What Is the U.S. Satellite Catalogue?

The Global Ledger

High above the Earth, traveling at speeds over 17,000 miles per hour, is a cloud of artificial objects. This cloud isn’t just composed of the thousands of active satellites that provide communication, navigation, and scientific data; it’s a sprawling, high-speed junkyard. It includes spent rocket bodies from the dawn of the Space Age, defunct satellites that have gone cold, and millions of tiny, lethal fragments from past collisions and explosions.

In this complex and hazardous environment, a single master list is perhaps the most foundational tool for space operations. That list is the U.S. Satellite Catalogue (SATCAT).

This article explores this monumental database. It examines who builds this catalogue, the sophisticated methods used to track tens of thousands of objects, its history, and the modern challenges that are pushing the system to its limits. It is the story of how a Cold War military file evolved into a global utility for collision avoidance, managing a domain that is becoming dangerously crowded.

A Universe of Data: What Is the Satellite Catalogue?

At its core, the Satellite Catalogue is a dynamic database of all human-made objects in Earth orbit. It is a ledger that attempts to account for every “resident space object,” or RSO. Each object, whether it’s the multi-billion dollar International Space Station or a discarded bolt from a 1970s rocket stage, is assigned a unique identifier and its orbital path is continuously monitored.

As of 2025, the public catalogue contains more than 45,000 tracked objects. These objects are generally 10 centimeters (about the size of a softball) or larger in Low Earth Orbit (LEO) and about 1 meter (or 3 feet) wide in higher, Geosynchronous Orbits (GEO).

These cataloged items fall into three main categories:

1. Payloads: This is the satellite itself – the object designed to perform a mission. This includes active satellites like the Hubble Space Telescope, satellites in the GPS constellation, and the thousands of Starlink internet satellites. It also includes “dead” satellites that are no longer functioning but remain in orbit.
2. Rocket Bodies: These are the large, upper stages of the rockets that carried the payloads into orbit. After deploying their satellite, these massive metal tubes are often left to tumble in their own orbits, becoming some of the largest and most dangerous pieces of debris.
3. Debris: This is the largest and most problematic category. It’s a catch-all for everything else. This includes fragments from satellites that have exploded due to old propellant, pieces from accidental collisions, lens caps, and even a spatula lost during a Space Shuttle mission.

The catalogue is not just a static list. It’s a predictive model. For each object, the catalogue stores a set of orbital parameters that, when fed into a physics model, can predict where that object will be in the near future. This predictive power is the system’s entire purpose. It exists to answer one question, thousands of times a day: “Is this object going to hit that object?”

It’s also important to note that the public catalogue is not the entire catalogue. The U.S. military maintains a separate, classified catalogue of objects. This file contains data on U.S. national security assets and objects belonging to other nations, which are tracked with a higher priority and whose data is not shared publicly.

The Sentinels: Who Manages the Catalogue?

The massive responsibility of maintaining the Satellite Catalogue falls to the United States Space Force, a branch of the U.S. Armed Forces. The specific unit is the 18th Space Defense Squadron (18 SDS), operating from Vandenberg Space Force Base in California.

The 18 SDS operates 24 hours a day, 7 days a week, in a mission that has evolved from a passive, Cold War-era “space surveillance” role into the modern, active mission of “Space Domain Awareness” (SDA). While “Space Situational Awareness” (SSA) was about knowing an object’s position, SDA is a broader military doctrine that includes understanding an object’s capabilities, its behavior, and the intent of its operator.

Historically, this mission was under the North American Aerospace Defense Command, which was famous for tracking Santa Claus but whose primary mission was (and is) detecting ballistic missile attacks against North America. As the number of satellites grew, the responsibility for the catalogue shifted through various Air Force and Strategic Command organizations before finding its current home with the Space Force.

The 18 SDS and its parent organization, United States Space Command (USSPACECOM), don’t just keep this data for themselves. They have become the world’s de facto space traffic coordinators. Through the public website Space-Track.org, USSPACECOM provides orbital data, collision warnings, and re-entry predictions to satellite operators, governments, and scientific institutions around the globe, free of charge.

This is an unusual arrangement – a single nation’s military providing a vital public service to the rest of the world, including commercial competitors and strategic rivals. This data sharing is based on a pragmatic understanding: a collision in orbit affects everyone. The debris from a collision between two satellites, regardless of their owners, will endanger all satellites in that orbital band for decades.

