Orbital Debris Tracking: How Space Agencies Do It

Space agencies track and monitor orbital debris using a combination of ground-based radar, optical telescopes, and space-based sensors. These systems detect, catalog, and predict the paths of objects ranging from defunct satellites to tiny paint flecks, helping to protect active spacecraft and the International Space Station. This guide explains the core methods and steps used by agencies like NASA and ESA to keep watch on the growing population of space junk.

What You’ll Need

• Access to public orbital data sources (e.g., Space-Track.org, ESA’s DISCOS database)
• Basic understanding of orbital mechanics (Keplerian elements, orbital period)
• A computer with internet access for data retrieval and analysis
• Optional: orbital analysis software (e.g., GMAT, STK) or a spreadsheet for calculations

Step-by-Step

1. Detect the debris. The first step is observing the sky. Ground-based radar systems (like the U.S. Space Force’s Space Fence) emit radio waves and listen for reflections off objects as small as 2-5 cm in low Earth orbit (LEO). Optical telescopes, such as those used by ESA’s Space Debris Office, are used for objects in higher orbits (geostationary, ~36,000 km) where radar is less effective. These telescopes capture light reflected from the debris.
2. Collect raw tracking data. Each detection provides a set of measurements: the object’s azimuth, elevation, range (distance), and range rate (how fast it’s moving toward/away from the sensor). This is recorded over several passes to get enough data points.
3. Determine the orbit. Using the raw measurements, analysts perform orbit determination. This is a mathematical process that fits a Keplerian orbit (defined by six elements: semi-major axis, eccentricity, inclination, right ascension of ascending node, argument of perigee, and true anomaly) to the observed positions. Software like the Simplified General Perturbations 4 (SGP4) model is commonly used for LEO objects.
4. Catalog the object. Once the orbit is determined, the object is assigned a catalog number and added to a database. The primary catalog is maintained by the U.S. Space Force’s 18th Space Defense Squadron, which tracks over 25,000 objects. Each entry includes the object’s name, international designator, launch date, and current orbital elements.
5. Update orbits regularly. Orbits decay due to atmospheric drag, solar radiation pressure, and gravitational perturbations from the Moon and Sun. Therefore, agencies re-observe each object periodically—daily for LEO objects, less frequently for high orbits—to update its orbital elements. This ensures predictions remain accurate.
6. Predict future positions. With updated elements, agencies propagate the orbit forward in time using models that account for drag and other forces. This produces ephemeris data: a table of predicted positions over the next days or weeks.
7. Screen for conjunctions. The predicted positions of all tracked objects are compared against the positions of active satellites (and the ISS). If any two objects come within a certain distance (e.g., 1 km in LEO) at the same time, a conjunction alert is issued. The probability of collision is calculated using the uncertainty in each object’s position.
8. Issue warnings and take action. If the collision probability exceeds a threshold (often 1 in 10,000), the satellite operator is notified. They may perform a collision avoidance maneuver, moving the satellite slightly to increase the miss distance. For the ISS, thrusters on the station or attached spacecraft are used.
9. Reassess after events. When a collision or fragmentation occurs (e.g., the 2009 Iridium-Cosmos crash), agencies immediately begin tracking the newly created debris cloud. This involves intensive observation to catalog as many new fragments as possible, which can number in the thousands.

Common Mistakes

• Ignoring atmospheric drag uncertainty. Drag varies with solar activity, which is hard to predict. Using a static drag model can lead to large position errors over time.
• Assuming all debris is tracked. Only objects larger than about 5-10 cm in LEO are tracked. There are hundreds of thousands of smaller pieces that are too small to detect but still dangerous.
• Mixing up coordinate systems. Orbital elements can be given in different reference frames (e.g., true equator vs. mean equator). Using the wrong one yields incorrect predictions.
• Rounding too early. When computing orbits, rounding to a few decimal places in the semi-major axis can cause errors of kilometers in position after a day.

Tips

• Use the publicly available TLE (Two-Line Element) sets from Space-Track.org for practice. They are updated daily for most tracked objects.
• For educational purposes, use the SGP4 model in Python (e.g., the sgp4 library) to propagate TLEs. This is the same model used by many agencies.
• Remember that tracking accuracy degrades with time. Always use the most recent TLE for any analysis.
• Combine radar and optical data for objects in medium Earth orbit (MEO) to improve accuracy, as each method has strengths and weaknesses.

Troubleshooting

If your predicted positions don’t match observed ones: Check the epoch of the TLE. If it’s more than a few days old for LEO, the error can be tens of kilometers. Download a fresh TLE.

If you can’t find an object in the catalog: It may be too small to track or it may have decayed and re-entered the atmosphere. Use the decay prediction tools on Space-Track to check.

If your conjunction screening shows too many false alarms: Increase the miss distance threshold or use a higher collision probability threshold. In practice, agencies use complex probability calculations that account for position uncertainty.

If your radar data is noisy: Apply filtering techniques like Kalman filtering to smooth the measurements and improve orbit determination.

Frequently Asked Questions

How many pieces of orbital debris are currently tracked? As of 2024, the U.S. Space Force tracks over 25,000 objects larger than about 10 cm in orbit. This includes active satellites, defunct satellites, rocket bodies, and fragmentation debris.

What is the minimum size of debris that can be tracked? In low Earth orbit, radar systems like the Space Fence can detect objects as small as 2-5 cm. Optical telescopes can track larger objects (typically >10 cm) in higher orbits. Smaller debris is not tracked but is still dangerous.

How often are debris orbits updated? For LEO objects, updates are typically done daily. For objects in higher orbits, updates may be weekly or less frequent. The exact schedule depends on the object’s size and orbit stability.

Can space agencies track all debris? No. Only objects above a certain size are tracked. There are estimated to be over 100 million pieces of debris smaller than 1 mm, which are too small to track but can still damage spacecraft.

What happens if a satellite is predicted to collide with debris? The operator is alerted with a conjunction warning. They may choose to maneuver the satellite to avoid the collision. The decision is based on the probability of collision and the cost of the maneuver.

How does the ISS avoid debris? The ISS performs collision avoidance maneuvers when the probability of collision with a tracked object exceeds 1 in 10,000. These maneuvers are done using thrusters on the station or on docked spacecraft, and they change the station’s orbit slightly.

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