Low Earth Orbit Congestion – What is it and what can we do about it?

LEO

There are three primary types of satellite orbits around Earth: Low Earth Orbit (LEO), Medium Earth Orbit (MEO) and Geostationary Earth Orbit (GEO).

The orbits are categorised by the altitude and speed at which satellites orbit the Earth. GEO satellites have a fixed altitude of approximately 35,788 km (22,236 miles) above the Earth and travel at approximately 7,000 miles per hour. Satellites in GEO orbit travel at a speed that exactly matches the rotation of the Earth, so that a GEO satellite remains above the same location on the Earth's surface.

Conversely, at the other end of the scale, LEO satellites have an altitude of 160 to 2,000 km (100 to 1,200 miles) above the Earth. The International Space Station (ISS) is in low Earth orbit, operating at an altitude of approximately 400 km. Satellites typically travel at approximately 17,500 miles per hour and orbit the Earth between 11 and 16 times per day. LEO satellites therefore move much faster than satellites in either MEO or GEO.

LEO is a very popular orbital regime. First, as the lowest orbital regime, its close proximity to Earth can reduce launch costs. More importantly, LEO satellites are capable of delivering high-bandwidth, low-latency communications, as well as enabling high-resolution Earth imaging. Consequently, modern satellite internet constellations consist of LEO satellites.

Technical and commercial interest in LEO is evolving at a rapid pace. In 2025, the LEO sector attracted more than $45 billion in investment across the globe. For comparison, in 2024, just under $25 billion was invested. There are currently over 16,000 active and inactive satellites and other tracked objects in LEO, with most being operational satellites. To put that into context, LEO satellites account for nearly 90% of all artificial objects in orbit around the Earth. The number of satellites in LEO is expected to increase dramatically.

Orbital congestion and the Kessler Syndrome

The LEO environment is becoming congested. Not only is there a large number of satellites in LEO, but there is also a significant amount of space debris. Space debris is any artificial object that is no longer in use or is broken but remains in orbit around the Earth. Space debris includes defunct satellites, rocket bodies and fragments of satellites created by collisions between satellites.

There is an estimated 500,000 objects between 1 and 10 cm in size, and more than 100 million particles larger than 1 mm. These particles travel at speeds of up to 7.8 km/s (28,000 km/h; 17,500 mph), meaning that even a small object impact can severely damage or destroy a spacecraft. This has caused growing concern in recent years.

Between 2019 and 2025, the number of satellites executing more than 10 collision-avoidance manoeuvres per month increased sevenfold. Each manoeuvre requires a fraction of the satellite’s total fuel. Congestion therefore requires satellites  to consume a higher amount of fuel to avoid damage or destruction. For launch companies, the congestion means insertion windows are more complex and flight paths require greater precision.

Collisions can produce additional space debris, and this additional space debris can in turn go on to collide with more satellites and other debris. This is known as the Kessler Syndrome, which describes the domino-like effect of ever-increasing space debris caused by repeated collisions. It was first proposed in 1978 by NASA scientists Donald J. Kessler and Burton G. Cour-Palais. Specifically, the Kessler Syndrome describes a scenario in which the density of objects in LEO reaches a critical threshold at which collisions between space objects cascade. The end result is an exponentially increasing amount of space debris over time. This could render the LEO environment unusable, as it would become filled with such a high density of space debris that newly launched satellites would be highly likely to collide with debris and be destroyed.

What can we do about it?

One way to reduce orbital congestion in LEO and avoid reaching the critical density at which the Kessler Syndrome is triggered is to reduce the amount of space debris in orbit. Several Active Debris Removal technologies are currently being developed to remove space debris from orbit:

• Laser Orbital Debris Removal (LODR): LODR uses the impulse generated by laser ablation of the debris surface by a focused, pulsed ground-based laser to change the debris orbit and cause it to re-enter the atmosphere.

• ClearSpace-1: An ESA mission designed to demonstrate the removal of space debris from Earth orbit. It will rendezvous with, capture and remove ESA's uncooperative 95 kg Proba-1 satellite from its valuable low Earth orbit by grasping it and altering its orbit.

• Robotic arms: Development efforts include a CubeSat debris de-orbiter prototype with dual robotic arms for space debris capture, as well as a NASA Johnson Space Center (JSC) innovation comprising computer vision software that rapidly determines target posture and then instructs an operator how to properly align a robotic end-effector with a target intended for capture. 

• Active Debris Removal Vehicle (ADRV): Removing debris from orbit involves several distinct challenges. The debris must be found and tracked so that its future position can be predicted. A removal spacecraft must then rendezvous with the target and determine how it is moving. This is particularly difficult where the target is an uncooperative, tumbling satellite or rocket body that was never designed to be captured. To address these challenges, NASA have designed an ADRV which can approach a debris object, assess its characteristics and motion, determine an initial capture trajectory, match its rotation rates, execute a capture manoeuvre, and control and deorbit the object [9].

Another approach to reduce orbital congestion is to prolong the useful life of functioning satellites. In-orbit servicing may involve correcting a satellite’s orbit, replacing or upgrading equipment, or supplying additional fuel. A spacecraft with sufficient fuel can continue to perform collision-avoidance manoeuvres. Refuelling of LEO satellites may therefore contribute to a sustainable space environment. Proposals for LEO refuelling include servicing spacecraft that collect propellant from dedicated supply tanks and deliver it to client satellites, potentially supporting both operational spacecraft and other in-orbit service vehicles¹. NASA is also funding a demonstration of technologies for storing and transferring liquid oxygen in LEO, which may be used for establishing refuelling capabilities for spacecraft reuse.

References 

1. Marie Tilbee, Astroscale: “Leading the development of LEO Refuelling”, October 2022