How does Space Debris Threaten the ISS?

Our Space Debris Expert is back to answer questions about protecting the International Space Station, the consequences of a debris cloud, and how we track space debris.

The Aerospace Corporation
Aerospace TechBlog


The International Space Station. Photo courtesy NASA.

Astronauts are at risk while inside the International Space Station (ISS) and while outside the station conducting spacewalks. Extra-vehicular activity, or EVA, suits can offer protection from extremely small debris particles and most of the ISS has protective shields to safeguard the station from objects up to one cm in diameter.

For larger debris objects, the Space Station must either navigate out of the way of its path or the use the auxiliary Soyuz and Dragon spacecraft as “lifeboats” in the event of a potential evacuation. During the ASAT incident during the early morning of November 15, the astronauts aboard the ISS evacuated to these lifeboats in anticipation of a conjunction with the Cosmos 1408 debris cloud.

Our Center for Orbital and Reentry Debris Studies (CORDS) at the Aerospace Corporation specializes in monitoring and tracking these events. We spoke with Marlon Sorge, CORDS principal engineer, about the impact of space debris on orbit.

How small and how large is space debris?

Space debris can be as big as a large rocket body (over 10 meters or 33 feet long) down to microscopic particles that are barely visible. The ones that we are most worried about are those larger than 3 mm (BB sized), although even impacts of very small debris can gradually degrade a solar panel or ruin a scientific instrument.

Why is small debris a problem?

Anything that is in orbit is moving very fast, typically 7.5 km/sec, or more than 17,000 mph in low Earth orbit (LEO). This is ten to twenty times faster than a bullet. If you are in orbit, you are also moving that fast, and the debris may be coming at you from the other direction, so the relative speed could be twice that. The result is that even tiny particles can be extremely deadly, and an object the size of a pinhead can pack a huge wallop.

What is an on-orbit breakup, how do they happen and what can be done about it?

Known orbit planes of Fengyun-1C debris one month after its disintegration by a Chinese interceptor missile in 2007. The white orbit represents the International Space Station

Most on-orbit breakups are caused by explosions which may be from leftover liquids and gases in the tanks of old rocket bodies, batteries on satellites and other leftover energy sources. A number of satellites have likely had batteries that have exploded. There have been five documented collisions between objects big enough to track, and several other suspected collisions. There have also been a number of deliberate explosions and collisions to destroy satellites. Collisions in certain orbits are predicted to become a major source of new debris in the future.

Are chain-reaction collisions or cascades real?

Unfortunately, yes. Mathematical modeling has repeatedly shown that the number of objects in low Earth orbit will likely grow from collisions. However, these cascades take place over decades and centuries, with a large collision happening currently only about once every five to ten years.

As an example, the collision between the dead Cosmos 2251 satellite and the active Iridium 33 satellite occurred in 2009. The next collision between two tracked objects occurred in 2021 between a Chinese satellite and a piece of debris. That is 12 years, statistically about what we expect.

So while the Kessler Syndrome — a scenario where space debris collisions cause a domino-effect resulting in an overwhelming amount of debris — is quite real mathematically, it is a slow-motion disaster that we have time to alter. If we limit the growth of space debris now, we can prevent it from being an unmanageable problem.

What does an on-orbit collision look like?

A hypervelocity collision, like those at orbital speed, doesn’t behave like collisions on Earth. The objects are moving so fast that they travel through each other faster than the shock waves can travel so it appears more like an explosion of each object — as if they passed through each other and exploded on the other side. The shock waves in the structures of each object then shatter them into fragments of varying sizes and, in the process, give each fragment a boost in a different direction. Each one of these fragments is then in a different orbit than the original object and will move away according to the laws of orbital motion. With thousands of fragments, each moving in slightly different directions, it looks a lot like an explosion.

Hypervelocity impact test to a cylinder by a 3.17mm aluminum projectile traveling at 7.03 km/sec. Video courtesy NASA WSTF

For dramatic purposes, movies, TV, and commercials tend to show space breakups at a much slower speed than they would happen at in real life. A breakup in space, especially a collision, can involve a lot of energy, and the pieces are flung away at extremely high speeds. Since there is no air to slow the pieces down the fragments would all fly away from one another and rapidly disappear from view. For many breakups, a softball-sized fragment would fly the length of the space station (a little less than a football field) in less than half a second. If you were watching it from nearby, you would see a flash, and the object that broke up would just disappear and be gone. It would be very unlikely for you to see pieces drifting away. Similarly, a low orbit space collision is unlikely to look much like a car crash — the speeds are much too high. The collisions would look like explosions to a nearby observer.

