Saving The Huygens Mission

In December 2000, the Huygens Communications Link Enquiry Board announced that while Cassini would receive the probe's tracking signal, the Doppler shift would degrade the telemetry subcarrier to barely 10 per cent of its intended strength. The problem was traced to an omission in the specification for the hardware.111

Cassini Mission Images

As originally planned, two days after releasing the Huygens probe on 6 November 2004, Cassini would have deflected the inbound leg of its 'capture orbit' to make a close fly-by of Titan on 27 November in order to receive the transmission from the probe as it entered the moon's atmosphere. However, due to a design oversight, the high speed of this straight-in approach would have Doppler-shifted the probe's transmission beyond the receiver's narrow frequency range.

As originally planned, two days after releasing the Huygens probe on 6 November 2004, Cassini would have deflected the inbound leg of its 'capture orbit' to make a close fly-by of Titan on 27 November in order to receive the transmission from the probe as it entered the moon's atmosphere. However, due to a design oversight, the high speed of this straight-in approach would have Doppler-shifted the probe's transmission beyond the receiver's narrow frequency range.

"We have a technical term for what went wrong,'' pointed out J.C. Zarnecki, the leader of the Surface Science Package team. ''It's called a cock-up!''

With the problem now understood, the Board set out to devise a solution that would allow the probe to be released as early as possible, while minimally changing the orbital tour in which so much planning had been invested. A number of actions were identified, each involving a degree of trade-off; these were not alternatives, they were options to be combined as deemed appropriate.

• Determine the prevailing wind direction and speed on Titan

Since the probe's drift would affect the Doppler shift, this option envisaged delaying the probe's release until the second orbit to enable the direction and speed of Titan's prevailing wind to be determined. Delaying the release until the third orbit offered the advantage of refining the moon's ephemeris in order to reduce the pointing errors of the spacecraft's high-gain antenna, and thereby improve the gain towards the end of the descent by a factor of 5 or more. Voyager 1 had measured the winds in the upper atmosphere as flowing from west to east at speeds of 200 knots, and this was recently confirmed by NASA's Infrared Telescope Facility on Mauna Kea in Hawaii.112 While there was a consensus that these conditions would probably persist, it was deemed unwise to risk the mission on a long-term weather forecast. It would be better to have Cassini verify the winds before releasing the probe.

• Exploit clock bias

The baseline mission called for a relative velocity of 5,534 metres per second during the relay. In-flight tests had revealed that the probe's transmitter clock was biased in a direction that would offset this by 1,200 metres per second. If the clock were to continue to drift, it might eventually compensate for the problem. However, the manufacturer could not confirm that the drift, which was temperature-driven, would continue at a predictable rate.

• Increase data transitions

The signal-to-noise ratio could be improved by up to 3 decibels by adding 'zero packets' in order to create a two-fold improvement in reception clarity, and because much of the data would be sent redundantly this would result in little or no data loss overall. This would be most effective at the beginning of the probe's entry, when Cassini would still be at its furthest point from its destination.

• Improved assumptions about the probe's antenna patterns

During its parachute descent, gusts in the wind would cause the probe to swing on its parachute like a pendulum. It would also spin on its parachute at a rate that would reduce from 15 revolutions per minute early in the relay to 1 revolution per minute towards its end, and the signal strength would vary with the angle to Cassini. The probe's antenna pattern was measured to determine the signal strength that could be assured for each 10- to 15-degree arc. The two redundant transmitters each had its own antenna. The signal via one transmitter was to be delayed by 5 seconds in order to preclude an overall loss of data in the event of a brief outage. One option was to endeavour to optimise this phasing to minimise interference between the antennas when the signal-to-noise ratio diminished towards the end of the descent.

• Reduce the probe's descent time

On the nominal timeline the probe's descent would take between 135 and 150 minutes. One option was to optimise this time for the performance of the radio link. Reducing the time the probe spent on its large parachute from 15 minutes to 5 minutes would sacrifice some of the early atmospheric data but would enable the probe to reach the surface more quickly and eliminate 10 to 15 minutes when the signal strength would be unusable.

