Space Debris Basics
Space Debris Basics:
What is Orbital Debris?
Orbital debris generally refers to material that is on orbit as the result of space missions, but is no longer serving any function. There are many sources of debris. One source is discarded hardware. For example, many launch vehicle upper stages have been left on orbit after they are spent. Many satellites are also abandoned at the end of useful life. Another source of debris is spacecraft and mission operations, such as deployments and separations. These have typically involved the release of items such as separation bolts, lens caps, momentum flywheels, nuclear reactor cores, clamp bands, auxiliary motors, launch vehicle fairings, and adapter shrouds.
Material degradation due to atomic oxygen, solar heating, and solar radiation has resulted in the production of particulates such as paint flakes and bits of multilayer insulation. Solid rocket motors used to boost satellite orbits have produced various debris items, including motor casings, aluminum oxide exhaust particles, nozzle slag, motor-liner residuals, solid-fuel fragments, and exhaust cone bits resulting from erosion during the burn.
A major contributor to the orbital debris background has been object breakup. As of August 2007 (the most recent time for which the data has been compiled), there have been 194 breakups and 51 events in which debris has been shed from an object. Since then, many more are believed to have occurred. Breakups can be caused by explosions or collisions with other objects, but the majority of breakups have been caused by explosions. Explosions can be caused by residual propellant, batteries that overheat, or in some cases, deliberate destruction of the satellite. Explosions can also be indirectly triggered by collisions with small debris.
Two events in recent years have greatly increased the amount of debris on orbit. On Feb. 10, 2009, the active Iridium 33 satellite collided with the defunct Cosmos 2251 satellite, and created about 2,000 tracked objects. On Jan. 11, 2007, the Chinese deliberately destroyed the FY-1C satellite in a test of an antisatellite weapon, creating more than 3,000 tracked objects. The tracked objects represent a small fraction of the debris objects created.
Several other collisions are known or suspected to have occurred since the beginning of the space age. In addition, the debris research community has concluded that at least one additional breakup was caused by collision. The cause of approximately 22 percent of observed breakups is unknown.
Approximately 70,000 objects estimated to be 2 cm in size have been observed in the 850-1,000-km altitude band. NASA has hypothesized that these objects are frozen bits of nuclear reactor coolant that are leaking from a number of Russian RORSATs.
At altitudes of 2,000 km and lower, it is generally accepted that the debris population dominates the natural meteoroid population for object sizes 1 mm and larger.
What Are the Risks?
Orbital debris generally moves at very high speeds relative to operational satellites. In low Earth orbit (altitudes lower than 2,000 km) the average relative velocity at impact is 10 km/sec (36,000 km/hr or 21,600 mph). At this velocity, even small particles contain significant amounts of kinetic energy. For example, NASA frequently replaced space shuttle orbiter windows because they were significantly damaged by objects as small as a flake of paint. An aluminum sphere 1.3 mm in diameter has damage potential similar to that of a .22-caliber long rifle bullet. The energy of an aluminum sphere 1 cm in diameter is comparable to a 400-lb safe traveling at 60 mph. A fragment 10-cm long is roughly comparable to 25 sticks of dynamite.
Debris particulates smaller than 1 mm in size do not generally pose a hazard to spacecraft functionality. However, they can erode sensitive surfaces such as optics and solar arrays. While the spacecraft may survive, degradation of certain components can still result in inability to complete the mission.
Debris fragments from 1 mm to 1 cm in size may or may not penetrate a spacecraft, depending on material selection and whether shielding is used. Penetration through a critical component, such as the flight computer or propellant tank, however, would result in loss of the spacecraft.
Debris fragments between 1 and 10 cm in size will penetrate and damage most spacecraft. If the spacecraft bus is impacted, satellite function will be terminated and, at the same time, a significant amount of small debris will be created. In large satellite constellations, this can lead to amplification of the local smaller debris population and its associated erosional effect.
If a 10-cm debris fragment weighing 1 kg collides with a typical 1,200-kg spacecraft bus, over one million fragments 1 mm in size and larger can be created. This collision results in formation of a debris cloud, which poses a magnified impact risk to any other spacecraft in the orbital vicinity (e.g., other constellation members).
At geosynchronous altitude, average relative velocity at impact is much lower than in low Earth orbit, about 200 m/sec (720 km/hr or 432 mph). This is because most objects in the geosynchronous ring move along similar orbits. Nevertheless, fragments at this velocity can still cause considerable damage upon impact. A 10-cm fragment in geosynchronous orbit has roughly the same damage potential as a 1-cm fragment in low Earth orbit. A 1-cm geosynchronous fragment is roughly equivalent to a 1-mm low Earth orbit fragment.
What are Debris Clouds?
Any concentration of debris particles or fragments in a well-defined region of space is referred to as a debris cloud. Debris clouds are formed whenever debris is being created by a single source. For example, discarded upper stages generally are surrounded by a cloud of particulates that are released over time by degradation of various materials such as paint and multilayer insulation.
