Tim Furniss / London

Mission will now continue its tour through the Saturn system after a communications flaw threatened the programme

The European Space Agency (ESA) has confirmed the final details of a recovery plan for the Cassini-Huygens mission to Saturn after a communications problem threatened the programme.

The revised mission has been devised to overcome a design flaw in the Huygens' communications systems which would have resulted in the spacecraft being unable to compensate for the Doppler shift between the signal emitted by Huygens and the one received by the Cassini mother ship.

Cassini-Huygens will enter orbit around Saturn on 1 July, 2004, as planned and will make two fly-bys of the moon Titan on 26 October and 13 December. On 25 December, 2004, Huygens will separate from Cassini to enter Titan's atmosphere on 14 January, 2005, seven weeks later than planned originally.

To reduce the Doppler shift in the signal from Huygens, the Cassini orbiter will fly over Titan's cloud tops at an altitude of 65,000km (40,400 miles) instead of the planned 1,200km.

This higher orbit offers the added advantage that Cassini will be able to resume its original orbital plan in mid-February and preserve its four-year tour through the Saturn system.

The new plan, however, will affect Cassini's propellant supply. About a quarter of the orbiter's fuel reserve will be used by the end of the recovery mission.

The moves also involve several modifications to ensure maximum efficiency of the Huygens communications system, including pre-heating the probe to improve tuning of the transmitted signal, continuous commanding by the orbiter to force the receiver into non-Doppler mode and changes in the probe's onboard software.

"I am happy we have found a good engineering solution," says Kai Clausen, the co-chairman of the Huygens Recovery Task Force, "but a lot more work still needs to be done. Now we need to complete the detailed design, implementation, validation and changes in the probe's onboard software."

Source: Flight International