What are debris clouds?

When an object in space breaks up or blows up, each of the pieces will fly in its own, independent orbit. These orbits are mathematically related to one another, and we analyze them collectively as a “cloud.” Since there is no air or other medium in which the cloud is suspended, the cloud grows and changes shape based solely on the laws of orbital motion. In models, you can see the cloud grow and change shape as the cloud forms into a ring around the Earth. But in real life, on a human scale, the pieces are too small and much too far apart to actually see debris as a coherent cloud.

Can we protect the ISS and other satellites?

Impact on Whipple shields. Image courtesy NASA.

Whipple shields are used on the ISS, and can protect against particles up to about 1 cm in size. A Whipple shield is a multi-layered shield designed so that the first layer breaks up the impacting object, the second layer breaks those fragments into smaller objects, and so on until the fragments are too small to penetrate the last layer.

The difficulty is that to shield against bigger objects, the shields must be bigger. Eventually they become too heavy for launch and must be spaced too far apart to be effective — one of the reasons why the ISS can’t be protected from every type of debris.

Are some orbits more dangerous than others?

Certain orbit altitudes are more crowded with space debris than others. Orbits between 800 and 1100 km in altitude are the most crowded and contain 40 percent of the tracked space debris. Orbits going near the Earth’s poles, for example, are more dangerous because they cross other orbits more frequently. Because space debris comes from human activity, the most useful orbits will also have the most space debris.

Who is responsible for creating and controlling space debris?

The nations that launch and operate satellites are responsible for the space debris from their satellites and rocket bodies. There is no one responsible for tracking it internationally, but the United States does track space debris to protect our own satellites and those of other nations, sharing orbit information with the rest of the world. Other nations also have tracking capabilities and perform similar services for their satellites. Satellite operators try to reduce space debris from recently launched satellites and rocket bodies by carefully designing them to prevent explosions, reentering them, or moving them to disposal orbits when their mission is over. Space debris from older objects, explosions, and collisions is not controlled at all.

Can you see space debris coming?

It is very unlikely you would see space debris. Relative to a person in orbit, space debris is moving about ten times faster than a bullet. The vast majority of debris is on that scale or smaller. Just as you can’t see a bullet coming, an object moving 10x faster would be impossible to see with the naked eye.How do we track space debris?

The Space Surveillance Network (SSN) is operated by the U.S. Space Force and tracks objects in space. The SSN has radar and optical sensors at various sites around the world as shown in the figure above. These sensors observe and track objects that are larger than a softball in low Earth orbits and basketball-sized objects, or larger, in higher, geosynchronous orbits. The sensors can determine which orbit the objects are in and that information is used to predict close approaches, reentries, and the probability of a collision. Other nations also run space object tracking systems.

The Space Surveillance Network of radar and sensors.

Is there an “air traffic control” equivalent for space?

The closest agency to an air traffic control system is the US Space Force’s Combined Space Operations Center. The CSpOC operates the Space Surveillance Network and maintains the most complete catalog of objects on orbit. If they predict a collision between a cataloged object and a known operational satellite, they usually attempt to notify the owner/operator. The CSpOC monitors the ISS and other NASA satellites for collisions in coordination with NASA. Other nations use CSpOC data as well as their own tracking data to protect their satellites. To date, there is no internationally recognized “Space Traffic Control” agency.

There is no way to control objects that are inactive. In addition, there is no way to control objects that may still be active but are unable to maneuver — such as the Hubble Space Telescope. And finally, even when we have a satellite that can maneuver, it’s not like air traffic control telling an airplane to climb, descend, or turn. A satellite coasting in an orbit adheres to the laws of physics and unless there is ample warning time, it can take a lot of energy to alter that orbit to avoid a collision.

If most debris is too small to be seen, how do we know it exists?

Some radar installations can see small objects pass through their beams but can’t track them for enough time to determine their orbit. We can determine the approximate amount of small space debris by counting these radar returns over a short period of time and then estimating the total. There are also special sensors that can be flown in space to measure small debris impacts or detect debris that passes nearby using lasers.

When a spacecraft is returned from orbit, we find that it is almost always pitted with debris strikes. The longer something has been in orbit the more hits it will have. Collecting data from multiple sources allows us to estimate the total number of objects in orbit. We can also estimate the number of particles created by collisions and explosions by modeling a simulation of the breakup.

