This weeks New Scientist article Cassini/Huygens

Titan's first visitor nears touchdown
08 January 2005
NewScientist.com news service
Stephen Battersby
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Explo earth
AFTER seven years riding piggyback on NASA's Cassini spacecraft, the
European Space Agency's Huygens probe is due to become the first craft from
Earth to arrive on an alien moon.

On 14 January the probe, which separated from Cassini on Christmas day,
will float down through the thick atmosphere of Saturn's moon Titan and
land on its mysterious surface. The Huygens team hopes to discover what
this world is made of and gain some insight into how life began on Earth.

Attempting to land on other worlds is risky, as ESA's Beagle 2 mission to
Mars showed a year ago. But Huygens's work will begin before it lands: in
fact, it will complete most of its goals while parachuting down to the
surface. Even so it will have to survive a number of perilous moments.

The most dangerous will come at 0907 GMT, when Huygens hits Titan's
atmosphere at 6000 metres per second. Its main shield will be blasted by up
to 1 megawatt of heat per square metre, scorching it to about 1800 °C.
Though the instruments inside have been designed to withstand a wide
temperature range, freak weather on the day could lead to a higher than
expected heat load which could fry them, warns mission analyst Michael
Kahn.

Three minutes after hitting the top of the atmosphere, assuming the probe
survives the heat, Huygens will slow to about 400 metres per second. About
180 kilometres above the surface, deceleration sensors will trigger
deployment of the pilot chute, which will quickly drag out the main
parachute. This will slow Huygens to about 80 metres per second, and the
heat shield will pop off, uncovering the probe's array of instruments.

This is where the mission really starts. For the next two-and-a-half hours,
as the probe floats down, it will sample the air and scan Titan's surface.
The imaging system's three cameras - one pointing straight down, one
sideways and one at 45 degrees - should build up panoramic pictures of the
surface, while also spotting any methane clouds.

The probe's main objective is to search for signs of the kind of chemistry
that could lead to life. Titan's atmosphere is the nearest thing in the
solar system to the atmosphere of the very early Earth. It is dominated by
nitrogen and laced with hydrocarbons and nitriles - organic molecules that
are assumed to have been among the precursors of life on Earth.

Huygens is carrying a mass spectrometer that will analyse the gases in the
atmosphere. Meanwhile another instrument will collect aerosols, which are
expected to contain the most complex chemicals, heat them up to break down
the molecules and feed them to the mass spectrometer for analysis.

Huygens will beam its measurements up to Cassini, which will relay them to
Earth. To minimise the chances of losing irreplaceable information, all the
data will be processed in parallel by two separate systems and sent to
Cassini using two antennas. The data streams will be transmitted 6 seconds
apart to ensure that no information is lost if, say, the craft is briefly
hidden behind its parachute or a gust of wind turns the antennas away from
Cassini.

About 125 kilometres above the surface, Huygens will ditch its main
parachute in favour of a third, smaller chute to speed its descent,
ensuring that it lands before Cassini has flown out of range.

Mission engineers have simulated Huygens's descent on a clone of the probe
at ESA's European Space Operations Centre in Darmstadt, Germany. They fed
fake signals to its sensors to check the probe was responding
appropriately. All the parachute systems were tested in 1995, when a scale
model of the probe was dropped from a balloon 38 kilometres above the
Earth's surface. And a model survived artificial lightning strikes in a
lab, simulating another hazard Huygens might encounter in Titan's
atmosphere. "We're very confident. We have done all the tests," says Joe
Wheadon, who leads the model-testing team in Darmstadt. "Everything humanly
possible has been done to double-check."

The most exciting moment will be when Huygens hits solid ground...or
liquid, or slush. The state of Titan's surface remains one of the moon's
greatest mysteries. Images from Cassini show complex patterns of light and
dark, and the hope is that Huygens will discover exactly what blend of ice,
tar, liquid, hydrocarbon snowdrift, or something else entirely, the landing
site is made of. That could be the key to decoding Cassini's maps of Titan.
"Huygens could give us the ground truth for one region," says John Zarnecki
of the Open University in the UK, principal investigator on a set of
instruments called the surface science package.

“The most exciting moment will be when Huygens hits solid ground...or
liquid, or slush. Titan's surface is a mystery”Twenty seconds before
impact, a lamp will illuminate the approaching surface, and an acoustic
sounder will measure the roughness of the terrain. A rod poking out of the
bottom of the probe will hit Titan before the body of the craft, and the
forces it senses should reveal how hard the surface is. The mass
spectrometer will be preheated so that it can vaporise a portion of the
surface and analyse any gases released. Other detectors will measure the
acoustic, optical, electrical and thermal properties of what Huygens lands
on.

Zarnecki is still hoping for a splashdown rather than a crash. "My dream
scenario is that we see a massive wave on the horizon," he says. Any such
wave might engulf Huygens but, in the low gravity on Titan, waves would be
slow-moving. If all goes well, the probe could be sending back images and
other data hours after landing. And even if Huygens is overwhelmed by a
giant wave, its final moments should give us some spectacular pictures.

From issue 2481 of New Scientist magazine, 08 January 2005, page 8

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Dave F