This closest moment of the approach would be at about the time Huygens is considered most likely to actually touch down on Titan's surface, although if it survives it may transmit data afterwards from the surface for 45 minutes or more.

Even at the beginning of Huygens' transmission period, Cassini would be approaching it at only about 12,000 km/hr - cutting the Doppler shift to only about 3/5 of what it would have been under the original plan.

And so - despite its greater distance from Titan throughout the relay period - the strength of the Huygens telemetry signal actually received and decoded by Cassini's mistuned receiver would be much greater.

The Task Force concluded that this strategy would do such an effective job of eliminating the Doppler problem that only one other measure need be taken: turning on Huygens' transmitter (though not the rest of the probe's systems) fully 4 hours before it enters Titan's atmosphere, so that its oscillator is substantially warmed up - somewhat lowering its operating frequency - by the time it starts to transmit scientific data.

But this scenario, as I've said, was considered "radical" - and originally rejected by the Task Force - for one simple reason.

Every time Cassini flies by Titan, Titan's gravity radically alters its next orbit around Saturn -- and in fact Titan flybys are used throughout the mission to keep altering the period, inclination and direction of Cassini's orbits around Saturn to achieve its long list of experiments dependent on specific orbital maneuvers. After analyzing thousands of possible orbital sequences, a four-year, 74-orbit tour of the Saturn system called "Tour 18-5" was created. This mission design process took years to develop, and calls for Cassini to make all its flybys of Titan at precise dates, times, and distances - starting with the very first close flyby of Titan it was supposed to make during the Huygens encounter on Nov. 27, 2004.

Replacing the close flyby during the Huygens encounter with a much more distant one would produce a radical effect on the period of Cassini's next orbit around Saturn, in turn affecting the date and time at which it next flew by Titan - and setting off a "domino effect" that might force extensive changes in all the rest of the orbital tour, which scientists were understandably extremely loathe to make.

So the problem was: how to incorporate that new, unplanned distant flyby of Titan into Cassini's orbital tour, while adjusting the timing and distance of its other Titan flybys immediately after (or before) that one to quickly compensate for the effects of that radically changed flyby and get Tour 18-5 back on track as soon as possible?

Fortunately, the Task Force succeeded in finding a strategy to do just that, which involves changing only the first 7 1/2 months of the Tour.

The plan for Cassini is still to have it arrive at the planet on July 1, 2004, and swoop a few thousand kilometers past the edge of the visible rings before descending to within 6000 km of the cloud tops in the southern hemisphere.

During this period, a protracted burn of its main engine will decelerate Cassini into a huge highly elongated orbit around Saturn with a period of several months.

The first SOI burn would then be followed by its opposite long burn at its first apoapsis. This will raise its periapsis out to about the orbit of Titan, 1.2 million km from Saturn, enabling its first close flyby of Titan, which will slow it down and greatly trim the length of its next orbit.

But now the details have changed considerably. Instead of braking into an initial elongated orbit with a period of fully 149 days, Cassini will brake into one with a period of only 117 days - and so it will return to Saturn and make its first close flyby of Titan on Oct. 26 instead of Nov. 27. And it will not release Huygens three weeks beforehand to land on Titan during that flyby, but will instead retain it.

This considerably shortened initial orbit will take a surprisingly small amount of additional braking to achieve - the engine will fire only 2 to 3 minutes longer than its originally planned 90 minutes to slow Cassini into orbit around Saturn, and may also be started a few minutes later than planned to increase its effectiveness in slowing the craft.

As in the original plan, that first flyby will occur at a distance of 1200 km from Titan and will trim the length of Cassini's next orbit to only 48 days - allowing it to make its second Titan flyby on Dec. 13 (at a range of 2400 km), rather than on Jan. 14, 2005.

And that flyby in turn will trim the length of its next orbit to only 32 days, so that it will make its third Titan flyby - rather than its second one - on Jan. 14. But in the new plan, that third Titan flyby - a flyby added to the original list of 44 such flybys - will be the distant one at which it finally releases Huygens to land on Titan.

And since that very distant flyby won't cause Titan's gravity to change Cassini's orbital period, it will then return to make its fourth Titan flyby 32 days later, on Feb. 15 - the date originally planned for its third Titan flyby.

