Ray A. Williamson, PhD, is editor of Imaging Notes and Executive Director of the Secure World Foundation, an organization devoted to the promotion of cooperative approaches to space security (SecureWorldFoundation.org).

Space Debris

Positional Data Needed Commercially

By Ray Williamson, PhD, editor

In the past, imaging satellite operators have generally not needed to worry much about the risk their satellites might face from collisions with other satellites or with orbital debris. More recently, however, they have begun to take notice of environmental conditions in Earth orbit. One immediate reason for increased interest in orbital safety is the February 10, 2009, collision of the Iridium-33 satellite with the defunct Russian Cosmos 2251 satellite. That collision created more than 800 pieces of debris 10 cm in length or above, in two clouds. Over time, these two clouds will slowly expand outward, threatening other working satellites that move in nearby orbits.

Because the destroyed Iridium satellite flew in a near polar orbit, debris from this collision now poses an additional hazard to some remote sensing satellites. The Canadian RADARSATS 1 and 2 have been cited as threatened by this debris.(1) Other Earth observing satellites are likely to be affected in the future.

Normal space operations unavoidably add debris to the space environment. In addition, fragmentations of spent rocket bodies in orbit and occasional explosions of old satellites add to the threat. Finally, anti-satellite tests by the Soviet Union and the United States in the 1970-80s and by China in January 2007 resulted in thousands of pieces of space debris. U.S. Air Force officials estimate that some 18,000 pieces of debris greater than 10 cm now circle Earth.

Collisions with debris are not the only concern, however. As more and more countries launch Earth imaging satellites into orbit, the sun-synchronous orbits that are valued for Earth observations will become ever more crowded, adding another concern to the safety question. As noted in an earlier column (Spring 2008), in July 2007, the orbit of NASA's Cloudsat satellite was shifted slightly to avoid the possibility of colliding with Iran's Singha remote sensing satellite.

NASA was able to accomplish this task because NASA debris experts have access to the classified database of satellite orbital positions maintained by the U.S. Air Force. However, commercial and non-U.S. satellite operators do not have access to such high quality data and can use only the much lower quality data provided by the Air Force on an openly available website: Space-track.org. These data, while important for crude assessments of conditions in Earth orbit, are derived from the accurate data set through a set of algorithms and are not of sufficient quality to allow so-called conjunction assessments, calculations of the chances of collision with other objects in orbit.

In fact, it turns out that Iridium, Inc., which operates a fleet of 66 satellites in low-Earth orbit, was not in the habit of calculating conjunction assessments because they do not have access to the high quality data that would make accurate assessments possible. Further, the Air Force, which routinely carries out conjunction assessments of its satellites and of NASA scientific satellites, does not have the resources to perform these calculations for all of the U.S. commercial satellites. The Air Force does, if requested, perform them for commercial communications satellite companies that operate in geosynchronous orbit when the companies need to maneuver or adjust the orbit of a satellite. However, the process, which is conducted under the so-called Commercial and Foreign Entities (CFE) program of the Air Force, is cumbersome and insufficient for handling all of the communications satellites, let alone all of the 900 or so satellites in Earth orbit.

Figure 1 On Feb. 10, 2009, an accidental collision occurred over 400 miles above Siberia between the privately owned communications satellite, Iridium-33, and an old, unused Russian satellite (Cosmos-2251). Celestrak provided tracking data for the debris, which was patched through to Google Earth by Robert Simpson using his Satellite KML code. The crash destroyed both satellites. The Iridium constellation of 66 satellites provides voice and data connections for satellite phones as well as other services. It has around 300,000 clients across the globe, including the U.S. Department of Defense and scientists at the South Pole. Image courtesy of Robert Simpson, www.orbitingfrog.com.

Although until now, apprehension over debris and orbital crowding has primarily concerned the commercial communications satellite companies, the Iridium collision has made other satellite operators take notice. Without accurate positional data and the analytical tools to perform conjunction assessments, both companies and non-U.S. operators are at a strong disadvantage in avoiding the threat that debris and orbital crowding pose.

Other countries collect positional data on their own satellites and perform conjunction assessments on them. However, they have relatively limited tracking capabilities and have nowhere near the tracking capacity of the U.S. Air Force, which operates both radar facilities and optical telescopes to carry out the task.

As I have argued before in this column, space systems, especially GPS and Earth observing systems, provide essential services that support human security, especially in disaster mitigation and response, and in food, water and environmental security. As the world community depends more and more on these systems, it will have to do much more to prevent future loss of Earth observing satellites from satellite-satellite or debris-satellite collision.

What can we do about the threat that debris poses to space operations? Unfortunately, at altitudes above 400 km, debris may stay in orbit for many years. At much higher altitudes, it will be there for centuries. Hence, the most effective ways to mitigate the threat that debris poses are to:

This March, Secure World Foundation partnered with the Eisenhower Center for Space and Defense, the Center for Defense Information, and the Marshall Institute at Intelsat Headquarters in Washington, D.C., to explore options for improving space situational awareness (SSA) in Earth orbit. Representatives from the U.S. Air Force, the National Security Space Office, the European Space Agency, the commercial telecommunication satellite industry and others shared ideas and plans for reaching an international consensus on an SSA construct. Although no shared conclusions were reached at the conference, participants generally agreed that the time has come for broader sharing of accurate satellite and debris positional data and for mechanisms to pursue conjunction assessments for all working satellites.

These developments are very promising. Yet, as several papers at an April European space debris conference in Darmstadt, Germany, warned, unless the space community begins to remove larger pieces of debris from orbit within the next few years, we may face conditions where debris will continue to build and make certain orbits unusable. The problem here is that no one yet knows how to accomplish this removal economically and in ways that do not just exacerbate debris production. Hence, in addition to instituting and strengthening all of the other debris-reducing mechanisms, the space faring states need to turn up their research on cost-effective methods to remove orbital debris, especially in sun-synchronous polar orbits, which face the greatest risk from destructive collisions.

Footnote 1. Megan Haynes, "Duo of Canadian Sats in Debris Danger," Satnews Daily, March 23, 2009.