On Christmas Day 2004, the Cassini spacecraft flawlessly released ESA’s Huygens probe, passing another challenging milestone for Cassini-Huygens mission. But, with no telemetry data from Huygens, how do we know the separation went well?
At 3:00 CET on 25 December, the critical sequence loaded into the software on board Cassini was executed and, within a few seconds, Huygens was sent on its 20-day trip towards Titan. As data from Cassini confirm, the pyrotechnic devices were fired to release a set of three loaded springs, which gently pushed Huygens away from the mother spacecraft. The probe was expected to be released at a relative velocity of about 0.35 metres per second with a spin rate of about 7.5 revolutions per minute. Telemetry data from Cassini confirming the separation were collected by NASA’s Deep Space Network stations in Madrid, Spain, and Goldstone, California, when the telemetry playback signal from Cassini eventually reached the Earth.
However, these data showed only that the Cassini systems had worked, and that the Cassini ‘attitude perturbation’ (how Cassini moved in reaction to the probe’s release) were as expected. Within hours, the preliminary analysis of this data confirmed that Huygens was on the expected trajectory and spinning within the expected range. The spin imparted to Huygens is vitally important to ensure that the probe remains in a stable attitude and on course when it enters Titan’s atmosphere. So how could we check the spin rate was correct?
Franco Bonacina | alfa
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