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Safety & Security

FAQ: Frequently asked questions

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ESA / Safety & Security / Space Debris

Frequently asked questions on space debris answered by the team at ESA’s Space Debris Office.

Q1: What is space debris?

Space debris is defined as all non-functional, human-made objects, including fragments and elements thereof, in Earth orbit or re-entering into Earth’s atmosphere. Human-made space debris dominates over the natural meteoroid environment, except around millimetre sizes.

Q2: How do we know that space debris exist?

A debris impact chip in a Space Station window
A debris impact chip in a Space Station window

Routine ground-based radar and optical measurements performed by the space surveillance systems of the United States and Russia allow the tracking and cataloguing of objects larger than 5–10 cm in low orbit, and larger than 0.3–1.0 m at geostationary orbit altitudes (36 000 km above the equator).

Each of these catalogued objects has a known orbit and many can be traced back to a launch event − to a unique owner. Ground-based search radars can detect smaller objects, down to a centimetre or less in size. Such objects, however, can generally not be correlated with specific launch events, nor can their orbits be determined with sufficient accuracy to be predictable in future.

The presence of smaller space debris, typically less than 1 mm in size, can be deduced from impact craters on returned space hardware, or from onboard impact detectors. The detection of objects larger than 1 mm is difficult using this technique owing to the limited data collection time span in combination with the reduced impact probability for larger objects.

Q3: What are the main sources of information on space debris?

The main source of information on space debris is the US Space Surveillance Network, which uses radar and other technologies to track, correlate and catalogue objects.

Additional data are collected by means of research radars and telescopes in several nations, including ESA Member States. Some of the observations are coordinated in common campaigns, e.g. within the Inter-Agency Space Debris Coordination Committee (IADC). For small debris, most information is deduced from the impact analyses of space-exposed surfaces that have been returned by US Space Shuttles.

Q4: What are the origins of space debris?

EISCAT radar tracking: increase in debris after Chinese a-sat test
EISCAT radar tracking: increase in debris after Chinese a-sat test

All human-made space objects result from the about thousands of launches conducted since the start of the space age. The majority of the catalogued objects, however, originate from in-orbit break-ups – more than 290 explosions – as well as fewer than 10 known in-orbit collisions.

See 'Space debris by the numbers' for the most recently updated figures from ESA’s Space Debris Office.

See 'About space debris' for a description of debris origins and sources.

Q5: How have deliberate satellite intercepts affected the space debris environment?

Space Debris
Space Debris

According to ESA’s Space Debris Office, in almost 60 years of space activities, more than 5200 launches have placed some 7500 satellites into orbit, of which about 4300 remain in space; only a small fraction − about 1200 − are still operational today

On 11 January 2007, China conducted an antisatellite test, intercepting their FengYun-1C satellite with a surface-launched medium-range missile. The intercept occurred at an altitude of 862 km on a near-polar orbit, adding more than 3300 trackable objects to the US Space Surveillance Network catalogue, increasing its size by 25% in just one incident.

This was by far the worst break-up event in space history − some 3.5 times worse than the worst previous event. Owing to the high altitude of the collision event and the low ambient air density, the fragments will have long orbital lifetimes.

Satellites in Sun-synchronous orbits orbits at around 800 km altitude now suffer from an increased number of close conjunctions with debris objects. Today, roughly 30% of such events are caused by FengYun-1C fragments.

On 21 February 2008, the United States intercepted their USA-193 satellite with a modified SM-3 missile. At the time of engagement, the target spacecraft was at an altitude of 249 km, on a near-circular orbit at 58.5º inclination. Owing to the low altitude, and the correspondingly high air drag, most of the generated fragments harmlessly reentered within one orbit.

Only 170 fragments entered the US catalogue within one month, and none were left by end-2008. In the short-term, however, the risk of penetration of the shields of the ISS manned modules by USA-193 fragments larger than 1 cm increased by about 30%.

Q6: Why does the Earth's atmosphere have a positive effect on space debris?

Solar activity for the last and upcoming two solar cycles
Solar activity for the last and upcoming two solar cycles

Earth’s atmosphere causes air drag that extracts orbital energy and leads to a reduction in the orbital altitude and final reentry of a space object. Upper layers of the atmosphere are supported by lower layers, which are compressed under the weight of the air column above them. The air density increases, and hence the increase in air drag with decreasing altitude is progressive.

