Ordinary Meeting, 2006 December 16
Farce and Fortuity in the Exploration of Space
Mr Ellison explained that his talk would outline a few of the turns of fortune which had affected the history of spaceflight. His first example was the Cassini-Huygens mission; he recalled how, after launch, its engineers had realised that they had over-looked the Doppler shift in the telemetry between the Huygens lander and the Cassini orbiter which resulted from their relative velocity. It had then been too late to change the frequencies to which the telemetric hardware was tuned, but with a stroke of genius, the ground team had re-designed the orbital trajectories of the two probes to minimise their relative velocities to within tolerable limits.
This had not been the only technical hitch in that mission; Mr Ellison further recalled how all of the data from one of the two communications channels between Huygens and Cassini had been lost as a result of a software bug on Cassini; the relevant receiver had never been turned on.
He turned next to the Jovian Galileo probe; he recalled its long history, having been designed by NASA in the 1970s, built in the 1980s, and launched after a delay in 1989; it had finally reached Jupiter in 1995. He recalled that its primary antenna, rather like an umbrella, had failed to unfold. The leading theory as to the cause was that grease from the antenna joints had shaken out during its many journeys between Florida and NASA Jet Propulsion Laboratory (JPL) in Pasadena, CA, making it too stiff to open.
Without this antenna, Galileo had been seriously crippled: its functional low-gain antenna could transmit only 10 bits/second; the lost antenna would have transmitted 138 kbit/sec. Mr Ellison explained that once again, engineers had devised ingenious work-arounds. By upgrading ground-based antennae and improving the data compression software used on the spacecraft, they had achieved a final communication rate of 120 bits/sec. Whilst many of the mission's planned activities had had to be cancelled, Galileo had still achieved many of its original aims.
Mr Ellison turned next to the NEAR Shoemaker probe, sent to asteroid 433 Eros. He recalled how, on its final approach to the asteroid in 1998, a 15-minute orbital-insertion burn had failed and NEAR had subsequently gone radio-silent for 27 hours. Upon realising the situation, the engineers had had little time to redesign the mission, yet they had been able to take data during a fly-by of Eros three days later, and then achieve orbital insertion upon its next close approach in 2000. The speaker noted that the subsequent mission had been a resounding success, ending in touchdown onto the surface of Eros in 2001.
He then turned to discuss the many turns of fortune which had affected missions to Mars. He recalled that the failure of Mars Observer (1992), three days prior to orbital insertion, had led to a feeling that NASA should not have risked all of its instruments on one spacecraft, but should instead have sent a larger number of cheaper probes. This philosophy had now grown to be known by the motto 'faster, better, cheaper'. It had had some early success with the fruitful missions of Mars Pathfinder and Mars Global Surveyor (1996), but less luck with Mars Climate Orbiter (MCO) and Mars Polar Lander (MPL) in 1998, both of which had been sunk by design flaws. He noted, however, that these had both had second lives: much of the technology from MCO had been reused in Mars Odyssey (2001), meanwhile Phoenix (scheduled 2007) would be similar to MPL.
In 2003, NASA had made the daring decision to send two identical rovers to Mars – achieving complete redundancy against spurious failure, but also risking the combined failure of both in the event of design failings. These, the Spirit and Opportunity rovers, had been tremendously successful: both remained to this day fully operational. Before launch, however, their mission had seemed in great doubt; 15 months beforehand, tests had revealed serious problems with both their parachutes and airbags; furthermore, the suitability of the chosen landing sites had been questioned. As in previous cases, Mr Ellison explained that mission engineers had effected a miracle in rectifying these issues within the time available.
He recalled that these had not been the only problems for the mission. Early in Spirit's explorations, an issue with its memory filing system had left it crippled for several days; more seriously, Opportunity's arm joint heater had always been jammed in the 'on' state, forcing engineers to power the rover down into a 'deep sleep' mode each night. In consequence, Opportunity's hardware was frequently exposed to temperatures well below its minimum rated tolerance; how it had survived this treatment remained something of a mystery.
The speaker closed by posing the open question of whether the space industry had learnt from these brushes with fortune, but praised the engineers for their often-ingenious fixes to problems.
Following the applause, the meeting broke for tea. The President then introduced the afternoon's second speaker, Prof John Brown, who would be presenting this year's Christmas Lecture. Prof Brown held a Regius Professorship in Astronomy at the University of Glasgow, and was also the current Astronomer Royal for Scotland.