Orbital Debris Tracking: How the Space Fence Monitors 25,000+ Objects

Every 90 minutes, the International Space Station completes an orbit around Earth at 28,000 km/h. Along its path lie thousands of pieces of debris (defunct satellites, spent rocket stages, fragments from collisions, even lost tools from spacewalks), each one a potential catastrophe traveling at 7 times the speed of a rifle bullet. In 2024 alone, the ISS performed four collision avoidance maneuvers. In 2025, the number rose to six.

Tracking these objects (knowing where each one is, where it's going, and whether it threatens an active spacecraft) is one of the most critical and least understood functions in the entire space enterprise. And at the center of it all is the Space Fence, a radar system on a remote Pacific atoll that represents the most significant upgrade to space surveillance capability in half a century.

The Scale of the Debris Problem

As of early 2026, the U.S. Space Force's 18th Space Defense Squadron (formerly the 18th Space Control Squadron) maintains the official catalog of tracked objects in Earth orbit. The numbers are staggering:

• Over 48,000 objects are currently tracked and cataloged, each assigned a unique catalog number and monitored continuously
• Over 25,000 of these are debris, not active satellites, but junk: dead satellites, rocket bodies, fragments from breakups and collisions
• An estimated 130 million+ pieces of debris smaller than 1 cm exist in orbit but are too small to track with current technology
• Over 1 million objects between 1 cm and 10 cm are large enough to destroy a satellite but too small to reliably track and catalog

The problem is getting worse. The Kessler Syndrome, a theoretical cascade where collisions create debris that causes more collisions, is no longer purely theoretical. The 2007 Chinese anti-satellite test (which destroyed the Fengyun-1C weather satellite) created over 3,500 trackable fragments, many of which remain in orbit today. The 2009 collision between the active Iridium 33 satellite and the defunct Russian Cosmos 2251 added another 2,300 fragments. Each new mega-constellation deployment (Starlink's 6,000+ satellites, OneWeb's 600+, Amazon Kuiper's planned 3,200+) increases the collision probability for everything else in orbit.

What Is the Space Fence?

The Space Fence is a ground-based S-band radar system located on Kwajalein Atoll in the Marshall Islands, operated by the U.S. Space Force and built by Lockheed Martin. It achieved Initial Operational Capability (IOC) in March 2020 and reached Full Operational Capability in 2023, replacing the legacy VHF Air Force Space Surveillance System (AFSSS), nicknamed the "VHF Fence", that had operated since the 1960s.

The improvements over its predecessor are enormous:

• 10x sensitivity increase: The Space Fence can detect objects as small as 10 cm in low Earth orbit (LEO), compared to the old system's approximately 1-meter threshold. This means it can track softball-sized debris that would be invisible to the legacy system.
• Much greater detection range: While the old VHF fence was primarily useful for LEO, the Space Fence's S-band radar can detect and track objects in medium Earth orbit (MEO) and geosynchronous orbit (GEO), up to 36,000 km altitude.
• Higher capacity: The system can process 1.5 million observations per day, dramatically increasing the number of objects that can be tracked simultaneously.
• Uncued detection: The Space Fence operates as an "uncued" sensor; it doesn't need to know where to look. It creates a radar "curtain" across a wide swath of sky, and any object passing through that curtain is automatically detected, measured, and cataloged. This means it discovers new objects that no one knew existed.

How Radar Tracking Works

The Space Fence uses a phased-array radar, a flat panel containing thousands of individual transmit/receive elements that can electronically steer the radar beam without physically moving the antenna. This allows the system to rapidly switch between tracking known objects and scanning for new ones.

The process works as follows:

1. Detection: The radar transmits S-band pulses (2-4 GHz range) upward in a broad "fence" pattern. When an object passes through this electromagnetic curtain, the radar return is detected by the receive array.
2. Measurement: From the return signal, the system extracts the object's range (distance), azimuth (direction), elevation (angle above horizon), range rate (velocity toward or away from the radar), and radar cross-section (which correlates to the object's size and reflectivity).
3. Orbit determination: Multiple observations over successive passes allow precise calculation of the object's orbital parameters: its inclination, eccentricity, altitude, and predicted future positions. A single pass through the fence provides initial orbit determination; subsequent passes refine the accuracy.
4. Cataloging: New objects are assigned a temporary designator and, once their orbits are confirmed through multiple observations, receive a permanent catalog number in the U.S. Space Catalog (also called the Satellite Catalog or SATCAT).
5. Conjunction assessment: The orbit data feeds into the Combined Space Operations Center (CSpOC) at Vandenberg Space Force Base, where automated systems compare every cataloged object's predicted trajectory against every other object's trajectory, identifying potential close approaches (conjunctions) and generating warnings.

