Huygens Sets Off With Correct Spin and Speed

http://www.esa.int/SPECIALS/Cassini-...0HQ3K3E_0.html

Huygens sets off with correct spin and speed
European Space Agency
11 January 2005

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?

When the Huygens probe was being designed more than 10 years ago, it
was
required that the probe had to be magnetically "clean" when switched
off, meaning that any residual permanent magnetic fields must not
interfere with the sensitive Cassini magnetometers. Later, when the
probe was built, it was found that there was still a weak magnetic
field
produced, but within acceptable limits for Cassini's magnetometer
sensors.

However, because magnetic fields have a "direction" as well as a
strength, and this weak field was slightly off-centre, it effectively
gave the probe a "left" and a "right" side (it behaves like a small
magnet with a north and south pole). With the implication being that if
you can detect this magnetic field, then you can also detect how it is
rotating.

Following an initial suggestion by Jean-Pierre Lebreton, the Huygens
Project Scientist, scientists on the Cassini Dual Technique
Magnetometer
(MAG) team, from Imperial College, London, and Braunschweig, confirmed
that their instrument should be able to detect this small rotating
magnetic field and plans were put in place to measure this during the
probe release period.

Magnetometers are direct-sensing instruments that detect and measure
both the strength and direction of magnetic fields in the vicinity of
the instrument. The Cassini MAG is measuring these fields while Cassini
is in orbit around Saturn as well as during the close Titan encounters.
But, just after separation on 25 December, the MAG scientists detected
fluctuations in the magnetic field around Cassini that could only have
come from Huygens rotating and moving away.

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."

Former MAG Principal Investigator David Southwood, who is now the
Director of Science at ESA, said, "Detecting the spin was immensely
reassuring - not only did it show Huygens was rotating correctly, but
also because the spin is directly related to the departure velocity,
that Huygens was headed off at the right speed. It was really great to
do it with an instrument I knew so well."