The Architecture of Awareness: The Space Surveillance Network

The 18 SDS doesn’t guess where objects are. It knows because it receives a constant stream of data from a global network of sophisticated sensors: the Space Surveillance Network (SSN). The SSN is the “eyes and ears” of the Satellite Catalogue, a collection of over 30 ground-based radars and optical telescopes, as well as space-based sensors, all feeding data into the central processing hub at Vandenberg.

Ground-Based Sentinels: Radar

The workhorse of the SSN is radar. Ground-based radar systems track objects by sending out a powerful beam of radio energy and listening for the faint echo that bounces off an object in orbit. By precisely timing this echo and measuring its frequency, technicians can determine an object’s range, direction, and velocity.

Radar is ideal for tracking objects in Low Earth Orbit, the most crowded region of space. It works 24 hours a day, is unaffected by cloud cover, and can “see” objects that are not reflecting sunlight.

Key radar systems in the SSN include:

• AN/FSY-3 (Space Fence): The newest and most advanced sensor in the SSN is the Space Fence, located on Kwajalein Atoll in the Marshall Islands. This is a massive, advanced phased-array radar system. Unlike a traditional radar dish that must be mechanically pointed, a phased-array radar steers its beams electronically. This allows it to scan huge swaths of the sky and track thousands of objects simultaneously. The Space Fence is powerful enough to detect objects as small as a marble in LEO, greatly increasing the fidelity of the catalogue.
• AN/FPS-85 Space Track Radar: Located at Eglin Air Force Base in Florida, this is another powerful phased-array radar that has been a cornerstone of the network for decades.
• Cavalier Space Force Station: A radar site in North Dakota, its location gives it a good view of satellites passing over the northern pole, a common path for Earth-observing satellites.

These systems “ping” objects as they pass overhead, generating a “tracklet” of data. This tracklet is sent to the 18 SDS, which uses it to refine the object’s known orbital path.

Ground-Based Sentinels: Optical Telescopes

Radar is very energy-intensive, and its power drops off rapidly with distance. To track objects in very high orbits, like the geosynchronous belt 22,236 miles (35,786 km) up, the SSN relies on optical telescopes.

These aren’t the same as astronomical telescopes used for studying distant galaxies. These are robotic telescopes designed to find and track faint, moving points of light. They work by taking a series of pictures of the night sky and looking for “stars” that are moving relative to the fixed background.

The primary optical system is the Ground-based Electro-Optical Deep Space Surveillance (GEODSS) system. GEODSS is a network of powerful telescopes in New Mexico, Hawaii, and Diego Garcia. These telescopes can detect objects as dim as a softball in GEO, but they have a major limitation: they can only be used on clear nights, and they can only see objects that are illuminated by the sun.

Eyes in the Sky: Space-Based Sensors

The best place to watch for satellites is from space. A sensor in orbit doesn’t have to worry about weather, the day-night cycle, or the filtering effect of Earth’s atmosphere.

The U.S. Space Force operates the Space Based Space Surveillance (SBSS) constellation. These satellites are essentially “neighborhood watch” patrols in orbit. They carry advanced optical sensors that can scan the orbital environment, looking for new objects, monitoring existing ones, and tracking satellites that may be maneuvering in an unusual way.

This combined network of ground-based radar, ground-based telescopes, and space-based sensors provides the 18 SDS with a continuous feed of observations. This data is the raw material used to build and maintain the Satellite Catalogue.

A History Written in Orbit

The Satellite Catalogue wasn’t created out of pure scientific curiosity. It was born from a moment of significant strategic shock.

The Opening Entry: Sputnik and Project Space Track

On October 4, 1957, the Soviet Union launched Sputnik 1, the first artificial satellite. The small, beeping sphere shocked the American military and public. The U.S. realized it had no reliable way to track this new object, or any future objects, that could be passing over its territory.

In response, the U.S. military quickly established Project Space Track. Using a hastily assembled network of radars and observing stations, they began the work of detecting and tracking objects in orbit.

The first objects in the catalogue tell the story.

• Satellite Number 00001 (or 1957-001A) was assigned to the R-7 rocket body that carried Sputnik into orbit.
• Satellite Number 00002 (or 1957-001B) was assigned to the Sputnik 1 satellite itself.

