Addressing the Dangers of Debris

The figure shows The Aerospace Corporation’s prediction for the Phobos-Grunt reentry on January 15, 2012. The red dots indicate where researchers predicted the surviving tanks and return capsule would land, although the objects could have landed anywhere along the track shown, with decreasing probability. As it turned out, Phobos-Grunt reentered earlier than predicted, just off the west coast of Chile. While it is possible that some fragments survived and landed on the ground, none have been found.

The potential dangers to space assets from the ever-increasing orbital debris field are numerous and present significant challenges in their mitigation.

by Glenn Peterson

 

Society is increasingly reliant on space-based capabilities to provide services used on a daily basis: navigation, weather prediction, and communications. An interruption or loss of these capabilities would have considerable impact on many different aspects of today’s technologically based world. A significant threat to the health and mission success of satellites—and therefore the capabilities they provide—is the potential for impact with orbiting space debris. Impacts between large, trackable objects are especially devastating, as they result in the destruction of the satellites involved and the attendant loss of capability. Such impacts are already known to have occurred. The collision of Iridium 33 and Cosmos 2251 is a notable example. While impacts with smaller debris are not sure to result in the destruction of the satellites involved—though that is possible— mission loss is still a possibility, should the debris damage or destroy a critical component. Even if capability is not completely lost, satellite performance can be affected through degradation of sensitive components. Of concern too is the fact that smaller debris cannot be tracked from the ground, making prediction and mitigation of collisions much more difficult.

While the risk to satellites from orbital debris is considerable, studies conducted by The Aerospace Corporation reveal actions that can be taken to make that risk more manageable. These mission assurance practices address four distinct areas of space system application: mission, satellite, and operational costs, and accessibility to space.

Mission Cost

The loss of capability or complete destruction of a satellite requires that satellite be replaced earlier than anticipated to ensure the continued functionality of the constellation. The increase in the satellite program’s budget necessary to cover that replacement is the mission cost.

The hazard posed by space debris to operating satellites is growing. In 2000, there were approximately 15,000 objects in space large enough to be tracked; that is, larger than 10 centimeters. Today, that number is more than 22,000. There are also several hundred thousand objects between 1 and 10 centimeters, which are too small to be tracked reliably, and literally millions smaller than 1 centimeter, which cannot be tracked at all. The size of a debris object dictates the amount of damage it can inflict in a satellite collision: large objects will likely destroy the satellite and create a cloud of debris that could impact other vehicles; objects on the order of 1 centimeter might also terminate a satellite’s mission if they strike a critical component, such as an electronics box or propellant tank, but are unlikely to create large secondary debris clouds; finally, very small particles can degrade the performance of components such as optics and solar cells. Over time, such degradation can lead to reduced mission effectiveness or even the loss of a satellite’s mission.

To determine the possible implications to mission cost resulting from debris collisions, The Aerospace Corporation conducted a study to examine the costs of maintaining three types of satellite constellation in the worst-case (most heavily populated) 850-kilometer sun-synchronous orbit environment, considering the timeframe of 2010 to 2030. The three constellations examined represented the range of possible satellites types: high-cost, high-reliability government-owned satellites; medium-cost, medium-reliability commercial satellites; and lower-cost, factory-produced satellites. Impacts by debris greater than 1 millimeter in diameter were assumed to degrade solar-panel performance; those larger than 1 centimeter led to damaged solar panels or, if the impact occurred in a critical area, termination of the satellite; and impacts with objects 10 centimeters or larger would terminate the satellite’s operation regardless of where they hit. Flux density projections from the Aerospace Debris Environment Projection Tool estimated the number of times a satellite would be impacted by objects of each size over time. The study then determined the number of launches required to maintain each constellation for the 20-year period.

It was found that the cost of maintaining the constellations due to the expected future debris environment increased by 3 percent for government satellites, 9 percent for the medium-cost commercial satellites, and 18 percent for low-cost factory satellites. Interestingly, a significant source of the increase was not satellite bus damage or destruction, but damage to the solar panels. The sandblasting effect from small particles degraded the panels’ functionality, eventually rendering the satellite unable to perform its mission. This study was for the near-term future and involved rather simplistic satellite designs, but does indicate that while debris will increase the cost of operations in general, one important area of satellite improvement that would mitigate the effect of future debris is in solar panel design. More detailed studies may show other satellite components can be similarly improved with subsequent debris mitigation benefits.

