Tucson

SEVEN years after blasting off from Earth, one of the most advanced spacecraft ever built will manoeuvre into orbit around Saturn. In June 2004, Cassini will begin to weave its way through the planet's 18-moon system at the start of the most difficult and complex planetary tour ever undertaken.

For four years, it will trace a seemingly chaotic trajectory around the ringed planet and its moons. Instead of following a simple circular or elliptical orbit, it will constantly switch from tight circular orbits lasting only 10 days to large elliptical "petals" that will send it on 100-day excursions more than 400 000 kilometres from the planet. At times it will hug the equator studying cloud formations, while at others it will soar to high latitudes and look down on Saturn's poles and its famous rings. Most exciting of all, Cassini will skim the surface of at least five Saturnian moons and fly within 50 000 kilometres of several others.

But there is a problem. Even though Cassini is due for launch next October, the scientists behind the project have yet to agree on what it should look at and when. Atmospheric scientists want Cassini to trace orbits that hang above the planet's dayside so that they can watch cloud patterns evolve—but magnetospheric physicists want to sweep deep into the nightside to study the tail of the planet's magnetic field. Scientists who study Saturn's famous rings favour orbits that will look down on them—but geologists prefer short orbits incorporating many encounters with the moons. Satisfying the sometimes mutually exclusive demands of competing research teams is a daunting task, and one which is bound to put a few noses out of joint.

In the past, such disputes never arose. Spacecraft such as Voyager and Pioneer used simple gravitational slingshots to send them from one planet to another. Galileo, which is currently in orbit around Jupiter, has embarked on a more complex tour but will remain more or less in the equatorial plane throughout its mission. "One of the key differences between Cassini and Galileo is that Cassini is a three-dimensional tour, whereas Galileo is mostly confined to the orbital plane of the moons," says John Smith, one of the tour designers at the Jet Propulsion Laboratory in Pasadena, California. And while the Galileo tour will last only two years, allowing 11 orbits around Jupiter, Cassini's four-year mission will allow as many as 60 orbits. This number of orbits and the three-dimensionality create a bewildering array of possible tours for the designers.

The driving force behind Cassini's tour is the gravitational field of Saturn's largest moon, Titan. The tour must be designed so that each orbit brings the spacecraft back to Titan, where the moon's gravitational field can be used to pull it into a new trajectory. Consequently, Cassini will visit Titan many times during the tour—on its first encounter in November 2004, the spacecraft will release a probe that will parachute through the moon's dense atmosphere.

Smith and his colleague Aron Wolf, also at JPL, can exploit Titan's gravity to change a trajectory in a number of ways. In the arcane jargon of celestial mechanics, altering the craft's inclination—the angle the orbit makes with the equator—is known as cranking, while moving the apoapsis—the furthest part of the orbit—is called petal rotation. The point at which Cassini crosses the plane of Titan's orbit is called a node, while changing the orbital period is pumping (see Diagram).

Of course, the researchers can always make minor adjustments to the spacecraft's velocity using its thrusters. This is called a delta-V manoeuvre but, because Cassini carries a limited supply of rocket fuel, it is strictly a last resort. "An elegant tour is one that meets a lot of the science requirements for a small cost in delta-V," says Smith.

In the process of designing more than 50 alternative tours for Cassini, Smith and Wolf have discovered some new manoeuvres. For instance, Wolf has perfected a way to increase the inclination while rotating the petals, a trick he calls cranking over the top. Some new techniques have arisen almost by luck. "I noticed that the node kept walking out close to Titan and thought it would be neat to force a flyby there," he says. The result was a way of rotating the petals using half an orbit rather than a whole one.

These techniques make the tours more flexible and efficient. A tour devised in 1992 had only 30 flybys of Titan, yet the tours now being considered have up to 40. "You have to be creative and open-minded. We're doing so many things for the first time," says Smith.

Bargains in space

There are other limitations on Cassini's route. For example the spacecraft must steer clear of the dust and ice in Saturn's rings. Titan's atmosphere also poses a problem because nobody knows exactly how far it stretches into space. Should the spacecraft pass through its upper reaches during a flyby, the drag could send it off course. Smith and Wolf must even ensure that periods of intense activity for ground control, such as flybys, do not occur during the Christmas holidays. Squeezing the maximum scientific return from a tour that is subject to such constraints is a major challenge. "It's like hunting for a bargain," says Smith.

Once the final tour has been decided, it will have to be divided up among the competing teams of researchers, a process which could turn into a bitter fight. In 1992, when US budget cuts threatened to kill off the mission, Cassini's $1.7 billion budget was slashed by $300 million and the spacecraft had to be slimmed down. The original design incorporated two movable platforms that allowed ground control to point the cameras and spectrometers in one direction while the dust counter pointed in another. At the same time, independently of the position of either platform, the main communications antenna could point either towards Earth or be used to take radar measurements of the surface of Titan.

In the new, leaner design, the cameras are fixed to the spacecraft's side and so can only be aimed by changing the attitude of the entire vehicle. This prevents the cameras and the radar from being trained on Titan at the same time, for example. Because of this constraint, each team of scientists will take turns to decide where to point the spacecraft. Naturally, each team would like to be in control for as long as possible, so some way has to be found for distributing "ownership" during the tour.

One of the most promising suggestions is to auction ownership. Each team will be given a notional currency, a pile of chips say, with which to bid. Control of the spacecraft during prime times such as close approaches will go to the highest bidder.

Satellite swapshop

This forces scientists to carefully consider the value of their observations. For example, how many Titan flybys are worth one good look at the mysterious moon Iapetus? One hemisphere of Iapetus is covered in a strange material that is among the darkest in the Solar System, while its other hemisphere is made of relatively bright material. Studying the border between the two is a priority for geologists who want to understand what this material is made of and where it came from.

A similar system has already been used to allow some flexibility for scientists developing Cassini's instruments. Normally, instruments must be developed to a fixed mass, on a fixed year-to-year budget. Instead, the project has operated a "resources exchange" in which the teams can trade the resources which they have been allocated, such as mass, money or electrical power. Should one team find that its instrument is lighter but more power-hungry than planned it can sell some of its mass allocation to another team in exchange for power.

The researchers have negotiated about a dozen sales of mass allocations and, interestingly, the price of mass has fallen sharply. In 1993, when the final mass of the instruments was difficult to predict, mass sold for about $100 000 per kilogram. Today, with the instruments built, mass is worth only $5000 per kilogram. Instrument teams could also sell the funding they would receive in one year in return for funding in another. During the project 16 such transactions have been made, totalling over $4 million.

The flexibility of this system seems to have paid off. The cost and mass of most missions tends to grow. For example, the budgets and mass allocations of instruments on the Mars Observer overran by about 20 per cent. However, many of the Cassini instruments are cheaper and lighter than planned. And the resource exchange system is catching on elsewhere. In southern California, where air quality is carefully managed, it is being used to trade "emission credits". And NASA is considering the system as a way of determining payloads on the space shuttle.

If this method is chosen to settle the ownership dispute, the first auction may take place next year and will finalise the first 15 months of the tour. The rest will be hammered out while Cassini travels to Saturn, when scientists will have a better idea of the amount of fuel that will be left after course corrections.

The study of Cassini's orbital mechanics will continue for four years or even longer—if any fuel is left, the mission could be extended. With the main objectives achieved, Smith and Wolf will develop riskier trajectories that skim Titan's atmosphere or fly closer to Saturn's rings, perhaps even inside them. Nobody knows whether the spacecraft can survive such encounters. But with a successful mission behind it, Cassini will have nothing to lose.

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