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Since the start of the space age in 1957, thousands of satellites have been launched. Many of them are now broken or discarded, and together with rocket stages, collision debris and lost tools, they form a growing problem: space debris. This space litter is closely monitored by several organizations around the world. In this article we explain who does this, how they work and why this is so important.
Who keeps track of space junk?
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The United States has the largest monitoring system in the world: the Space Surveillance Network (SSN). This network is managed by the US Space Command and consists of more than thirty radar and telescope stations spread across the globe. They can track objects of about 10 centimeters in size in low orbits around the Earth and objects of about one meter in high orbits such as the geostationary orbit where communications satellites orbit. The SSN maintains a public catalog of more than 25,000 space objects. This data is updated daily and made available to satellite operators worldwide. This helps prevent satellites from colliding with space debris.
The European Space Agency (ESA) coordinates European efforts through the Space Debris Office in Darmstadt, Germany. European countries work together in the Space Surveillance and Tracking program, with each participant contributing their own radars and telescopes. For example, Germany has the powerful TIRA radar, while other countries operate optical telescopes. ESA also plays an important role in predicting how the space debris problem will develop in the future. They have developed computer models that calculate how much waste there will be in decades if we continue as we are now, or if we take measures.
Russia operates the Russian Space Surveillance System with powerful radar installations such as the Don-2N. They share information less openly than Western countries, but maintain their own catalog of space objects. China has invested heavily in tracking space debris in recent years. They have built modern radars, like the one in Xuyi, and have several telescopes. As China launches more satellites, their monitoring system becomes more and more extensive. Japan, France, Germany and the United Kingdom also have their own facilities. For example, Japan operates the Bisei Space Guard Center with telescopes that mainly look at higher orbits. These national systems all contribute to a more complete picture of what is happening in space.
How is space debris detected?
There are two main techniques used to detect space debris, each with their own strengths: radars and telescopes. Radar systems work by emitting radio waves that reflect off space objects. By picking up the echoes, they can determine where an object is, how fast it is moving and approximately how big it is. Radar works day and night and can see through clouds. Large radars such as the AN/FPS-85 in Florida can track thousands of objects per day. They are especially effective for objects in low orbits, up to about 1,000 kilometers altitude. The disadvantage is that radar can only detect objects from about 10 centimeters, while smaller chunks can also cause damage.
Telescopes detect sunlight reflected from space objects. They work best for objects in high orbits that move more slowly, allowing longer exposure times. The disadvantage is that telescopes can only work when the object is illuminated by the sun while the telescope itself is in the dark. Modern telescopes use sensitive digital cameras that systematically scan large areas of the sky. For example, the American GEODSS (Ground-Based Electro-Optical Deep Space Surveillance) system has several telescope stations worldwide that mainly track objects in high orbits. For objects with special reflectors, laser systems such as the International Laser Ranging Service (ILRS) system can make extremely accurate measurements. They shoot laser pulses at a satellite and measure how long it takes the light to return. This provides position determinations at centimeter level. This technique is mainly used for active satellites, but sometimes also for large pieces of space debris. Recently, satellites with cameras have also been used to detect space debris. The advantage is that they do not have to deal with clouds, daylight or geographical limitations. They can continuously monitor certain areas and are especially useful for tracking objects in high orbits.
What exactly is tracked?
Tracking space debris provides several types of information, all of which are important for spaceflight safety. The basic information consists of the trajectory of each object: how high it flies, how round or elongated its orbit is, at what angle to the equator, and how fast it moves. This information is recorded in standardized files called Two-Line Elements (TLEs). These files are regularly updated if there are new observations. This trajectory data allows computer programs to predict where an object will be in the future. This is essential to determine whether two objects are coming dangerously close to each other.
Every day, computers perform millions of calculations to check whether objects are coming too close to each other. If two objects are predicted to be within approximately one kilometer of each other, a warning is issued. At even closer distances, the satellite operator receives detailed information about when and where the closest approach will take place. These warnings also contain information about how certain the prediction is. Sometimes the orbits are not known very accurately, causing great uncertainty. In other cases, the orbits are known very precisely and the prediction is reliable. Organizations keep track of how many objects are floating around at different heights. They distinguish between active satellites, disabled satellites, rocket stages, collision debris and lost objects. By tracking these statistics for years, scientists can see how the problem is growing. There are currently more than 36,000 objects at least 10 centimeters in size in orbit around the Earth. It is estimated that there are millions of smaller pieces that cannot be systematically tracked, but can cause damage in the event of a collision.
