Collision Warning and Avoidance
The ISS program plans to maneuver the space station to avoid collision with objects tracked and cataloged by the U.S. space surveillance network (SSN). A modified version of the scheme currently used in the space shuttle program will be employed. In support of that program, the SSN routinely screens the catalog for objects predicted to approach the orbiter within a defined “warning box” approximately 25 km along the track of the orbit (either leading or trailing), 5 km across the track of the orbit, and 5 km out of the plane of the orbit. The estimated 10 to 30 objects per day that come within the warning box are reassessed using a more accurate algorithm to determine whether any come within a “maneuver box” of 5 km along track × 2 km across track × 2 km in the radial direction. If an object does, the ISS may initiate a maneuver to avoid impact (Schultz, 1996). The maneuver box is many times larger than the ISS to provide a safety margin because the locations of tracked objects are not precisely known.
To reduce disturbance to microgravity experiments, the ISS program requires that the avoidance scheme require less than six maneuvers per year. The SSN, however, projects that 200 trackable objects will enter a 5 km × 2 km × 2 km box around the ISS each year by 2005. To decrease the number of required maneuvers, the ISS program plans to use the global positioning system (GPS) to increase the accuracy with which the position of the ISS is known. This information will be fed into an algorithm to determine the probability that an object entering the maneuver box will collide with the station. If the probability exceeds a certain threshold, the station will maneuver to avoid collision. The ISS program has not yet set this threshold probability, but it predicts that fewer than 10 maneuvers will be conducted each year.
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
during ISS assembly and operations could help reduce the number of debris avoidance maneuvers the ISS would need to perform.
Avoiding More Objects
If the gap could be reduced between the population of objects that can be tracked and the population of objects that can be shielded against, then the risk to the ISS could be cut drastically. (Shrinking the maneuver box is a prerequisite, however, because the number of false warnings will increase greatly as the size of the debris the tracking system is able to catalog decreases.) The SSN has difficulty tracking smaller objects, however, because it was designed to detect and track space objects that reflect a large radar cross section.
The current catalog at ISS altitudes is essentially complete for objects larger than 100 cm in diameter and about 95 percent complete for objects larger than 30 cm in diameter. Although some objects as small as 10 cm are cataloged, somewhere between 15 and 50 percent of the objects between 10 cm and 20 cm may be missing from the catalog (Kessler, 1996; Lord, 1996). Improving the system to allow it to catalog debris smaller than 10 cm would require adding new sensors, retasking current and future sensors, developing new procedures and algorithms, and improving computational capability.
Although NASA would like to see the SSN sensitivity threshold improved, the DoD does not have a mission to achieve tracking accuracies for objects smaller than 10 cm in diameter. The DoD, therefore, cannot be expected to exert great effort to improve its capabilities in this direction. Indications are that network sensitivity is more likely to decline than improve in the near future as a result of sensor closures or other actions.
In the early 1990s, NASA assessed the feasibility of developing a system to detect and track a greater number of debris objects in the 1 cm to 30 cm range (Loftus and Stansbery, 1993). The scheme, which involved building a number of short-wavelength radars, was estimated to cost approximately one billion dollars, plus a hundred million dollars a year to operate. Assuming its only function were to support the ISS, building such a system would probably not be a cost-effective method of reducing the risk to the ISS from meteoroids and debris.
Finding 6. Although it would be technically feasible to track a larger population of objects and warn the ISS about more potential collisions, doing so would require a significant effort. If this effort were made, additional steps would need to be taken to reduce the number of false alarms generated by this larger population.
Maneuvering the International Space Station
The SSN is believed to be capable of delivering warnings of the potential hazards it is able to track at least six hours (three to four orbits) in advance. This
should provide sufficient time for maneuvers to be executed. A number of issues, however, need to be resolved before the effectiveness of the avoidance maneuver scheme can be fully evaluated. Currently, for example, the ISS often will be unable to maneuver because a radio link to the Russian ground stations (which will coordinate the maneuvers) will be unavailable for at least part of every orbit. In addition, the ISS program currently has no plans to maneuver the ISS during the estimated one week out of every ten that the shuttle orbiter is docked with the station because of the effect the shuttle would have on the behavior of the ISS under acceleration. For similar reasons, the ISS currently has no plans to maneuver to avoid debris during some periods of the assembly sequence.
Use of On-Board Sensors
ISS collision warning systems that rely solely on on-board sensors are currently infeasible. There are two underlying problems. First, it is difficult to track debris from an orbiting spacecraft. On-board sensors would be unable to identify and track most objects for multiple orbits. Thus, the sensors would have to detect and obtain accurate knowledge of the orbital characteristics of an incoming object only from data obtained as the object approaches the station at average closing speeds of about 9 km/s. (Such a sensor capable of reliably detecting oncoming debris from a wide variety of angles, without consuming the majority of the electric power generated by the station, is well beyond current capabilities.)
Second, even if such sensors were available, the space station would still be unable to maneuver fast enough to avoid a collision. At typical speeds, a hypothetical advanced sensor capable of detecting and tracking an incoming object on its final impacting orbit at a range of 500 km (or about half the distance to the horizon) would provide less than a minute of warning at the expected approach velocities. Even if the ISS program could cut the time to prepare for a maneuver from two hours to two seconds, it would still take the ISS about 30 seconds to achieve a velocity of 1 m/s and 100 seconds at that velocity to move a distance equal to its own length.
Finding 7. Barring major leaps in technology, on-board sensors will not be effective in providing the ISS with a collision avoidance capability.
Recommendation 17. The efforts of the International Space Station program to reduce the disturbances to microgravity experiments from collision avoidance schemes should concentrate on reducing the number of false warnings, rather than on accepting a higher level of risk.
Recommendation 18. The International Space Station program should take particular care to avoid producing debris during the operation and assembly of the space station.
Recommendation 19. The International Space Station program should continue to work on ensuring that the International Space Station is able to maneuver when threatened by debris. Efforts should be made to reduce the time between receiving a warning and executing an effective maneuver.
Recommendation 20. NASA should work closely with the Space Surveillance Network to determine what Space Surveillance Network support will be available to the International Space Station over its lifetime and to determine whether improvements in that support are possible.
Kessler, D.J. 1996. Private communication to Robert Culp, April 17, 1996.
Loftus, J.P. Jr., and E.G. Stansbery. 1993. Protection of Space Assets by Collision Avoidance. Presented at the 44th Congress of the International Astronautical Federation, Graz, Austria, October 16–22, 1993.
Lord, Maj. Gen. L.W. 1996. Memorandum to the NRC Committee on International Space Station Meteoroid/Debris Risk Management. April 3, 1996.
Schultz, E.D. 1996. Planned Operations for Orbital Debris Collision Warning/Avoidance. Briefing presented to the NRC Committee on International Space Station Meteoroid/Debris Risk Management, Houston, Texas, April 3, 1996.