ALTHOUGH the recent prediction of a near collision between Earth and asteroid XF11 turned out to be inaccurate, hazards from asteroids and other near-Earth objects are out there. After all, just a few years ago, the Shoemaker-Levy comet hurtled onto Jupiter, leaving Earth-sized scars on the planet's face, and a similar event is believed to have caused the extinction of the dinosaurs on Earth. The few nervous moments we Earthlings had over XF11 serve as a reality check on the hazards that await from space.|
Scientists and engineers at Lawrence Livermore have been engineering small, agile satellites that can help deal with potential space calamities. Called microsatellites (microsats, for short), they are an outgrowth of research performed for the Laboratory's Clementine satellite program, which mapped the moon and then discovered the first evidence that water may exist there. The microsatellites are envisioned as operating autonomously in orbit to serve a variety of future space-exploration needs in addition to probing near-Earth asteroids. Microsatellites would be able to strike or probe the potentially hazardous objects that threaten Earth. In addition, they might be handy rescue vehicles used to inspect disabled satellites and relay observations about them to ground stations; they might also dock with and repair satellites. Microsatellites could also be part of a control system that protects and defends U.S. assets in space.
The capability for such uses will come through integrating a complex array of advanced technologies in the microsatellite vehicle. Sensors, guidance and navigation controls, avionics, and power and propulsion systems--all must perform precisely and in concert so the vehicles can find, track, lock onto, and rendezvous with their targets, even though those targets are also on the move. The rigorous ground testing of microsatellites' integrated technologies is essential; these tests produce data needed for effective flight testing.
The best ground-testing environment is one that mimics, as much as possible, the free-floating environment of a space flight. Finding a way to emulate such an environment was one of the important tasks facing microsatellite developers.
Inspired by a Game
Scaling Down Space Maneuvers
The intercept geometry, comprising the vehicle positions, target positions, and the changing line of sight between them, is scaled for the indoor table experiment to preserve the intercept geometry of an actual flight. For example, for a successful interception, the line-of-sight rate must approach zero; that is, the vehicle and target must both arrive at the same point at the same time. To preserve that line-of-sight requirement in the test, the test maneuvering distance is scaled down in relation to the target that is projected on the screen. The target location and interception point are predetermined, and these values, used in conjunction with the precise measurements of vehicle position (from the laser measurement system), allow experimenters to determine the ability of the onboard guidance and control software to maneuver the vehicle to the point of interception.
Taking Testing to the Next Steps
To eliminate some of the indoor setup's limitations, an outdoor version of the device is being developed. In this version, the test vehicle "floats" on a smooth rail 100 to 200 meters long and "views" a tilted board on which an incoming target is projected (Figure 3). The rail air-bearing system can move in only one linear direction, but because of its larger scale, it provides an improved replication of flight maneuvers and a more accurate tracking of vehicle position. Both improvements lead to a more precise reconstruction of line-of-sight angles, which is key to correctly predicting the point at which the microsat maneuvers to its target.|
The air-bearing team's work on ground testing techniques continues. To date, a 17-meter-long rail has been used to "fly" the newest generation of the microsat vehicle. Longer range outdoor docking experiments that incorporate both an onboard Star Tracker camera, which uses stars to calculate the orientation of the microsats, and a global positioning system receiver are in the planning stages.
Key Words: AGILE (air-table guided-intercept and line-of-sight experiments), dynamic air-bearing table, dynamic air-bearing rail, ground testing, microsat, microsatellite, spacecraft interceptor, space vehicle.
For further information contact Arno Ledebuhr (925) 423-1184 (firstname.lastname@example.org).