The catalogue had begun. Every object launched into orbit, or discovered in orbit, from that day forward has been added to the list.

The Cold War Context

Throughout the 1960s and 1970s, the catalogue’s primary purpose was military intelligence. As part of NORAD, the Space Surveillance Network was expanded to track the orbits of Soviet reconnaissance satellites, communication satellites, and early anti-satellite (ASAT) weapon tests. Knowing the precise orbital path of a Soviet photo-reconnaissance satellite allowed the U.S. military to know when it would be passing over sensitive sites, such as military bases or missile silos.

During this period, the “Space Race” and military competition rapidly populated the orbital environment. Every launch added at least two new objects to the catalogue: the payload and the rocket body that got it there. Sometimes, launches failed or rocket stages exploded, adding dozens or hundreds of new fragments at a time.

Waking Up to the Threat: Key Debris Events

For decades, space was seen as infinitely large. The idea of “orbital crowding” seemed absurd. That perception began to change with a 1978 paper by a NASA scientist named Donald J. Kessler.

Kessler proposed a theory that became known as the Kessler syndrome. He pointed out that as the number of objects in orbit increases, the probability of collisions also increases. When a collision occurs, it creates a cloud of new debris fragments. Each of those fragments increases the probability of more collisions, which create more debris, in a cascading chain reaction. This cascade could, in theory, make certain orbits so hazardous that they would become unusable for future generations.

At the time, Kessler’s theory was a distant, abstract concern. Two major events in the 21st century made it a terrifyingly immediate reality.

1. The 2007 Chinese Anti-Satellite Test: On January 11, 2007, China conducted an anti-satellite missile test. It launched a missile that purposefully slammed into one of its own defunct weather satellites, Fengyun-1C. The hypervelocity collision was a success, but it was a catastrophic event for the orbital environment. The satellite, which had a mass of over 1,600 pounds, was obliterated. The 18 SDS (then known as the 18th Space Control Squadron) watched in horror as a single cataloged object instantly became more than 3,000 new pieces of trackable debris. This single event increased the total number of tracked objects in the catalogue by more than 25% and created a dense cloud of debris in a very busy orbit.
2. The 2009 Satellite Collision: If the 2007 test was a deliberate act of destruction, the event on February 10, 2009, was the accident everyone had feared. Over Siberia, an active U.S. communication satellite, Iridium 33, collided with a defunct Russian military satellite, Kosmos-2251. The two satellites, with a combined mass of over 3,000 pounds, hit each other at a relative velocity of over 26,000 miles per hour. Both were instantly vaporized, creating two new, massive debris clouds totaling more than 2,300 new trackable objects.

These two events, which are still the largest debris-generating incidents in history, fundamentally changed the purpose of the Satellite Catalogue. Its primary mission shifted from military surveillance to environmental protection and collision avoidance. The Kessler syndrome was no longer a theory; its mechanism had been demonstrated. The debris from these two events still poses a regular threat to the International Space Station and other satellites.

The Language of Orbit: How Data is Structured

To manage tens of thousands of objects, the 18 SDS needs a standardized way to identify and describe them. The catalogue uses a few key identifiers.

The Satellite Number (SSN)

The simplest identifier is the Satellite Catalog Number, or SSN. This is a simple, sequential 5-digit number (now expanding) assigned to an object when it is officially cataloged. As noted, Sputnik’s rocket is 00001, and Sputnik itself is 00002. The Hubble Space Telescope is 20580. The first piece of debris cataloged from the Iridium/Kosmos collision is 33753. This number is the object’s primary “license plate.”

The International Designator (COSPAR ID)

There is also a scientific naming convention called the International Designator, or COSPAR ID, managed by the Committee on Space Research. This name describes the object’s launch. For example, the Hubble Space Telescope’s COSPAR ID is 1990-037B.

This is read as:

• 1990: The launch year.
• 037: It was the 37th successful launch of that year.
• B: It was the second piece (B) cataloged from that launch. (Piece A was the Space Shuttle Discovery, which deployed it but returned to Earth).

The Two-Line Element Set (TLE)

The most important piece of data for each object is its Two-Line Element Set, or TLE. For a non-technical user, the TLE is the most confusing part of the catalogue, but it’s also the most powerful.