• Reduce the Orbiter Delay Time

If the Orbiter Delay Time was reduced from 4 hours (as planned) to 3 hours, this would reduce the initial communication distance from 77,000 to 57,000 kilometres. This would increase the signal strength by 3 decibels at the start of the transmission, but complicate the final phase since Cassini would have to swing around to ensure that its high-gain antenna remained pointing at the probe. However, shortening the descent time would compensate for this complexity. If Cassini had to manoeuvre to point its antenna, it would be advisable to delay the probe's mission until the winds were confirmed, since refined antenna pointing would yield a 5-fold increase in the received signal strength during the final part of the descent.

Reduce the probe's flight time

As planned, the probe was to be released 22 days distant from Titan. Reducing the flight time would enable Cassini to enter a shorter orbit, which would reduce the relative velocity during the relay. However, the additional manoeuvring to establish such an orbit would cost propellant that would otherwise be available to facilitate extending the orbital tour beyond the primary mission.

Undertake the probe's mission on a later orbit

With so many fly-bys of Titan on the tour, postponing the probe's release to a later orbit would preserve the early part of the tour, and therefore exploit the detailed planning already in hand. Orbits in the middle of the tour would permit a high-altitude fly-by that would reduce the Doppler issue. However, as long as the probe remained in place, it would partially obscure the field of view of several of Cassini's instruments. Postponing the probe's mission for so long would also cost up to 15 per cent of the available propellant averaged over the tour. The nominal plan included the option of delaying the probe's release until the second orbit. If this was pursued, it would enable Cassini to verify Titan's winds during its first fly-by, and because the plan was to use this fly-by to trim the apoapsis, the shorter second orbit would reduce the relative velocity during the relay.

Redesign the first two orbits

A shorter capture orbit would enable Cassini to release the probe on its third fly-by from an orbit having a period of about 32 days (twice that of Titan) and the slower fly-by would be acceptable. With careful planning, the fly-by geometry would enable Cassini to 'rejoin' the planned tour.

Raise Cassini's fly-by altitude

On the plan, Cassini was to receive the probe's transmission while inbound, then skim Titan's leading hemisphere at an altitude of 1,200 kilometres. During the relay, the relative velocity would be 5.5 kilometres per second. If the range was opened to 50,000 kilometres, the relative velocity would be cut to 3.4 kilometres per second and the Doppler shift would be acceptable. The increased range would significantly decrease the signal strength, however. If this option was pursued on the first orbit, then Cassini would be required to increase its deflection manoeuvre just after the probe's release, which would cost propellant. The great advantage of this option was that it would solve the entire problem, even if no other measures were pursued. However, it would require the early part of the tour to be revised to contrive an additional 'short' orbit to provide a Titan slingshot that would re-establish the planned tour.

A joint ESA/NASA Huygens Recovery Task Force was established to study the possible recovery options and decide how to proceed. It met at the European Space Technology Centre in Noordwijk in the Netherlands on 10 January 2001 to consider the various proposals for improving the performance of the relay link, such as better ground processing and error correction, and changes in the software for the probe and orbiter. A further meeting at JPL was arranged for the following week. Later in the month, a Probe Relay Test 'mini-sequence' was uplinked to calibrate the Huygens receivers on board Cassini. During the test, which was conducted between 31 January and 5 February, the Deep Space Network commanded Huygens through Cassini and received the probe's response via the relay link. Each session ran about 10 hours per day, while the spacecraft was above the Goldstone antenna's horizon. In essence, this was a repeat of the test of February 2000, but this time the effort was driven by the need to fully characterise the performance of the relay receivers, to provide the Task Force with precise engineering data and to enable the engineering test vehicle in Darmstadt to be adjusted to precisely mimic its spacefaring partner. The Huygens team expressed themselves as being ''very delighted'' with the results. A routine check on 22 March showed the probe to be healthy. It was awakened every 6 months or so. ''As nearly as possible,'' explained Shaun Standley, an ESA systems engineer, ''we put the probe though all the stages of the real descent system.'' Of course, with the probe cocooned inside its aeroshell some equipment could not be exercised, and none of the once-only devices could be fired. Nevertheless, each instrument was tested in real mode and its engineering data routed to Cassini for transmission to Earth. In April, members of the Spacecraft Operations Office attended a meeting convened at the Alcatel factory in Cannes, France, and the Huygens Science Working Team participated in the process for the first time. The primary task was to review the Task Force's progress in determining how the relay receiver's performance would be influenced by the signal-to-noise ratio, the received frequency, and the data bit transition probability. On this basis, the meeting then selected recovery scenarios for further study in the coming months.