Whenever an orbital breakup occurs, a debris cloud is instantly formed. Such debris clouds first take on the form of an expanding three-dimensional ellipsoid. The center of the debris cloud moves along a well-defined orbit, which for explosions is identical to the orbit of the original object. The debris cloud gradually spreads around this orbit in a spiral pattern. As time passes, the debris cloud eventually envelopes the entire orbit and any other satellites in the nearby vicinity.
Due to the laws of orbital motion and to physical processes involved in an explosion or collision, fragments are not spread uniformly throughout a debris cloud. At some locations, spatial density of fragments is much greater than at others. When spatial fragment density is high, the collision risk posed to satellites that fly through the cloud is greatly increased.
Certain regions of the debris cloud are constricted to nearly one or two dimensions. There are three types of debris cloud constrictions: pinch points, pinch lines, and pinch sheets. Spatial fragment density is, relatively, very high at these constrictions.
Pinch points and pinch lines are particularly important to satellite constellations. Neither of these constrictions moves with the debris cloud around its orbit. They remain nearly fixed in inertial space while the debris cloud repeatedly circulates through them. In many satellite constellations there are multiple satellites in each orbital ring. If one of these satellites breaks up, the remaining satellites in the ring will all repeatedly fly through the pinch point and pinch line. If many fragments are produced by the breakup, the risk of damaging another satellite in the ring may be significant.
Animations of debris particles can look very alarming and can give the impression that space is enormously crowded. Indeed, the pinch points can appear extremely hazardous due to the relative concentration. But space is very large and the distances are vast. The smallest pixel on a typical monitor used to represent a debris particle would be many miles across if it were drawn to the same scale as the Earth. The risks from these “clouds” of debris may be much higher than the risk of flying through other parts of the orbit, but in an absolute sense, the risks are still low.
If satellites from two orbital rings collide, two debris clouds will be formed, one in each ring. The constrictions of each cloud then pose a hazard to the remaining satellites in both rings.
How Can Risk Be Controlled?
Risk is best controlled by limiting the creation of debris through mitigation. Unfortunately, debris mitigation usually increases mission cost. Some debris mitigation procedures have minimal impact on mission cost if they are specified early in the development phase. For example, deployment procedures can be designed to prevent ejection of objects. Tethered lens caps and bolt catchers for explosive bolts are examples of preventive design.
To prevent explosions, satellite components that store energy can be passivated at the end of useful life. For example, propellant in upper stages and satellites can be eliminated by either venting or burning to depletion. Batteries can be designed to reduce risk of explosion. Passivation may entail moderate cost during the nonrecurring phase of the mission, but cost during operation should be low.
To prevent debris accumulation from collisions in common orbits, satellites and other objects must be removed from the orbit at the end of life before collisions are likely to occur. This practice is called post-mission or end-of-life disposal. International guidelines for limiting orbital debris recommend that objects in low orbit be moved to a lower orbit when near their end of life such that they will reenter within 25 years.
Satellites, upper stages, and deployed objects in low Earth orbit can take advantage of Earth’s atmosphere to reduce time spent on orbit. At sufficiently low altitudes, atmospheric drag on the object will cause the object’s orbit to decay and result in reentry within 25 years. Vehicles at higher altitudes can perform post-mission maneuvers to drop the perigee (the point closest to Earth) further down into the atmosphere. Propellant must be reserved for this maneuver. Hence, the cost to satellites is reduced mission life and reduced performance to upper stages. If atmospheric decay is exploited to remove an object from orbit, then the risk posed to the ground by reentry of the object must be considered. Current guidelines call for a risk of no more than 1 in 10,000.
At altitudes above 2,000 km, it is often not feasible to force reentry within 25 years using current space technology. At this time, it is generally recommended to place vehicles in disposal (or “graveyard”) orbits. Spacecraft in geosynchronous orbits are typically boosted into a higher disposal orbit at the end of their mission life.
For many missions, it may be necessary to perform collision avoidance. The space station frequently maneuvers to avoid collisions with other objects. Many satellite operators routinely monitor future close approaches and sometimes maneuver to lower the risk of a collision.
Satellite operators can also manage risk by increasing redundancy in their designs, particularly relative fragile components such as solar arrays. Extra solar cells and alterations in the wiring of the arrays can reduce the effect on a mission if there is a small debris strike. Aerospace studies indicate that for most constellations of multiple spacecraft, it would be prudent to plan for a spare spacecraft.
In the future, it may be necessary to completely remove all non-operational satellites and upper stages from orbit.
How is Aerospace Helping to Manage Risk?
Aerospace is working to help satellite and launch vehicle designers and operators address the problems that come with space debris. In the design phase, Aerospace works with the government to design satellite architectures that consider debris risk and replenishment. Orbit choices and operation plans take into account both on-orbit risk and post-mission disposal that will minimize future debris. Aerospace works with the government to set design requirements, and with vehicle contractors to design spacecraft that can maneuver to avoid collisions, withstand small debris strikes, and to move to disposal orbits or reenter at end of life. If a vehicle is intended to be reentered, we consider design changes that will minimize the risk to people on the ground from falling debris.