LeoLabs tracks current satellites in Low Earth Orbit with their visualization tool. Image Courtesy LeoLabs

Additionally, the private sector is now working to provide low Earth orbit and space situational awareness services to the growing private space market. LeoLabs, founded in 2016, maps smaller debris that can be devastating to small satellite constellations operating in LEO. Their visualization tool tracks satellites and debris currently on-orbit.

How far in advance can you predict a collision or a reentry?

We can reasonably predict close approaches in space beginning a week prior to the event. Predicting where space debris will reenter and land is much harder to do because we don’t know when the object will actually enter the denser atmosphere and begin its final dive. Reliably predicting a reentry location more than a day ahead of the event is difficult, and even then our margin of error could be several hours, which is equivalent to multiple orbits.

If we know about it in advance, can space debris be identified and moved to avoid a collision?

The debris can’t be moved with other technology, but if the other object is a maneuverable satellite, it may be able to move out of the way in time to avoid a collision. Most satellite operators require hours or days to plan and execute a collision avoidance maneuver.

Will the space debris problem take care of itself?

In low Earth orbit (below 600 km or 370 miles), the little atmosphere that is there will, over weeks, months, and years, drag the space debris low enough to reenter. Between 600 km and 1000 km (620 mi) it may take tens to hundreds of years for the debris to reenter. For orbits from 1000 km to 2500 km (620 to 1550 miles) re-entry may eventually occur, but it may take thousands of years. Above 2500 km, lifetimes can be much longer.

Some very high orbits can also reenter over decades to centuries due to the Sun and Moon’s gravity “stretching” the orbits out until their low point is in the atmosphere. While it’s possible that some of the debris will be removed naturally, the problem is that space debris objects can collide with each other and produce more debris.

The most straightforward thing to do is to not leave objects in orbit once their useful life is over. There are international guidelines from the Inter-Agency Space Debris Coordination Committee for handling debris. Many nations, including the United States, have rules about disposing of satellites and rockets but can be difficult and expensive to eliminate old spacecraft, especially if the satellite or rocket was not designed for disposal. Furthermore, this doesn’t work for objects that are already in orbit and uncontrollable.

How much debris is there? How many objects are in orbit?

The Space Age began in October of 1957, with the launch of Sputnik 1. In the last 56 years there have been over five thousand launches. Each launch typically has several separate objects associated with it that may remain in orbit. In addition, there have been explosions and other violent breakups of vehicles that have resulted in thousands to hundreds of thousands of fragments.

The U.S. military is currently tracking about 20,000 objects, and has cataloged more than 40,000 objects over the years — a number of which have reentered the atmosphere. Most space debris is too small to be tracked, but large enough to damage spacecraft. We estimate that there are hundreds of thousands of objects that could be fatal or catastrophic to a space mission, and millions of objects that are capable of causing damage.

Can space debris be cleaned up?

There is no easy or cheap solution to space debris. Cleaning it up will be very expensive and time-consuming. Big debris, like used rockets, is most likely to create more debris and is heavy and difficult to move. Small debris that can still damage a satellite is very hard to find and track and there are very large quantities of it. Both types of debris are difficult to remove for their own reasons. There are on-going efforts by both governmental organizations and commercial companies to develop the technologies needed to remove debris.

To remove space debris, particularly the large and more dangerous objects, requires getting close to it and maintaining the same speed as the object. We then must attach to it and move it into a lower orbit or reenter it directly into the ocean. If the object is a rocket stage with propellant still on-board, there is explosion risk, so it needs to be an unmanned task. There is also the issue of property rights; you can’t grab a satellite or rocket that belongs to another country without their permission. There is no easy way to control the small-but-dangerous objects that are not well tracked or not tracked at all.

Will space debris eventually make it impossible to fly or operate in space?

No, this is very unlikely. Over many decades, the growth in space debris will make orbit operations more hazardous, and more costly. The growth of debris will make tracking and avoiding the debris more complicated, costly, and operationally difficult. It might be tough to perform a mission if frequent maneuvers are required to avoid debris. A satellite would have to carry extra fuel for these extra maneuvers and would likely need to shield critical areas from collisions with small debris. Space debris can make space missions more costly and difficult, but it won’t make them impossible.

Marlon Sorge is a principal engineer for the Space Innovation Directorate of The Aerospace Corporation. For more than 30 years, he has conducted space debris research and analysis in a broad range of fields including debris risk assessment, fragmentation analysis, operations support, debris mitigation technique implementation, debris event reconstruction, satellite design for debris survivability, orbital and suborbital range and space safety, ballistic debris management, debris environment projection, collision avoidance, orbital reentry prediction, and national and international mitigation guideline and standards development.