The DISR, which weighs only 8 kg, is actually a package containing an amazing variety of different instruments -- including sensors to measure the visible and UV light levels at different altitudes in Titan's hazes and the hazes' optical properties, and visible and near-IR spectrometers which will look both upward to measure the spectra of the sunlight streaming down through the haze, and downward to measure the visible and near-IR reflectance spectra of Titan's surface itself, providing data on its composition.

The DISR's central sensors, however, are three miniature CCD cameras -- one peering off to the side, one down at an angle, and one almost straight own -- which together will radio back over 1100 photos of Titan's surface.

Since they have overlapping view fields, together they can provide a continuous 715-line photo extending from above Titan's horizon all the way down to within 6 degrees of the point directly beneath the probe. And Huygens will spin during its descent -- it hangs from its parachutes by a swivel, and has 32 tiny vanes on its bottom so that the air rushing past it during its descent will make it rotate.

It will precisely measure its pointing direction at every moment during each rotation, and during many of them the three cameras will take 12 such three-image sets per each rotation, thus providing a complete 36-photo panorama of the Titanian landscape below.

Huygens radios back its data to Cassini at almost 8200 bits per second just to send back these photos -- without them, its communications rate would only need to be a few hundred bits per second. They will definitely be worth it, though.

Since Saturn is almost 10 times farther from the Sun than Earth, sunlight is only 1/100 as bright there, and much of the remainder will be blocked by Titan's smog -- but the surface light will still be 350 times brighter than on a full-Moon night on Earth, allowing the cameras easy visibility.

A bigger problem may be that the smog will tend to diffuse the sunlight's direction and thus somewhat blur the shadows of surface features, but there should still be more than enough shadows to distinguish most of them.

The photos themselves will be black-and-white. But as the probe rotates, Huygens' downward-aiming visible-light spectrometer will frequently map the color of Titan's surface in 20 different "zones" at angles of 10 to 50 degrees from the probe's bottom -- and this color map can be superimposed on the surface photos to construct highly realistic full-color photos of Titan's surface.

Color test photos of Earth landscapes taken with this strange constantly panning camera setup are spectacularly sharp and clear. (One of the few things we can say with confidence about Titan's surface is that it will be mostly reddish.)

Huygens will take 15 of these 36-photo panoramas, at altitudes from 150 km down to only 5 km. After that, it will transmit a series of about 500 isolated photos, ultimately just from its downward-pointing camera -- with the last one only 200 meters above Titan's surface, covering a patch of ground only about 100 meters across with a resolution of 1/2 meter.

After that, Huygens will switch on a 20-watt searchlight -- not for its cameras, but for the benefit of its visible and near-IR spectrometers. Titan's smog, and the methane in its air, only allow sunlight in a few limited spectral bands to pierce all the way through to the surface, seriously limiting the spectra that the spectrometers can take of the surface composition -- but the searchlight, tiny though it is, will allow the sensitive spectrometers to obtain full surface spectra at least during this final part of the descent.

It was long ago decided that Huygens' aiming point would be 18 degrees north of Titan's equator, at a spot which -- by agreeable coincidence -- has turned out to be at the edge of that continent-sized "white patch" (probably consisting of mountains) which is by far Titan's most prominent surface feature.

However, its landing-point accuracy will be only about 1200 km, and during its descent Titan's high-speed upper-atmospheric winds may blow it several hundred kilometers east or west - we don't even know yet which direction they blow - so its actual landing site is very uncertain.

On a side note, despite the many stunning artworks of Huygens descending with a ringed Saturn as the backdrop, the sad truth is that Huygens will land on the side of Titan opposite Saturn, and it will be many decades before such a view of Saturn from Titan is photographed.

Meanwhile, Huygens will be hitting the surface at around 20 km per hour. Scientists would be ecstatic even if it doesn't survive that landing, but there's a fair chance that it will, giving them some optional data from the surface of Titan itself.

Its batteries are designed to work for at least 153 minutes (and probably considerably longer), and Cassini will listen for its signals for 3 hours -- so depending on how fast it descends, if it survives it could send data back from the surface for anywhere from a few minutes to about an hour.

When Huygens was first designed, it was thought likely that it would splash down in a sea of liquid ethane and methane -- in which it's light enough to float.

In that case, the GCMS inlet on its bottom -- which is heated -- will draw in a vaporized sample of Titan's ocean for detailed analysis; and Britain has also provided a small well on its bottom containing an array of little sensors to measure the density, heat and electric conductivity, sound speed, and light-refraction index of the liquid, providing more data on its composition (the ratios of methane, ethane, and dissolved nitrogen in it).

The British have also provided a tiltmeter to gauge the size of any waves rocking Huygens (in Titan's light gravity, they should be several times larger but also several times slower than Earth's), and even a tiny echo sounder to sense the bottom and thus gauge the depth of any Titanian sea up to a kilometer.

However, it now seems much more likely that liquid covers only a small part of Titan's surface, and that Huygens will hit some kind of solid or semi-solid surface -- although we have no idea just how hard that surface may turn out to be.

For billions of years, flecks of liquid ethane and smoke-sized particles of more solid organic compounds - especially frozen acetylene - have been sprinkling down on Titan's surface at an extremely slow rate -- only about 2 to 8 centimeters of solid material every million years.

The solid specks are each likely to be wrapped in a film of liquid ethane, so (given the long time in which each particle sits undisturbed on the surface) the ethane may have drained away underground, leaving a layer of dry, loose powder on the surface -- or it may make them into a slushy or sticky layer, or even have fused them together into a hard pavement.

Alternatively, Huygens may touch down in an area where a big meteor impact has blasted away the overlying organic layer to expose the hard water ice underneath.

In any case, the impact is likely to be hard enough that the GCMS and its inlet will be wrecked, and if the surface is solid all the British sensors will be useless except for two landing-shock accelerometers that could gauge surface hardness.

There's a good chance, though, that Huygens will continue to transmit weather data, and its DISR cameras -- which are mounted on the upper deck -- will continue sending back occasional pictures of the Titanian landscape off to one side of the probe (unless its still-attached parachute drapes over them).

Huygens' internal electronics packages are in two layers separated by a layer of crushable aluminum honeycomb, so the probe has some ability to "flatten" out on landing and absorb the shock.

Click For Part Three

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