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RINGWORLD
Ringing Out The Bugs On Route To Saturn And Titan

Cassini is a very expensive mission to deep space that must work
by Bruce Moomaw
Cameron Park - June 26, 2001
Working the problem has been a keystone of the space program for decades. Whatever the cause of a problem, if at all possible, a solution must be found. And for the past year a deep space mission team has been working to solve a communication problem that will happen in 2004 when for a few hours two spacecraft communicate as they set about exploring Sol's biggest lightweight Titan during the first phase of Cassini's four-year mission to Saturn.

As previously reported, a design error with Cassini went unnoticed until last year, when it was found that the two billion dollar probe's radio receiver wasn't properly tuned for the Huygens mission. The design fails to take into account that Cassini -- if its original mission design is followed -- will be hurtling towards Titan at fully 21,000 km per hour while Huygens parachutes down through Titan's 300-km high atmosphere.

The result of this approach speed would be a Doppler shift in the RF signal big enough to significantly interfere with Cassini's ability to receive scientific telemetry from Huygens. Without correction, this could lead to the loss of fully half of Huygens' scientific data.

Since the discovery of the problem last fall, the European Space Agency (which built both Huygens and its data receiver on Cassini) has organized a "Huygens Recovery Task Force" to consider various schemes to modify the mission to recover most or all of Huygens' data.

Last week, the Cassini mission's Project Science Group held a meeting in Britain which considered the Task Force's final recommendations as to the best plan - which the Group has tentatively adopted.

The plan is a bit of a surprise, in that it involves a bigger change in the early part of Cassini's mission than was initially thought likely.

Previously, it was hopped that some relatively small corrective measures could be followed such, as having Cassini follow Huygens toward Titan at a closer distance to double its received signal strength.

However, the Task Force's more recent tests indicated that the problem was more severe than had been thought - and so, in the end, it recommended a more radical scheme.

The original plan was for Cassini to follow Huygens on a path leading almost straight toward the edge of Titan, with the spacecraft finally flying past Titan at a distance of only 1200 km four hours after Huygens entered Titan's atmosphere.

This would mean that Cassini would be hurtling on a virtual straight-line path toward Huygens during the probe's entire three-hour data-transmission period - which would simplify its operations by allowing Cassini to keep pointing its big high-gain communications dish (which, during this period, will work instead as the receiving antenna for Huygens' data) straight forward, without Cassini ever having to slew around to change its pointing direction.

But this scheme, as I've said, would also mean that Cassini would be hurtling straight toward Titan - and Huygens - at 21,000 km/hour, with a resultant Doppler shift that raises the frequency of the received Huygens radio signal to move much of it out of Cassini receiver's preset frequency range.

However, if - after releasing Huygens on its collision course with Titan three weeks earlier - Cassini then changed its course so that it flew past Titan at a range of several tens of thousands of kilometers, this would, paradoxically, greatly improve the receiver's ability to pick up Huygens' signal.

Instead of racing straight toward Titan and Huygens, Cassini would be swinging past them at considerable distance - and so, during that same three-hour communications period, the actual straight-line distance between Cassini and Titan would shrink at a considerably slower rate (although Cassini would have to slowly slew around to keep its antenna pointed at Huygens).

Thus the Doppler shift in the received signal would also be much less, keeping it in the receiver's full operating range.

The Task Force finally decided that Cassini, during Huygens' arrival, should fly past Titan at a distance of fully 65,000 km farther from Saturn (since Huygens will be landing on the side of Titan pointing away from Saturn).

Also, it should follow much closer behind Huygens, so that Cassini would make its closest approach to Titan only about 2.1 to 2.3 hours after Huygens began parachuting down through Titan's atmosphere.

By the simple law of the Pythagorean Theorem, this means that when Huygens arrived at Titan and began transmitting back its data, Cassini would be only about 80,000 km from Titan - and, 2.2 hours later, it would make its closest approach at 65,000 km range.

Thus, during those 2.2 hours, it would be approaching Titan at an average speed of only 6800 km/hr, rather than 21,000.

  • Click For Part Two




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