Getting a Clear View), which could provide the raw materials for life.

The scientists might be in for a disappointment, however. A communications problem, first detected in February and confirmed last month, could prevent Huygens from relaying some of its data.

The European Space Agency (ESA) announced Oct. 5 that the radio frequencies Huygens will transmit during its descent won’t perfectly match those that can be received by Cassini, orbiting overhead. If technicians can’t solve the problem, a precious 20 minutes or so of the probe’s 2.5-hour descent into Titan’s atmosphere could be lost.

“It’s a serious hit to the mission,” says Earl Maize, Cassini spacecraft operations manager at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. NASA operates Cassini, which during its tour of Saturn will briefly act as a relay for Huygens. ESA built the probe.

The communications mismatch will become problematic when Huygens parachutes through Titan’s atmosphere and the relative motion between the probe and the orbiting Cassini rapidly hanges. Akin to the changing pitch of an ambulance siren as the vehicle approaches and speeds away, the difference in the probe’s speed relative to Cassini shifts the radio signals it transmits to higher or lower frequencies. Somehow the consequences of this well-known Doppler shift were not fully taken into account by engineers before Cassini’s 1997 launch.

This week, at ESA’s operations center in Darmstadt, Germany, scientists met to discuss the communications flaw. One solution may be to slow the Cassini craft during the 2.5 hours in which the probe descends. That would reduce the Doppler shift, permitting the craft to receive more of the probe’s radio signals. An ESA investigations board will release a full report on Dec. 15.

The flaw “is not fatal and can be rectified with modifications,” says Cassini scientist Carolyn C. Porco of the University of Arizona in Tucson. The challenge, she notes, is to make modifications that don’t compromise other parts of the Saturn mission.

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