A TLE is a set of two 69-character lines of text and numbers. It is not a real-time coordinate, like a GPS location. Instead, a TLE is a “recipe” that describes an object’s orbit at a specific moment in time (called the “epoch”). It contains all the ingredients a computer program needs to calculate that object’s position in the past or, more importantly, predict its position in the future.

You can think of a TLE as a snapshot. Imagine you take a snapshot of a car on a highway that records its exact position, speed, and direction. You could use that snapshot to make a good guess where the car will be in five minutes. That is what a TLE does for a satellite.

The problem is that this “recipe” goes stale. Orbits are not perfectly clean. They are constantly disturbed by several forces:

• Atmospheric Drag: In LEO, there is still a tiny amount of atmosphere, which creates drag and causes satellites to slow down and lose altitude.
• Solar Activity: When the sun is active, it heats the Earth’s upper atmosphere, causing it to “puff up.” This increases drag on satellites, making them decay from orbit faster.
• Gravitational Pulls: The moon and sun pull on satellites, slowly warping their orbits.
• Earth’s Lumpy Gravity: The Earth is not a perfect sphere, and its gravity is not uniform. This also affects orbital paths.

Because of these perturbations, a TLE is only accurate for a few days, or even just a few hours for objects in low, high-drag orbits. This is why the Space Surveillance Network must constantly re-observe objects. Every new observation from a radar or telescope is used to generate a new, updated TLE for that object. The Satellite Catalogue is not a static book; it’s a “living” database that is updated thousands of times every day.

This data is made available to the public on Space-Track.org. Any satellite operator, astronomer, or even a hobbyist satellite tracker can create a free account, download the latest TLEs, and use them to predict satellite passes or check for potential collisions.

Why the Catalogue Matters: The Mission of Collision Avoidance

The primary, day-to-day job of the 18 SDS is to use the catalogue to prevent collisions. This process is called “conjunction assessment.”

Protecting High-Value Assets

The highest priority is protecting human life, specifically the astronauts aboard the International Space Stationand other crewed vehicles. The ISS exists in a busy orbit and is a massive target.

Several times a year, the 18 SDS will warn NASA and its international partners of a “high-probability conjunction” – a piece of debris whose predicted path will take it dangerously close to the station. If the warning comes early enough, and the risk is high enough, the ISS will fire its thrusters in a “Debris Avoidance Maneuver” to move out of the way. If the warning comes too late, astronauts are instructed to “shelter in place” by moving into their docked Soyuz or Crew Dragon capsules, which can act as “lifeboats” in the event of a depressurizing impact.

The same protection is extended to high-value robotic assets, like the Hubble telescope, multi-billion dollar national security satellites, and the scientific satellites run by NASA, the European Space Agency (ESA), and others.

The Conjunction Assessment

The 18 SDS performs this check by running the predictive orbital paths of all active satellites against the paths of all other 45,000+ objects in the catalogue. This massive computational task flags any time two objects are predicted to come too close to each other.

When a potential conjunction is found, the 18 SDS issues a “Conjunction Data Message” (CDM) to the satellite’s operator. This message essentially says, “At this time, our models show object X will come within this many-mile/kilometer radius of your satellite.”

This presents a difficult choice for the operator. The TLE data is not perfectly precise; it comes with an “uncertainty box,” often described as a 3D “pizza box,” that represents the statistical probability of where the object is. The operator must decide if the risk of collision is high enough to warrant a maneuver. Maneuvering is a last resort. It consumes precious fuel, which shortens the satellite’s operational lifespan, and it temporarily takes the satellite out of service.

Tracking the Unpredictable: Atmospheric Re-entry

What goes up must eventually come down. For objects in LEO, atmospheric drag is a constant, inescapable force. Over years or decades, it pulls objects lower and lower until they finally re-enter the atmosphere.

The vast majority of objects – especially small debris – burn up completely, appearing as nothing more than a momentary shooting star. But large objects, like defunct satellites or massive rocket bodies, can partially survive re-entry, with large, super-heated fragments reaching the ground.

The Satellite Catalogue is the primary tool used to predict these re-entries. The 18 SDS monitors the orbital decay of large objects. As an object gets close to re-entry, they issue regular updates. However, predicting exactly where a large object will land is extremely difficult. The main variable is the sun. A solar flare can cause the atmosphere to expand, dramatically increasing drag and causing an object to re-enter days earlier – and thousands of miles away from – its original prediction.