Satellite Cost

Smaller debris objects (less than 1 centimeter) cannot be tracked and hence are unavoidable. Satellites will be hit by small particles during their lifetime. One way to mitigate this is to add shielding to protect a spacecraft from this small debris, but the sheer number of such small debris objects in orbit requires hard decisions in terms of tradeoffs: as the amount of debris grows, greater amounts of shielding will be required. Extra shielding takes extra mass, which lowers the amount of functioning payload mass that can be delivered to orbit. This extra mass and/or reduced functionality is referred to as satellite cost.

When a small particle hits a spacecraft, the damage can range in severity from minor surface degradation to inhibiting or ending the mission by hitting a critical component. An object that penetrates a satellite wall may continue on into the spacecraft interior, possibly damaging internal components; further, the object will break up on penetration, creating more objects from that breakup and through creating fragments from the punctured satellite wall. In some instances, the particle does not even have to penetrate the wall; the impact can cause material on the back of the wall to come off and spread further. All of these secondary particles generated by an impact are called “spall” and can damage the interior of the spacecraft.

One way to combat this small particle damage is to shield the spacecraft surface. The most simplistic shielding consists of thickening the spacecraft wall until debris objects can neither penetrate nor create spall. However, the better the single-wall shield is in terms of resisting impact energy, the greater its mass becomes. This greater shield mass means either increased launch costs or reduced payload mass.

Another shield type is the so-called Whipple shield. There are a number of variations, but the basic design consists of using one or more thin layers of a substance like aluminum placed at a certain standoff distance from the main spacecraft wall. Multiple layers of material are often used, with the space between sometimes filled with materials like Kevlar, to provide protection of the spacecraft interior. The Whipple shield works by dissipating the energy of an impacting debris object into the layers; the object fragments, and the fragments spread over a larger area until they lose momentum and cannot penetrate further. While the required mass for Whipple shields is less than that for single-wall shields, the spacing between the layers results in an increase in overall spacecraft size, which has implications for the spacecraft’s launch fairing. Specific designs can be complex; for example, the International Space Station has more than 100 different Whipple shielding configurations for different parts of the vehicle.

The cost of shielding must be balanced against the benefit it delivers. The amount of shielding cannot be so great that the vehicle’s mission is impaired or its overall cost becomes prohibitive. In a study performed by the European Space Agency, the lower replacement cost and lower failure probability for a shielded versus an unshielded spacecraft resulted in an overall savings greater than the cost of the added shielding; total savings were approximately 1 percent of the original satellite cost. Thus there was a small but noticeable benefit to be had by shielding this sample spacecraft, but the benefit for each individual satellite design must be evaluated for that particular vehicle.

Operational Cost

Operational cost is determined by the number of times a spacecraft has to maneuver to avoid potential collisions (known as collision avoidance, or COLA). Each such maneuver requires fuel, reducing the amount of fuel available for mission-specific tasks and thereby shortening the functional lifetime of the vehicle. COLA maneuver opportunities can only be determined relative to large debris objects that can be tracked, and not the many smaller debris objects in orbit.

The projected growth in the debris environment will have a significant impact on future satellite operations. As the number of tracked debris objects grows, there will be a correspondingly greater number of conjunctions (close approaches) between operational satellites and debris objects. Each of these conjunctions must be evaluated, and, if the threat is deemed serious enough, some type of operational response must be determined. There are two possible response scenarios. First, there can be a deliberate tasking of the sensor network to track the debris resulting in a more accurate determination of its orbit, and thereby hopefully eliminating the conjunctions as a threat. Such additional tasking uses resources in terms of personnel and equipment, increasing cost. The second possible response is to maneuver the at-risk satellite to avoid the potential collision. However, each such maneuver reduces the satellite’s lifetime and impacts the achievement of its mission goals.

When a conjunction occurs, the probability of collision is computed using the estimated miss distance between the two objects and their associated orbit uncertainties. A response is indicated when that probability exceeds a predetermined maximum, called a threshold value. Because the goal of this process is to reduce the likelihood of a collision with the satellite, the threshold must be low enough that an acceptable level of mission risk reduction is achieved, but not so low that too many responses are indicated and overwhelm analysts. Therefore, the number of potential responses facing a satellite controller is determined through consideration of the debris environment, the acceptable level of risk reduction, and the uncertainties inherent in tracking space debris.

As tracking of orbital debris improves, by, for example, the proposed Space Fence radar, and smaller objects are tracked than is currently possible, even more threshold violations warranting some type of response will occur.

