Phase I begins with Cassini's insertion into Saturn orbit on July 1, 2004, followed by its first elongated 5-month-long orbit and its release of the Huygens probe.

During Cassini's first Titan flyby on Nov. 27, the Huygens probe plunges into Titan's atmosphere, and Cassini spends threee hours of its approach to Titan simply recording Huygens' signals for repeated transmission to Earth during the remainder of the next orbit.

(That Cassini Titan flyby -- at an altitude of 1200 km -- will also greatly trim down the apoapsis of Cassini's next orbit, so that it lasts only 48 days instead of 148.) While it is still an hour out from Titan, Cassini will stop listening for Huygens with its big high-gain antenna, then slew around and spend the remaining hour observing the weather patterns around Huygens' landing area with its side-mounted cameras and spectrometers.

During that next 48-day orbit, it will repeatedly transmit Huygens' recorded data back to Earth.

Then, during its second Titan flyby on Jan. 14, 2005, Cassini will use its onboard radar system for the first time to probe beneath Titan's haze -- which it couldn't do during its first flyby because the big fixed high-gain communications antenna dish on its top also doubles as both the antenna to receive Huygens' signals and as Cassini's radar antenna, and naturally can't be used for any two of these functions simultaneously.

This system -- built largely by Italy -- is strikingly similar to the radar system that the Magellan spacecraft used to map Venus, and has several different modes.

During Cassini's initial approach to Titan, from a range of about 22,000 km in to about 9000 km, Cassini will tilt back and forth and use its radar to map Titan's surface radar reflectivity ("backscatter"), which can provide data on its surface texture and composition.

(For instance, water ice is far more radar-bright than liquid hydrocarbons -- which was one of the first clues that allowed Earth-based astronomers to conclude in 1990 that most of Titan's surface was NOT covered with a liquid ethane-methane ocean as they had thought.) During all its operations, the radar will also frequently listen to the microwave emissions of Titan's own surface rather than its own pulses; the strength of these emissions depends both on the surface temperature and, again, on the types of substances making up the surface.

(For instance, it may be able to distinguish liquid ethane from more complex solid hydrocarbons, and water ice from frozen ammonia.)

Then, from 9000 km range in to 4000 km, Cassini will start pointing its antenna constantly at the point on Titan's surface directly under it, and make high-quality profiles of the altitude of Titan's surface features, with a spot wideth of about 25 km and a height accuracy of only 90 to 150 meters. This technique allows it to study very subtle and wide altitude variations -- such as the difference between continents and seas on Earth -- which can't be detected well by photograph-type imaging techniques, but are invaluable for understanding a world's geology.

For instance, Titan -- for reasons we don't understand -- has a slightly eccentric orbit; it's about 6% farther from Saturn at its apoapsis than at its periapsis.

And if, as some speculate, Titan does have a liquid layer of water and ammonia under 30-100 km of its solid surface ice, the slight tidal flexing of its surface produced by those slight differences in Saturn's gravitational pull on it will probably be big enough for the radar altimeter to detect.

Finally, from 4000 km all the way in to the closest point of the flyby, Cassini's radar will switch to its "synthetic-aperture imaging" mode, in which it will send two pairs of radar beams off at angles to the left and right sides of its orbital track, and precisely record both the timing and the Doppler frequency of the complex return echoes from their pulses -- providing data which can be used by its Earth controllers to reconstruct "photographic" images of Titan's surface, complete with bright hills and slopes facing the "light" source and shadowed slopes pointing away from it. The difference, of course, is that the "light" in this case will be radio waves, and its source will be the spacecraft itself rather than the Sun.

This is the technique Magellan used to construct photographic maps of almost all of Venus' surface with a resolution of a few hundred meters.

Cassini's radar resolution will be similar -- its radar echoes will map the surface at about 1.5 km resolution until the craft is within 1500 km of Titan; and for the short period in which it's closer than that, it will switch to a high-resolution mode in which it can detect features only 500 meters wide. (After the closest approach, as it pulls away from Titan again, it will run through the whole radar sequence in reverse.) But since Cassini makes only fast flybys of Titan -- rather than orbiting it and mapping every bit of it as Magellan did -- the area of its mapping coverage will be far smaller.

On each Titan flyby, it can map less than one percent of Titan's surface using the synthetic-aperture technique, and so even at the end of that 4-year orbital tour only about 35% of Titan's surface will have been covered.

And when it comes to mapping Titan, the radar has competition.

The TV cameras of the Voyagers couldn't detect any longer-wavelength light than orange, and thus were hopeless at trying to pierce Titan's smog.

But longer-wavelength light -- on the border between red and infrared -- punches through the smog fairly well; both the Hubble Telescope and earth-based telescopes have made fuzzy but intriguing maps of Titan's surface features by using light at 0.94 microns and longer wavelengths.

And since Cassini's cameras can detect near-IR light all the way up to 1.1-micron wavelengths, they will probably have the ability to get moderately good visible-light photos of Titan's surface.

Without the smog, the narrow-angle camera could take pictures with only 6-meter resolution at 1000 km altitude -- but the blurring from the smog will probably reduce its sharpest pictures to only 100 or 200 meters resolution, and the fact that the smog diffuses the sunlight hitting the moon will blur the shadows of its surface features to the point that in many ways the radar pictures will probably be better.

There's also Cassini's VIMS -- which can not only take imaging maps of Titan's surface, but visible and near-IR spectra of it from which its surface composition can be mapped.

The smog particles don't fuzz up IR light at all at wavelengths of more than about 1.2 microns -- but when it comes to studying Titan, VIMS has another problem.

The methane gas itself in Titan's atmosphere absorbs all the sunlight streaming down onto Titan's surface except in half a dozen wavelength "windows" in VIMS' spectral range, so it can only take spectra of Titan's surface color in those limited-width bands, which greatly cuts down on its ability to identify different substances there.

(This is the same reason that the Huygens probe -- which has its own near-IR spectrometer -- will turn on a searchlight during the last 200 meters of its descent, to provide the spectrometer with full-spectrum lighting for a more complete analysis.) Still, VIMS should have the ability to distinguish several different types of frozen ices and organic substances -- and its surface resolution will be as little as 500 meters for complete spectra, and only about 200 meters if it uses its "high-resolution" mode for sharper photographic maps in selected wavelengths.

The problem, however, is that all the cameras and spectrometers point off to Cassini's side, so it can't use them and the radar to map Titan's surface simultaneously -- and during each flyby of Titan, ground controllers will have to choose which type of mapping they want to do.

The current plan calls for most of the 44 flybys to be used for radar mapping, but there will likely be some changes made as a result of the early scientific results.

As for Cassini's mapping targets, they are fairly rigidly set (especially for the radar) by its mandatory flyby paths past Titan -- but they cover a wide range of likely features.

(On its first radar-mapping flyby Cassini will map the general area in which the Hugyens probe landed.)

That second Titan flyby will shorten Cassini's orbit to only 32 days, so it comes swooping back in for its third Titan flyby on Feb. 15, 2005.

Part One - Two - Three

Thanks for being here; We need your help. The SpaceDaily news network continues to grow but revenues have never been harder to maintain. With the rise of Ad Blockers, and Facebook - our traditional revenue sources via quality network advertising continues to decline. And unlike so many other news sites, we don't have a paywall - with those annoying usernames and passwords. Our news coverage takes time and effort to publish 365 days a year. If you find our news sites informative and useful then please consider becoming a regular supporter or for now make a one off contribution.
SpaceDaily Contributor $5 Billed Once credit card or paypal		SpaceDaily Monthly Supporter $5 Billed Monthly paypal only