The Crowded Sky

1. Summary

The population of artificial objects in low Earth orbit has increased faster in the past six years than in the preceding sixty. Existing coordination arrangements were designed for a regime of a few hundred operational satellites managed by a small number of state agencies. They are being applied to a regime of tens of thousands of objects operated commercially, and the mismatch is now the principal risk to the sustained use of the orbital environment.

2. The physical situation

An object in low Earth orbit travels at approximately 7.8 kilometres per second. Two objects in crossing orbits may therefore meet at a relative velocity approaching 15 kilometres per second — some fifteen times the muzzle velocity of a rifle.

At those speeds the relevant quantity is kinetic energy, which rises with the square of velocity. A fragment of aluminium one centimetre across carries energy comparable to a small explosive charge. It cannot be shielded against on any spacecraft of practical mass, and it will destroy whatever it strikes.

The catalogued population is approximately as follows:

Size	Estimated number	Trackable	Consequence of impact
Greater than 10 cm	~36,000	Yes	Catastrophic; total loss
1–10 cm	~1,000,000	Largely not	Catastrophic; unavoidable
1 mm – 1 cm	>100,000,000	No	Subsystem damage, degradation

The central difficulty is visible in the second row. Objects in the one-to-ten-centimetre range are lethal and, for the most part, cannot be detected reliably from the ground. They cannot be avoided because they cannot be seen.

3. The cascade mechanism

A collision between two large objects does not merely remove them. It converts them into thousands of fragments, each capable of causing a further collision.

Where the population density is sufficiently high, the rate at which collisions generate new fragments may exceed the rate at which atmospheric drag removes them. The debris population then grows without further launches — a self-sustaining process first described in 1978 and generally referred to as collisional cascading.

Two events dominate the present catalogue. A destructive anti-satellite test conducted in 2007 generated in excess of three thousand trackable fragments, a substantial proportion of which remain in orbit and will do so for decades. An accidental collision between an operational communications satellite and a defunct spacecraft in 2009 produced approximately two thousand three hundred further trackable objects.

Together, these two events account for a significant share of all catalogued debris. Neither involved a launch.

4. Current operational practice

Conjunction assessment is conducted by correlating tracked objects and computing close-approach probabilities, typically several days in advance. Operators receive warnings and may execute avoidance manoeuvres.

Three limitations of this process should be noted.

Positional uncertainty. Orbital predictions carry error ellipsoids extending hundreds of metres to kilometres, driven principally by uncertainty in atmospheric density, which varies with solar activity. Warnings are consequently probabilistic, and the great majority of manoeuvres are performed against approaches that would not in fact have resulted in collision.

Manoeuvre cost. Each manoeuvre consumes propellant, shortening mission life, and interrupts service. Operators face a direct commercial disincentive to act on marginal warnings.

Coordination. Where both objects are manoeuvrable, there is no binding protocol determining which gives way. Coordination is conducted bilaterally, by direct contact between operators, using contact details that are not universally published. Instances have been recorded in which one party was unable to reach the other before the point of closest approach.

5. Regulatory position

The governing treaty framework predates the situation it is being applied to. Liability for damage caused by a space object is established in principle, but attributing a collision to a specific fragment requires identification that is generally not achievable.

Mitigation guidelines adopted internationally recommend that objects in low Earth orbit be removed from orbit within twenty-five years of end of mission. Compliance has historically been partial. Several national regulators have since reduced the requirement to five years for objects under their licensing authority, which is a material improvement in the regimes to which it applies but does not bind operators licensed elsewhere.

There is at present no international body with authority to allocate orbital slots in low Earth orbit, to require coordination, or to enforce disposal. Participation in the existing arrangements is voluntary.

6. Remediation

Active removal of existing debris has been demonstrated at small scale using capture mechanisms including nets, harpoons and magnetic docking plates. Several missions targeting specific large objects are in preparation.

Two constraints apply. First, cost: each removal presently requires a dedicated mission, and the economics are unattractive to any commercial party, since the benefit accrues to all operators while the cost falls on one. Second, selection: analysis indicates that removal has a meaningful effect on long-term population growth only if it targets the largest and most collision-prone objects, most of which are defunct upper stages that were never designed to be captured and are frequently tumbling.

7. Recommendations

1. Mandate publication of operator contact and manoeuvre-capability data as a condition of licensing.
2. Establish a common conjunction data format and a defined right-of-way convention determining which operator manoeuvres.
3. Extend the five-year disposal requirement across licensing jurisdictions and require demonstrated disposal capability at the design review stage.
4. Fund removal of high-consequence derelict objects as public infrastructure, on the reasoning that the benefit is non-excludable and will therefore not be financed privately.
5. Improve tracking sensitivity in the one-to-ten-centimetre range, which is presently the largest gap between what is lethal and what is observable.

8. Assessment

The orbital environment is a shared resource with no allocation mechanism, no enforcement authority and no price attached to its degradation. The economic structure is therefore identical to that of every historical fishery and grazing commons, and the outcome in those cases is well documented.

The distinguishing feature here is irreversibility on human timescales. A depleted fishery may recover within a generation. Debris at eight hundred kilometres remains in orbit for centuries, and if cascading begins in a heavily used band there is no technical means of clearing it. The window for coordination is open, and it is a function of population density rather than of anyone's intentions.