WEEKLY WHINE

Systems disintegration

Yes, you should always test before you leave. That's what they do in high school, the European Space Agency, and the National Aeronautics and Space Agency. But they didn't run one test that would have been very helpful right about now.

The Cassini mission to Saturn and the Huygens mission to Titan are travelling to the outer solar system together, and Huygens's data will come back to Earth via Cassini. The problem here was that neither Cassini nor Huygens sufficiently took into account the Doppler shift between the two spacecraft.

In the missions' original plan, Cassini would have released Huygens on SAT 06 NOV 2004, and the probe would have encountered Titan on SAT 27 NOV 2004. In that plan, Huygens would have been descending through Titan's atmosphere whilst Cassini was approaching Titan. Hence Huygens's signal would have been blueshifted enough that Cassini wouldn't have been able to pick it up.

The Huygens Recovery Task Force therefore had to choose a new flight plan with a smaller relative velocity between Cassini and Huygens. In this new plan, Cassini's first two orbits around Saturn are shorter, and the probe doesn't separate until the third orbit. Huygens will now be released on SAT 25 DEC 2004, less than two weeks after Cassini's second Titan flyby on MON 13 DEC 2004. That pushes Huygens's drop through Titan's atmosphere back to FRI 14 JAN 2005.

Huygens will also be preheated before its deployment, because the communications subsystem works best when it's nice and toasty. When it flies through the atmosphere of Titan, it will be roughly 65,000 km [40,000 mi] from Cassini, and they will be moving at a smaller relative velocity.

They might have noticed the issue much earlier had they performed the Relative Velocity Test. In this test, the two spacecraft are put in a clean room at the integration facility. One is then accelerated to the same velocity that the original plan called for, and the communication between the two is then tested. Had they done this, the project teams would easily have noticed Huygens's inability to communicate with Cassini.

WARNING: This test does not actually exist. Nobody has ever done it, or even considered it.* Reports GoobNet's crack investigator Chris M Hirata, the relative velocity between Cassini and Huygens in the original plan was 6.5 km/s, a velocity that is not practical in most systems integration facilities currently in existence. [Not even the Vehicle Assembly Building.] Nonetheless, perfect hindsight shows a few tests that would have exposed this problem before launch.

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* Except us. Therefore we decree that this test hereafter be called the GoobNet Relative Velocity Test.