When Cassini's prime Command and Data System detected that its backup had initiated a series of resets on 10 May, it 'safed' the vehicle by halting all sequences, powering off of its science instruments, adopting an attitude in which its high-gain antenna would act as a Sun shade, selecting the low-gain antenna for communication, and reducing the downlink data rate to 20 bits per second -precisely as required by its fault-protection software. The fault was found to be a missing 'telemetry mode' table on the backup computer. The timing was unfortunate, as an important test was scheduled to provide information for the Task Force, but rather than cancel the test it was decided to proceed by issuing real-time commands. The test was to evaluate the ability of the thrusters to maintain a specific orientation within the requisite deadband of 0.5 milliradians, as a preliminary to deciding whether to have Cassini slew around to keep its high-gain antenna pointing at the probe during the final phase of the descent. The test was successful. Meanwhile, having overcome the CDS issue, the engineers restored Cassini to normal operation.

The Task Force's meeting in Pasadena in mid-May was attended by members of the Orbiter Science Team to enable them to be briefed on the recovery options and their impact on the orbital mission. The Huygens team presented the results of their analysis of the various data-return scenarios, and how they would affect the probe's science. The low-altitude and high-altitude relay options were refined and issues for follow-on studies were defined. The Task Force met again in Noordwijk two weeks later. A summary report on progress towards solving the problem was presented to the full Project Science Group held in Oxford, England, in mid-June. Meanwhile the Atmospheres Working Group considered the implications of the possible trajectory changes identified by the Task Force, and an 'Apoapsis Splinter Group' was formed to investigate integration issues in apoapsis periods. The Titan Orbiter Science Team finalised the integration of the first ten Titan fly-bys, and its plan was presented to the Oxford meeting.

The large number of actions that might improve the situation were encouraging, since they provided flexibility. In essence, however, three different scenarios were considered: (1) slower low-altitude fly-bys, (2) low-altitude fly-bys with improved navigational performance and (3) high-altitude fly-bys integrated into the planned orbital tour. It soon became apparent that the propellant cost of contriving a slower low-altitude fly-by was prohibitive.113 Furthermore, to have pursued such a solution would have required some (or all) of the subsequent orbital tour to be redesigned. To postpone the probe's mission to the second or third orbit of the original tour would enable Titan's ephemeris to be improved and increase the accuracy of the delivery and antenna-pointing, but it was decided that this would not produce a satisfactory mission.114,115 Nevertheless, these studies established that it would be possible to deliver Huygens from an orbit with a 32-day period, and identified the manoeuvre locations and tracking requirements.

Four 'high-altitude delivery' options were assessed in the context of the planned orbital tour. Flying the early part of the tour as planned would enable the completed detailed planning to be pursued. Thereafter, a high-altitude fly-by could be contrived to deliver the probe. Doing so after the 10th Titan encounter would guarantee the early fly-bys of the icy moons, but the rest of the tour would have to be redesigned and it might not be practicable to arrange the anticipated later encounters with the icy moons.116 Contriving a high-altitude fly-by after the 35th Titan fly-by would protect the Iapetus fly-by, but only, unfortunately, at the cost of the third and final inspection of Enceladus.117 Another study showed that it would be possible to arrange a high-altitude fly-by on the second Titan fly-by and then initiate a series of 'clean up' manoeuvres in order to resume the tour at the sixth encounter, but this could only be done at the expense of valuable propellant.118 An alternative would be to reconfigure the early part of the tour to arrange a high-altitude fly-by, and then rejoin the tour at the sixth encounter without such extensive manoeuvring.119 An overriding factor was the two years that had been invested in developing a tour that would address all the science objectives, as this could not be lightly discarded. The Task Force published its decision on 29 June 2001. It chose a compromise that would not only overcome the communication issue but also rejoin the original tour within 8 months.

nominal tour

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