To determine the effect the future environment will have on satellite operations, Aerospace conducted a study for a sample satellite at an altitude of 850 kilometers, the expected worst-case altitude for debris. For the purposes of the study, tracking accuracy was at first fixed to the current levels and desired risk reduction was set at 50 percent. Tracking-system resolution was set to the current level of 10 centimeters. Under this scenario, a threshold violation occurred for the sample satellite roughly once every three days, but by the end of the century, due to projected increases in debris objects, violations were occurring between once or twice a day. Lowering the tracking-system resolution so that objects down to 5 centimeters were observed (the potential resolution of the Space Fence) increased the number of violations to approximately 5 per day. However, also projecting that tracking accuracy would be similarly improved (that is, orbit uncertainties would be reduced), in this scenario by a factor of 3, then the number of violations for 5-centimeter objects dropped to fewer than 1 per day.

The results of this study have significant implications for future tracking system requirements. Simply creating a system with improved resolution where smaller and smaller objects are seen will create an operational environment that becomes dominated by responses to potential collisions. However, if a future proposed tracking system is able to pick up smaller particles and also produce better observations, then the number of violations would be lower and could even improve over the current situation. While the debris environment does (and will) have operational consequences, these are manageable.

Accessibility to Space

When new satellites launch, COLA analysis is performed to reduce the likelihood that the launch vehicle upper stage and the satellite payload will collide with an orbiting tracked object. The more objects that are being tracked, the higher the likelihood becomes that launch opportunities within the launch window will be closed, preventing easy accessibility to space. The reduction in accessibility to space due to debris is separate from the issue of on-orbit maneuver frequency. In maneuver-frequency analysis, a satellite is on orbit for years, and the goal is reduction of the overall probability of collision that the satellite must face during its mission lifetime. Therefore, the satellite’s operators need to be concerned about every conjunction that violates the threshold probability.

However, in launch operations, there is a launch window that contains multiple launch opportunities and each opportunity can be selected for launch with equal viability for mission success. Therefore, when any conjunction occurring during an individual opportunity exceeds the threshold, that opportunity is closed. As long as some opportunities remain in the window without high probability conjunctions, those low-risk opportunities can be easily selected, and access to space is not inhibited.

Studies performed by Aerospace have shown that the current operational procedure of reporting all closeouts to a collision probability down to one in ten million is acceptable for the short term; that is, aside from certain rare instances, a large number of opportunities for a given window are not being closed. In the long term, this will change. Considering the combination of a Space Fence-like tracking system with improved resolution capability along with the projected orbital debris growth results in a situation where high-probability conjunctions will close out entire windows within a few decades for launches to low Earth orbit (LEO). Launches to geosynchronous Earth orbit are less affected by this phenomenon, because most debris resides in LEO. Therefore, relying on a fixed threshold value rather than one based on the desired level of overall probability reduction could be problematic; threshold determination should be based on the characteristics of an ever-evolving debris environment, a changing tracking system, and the desired amount of overall risk reduction.

In maneuver-frequency analysis, improving the tracking accuracy of orbiting objects helps alleviate the effect of debris growth. However, improvements in tracking accuracy will not impact launch COLA, simply because the along-track uncertainty in the launch vehicle trajectory is so much larger than orbital accuraciesLaunch vehicle uncertainties are dictated by factors outside of easy control, for example, the ability to predict upper atmospheric winds. Therefore, the issue of access to space will be affected by debris growth sooner than it will be by on-orbit maneuver frequency.

Conclusion

Space debris is an issue that is currently affecting spacecraft and mission designers as well as the operational community. This problem will become more serious in the future as more debris-generating collisions occur, but with proper anticipation and knowledge, the effect that the growth of debris will have on the satellite community is manageable for the foreseeable future.

Further Reading

Ailor, J. Womack, G. E. Peterson, E. Murrell, N. Lao, “Effects of Space Debris on the Cost of Space Operations,” 61st International Astronautical Conference, (Prague, Czech Republic, Sept. 27, 2010).

E. Peterson, “Effect of Future Space Debris on Mission Utility and Launch Accessibility,” AAS 11-414, AAS/AIAA Astrodynamics Specialist Conference, (Girdwood, Alaska, Aug., 2011).

Wiedemann, M. Oswald, S. Stabroth, D. Alwes, and P. Vorsmann, “Cost and Benefit of Satellite Shielding,” Acta Astronautica, Vol. 63, pp. 136–145 (2008).

Related article:  What Is the Risk of Being Hit by Reentering Debris?

Related publication:  Crosslink, Fall 2015, Understanding Space Debris