Machine Design

Tunes for spacecraft lovers

Scientists have produced an audio soundbite that captures what the Cassini orbiter heard from the Huygens probe as it descended on Saturn's Titan moon in January.

The sounds are important to investigators who are reconstructing the probe's exact position and orientation throughout its parachute dive to Titan's surface.

"The minute-long sound file covers about 4 hours of real time, from when the Huygens probe deployed its main parachute, down to ground impact 21/2 hours later, and then for about another hour on the surface," said Ralph D. Lorenz of the University of Arizona.

Lorenz, an assistant research scientist at UA's Lunar and Planetary Laboratory and a coinvestigator on Huygens' Surface Science Package, made the sound file from data formatted by Miguel Perez of the European Space Research Technology Centre, Noordwijk, the Netherlands.

To hear the audio file, go to the European Space Agency Web site at,or Lorenz' home page at,or the UA News Services science Web page at sound is a tone which has a frequency that depends on the strength of the Huygens signal picked up by the Cassini orbiter's receiver. Signal strength depends on distances and angles between the orbiter and probe.

Huygens' antenna emits radio energy unevenly, Lorenz said, "like the petals of a flower rather than the smooth shape of a fruit." The rapid changes in the tone reflect Huygens' changing orientation caused by its slowing spin rate during descent and its swinging beneath the parachute.

"You can hear how the motion becomes slower and steadier later in the descent," Lorenz said.

The tone changes dramatically at 43 sec into the 1-min soundbite, when the decelerating, choppy whistle suddenly becomes a steady whistle, generally rising in pitch. That sound change is when the probe landed.

"After landing, the tone is far less rich because the probe has stopped moving," Lorenz said. "But you still hear slight changes as Cassini flies through the lobes or 'petals' of the antenna pattern. Just before the end, you hear the weak signal drop out for a moment and then return. Overall, the signal was very robust. Cassini was locked on the Huygens signal throughout descent."

"Sounds are an interesting way of evaluating one-dimensional data like this," Lorenz said. "The human ear is very good at detecting small changes in sound."

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