Usually, predictions can only be narrowed down to a specific time and location within the last few hours of the object’s life. This was seen in the famous re-entries of Skylab in 1979 and the Mir space station in 2001. The catalogue’s re-entry-tracking mission is for public safety, helping to provide as much warning as possible to air traffic and populations on the ground.

The New Space Race: Modern Challenges for the Catalogue

The Satellite Catalogue and the SSN were designed in the 20th century for a space environment that was relatively sparse. The 21st century has introduced new challenges that are stressing this legacy system to its breaking point.

The Rise of the Mega-Constellations

The single greatest challenge to the modern catalogue is the deployment of “mega-constellations.” These are vast networks of thousands of small, mass-produced satellites designed to provide global internet coverage.

The most prominent is Starlink, operated by SpaceX. SpaceX has already launched thousands of satellites and has approval for tens of thousands more. Other companies, like OneWeb and Amazon’s Project Kuiper, are building their own constellations.

This new model has completely changed the math of orbit. In a few short years, these companies have more than doubled the total number of active satellites in orbit. The 18 SDS has reported that conjunction alerts have increased exponentially. A large percentage of all collision warnings are now generated by the satellites within the Starlink constellation, either screening against each other or against other objects in the catalogue.

This places an incredible burden on the catalogue. The SSN must track all these new satellites, and the 18 SDS must adjudicate an ever-growing flood of conjunction alerts. The TLE-based system, which requires days-old data, is too slow for an environment where active satellites are maneuvering daily. This has led to “close calls” where automated collision avoidance systems on two different satellites have nearly caused a collision.

The Commercialization of Space Awareness

The U.S. government is no longer the only entity with a sophisticated network for tracking space objects. The challenges of the new space environment have created a booming commercial market for Space Situational Awareness.

Companies like LeoLabs and ExoAnalytic Solutions have built their own global networks of sensors.

• LeoLabs has built a network of advanced, ground-based phased-array radars specifically to service the LEO market. They offer a data service, often faster and with higher fidelity than the public catalogue, that operators like Starlink can subscribe to.
• ExoAnalytic Solutions has built a massive global network of optical telescopes to track objects in GEO, providing a persistent “neighborhood watch” on high-value satellites.

These commercial services are changing the role of the U.S. Space Force. The government is moving from being the sole provider of SSA data to being a customer and integrator. The U.S. military now buys data from these commercial companies and fuses it with its own SSN data to create a richer, more accurate “fused” catalogue.

The Problem of “Un-trackables”

The catalogue’s tracking limit of 10 cm in LEO is a practical one, not a safe one. An object doesn’t have to be the size of a softball to be dangerous. A 1 cm piece of debris (the size of a marble), traveling at 17,000 mph, has the kinetic energy of a bowling ball dropped from a skyscraper. A 1-millimeter paint fleck can-and-has-pitted Space Shuttle windows.

There are an estimated 1 million pieces of debris between 1 cm and 10 cm in orbit. They are large enough to be lethal to a satellite, but too small for the Space Surveillance Network to reliably track and catalogue.

This means that even the best collision avoidance system, based on the most perfect catalogue, can only protect against a fraction of the total risk. For the millions of “lethal non-trackables,” the only defense for satellite operators is armoring, luck, and statistical modeling.

A Sample of the Catalogue

To illustrate how the catalogue is structured, here is a small sample of some of the most famous objects and their identifiers.

Summary

The U.S. Satellite Catalogue is far more than a simple list. It is a foundational, dynamic system that makes modern space operations possible. Born from the anxieties of the Cold War, it has evolved into a global utility for public safety and environmental protection, managed by the United States Space Force.

It is a system built on a global network of powerful sensors and complex predictive models, all designed to answer the constant, high-stakes question of “what’s going to hit what.” Today, this vital system is facing its greatest challenge: the rapid, exponential growth of commercial space activity.

The catalogue’s future will depend on its ability to adapt. It must integrate new commercial data, adopt faster and more accurate tracking methods, and find a way to manage an orbital environment that is on the verge of being overcrowded. The ledger that began with a single beeping sphere in 1957 is now the one and only master list for the entire human enterprise in space.