Kessler syndrome

The Kessler syndrome is a possible situation in space where there are so many pieces of trash moving around Earth that they keep crashing into each other. This idea was first talked about in 1978 by scientists from NASA, Donald J. Kessler and Burton G. Cour-Palais. They worried that if too many objects were flying close together in low Earth orbit, the crashes could cause even more pieces to fly around, making the problem grow very fast.

This growing amount of space junk can be very dangerous. It can hit satellites, space missions, and the International Space Station. If this happens, some parts of space might become too risky to use, which could stop people from exploring space for a long time.

In 2009, Kessler said that studies showed the amount of space trash was already growing too fast. New pieces from crashes would appear quicker than natural forces could clean them up. This shows how important it is to manage space better and avoid crashes to keep space travel safe for the future. That’s why scientists work on space traffic management and collision avoidance to protect our ability to go into space for many years to come.

History

NORAD, Gabbard and Kessler

After the launch of Sputnik 1 in 1957, the US North American Aerospace Defense Command (NORAD) began keeping a list of all known objects in space, like satellites and parts of rockets. Scientists studied these objects and noticed that sometimes they exploded, creating more pieces of space junk.

In 1978, a scientist named Donald J. Kessler studied this problem and found that if too many pieces of junk were in space, they could start hitting each other and creating even more junk. This could make some parts of space very dangerous for satellites and space missions.

Follow-up studies

More studies showed that there was more space junk than people thought. Small pieces, like bits of paint or broken parts, were hitting satellites and creating even more junk. Scientists realized that space junk was a bigger problem than they first believed.

A new Kessler syndrome

In the 1980s, more studies showed that big pieces of junk could break satellites apart and create many new pieces. This made the problem even worse.

Anti-satellite missile tests

Main article: Anti-satellite weapon

In 1985, a test destroyed a satellite and created a lot of space junk. In 2007, another test by the People's Republic of China (PRC) destroyed a weather satellite and created even more junk that will stay in space for many years.

Chinese rocket explosion

A Chinese Long March 6A rocket broke apart in 2024, creating hundreds of pieces of space junk. This junk could stay in space for years and make it harder to use certain parts of space.

Debris generation and destruction

Further information: Space debris

Every satellite, space probe, and crewed mission can create space debris. As more satellites are sent into space, the chance of collisions increases. In 2025, there were over 11,800 objects in orbit, with most belonging to Starlink, a group of satellites from SpaceX. By 2021, there were about 600,000 pieces of space junk between 1 to 10 cm big, and 23,000 larger pieces. On average, one satellite is destroyed each year by hitting another satellite or space junk. As of 2009, there had been four collisions between known objects, including a collision between two satellites in 2009.

Orbital decay, or the slow falling back to Earth, is very slow at high altitudes. Small effects like air drag high up, the pull of the moon, and the stream of particles from the sun can slowly pull debris down to lower heights where it eventually falls back to Earth, but this can take thousands of years at very high altitudes.

Implications

The Kessler syndrome is a big problem because when objects in space crash into each other, they break into pieces. These pieces can then hit other objects, creating even more pieces. If a really big crash happens, like between a space station and an old satellite, it could create so many pieces that it becomes very hard to keep satellites working in some parts of space.

Some scientists think this could explain why we haven’t found signs of alien life. If an alien civilization started using space, they might have created so much space junk that they trapped themselves on their planet. Humans need to be careful to avoid this problem.

Avoidance and reduction

When building new satellites or spacecraft, designers must show that they can safely get rid of the satellite when it stops working. This can be done by bringing it back into the atmosphere to burn up or moving it to a special orbit far away from busy space paths. Rules in the United States require satellites that send TV and radio signals to move to a safe orbit when they stop working.

One idea to bring satellites closer to Earth is to shift their orbit so that the Sun or Moon helps pull them down faster. Scientists also suggest using a strong laser broom to push small pieces of space junk so they fall out of orbit and burn up safely.

The European Space Agency ESA and a Swiss company called ClearSpace plan to bring a satellite called PROBA-1 back from orbit.

Potential concerns

Scientists have looked at the space junk in orbits close to Earth and found that many of the most worrisome pieces come from old rocket parts called SL-16 stages.

One big satellite called Envisat, which is no longer working, could become a big problem in the future if it crashes into other space junk. There are also concerns about programs like SpaceX's Starlink, which plans to put many new satellites into space. This could make the problem of space junk much worse. To help with this, SpaceX now launches many of its satellites lower in the sky so they will fall back to Earth faster if they stop working.

Current status

In 2023, experts studied how many satellites can safely stay in space without causing too many pieces of old satellites and other space junk to crash into each other. They found that about 72,000 satellites might be the most that can be kept safely.

In 2024, another expert said scientists still don't all agree on whether too many space crashes have already started or how bad they might get someday. But they all agree that space needs to be cleaner.

In 2025, a report said a big storm from the Sun could stop us from moving satellites out of the way for a few days, which could cause many crashes in space. This would be a bigger problem now than in 2018 because there are more satellites in space today. A special tool was made to help figure out the risks from these solar storms.

In fiction

The idea of the Kessler syndrome has been used in many stories. In the 2003 manga and anime Planetes, a group tries to cause big problems in space by crashing a satellite into a space station. The 2013 film Gravity starts with an event where an old satellite is destroyed, leading to many pieces flying around in space. In the 2015 novel Seveneves by Neal Stephenson, the Moon breaks apart, creating a cloud of debris that affects Earth. The 2019 game Ace Combat 7: Skies Unknown shows how battles in space can cause satellites to break apart, making it hard to talk to faraway places.