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鈥檚 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 鈥減etals鈥� 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鈥檚 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.
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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鈥檚 dayside so that they can watch cloud patterns evolve鈥攂ut
magnetospheric physicists want to sweep deep into the nightside to study the
tail of the planet鈥檚 magnetic field. Scientists who study Saturn鈥檚 famous rings
favour orbits that will look down on them鈥攂ut 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. 鈥淥ne 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,鈥� 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鈥檚
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鈥檚 tour is the gravitational field of
Saturn鈥檚 largest moon, Titan. The tour must be designed so that each orbit
brings the spacecraft back to Titan, where the moon鈥檚 gravitational field can be
used to pull it into a new trajectory. Consequently, Cassini will visit Titan
many times during the tour鈥攐n its first encounter in November 2004, the
spacecraft will release a probe that will parachute through the moon鈥檚 dense
atmosphere.
Smith and his colleague Aron Wolf, also at JPL, can exploit Titan鈥檚 gravity
to change a trajectory in a number of ways. In the arcane jargon of celestial
mechanics, altering the craft鈥檚 inclination鈥攖he angle the orbit makes with
the equator鈥攊s known as cranking, while moving the apoapsis鈥攖he
furthest part of the orbit鈥攊s called petal rotation. The point at which
Cassini crosses the plane of Titan鈥檚 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鈥檚 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. 鈥淎n elegant tour is one that meets a lot of the science
requirements for a small cost in delta-V,鈥� 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. 鈥淚
noticed that the node kept walking out close to Titan and thought it would be
neat to force a flyby there,鈥� 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. 鈥淵ou have to be creative and open-minded. We鈥檙e doing so many things for
the first time,鈥� says Smith.
Bargains in space
There are other limitations on Cassini鈥檚 route. For example the spacecraft
must steer clear of the dust and ice in Saturn鈥檚 rings. Titan鈥檚 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. 鈥淚t鈥檚 like hunting for a
bargain,鈥� 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鈥檚
$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鈥檚 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
鈥渙wnership鈥� 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鈥檚 instruments. Normally, instruments must be
developed to a fixed mass, on a fixed year-to-year budget. Instead, the project
has operated a 鈥渞esources exchange鈥� 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 鈥渆mission credits鈥�. 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鈥檚 orbital mechanics will continue for four years or even
longer鈥攊f any fuel is left, the mission could be extended. With the main
objectives achieved, Smith and Wolf will develop riskier trajectories that skim
Titan鈥檚 atmosphere or fly closer to Saturn鈥檚 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.