That Feb. 15 flyby (still called "Titan-3" in the flight plan) will go exactly the way it was originally planned to do so, at a distance of only 1194 km from Titan - and thereafter Tour 18-5 will be perfectly back on track and proceed as originally planned (with Cassini making its first flyby of the strange little moon Enceladus on March 9).

Finally, this delay in Huygens' release from Cassini's very first Titan flyby to its third one has another advantage: it allows Cassini to use its own instruments to study Titan in detail during two close flybys before the Huygens landing. This will allow Huygens' landing area to be radar-mapped in detail before - rather than after - its landing.

But a more important consequence may be that it will allow Cassini's instruments to give us a better estimate of the speed of Titan's powerful upper-atmospheric winds, which may blow the probe several hundred km to one side during its parachute descent - and such an estimate, in turn, will allow ground controllers to decide more exactly where to point Cassini's antenna dish during Huygens' descent, further increasing the strength of the received signal.

Indeed, until this month, we didn't even know whether Titan's winds blew in an easterly or westerly direction! Theoretical models of its weather suggested that either was possible.

However, a just-published report based on ground-based observation by telescope of the slight Doppler shift in the infrared radiation given off by the haze on opposite sides of the moon as it's blown around the planet now indicates strongly that it blows west - and that, at an altitude of 200 km (where Huygens will begin its parachute descent), it may be blowing at an astonishing 750 km per hour!

While it should quickly slow down at lower altitudes, the more advance information we can get on the precise speed of Titan's winds at various altitudes will be welcome - and Cassini's weather-pattern photos and IR observations during its first two flybys may give some to us.

In short, Cassini's continuing run of good luck - as striking as the incredible run of bad luck that dogged the Galileo Jupiter project from its beginning - is continuing.

The only scientific price to be paid for the correction of a very serious problem will apparently be a reduction in the amount of extra fuel it has remaining after its 4-year primary orbital tour is complete, which could force any later extended mission to be somewhat scaled down.

Its huge collection of scientific instruments continue to work perfectly, except for a small part-time problem with an auxiliary transmitter that's part of its radio science experiment.

Whether you ascribe this to plain good luck or the $2 billion that was spent on this mission - the most expensive planetary mission ever, except for the twin Viking Mars landers - it's very welcome news.

Back To Part One

Related Links
Main Cassini Portal NASA/ESA
Titan and the Origin of Life on Earth
Some Speculations on Titan's Past, Present and Future
Weather on Titan
Beneath the Clouds of Titan
Hubble Titan Surface Photos and Map
SpaceDaily
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RINGWORLD

Planetary Scientist Looking Forward To Saturn's Splendor
Cameron Park - Jan 27, 2001
The Cassini spacecraft has now successfully rounded Jupiter, and used a gravity-assist from that world to catapult itself on its way outward to Saturn -- a voyage of over six and half years of which it is now approaching the halfway point.

Exploring a Distant World of Ice and Hydrocarbons
Cameron Park - February 27, 2001
In the first of our series on the activities of the Cassini spacecraft at Saturn, we ended with the spacecraft having entered orbit and firing its main engine to fly by Saturn's moon Titan, where it will parachute its European-constructed probe Huygens into that moon's atmosphere, giving us our first close up look at Saturn's most enigmatic moon.

Titan: Sol's Biggest Lightweight
Cameron Park - March 20, 2001
In our continuing series on the upcoming Cassini mission to Saturn and Titan, Bruce Moomaw provides SpaceDaily readers with an insight into the science and technology that will make or break our first expedition to the surface of a gas giant's moon.

Cassini's Epic Tour of the Rings
Cameron Park - May 15, 2001
In our previous installment on Cassini mission to Saturn we looked at the activities the Huygens entry probe would conduct during its active phase as it parachutes into the atmosphere of Titan. but this brief visit to Titan is only the beginning of Cassini's epic survey of Saturn, its rings, and its moons as it orbits the Sol's mightiest ringed world.

Cassini's Tour de Saturn
Cameron Park - May 25, 2001
In my last article on the Cassini Saturn probe, I described the first of the four "phases" of its four-year orbital tour of Saturn and its rings and moons, during which it will use a constant sequence of gravity-assist flybys of Titan - 44 flybys during its 74 orbits around Saturn during those four years - to keep radically modifying its orbit to study different parts of the Saturn system. Each phase represents a period in which Cassini will carry out orbital maneuvers of a different general type to make a particular kind of scientific observation.

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