ESA provides an interesting explanation of a question that had puzzled more than one reader: given that the Huygens probe provided no telemetry at the time, how did controllers know that its Christmas Eve separation from the Cassini orbiter had been nominal? It’s true that Cassini was able to send data showing its own change in attitude due to the Huygens release, but how did we learn that Huygens was spinning at just the right rate (vital for a correct entry into Titan’s atmosphere)?
The answer: Huygens houses a weak magnetic field, despite attempts to render it ‘magnetically clean’ when it was constructed. The field is weak enough not to interfere with Cassini’s sensitive instruments, but turns out to be measurable by its magnetometers. Cassini’s Dual Technique Magnetometer (MAG) team used this data to establish the rate of Huygens’ spin.
From a news release from ESA:
Professor Michele Dougherty, Principal Investigator for MAG, said, “What was observed by MAG just after the probe separation on 25 December 2004, were weak but clear fluctuations in both magnetic sensors which reside on the 11-metre magnetometer boom. These fluctuations were a clear indication of the Huygens probe moving away from the Cassini orbiter. This signature confirmed the spin rate of the probe at 7.5 revolutions per minute, the ideal rate which was predicted, and that Huygens is well on its way to Titan.”
