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Days from nominal tour start

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Days from nominal tour start

The primary and backup delivery timelines for the Huygens probe to Titan, which rejoin the original planned orbital tour with either the T3 or T6 fly-by, respectively.

The Saturn Orbit Insertion burn would be slightly longer than originally planned, in order to lower the apoapsis of the capture orbit and advance it by one month. The burn to raise the periapsis would be made on 23 August rather than 25 September, and be increased by 57 metres per second to 393 metres per second. As a result of these changes, the encounter with Titan on the inbound leg would be advanced by 32 days, from 27 November to 26 October. Cassini would then fly past the moon at an altitude of 1,200 kilometres with the probe still in place, and make observations to measure the winds to refine the antenna-pointing requirements for the final phase of the probe's relay. It would have another an opportunity to verify the winds during its second fly-by on 13 December.

After refining its trajectory on 17 December, Cassini was to release the probe on 24 December. Five days later, Cassini would perform the deflection manoeuvre in order to open the range of its fly-by on 14 January 2005 - the day the probe was to reach Titan - to 60,000 kilometres. Although the increased range would marginally diminish the received strength of the probe's signal, the offset fly-by would greatly constrain the Doppler shift because, although Cassini would still cross Titan's orbit at 6,800 metres per second, the radial component responsible for the Doppler shift would peak at 2,500 metres per second. It would decrease as the spacecraft drew alongside the moon, pass through zero, then increase in the opposite sense as the spacecraft withdrew towards the end of the probe's descent. Thus, the frequency drift ought never to leave the narrow band to which the receiver was tuned. Cassini was to rotate to maintain its high-gain antenna pointing at the probe's predicted position, taking into account both the spacecraft's fly-by and the probe's motion in Titan's atmosphere. Within the constraints of its firmware, which could not be altered,

An artist's impression of Cassini releasing the Huygens probe. Note that a certain licence has been taken concerning the range from Titan when this occurs.

In the revised fly-by to receive the data from the Huygens probe, Cassini was to make a distant pass in order to reduce the radial component of its relative velocity and thus minimise the Doppler effect, and slew around to keep its high-gain antenna aimed at the probe's predicted position during the descent in order to maximise the signal-to-noise ratio.

In the revised fly-by to receive the data from the Huygens probe, Cassini was to make a distant pass in order to reduce the radial component of its relative velocity and thus minimise the Doppler effect, and slew around to keep its high-gain antenna aimed at the probe's predicted position during the descent in order to maximise the signal-to-noise ratio.

Huygens's software was to be rewritten to optimise its transmission characteristics. Taken together, it was hoped that these measures would significantly improve the signal-to-noise ratio, and result in little or no loss of data. On the original plan, the first Titan fly-by would have occurred on 27 November and the second on 14 January. Tightening the capture orbit enabled the insertion of a 'short' orbit for the probe's mission. After a series of manoeuvres, Cassini would be able to return to Titan on 15 February 2005 and resume its primary mission just in time for the first encounter with Enceladus.120

''This plan will allow us to meet all of the mission's scientific objectives,'' noted Robert Mitchell, ''and has the additional advantage of giving us a close look at Titan before releasing Huygens.''

''It's a fantastic solution,'' agreed J.C. Zarnecki.

''We were in hell, now we're in paradise,'' said Marcello Fulchignoni, a Huygens scientist and astronomer at the Paris Observatory.

The probe was required to dive into the atmosphere at a depressed angle of 65 degrees below local horizontal in order to establish a deceleration profile that would enable the onboard sensors to initiate the sequence of events for the programmed descent. The landing site was defined by the approach geometry. The target ellipse was nominally 200 by 1,200 kilometres, but would be able to be reduced if the winds were measured in advance. Although the longitude of the target would remain 190°W, the geometry of the revised fly-by caused it to be moved from about 10°N to 10°S and the timing would be such that Cassini would cross Titan's orbit behind, rather than ahead of the moon.

The cost of this elegant solution was the propellant for manoeuvres equivalent to an overall velocity change of about 95 metres per second, but owing to the excellent navigation a substantial reserve had been husbanded, and consuming some of this to exploit the planning for the primary mission was considered to be acceptable tradeoff. In the event of a problem that prevented the probe's release, there was provision for postponing it by one revolution, to about 25 January, for the Titan encounter on 15 February. But if this was required, it would mean using more propellant - due in part to the probe still being in place during the manoeuvres - and it would impose a significant delay in rejoining the original tour. If all went as planned, however, the recovery plan would consume 25 to 30 per cent of Cassini's propellant reserve, and unless a further problem occurred that imposed additional manoeuvres, there was every possibility that the orbital tour would be able to be extended beyond 2008.

After Edward Weiler, NASA's Associate Administrator for Space Science, and David Southwood, ESA's Director of Science, accepted the revised plan, the Task Force met in Noordwijk once more to prepare its final report, which was issued on 27 July 2001.121 The reinvigorated science teams set about adjusting their plans for the revised portion of the orbital tour. By the end of August, the navigators had an accurate trajectory for the first few orbits, including the changes required to execute the probe's mission.

In November, the Goldstone Deep Space Network antenna played the role of the probe in a full rehearsal of Huygens's descent to Titan's surface, transmitting to the relay receivers on Cassini a stream of data in precisely the same format as the probe would utilise, and continuously adjusting the frequency to simulate the Doppler effect of the fly-by, while simultaneously factoring out the spacecraft's actual motion during the test. As Jean-Pierre Lebreton observed, ''We need to be certain that the modified mission will allow the receivers on board Cassini to operate within the narrow range of frequencies available.'' The tests took four days to complete. ''The whole test was very smooth,'' reported Julie Webster, the deputy manager of the Cassini Spacecraft Operations Office at JPL, who was responsible for interfacing between the JPL and ESA relay teams. ''We've taken major steps towards the validation of the Huygens Recovery Task Force design,'' agreed Earl Maize, her boss. ''We tested a nominal mission scenario, and several deviations from it,'' pointed out Claudio Sollazzo, the Huygens Mission Operations Manager in Darmstadt. ''We appear to have met all of our objectives.'' The data analysis would take months, but it was clear that the relay would work. Jean-Pierre Lebreton was very optimistic: ''Even under the worst-case conditions tested - with significant deviations from the nominal parameters - we've shown that we will retrieve all the data that the probe will send to Cassini during its descent, and for at least 15 minutes on the surface.''

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