Tim Furniss/LONDON
A European Space Agency (ESA) investigation board has identified 10 recovery options to overcome a communications error on the Huygens mission to Saturn. The fix is intended to ensure that all the Huygens probe's data transmissions from the vicinity of Saturn's moon Titan in 2004 can be collected by the receiver on the NASA Cassini mother ship, which will transmit the data to Earth. A decision on which option to take has to be made by May to enable NASA to change Cassini's mission.
The board, established following the discovery of the communications error during tests last February, has criticised the "entire structure" of the European Huygens project.
During the tests it was discovered that the Doppler shift - the change in the apparent frequency and the wavelength of sound, light and other waves due to the motion between the Huygens lander and Cassini - caused the data signal from Huygens to fall outside the bandwidth of a component in the Cassini receiver. The Cassini receiver's bandwidth is too narrow to collect all of the data from Huygens, which will change its velocity during a planned 2.5h Titan descent.
Huygens and Cassini were launched in October 1997. Cassini is due to enter an initial orbit of Saturn on 1 July 2004 for an extended orbital tour of the planet and some of its 28 known moons.
Huygens will be released on 6 November 2004, with Cassini in a slightly higher orbit. Huygens is scheduled to descend into Titan's atmosphere on 27 November, while Cassini flies past, collecting atmospheric and surface data from Huygens.
Options include raising Cassini's final approach to Saturn to at least 50,000km (31,000 miles) instead of the planned 1,200km, which would "solve the entire probe receiver Doppler problem". However, this would hamper Cassini's multi-orbital "tour" of Saturn's rings and moons.
Other options would not entirely correct the error. One is to ensure that wind direction and velocity in Titan's atmosphere which might affect Huygens' speed and direction is known before the landing attempt, and that modifications are made to communications parameters.
Other options are: altering the clock bias on Cassini's receiver; reducing the planned pendulum motion of the Huygens lander's descent under a parachute; reducing the communications distance between the two spacecraft and delaying the release of Huygens; performing the mission during one of the later orbits of Saturn; and redesigning the first Cassini orbits of Saturn.
The investigation board was unable to find any direct reference to Doppler shift in original mission specifications and the issue did not surface in any ESA, NASA or independent project reviews, it says.
"Had anyone on the receiver design team at Alenia, Alcatel, ESA or NASA's Jet Propulsion Laboratory asked about the effect of Doppler on the data stream, the problem would probably have been discovered in time," it says.
Source: Flight International
