IT SOUNDS like a recipe for frying a spacecraft to a crisp, but scientists in
Illinois reckon orbiting space probes will one day be able to fly into and out
of a planet鈥檚 atmosphere just like an aircraft鈥攚ithout burning up.
Using new trajectory-planning software they have developed, the researchers
say spacecraft will dip into a planet鈥檚 atmosphere, harness aerodynamic forces
to fly like an aeroplane, and then zip back into space, having steered into a
new orbital trajectory.
The idea is to save precious fuel, says Bruce Conway and his colleague
Kazuhiro Horie of the University of Illinois at Urbana-Champaign. Today, says
Conway, spacecraft must carry enough propellant to carry out most of the
position and orbit changes they might be expected to make during their
lifetime.
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The fuel鈥檚 weight cuts the craft鈥檚 useful payload, so space scientists have
long pondered the possibility of building craft that can use a planet鈥檚
atmosphere to steer onto a new course. Until now, space probes have only used
atmospheres for 鈥渁erobraking鈥濃�攚here the craft skims the outer edge of an
atmosphere to slow down and gradually change the shape of its orbit. The NASA
Mars Global Surveyor probe did just this in 1997.
鈥淚n aerobraking, you鈥檙e only interested in getting drag, reducing the amount
of energy that the spacecraft has,鈥� Conway says. But the 鈥渁ero-assisted鈥�
vehicles he and Horie envisage would use aerodynamic lift to take a probe into a
new orbit rather than merely slow it down. 鈥淚f you have some control surfaces,
you can steer the craft,鈥� he says.
An articulated solar panel might do the
trick. 鈥淚t would be like moving the ailerons on an airplane,鈥� Conway says.
Alternatively the craft itself鈥攐r its heat shield鈥攃ould have an
aerodynamic 鈥渓ifting body鈥� shape.
The Illinois work was focused on trajectories for the interception of one
satellite by another, determining the best route for an aero-assisted spacecraft
to take in order to fly safely through an atmosphere and then exit to a given
orbit, depending on the craft鈥檚 lift and drag capabilities, and the
characteristics of the atmosphere. 鈥淭he program finds the trajectory from a
given initial position that enables the vehicle to intercept the target in
either the minimal time or with minimal fuel,鈥� Conway says (see 鈥淯p a
bit鈥︹��, below). Crucially, it does so while taking into account how hot
the craft can safely get. 鈥淭he heat it can stand determines how low in the
atmosphere it can fly,鈥� he says. 鈥淭hat鈥檚 the critical factor.鈥�
Charles Whetsel, chief project engineer for Mars Global Surveyor at NASA鈥檚
Jet Propulsion Laboratory in Pasadena, California, says of the Illinois project:
鈥淚t鈥檚 definitely valuable work that will help us launch more efficient missions
that use less fuel and which can carry larger scientific payloads.鈥�
To move a craft from one orbit to another, there are many possible
trajectories but only one 鈥渙ptimal鈥� trajectory that uses either the least fuel
or takes the least time. The conventional way to find this, says Bruce Conway,
is called 鈥渟hooting鈥�. A computer starts from guessed initial conditions and uses
equations of motion to work out where the craft will end up after, say, an
aerodynamic pass through a planet鈥檚 atmosphere.
If the craft does not end up where it is wanted, the guessed initial
conditions are adjusted and the process is repeated鈥斺�漧ike adjusting the
aim of a cannon to hit a target鈥�, says Conway. But this 鈥渋terative鈥� process is
very slow, and may never find an optimal solution at all. So Conway and Kazuhiro
Horie have worked out how to divide the trajectory into a curve and then chop it
up into many discrete, mathematically malleable chunks. Conway says these can be
adjusted to give 鈥渁 correct optimal trajectory even from a very bad guess鈥�.