The ISS will maneuver itself to avoid debris by firing thrusters to raise the orbital altitude with a velocity increment of less than 1 m/s. The ISS is expected to execute similar maneuvers about once a month to maintain orbital altitude. Thus, it is expected that avoidance maneuvers will simply change the scheduling of reboost maneuvers, and no extra propellant will be required. Because the thrusters are in the Russian part of the ISS, the maneuvers will be controlled by the Russian crew or by Russian ground stations. The ISS program estimates that, once a warning is received, it will take two hours to coordinate the maneuver through the RSA, communicate instructions to the crew, prepare the ISS to perform the boost maneuver, fire the thrusters, and have the ISS actually move the required distance.

ANALYSIS AND FINDINGS

Reducing False Warnings

To minimize disturbances to microgravity experiments, the ISS program needs to reduce the number of unnecessary collision avoidance maneuvers. One approach to reducing the number of false warnings would be to accept a higher level of risk. A better approach would be to determine with greater accuracy the location of the object threatening the ISS. For normal LEO objects, the SSN has demonstrated its ability to reduce tracking errors to less than 500 m for periods of up to 24 hours. New technologies under evaluation by the U.S. Space Command and NASA may further improve this capability. If the SSN can reliably sustain such an effort, the need to maneuver the ISS will be markedly curtailed.

However, there are difficulties in achieving this goal. Some approaches to increase the accuracy with which the positions of incoming debris are known will require the development and deployment of new sensor systems, as well as the retasking of current and future sensor systems. Although this may be technically feasible, the U.S. Space Command is not currently funded or responsible for providing this type of support or for retasking or upgrading SSN sensors. In addition, because of uncertainties in atmospheric density, ballistic coefficients, and gravity models, the validity of these procedures for objects with large area-to-mass ratios and for periods of high solar activity has yet to be verified. Reliably tracking objects in eccentric orbits will also require further demonstration.

The ISS itself might produce debris that could force the station to perform maneuvers. An unpublished Air Force Space Command analysis of close conjunctions of debris with the Russian Mir space station showed that 5 of the 16 objects that entered a 5 km × 2 km × 2 km box around Mir in 1995 were originally associated with Mir operations. Although the collision velocities between the ISS and debris from ISS operations would be low, the station would still need to maneuver itself to avoid trackable items. Minimizing the production of debris



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