The Space Catalog and Conjunction Warnings

The U.S. Space Catalog is the world's most comprehensive database of objects in Earth orbit. Maintained by the 18th Space Defense Squadron, it contains orbital element sets (called Two-Line Elements (TLEs) or, increasingly, more precise ephemeris data) for every tracked object.

When the conjunction assessment process identifies a potential collision, a Conjunction Data Message (CDM) is generated and distributed to the affected satellite operator. CDMs include:

• Time of closest approach (TCA)
• Miss distance (predicted separation at TCA)
• Probability of collision (Pc): typically, operators consider maneuvers when Pc exceeds 1 in 10,000
• Covariance data (uncertainty in the prediction)

In practice, the process generates thousands of CDMs daily. The vast majority involve close approaches with very low collision probabilities. But several times per year, an active satellite operator receives a CDM with a probability high enough to warrant a collision avoidance maneuver, burning propellant to adjust the satellite's orbit and increase the miss distance.

Beyond the Space Fence: The Full SSA Network

The Space Fence is the crown jewel, but it's not the only sensor in the Space Surveillance Network (SSN). The full network includes:

• Ground-based radars: Including the AN/FPS-85 phased-array radar at Eglin Air Force Base and the Globus II radar in Norway
• Ground-based optical telescopes: The Ground-based Electro-Optical Deep Space Surveillance (GEODSS) system, with sites in New Mexico, Diego Garcia, and Maui, tracks objects in deep space (MEO and GEO) using reflected sunlight
• Space-based sensors: The Space-Based Space Surveillance (SBSS) satellite and the upcoming next-generation systems observe objects from orbit, avoiding weather and atmospheric distortion
• Allied contributions: Sensors from the UK, Canada, Australia, and other Five Eyes nations contribute observations through data-sharing agreements

Commercial space situational awareness (SSA) providers are also growing rapidly. Companies like LeoLabs, ExoAnalytic Solutions, and Numerica Corporation operate independent radar and optical sensor networks that supplement government data. LeoLabs, for example, operates phased-array radars in Texas, Costa Rica, New Zealand, and Australia, providing global LEO coverage with rapid orbit determination for commercial satellite operators who need faster-than-government updates.

Future Challenges

Despite the Space Fence's capabilities, significant challenges remain:

• Sub-centimeter debris: Objects smaller than 10 cm remain largely untrackable, yet a 1-cm aluminum sphere at orbital velocity carries the kinetic energy equivalent of a hand grenade. There is currently no practical way to track and catalog the estimated 130 million+ pieces of debris in this size range.
• Mega-constellation management: With Starlink alone planning up to 42,000 satellites, the number of active objects in LEO is growing faster than conjunction assessment systems were designed to handle. SpaceX's satellites perform autonomous collision avoidance using onboard GPS and inter-satellite communications, but coordination between operators remains largely manual.
• Active debris removal (ADR): Tracking debris is only half the problem; removing it is the other half. Missions like ESA's ClearSpace-1 (targeting a Vega rocket adapter for removal) and Japan's Astroscale (which demonstrated proximity operations with a defunct satellite) are pioneering ADR technology, but scaling from one-off demonstrations to routine cleanup of thousands of objects remains a massive engineering and economic challenge.
• Geopolitical tensions: Russia's 2021 anti-satellite test (destroying the Cosmos 1408 satellite) created over 1,500 trackable fragments in orbits that threatened the ISS and active satellites. China's expanding space program adds complexity. Not all nations share tracking data equally, creating blind spots in the global catalog.

Why It Matters for Every Satellite Operator

If you operate satellites, whether you're SpaceX managing 6,000+ Starlinks or a university running a single CubeSat, orbital debris tracking directly affects your mission. Collision avoidance maneuvers consume propellant, reducing satellite lifetime. Insurance premiums factor in debris risk. Regulatory requirements for post-mission disposal (the FCC now requires deorbit within 5 years of end of life) add cost and complexity.

The Space Fence's expanded catalog has already changed operations. Since reaching full capability, it has discovered thousands of previously untracked objects, debris fragments that operators didn't know were near their satellites. More data means more conjunction warnings, which means more informed decisions about when to maneuver and when to accept the risk.

As the orbital environment grows more congested, the ability to track, predict, and avoid debris isn't optional; it's existential for the space industry. The Space Fence is the most powerful tool we have for maintaining that awareness, and its data underpins every conjunction assessment, every collision avoidance maneuver, and every launch window calculation for objects passing through its coverage area.

Monitor orbital debris, conjunction alerts, and space environment conditions on the SpaceNexus Space Environment Dashboard.