To help assure safe launches, Aerospace led the development of probability-based screening of launch trajectories to ensure that the new launch will not collide with any objects currently in the resident space object catalog. Aerospace performs launch collision avoidance (LCOLA) for most government launches by generating real-time reports that tell the mission director when it is safe to launch. We do this by comparing all of the potential launch trajectories at all of the potential launch times to the calculate positions of all of the objects in the resident space object catalog.
Routine on-orbit collision avoidance is done in a similar manner. Operators compare the calculated future positions of their satellites against all of the other objects in space, and look for close approaches, or “conjunctions.” If a future conjunction is too risky, the operator can consider a small maneuver to lower the risk. Aerospace assists the government by developing better methods of computing these conjunctions and developing better processes to ensure smooth operations.
If a collision does occur, there is an immediate unknown risk to operational satellites from this new debris cloud. Aerospace formed the Debris Analysis Response Team (DART) to pioneer new tools and techniques to compute that risk. The DART is on call to assist the government in determining the immediate risk to critical national assets.
What is the Future Trend?
The amount of debris on orbit in the future will depend upon whether the creation or removal rate dominates. Currently, the only mechanism for removal of uncontrolled objects is orbital decay through atmospheric drag, which ultimately leads to reentry. This mechanism is only effective in a restricted range of low Earth orbits. At higher orbits, it takes hundreds to thousands of years for objects to reenter, so there is no effective removal mechanism. Historically, the creation rate of debris has outpaced the removal rate, leading to a net growth in the debris population in low Earth orbit at an average rate of approximately five percent per year.
A major contributor to the current debris population has been fragment generation via explosions. As the debris mitigation measure of passivation becomes more commonly practiced, it is expected that explosions will decrease in frequency. It may take a few decades for the practice to become implemented widely enough to reduce the explosion rate, which currently stands at about four per year.
Beginning in the late 1990s, many space operators began adopting practices to minimize space debris, and progress was clearly being made. The deliberate destruction of the FY-1C satellite by the Chinese, and the later collision of Iridium 33 and Cosmos 2251, undid a decade of progress in reducing the number of objects in orbit.
It is predicted that the main contributor to the future growth of the debris environment in LEO will be debris created by collisions. The most effective way to reduce this growth is through the reduction in the number of large objects (satellites and upper stages) left in orbit. Due to their large masses these objects become the major source of debris from collisions. One of the most cost-efficient ways to do this is through post-mission or end-of-life disposal. After the end of a mission the satellite or upper stage is moved into an orbit with a reduced lifetime. That orbit may allow the object to reenter the atmosphere within 25 years or it may reentry the object within the next orbit. Reducing the amount of time large objects are in orbit reduces the chance that they will be hit and produce more debris.
Because of the increased number of objects, lack of sufficient debris mitigation efforts such as collision avoidance could eventually result in collision-driven population growth. Various technical models for population growth have been developed by the international community. Most models agree that rapid population growth can occur in the absence of appropriate debris mitigation. They also agree that the population level required to trigger rapid growth in a given orbital region will be achieved before rapid growth is observed. In low Earth orbit (LEO), we are already at that level.
Active Debris Removal
A number of studies have concluded that the debris environment in low Earth orbit will grow over time even if strict end-of-life disposal measures are followed. The biggest source of this debris is from collisions between large objects such as spent rocket upper stages and unused satellites. Since these objects are large they can produce more fragments which are also large enough to produce additional fragments in future collisions.
A way to prevent this growth is to remove large existing objects from the parts of low Earth orbit where collisions are most likely to occur. This is called active debris removal. By removing large objects there is less material in orbit from which new debris can be created.
Removing large objects from orbit, usually by causing them to re-enter the Earth’s atmosphere, is difficult. Almost no spacecraft are designed to be physically grappled once they are in orbit, and they may have antennas, solar arrays, or other fragile projections. They may be tumbling or spinning, making them difficult to grapple and control. Many of these old satellites and rocket upper stages weight thousands of pounds, making them difficult to move. Some of the objects have been in orbit for decades and so may not be as sturdy as when launched or may contain fuel that could be triggered to explode.
A number of concepts have been proposed to remove large space objects from orbit, including flying up to them and grabbing them to pull them out of orbit, or attaching a device, such as a balloon, that will increase the speed with which the Earth’s atmosphere causes their orbits to decay and re-enter the atmosphere on their own. Regardless of the approach, a number of technologies must be developed before these concepts can be made to work.
Another challenge for active debris removal is international treaties. Currently ownership of a satellite or rocket upper stage remains with the country that launched it even after the satellite or upper stage is no longer used. This means that one country cannot remove the debris launched from a second country without that second country’s permission.