Changes in air density at a given orbital altitude are mainly driven by the Sun, which varies its activity in an 11-year cycle. Thus, every 11 years, lower parts of the atmosphere are heated and expand towards higher altitudes, where the air density increases, causing higher air drag on objects in space. As a consequence, space debris is periodically cleaned from the lower orbital regions (but these are subsequently refilled by objects descending from higher orbits).

After sufficient exposure to air drag the orbit decays, and the object enters Earth’s denser atmosphere, where the air drag converts orbital energy into heat. This heating process is normally sufficient to destroy an object. Approximately 20–40% of the mass of larger-size spacecraft or rocket bodies, or parts made of particularly high-melting steel or titanium alloys, may survive the reentry.

Q7: How many space debris objects are currently in orbit?

See “Space debris by the numbers” for the most recently updated figures from ESA’s Space Debris Office.

Any of these objects can cause harm to an operational spacecraft. For example, a collision with a 10 cm object would entail a catastrophic fragmentation of a typical satellite, a 1 cm object will most likely disable a spacecraft and penetrate the ISS shields, and a 1 mm object could destroy subsystems on a satellite.

Scientists generally agree that, for typical satellites, a collision with an energy-to-mass ratio exceeding 40 J/g will be catastrophic.

Q8: What is the Kessler Syndrome, and how can it be avoided?

At present, the majority of all space debris that can cause a catastrophic collision (i.e. larger than 10 cm) results from more than 290 in-orbit fragmentation events in the course of space history.

However, simulations of the long-term evolution of the space debris environment indicate that within a few decades, generated collision fragments will start to dominate, at least in orbits around 800–1400 km altitude. This will be true even if all launch activities were to be discontinued now, which is an extremely unlikely development.

In the most probable scenario, fragments will initially collide with large, intact objects. Then, the resulting collision fragments will start to collide with such objects, and ultimately collision fragments will collide with collision fragments until all remaining objects are reduced to subcritical sizes. This self-sustained collisional cascading process is most likely to set in at altitudes with high debris population densities and insufficient cleansing by air drag, i.e. around 900 km and 1400 km.

This runaway scenario is the ‘Kessler syndrome’ because it was first postulated by NASA’s Don Kessler in 1978.

Q9: What risks to space operations are caused by space debris?

Giotto in 1985, with Whipple shield at bottom
Giotto in 1985, with Whipple shield at bottom

One must distinguish between debris-related risks in orbit and risks due to reentries.

In-orbit risks are due to collisions with operational spacecraft, or with decommissioned spacecraft or rocket bodies. Impacts by debris larger than 10 cm are assumed to cause catastrophic break-ups, which cause the triggering of a collisional cascading process − the Kessler syndrome. Collisions with debris larger than 1 cm would disable an operational spacecraft, and may cause the explosion of a decommissioned spacecraft or rocket body. Impacts by millimetre-size debris may cause local damage or disable a subsystem of an operational spacecraft.

Large space-debris objects (e.g. spacecraft, rocket bodies or fragments thereof) that reenter into the atmosphere in an uncontrolled way can reach the ground and create risk to the population on ground. The related risk for an individual is, however, several orders of magnitude smaller than commonly accepted risks, such as driving a car, that we all accept in day-to-day life.

Q10: How does the ISS protect itself against space debris?

ATV-CC teams during rendezvous
ATV-CC teams during rendezvous

The International Space Station has debris shields deployed around the crewed modules. These shields are composed of two metal sheets, separated by about 10 cm. The outer bumper shield exploits the impact energy to shatter the debris object, such that the inner back wall can withstand the resulting spray of smaller-sized fragments.

Between the walls, fabric with the same functionality as in bullet-proof vests is deployed. This design enables the shield to defeat debris objects up to 1 cm in size.

The orbits of debris objects that are large enough to be contained in the US Space Surveillance catalogue can be predicted and compared with the Space Station’s orbit to determine whether a close approaches will occur. Assuming that both orbits can be determined with sufficient accuracy, then a predicted flyby, or conjunction, distance can be translated into a specific in-orbit collision risk.

If this risk exceeds the ISS threshold level, as set under the flight rules, then the Station performs an avoidance manoeuvre. By the end of 2012, the Station had performed more than 15 of these manoeuvres, some by using the engines of ESA’s Automated Transfer Vehicle when one happened to be docked to Station.