Huygens Release

On 16 December 2004 the Huygens management team reviewed the status reports for both Cassini and Huygens and the final report of the Mission Risk Review, then took the formal decision to proceed with the probe's release as per the plan. Later in the day, the Probe Targeting Manoeuvre was executed. It was a main engine burn of 84.9 seconds for a delta-V of 11.937 metres per second, and it placed Cassini on a collision course with Titan. The critical sequences for the Probe Relay were uplinked on 19-20 December, and the requisite value was loaded into the Master Timer Unit on Huygens. This triple-redundant 'alarm clock' - which was to power up the probe's key systems several hours prior to atmospheric entry - was started on 21 December. The next day, Cassini refined its trajectory by a burn of 20 millimetres per second. On 23 December the Probe Release Approval Meeting gave permission to proceed. The probe was to be released on 24 December (02:00 GMT on 25 December). As Huygens had no means of controlling its attitude, Cassini was to release it at precisely the angle for safe entry into Titan's atmosphere. The release mechanism was to spin the probe up to a rate of 7.5 revolutions per minute for stability in free-fall. Pyrotechnics would then sever the physical and electrical links to the probe, and a set of three springs would push it away at 0.35 metre per second - a delta-V that had been allowed for in calculating the Probe Targeting Manoeuvre. As soon as the probe was away, Cassini turned to point its high-gain antenna at Earth to report the event. Although the probe was magnetically 'clean' in its powered down state, it had a small residual magnetic signature. The fact that the Dual-Technique Magnetometer could detect this field enabled the instrument to measure the rate at which the probe was made to spin, and then verify its departure. ''Detecting the spin was immensely reassuring,'' said David Southwood, director of science programs for the European Space Agency. ''Not only did it show Huygens was rotating correctly but, because the spin is directly related to the departure velocity, that it was ejected at the right speed.'' A 5-by-5-frame mosaic of wide-angle images was taken 12 hours later, and to everyone's delight the probe was visible as a dot on the central frame. The navigation team used this and a narrow-angle image taken on 26 December to confirm that the probe was on course to penetrate the atmosphere of Titan at the requisite angle. On 27 December Cassini executed a main engine burn of 153.4 seconds for a delta-V of 23.7 metres per second. This Orbiter Deflection Manoeuvre was designed to adopt the trajectory required for the Tc fly-by on 14 January 2005 at a range of 65,000 kilometres, during which it was to receive the transmission from the probe. Revolution 'c' began with apoapsis on 31 December at 59.7 planetary radii. Following instructions, 7 days prior to the probe's arrival Cassini terminated all non-essential activities, including science observations.

An image taken by Cassini's wide-angle camera on 25 December 2004 showing the departing Huygens probe at a distance of 18 kilometres.

Jagelos

Trajectory correction

OrMtrlrn maneuver 21 Nov 2004

Orbitei deflection maneuver 27 Dec J

Probe targeting maneuver 16 Dec Jt

Huygens separation 25 Dec /

Saturn ortil insertion

Huynens

Hev e probe relay

1.1 Jan 2005

mans iiver

15 Jun 2004

Pirtspsts

23 Aug 20W

A diagram of the manoeuvres made by the Cassini spacecraft following Saturn orbit insertion in order to facilitate the Huygens mission.

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