Thousands of satellites have been launched into earth orbit, with nearly 1,000 of them still active and providing valuable services such as military and intelligence data collection, global positioning, telecommunications, weather and climate monitoring, and so on. These active satellites, however, are increasingly at risk of colliding with “space junk,” also referred to as “space debris.” Space junk (“junk objects”) is the collection of now-useless, human-created objects in earth orbit such as spent rocket stages, defunct satellites, mission-related debris, and fragments from collisions and on-orbit breakups. The risk of a collision occurring is more than a theoretical possibility as in February 2009 a collision occurred between an American Iridium satellite and a defunct Russian Cosmos satellite.
The risk of future collisions occurring is increasing because the density of objects in earth orbit is increasing. Although space junk will eventually be removed from orbit by the frictional forces of the earth's atmosphere, space junk can remain in orbit a very long time as those frictional forces are small. Space objects (e.g., active satellites and space junk) of certain types tend to orbit in certain orbital regions. For example, low-earth orbit (i.e., 160 km to 2,000 km) contains space stations, upper rocket stages, and amateur satellites; middle-earth orbit (2,000 km to 35,876 km) contains navigation satellites; and geostationary-earth orbit (36,000 km) contains geostationary satellites. Eventually, some orbital regions may become so crowded that placing new satellites in orbit will be impractical as the risk of a collision will be too high.
Estimates place the number of junk objects at tens of millions. The vast majority of the junk objects are very small particles such as dust from solid rocket motors or paint that flakes off of satellites. A collision between an active satellite and such small junk objects can have an erosive effect, similar to sandblasting, on the satellite. Estimates place the number of junk objects in low-earth orbit that are larger than 1 cm to be around 300,000. A collision between an active satellite and such junk objects can have a more serious effect, but not necessarily a catastrophic effect. A significant number of junk objects, however, are larger than 5 cm, and a collision between an active satellite and such large space junk can indeed be catastrophic.
A collision between large space objects (e.g., an active satellite and large space junk) may result in the creation of hundreds or thousands of fragments, some of which could be larger than a softball. The collision between the Iridium satellite and the Cosmos satellite produced thousands of junk objects that are still in orbit. Actions taken by certain countries have resulted in significantly increasing the amount of space junk. In 2007, China performed an anti-satellite weapons test that destroyed an aging weather satellite using a kill vehicle launched on board a ballistic missile. The destruction of the weather satellite created 2,000 baseball-sized or larger junk objects that could destroy a satellite and over 2 million junk objects that could cause damage to a satellite.
Once a collision between large space objects occurs, the likelihood of additional collisions occurring increases because of the new space junk created by the breakup of the space objects. If new space junk collides with a large space object, then even more space junk may be created, further increasing the probability of more collisions. This “collision cascading” problem is referred to as the “Kessler Syndrome.” Kessler characterized this problem using analytic calculations using density of objects as a function of altitude to represent the low-earth orbit (“LEO”) environment. (See Kessler, D. J. and Cour-Palais, B. G., “Collision Frequency of Artificial Satellites: The Creation of a Debris Belt,” J. of Geophysical Research 83:63 (1978), which is hereby incorporated by reference.) Recently, NASA implemented a debris evolutionary model referred to as “LEGEND” to study the debris environment over the long term. (See Liou, J. C., “An Updated Assessment of the Orbital Debris Environment in LEO,” Orbital Debris Quarterly News, vol. 14, issue 1 (2010), which is hereby incorporated by reference.) Although LEGEND propagates individual objects, it evaluates collisions at coarse sampling intervals. However, because of this coarseness, LEGEND may miss collisions that occur in between the sampling times. The rate of collisions cannot be scaled up by a ratio of sampling intervals as the rate of collisions is non-linear due to collision cascading.
Many applications can benefit from the accurate modeling of the evolutionary environment of space junk. One application is the analysis of various techniques to minimize space junk such as including a mechanism on a booster rocket to speed up its deorbit after being spent. Accurate modeling of space junk can be used to predict the effect of these techniques on the overall risk of collision, which can be used in a cost/benefit analysis. Another application is the assessing of the risk of collision for purposes of insuring a satellite. Accurate modeling can be used to aid in an actuarial analysis of the risk of loss of the satellite. Another application is performing a cost/benefit analysis of various potential orbits for a satellite. Accurate modeling may indicate that an orbit that is slightly suboptimal for the primary function of the satellite should be used because the risk of collision is significantly less than an orbit that is optimal. It would be desirable to accurately model the evolution of space junk over a long term (e.g., 100 years) to analyze the effects of various assumptions (e.g., rapid deorbiting of spent booster rockets or increase in satellite launch frequency).