When a satellite or rocket stage disintegrates, monitoring systems attempt to quickly detect and catalog any new debris. This is important to understand what happened and to warn other satellites of the new debris. The analysis of such an event also provides insight into how objects break apart and helps improve computer models. For example, in 2009, the American satellite Iridium 33 and the Russian Kosmos 2251 collided, resulting in more than 2,000 new, traceable pieces of debris. Such an event keeps monitoring systems busy for weeks.
Objects in low orbits are gradually slowed down by the thin atmosphere and eventually fall back to Earth. For large objects, predictions are made when and where they are likely to land. This is difficult because the density of the atmosphere varies and because it is often not known exactly how heavy and large an object is. For controlled rollbacks, where a satellite operator deliberately crashes the satellite, the predictions are more accurate. For uncontrolled return, the forecasts may still have uncertainties of thousands of kilometers several hours before the return.
What do we learn about this data?
In addition to simply tracking where objects are, scientists are also trying to learn more about the properties of space junk. By measuring how an object’s brightness changes as it moves across the sky, researchers can determine how fast it is spinning or tumbling. A satellite that is stable in space will have a different light curve than an object that is tumbling uncontrollably. This information helps identify unknown objects and understand their stability. By carefully analyzing reflected sunlight, scientists can sometimes determine what an object is made of. Aluminum, multi-layer insulation and solar panels each reflect light differently. This helps identify where the object came from and makes models more accurate. The strength of a radar signal returning from an object (the radar cross-section) provides information about the effective size. By combining measurements from different angles, researchers can better estimate the actual size and shape, although this remains difficult for small objects.
Is there collaboration?
Because space waste is a global problem that knows no borders, international cooperation is essential. The Space Data Association is a partnership of satellite companies and government organizations. They share information about where their satellites fly and warn each other of possible collisions. This makes the system more robust than if everyone were dependent solely on government information. The Inter-Agency Space Debris Coordination Committee (IADC) brings together thirteen major space agencies, including NASA, ESA, JAXA, Roscosmos and CNSA. They jointly develop guidelines to reduce space waste and share technical knowledge about monitoring and modeling. Through Space-Track.org, the US military shares free trajectory data and collision warnings with registered users worldwide. Thousands of satellite operators, scientists and hobbyists use this. This openness has significantly improved the safety of space travel. Within the European Union, member states work together through the Space Surveillance and Tracking Support Framework. This program brings together radars and telescopes from different countries into one integrated system that issues warnings to European satellite operators. The goal is to become partially independent of non-European data sources.
Space waste is becoming an increasing problem
Despite all the technology, significant challenges remain in monitoring space debris. The biggest limitation is that objects smaller than about 10 centimeters in low orbits simply cannot be seen with current systems. Yet a piece of debris as small as one centimeter can cause enormous damage in the event of a collision because the speeds in space are so high (up to 15 kilometers per second). Scientists therefore use computer models to estimate how many small objects there are. They base this on the larger objects that are seen, on theories about how explosions and collisions occur, and on analysis of impact damage on returned spacecraft. The further you look into the future, the more uncertain predictions become. This is mainly because the atmosphere is not equally dense everywhere. Solar activity, geomagnetic storms and the difference between day and night all cause variation in the air resistance that satellites experience. For space waste, it is often unknown how heavy it is and how large its surface area is. This ratio determines how quickly an object is slowed down. Without this information, it is difficult to accurately predict where an object will be in a few days.
A difficult dilemma is how many warnings you want to issue. If you’re too careful, satellite operators will receive dozens of alerts a day, most of which turn out to be false alarms. This costs time and money. If you are too strict and only warn at very close distances, you may miss a real threat. The trick is to find the right balance, taking into account how accurate the track data is and how much risk is acceptable. When hundreds of debris are suddenly created by an explosion or collision, identifying and cataloging all the new objects is a daunting task. The debris initially has similar trajectories, which makes it difficult to link observations to specific objects. It may take days or weeks before all new objects are reliably tracked.
What does the future bring?
The coming years will bring major improvements in how we can track space debris. There are more and more satellites specifically intended to detect space debris. Small satellites with cameras or radars can continuously monitor certain areas without being affected by clouds, daylight or geographical limitations. This will be especially useful for tracking objects in busy orbital areas. Artificial intelligence (AI) and machine learning are also increasingly being used to monitor satellites and space debris. Computers can automatically recognize objects in sensor data, link observations to known objects, predict satellite maneuvers and detect anomalies that may indicate explosions. This saves time and makes the system more reliable. More powerful computers and improved models of the atmosphere will also eventually lead to more accurate predictions. By using more satellite data, the density of the atmosphere is better mapped, which is especially important for objects in low orbits. Standardized ways of exchanging information are also being developed. If everyone uses the same data formats, collaboration becomes easier. This applies to both governments and commercial companies.
