Frequently asked questions on space debris prepared by the ESA Space Debris Office team.

Q1: What are space debris?

Space debris are defined as all non-functional, man-made objects, including fragments and elements thereof, in Earth orbit or re-entering into the Earth atmosphere. Man-made space debris dominate over the natural meteoroid environment, except around millimetre sizes.

Q2: How do we know that space debris exist?

Protected orbital regions as developed by the IADC

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 that 5 to 10 cm in low Earth orbits, and larger than 0.3 to 1.0 metres at geostationary orbit altitudes (i.e. 35 786km above the equator).

Each of these catalogued objects has a known orbit and can be traced back to a launch event, i.e. to a unique owner. Smaller objects, down to a centimetre or less in size, can be detected by ground-based search radars. Such objects, however, can generally not be correlated with specific launch events, nor can their orbits be determined with sufficient accuracy for re-acquisition.

The presence of smaller space debris, of typically less than 1 millimetre in size, can be deduced from impact craters on returned space hardware, or from in-situ impact detectors. In this case, the detection size limit is a consequence of the limited data collection time span in combination with the reduced impact probability with increasing impactor size.

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

Impact crater (size 4 mm) on solar cell retrieved from space

The main source of information on space debris is the Space Surveillance Network of the United States, which tracks, correlates and catalogues approximately 12 500 space objects larger than 5 to 10 cm in Earth orbits (as of January 2009).

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

Q4: What are the origins of space debris?

All man-made space objects result from the 4900 launches, as of January 2009, conducted since the start of the space age. The majority of the catalogued objects (about 50 percent), however, originate from in-orbit break-ups - about 200 explosions - as well as fewer than 10 known collisions.

Approximately 25 percent of the catalogued objects are payloads (some six percent of these are still operational), and about 12.5 percent each comprise spent rocket bodies and other mission-related objects. Fragmentation debris dominate the smaller size regimes down to 1 mm. Below 1 mm, slag and dust residues from more than 1000 solid rocket motor firings prevail.

Other debris sources can be associated with the release of coolant liquid from 16 Buk reactors in the 1980s (used by Russian Radar Ocean Reconnaissance Satellites), and with the release of surface materials from old satellites and rocket bodies due to impacts and/or due to surface degradation.

In addition, the first-ever accidental in-orbit collision between two satellites occurred at 16:56 UTC, 10 February 2009, at 776 km altitude above Siberia. An American privately owned communication satellite, Iridium 33, and a Russian military satellite, Kosmos-2251, collided at a relative speed of 11.7 km/second. Both were destroyed, and a large amount of debris generated.

Q5: How did recent deliberate satellite intercepts affect the space debris environment?

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

On 11 January 2007, China conducted an anti-satellite test, intercepting their Feng Yun 1C satellite with a surface-launched medium-range missile. The collision occurred at an altitude of 862 km on a near-polar orbit, adding more than 2500 trackable objects to the US Space Surveillance Network catalogue, increasing its size by 25 percent.

This was by far the worst break-up event in space history - some 3.5 time worse compared to the worst previous event. Due to the high altitude of the collision event and the low ambient air density, the fragment orbits will have a long lifetime.

In the lists of high-risk fly-bys of catalogue objects with ESA's Envisat and ERS-2 satellites, an average of 30 percent of such events are caused by Feng Yun 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 58.5 degrees inclination. Due to the low altitude, and the correspondingly high air drag, most of the generated fragments re-entered within one revolution.

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

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

Distribution of catalogued objects in space - close-up of the LEO region

The Earth's atmosphere causes air drag that extracts orbital energy and leads to a contraction and final re-entry 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 toward higher altitudes, where the air density increases, causing higher air drag on space objects. As a consequence, space debris are periodically cleaned from the lower orbital regions (but these are subsequently re-filled by objects descending from higher orbits).

After sufficient exposure to air drag the orbit decays, and the object re-enters into the denser Earth atmosphere, where the air drag converts orbital energy into heat. This heating process is normally sufficient to destroy an object. Only for larger-size spacecraft or rocket bodies, approximately 20 to 40 percent of the mass, particularly high-melting steel or titanium alloys, may survive to ground impact.

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

Is is estimated that the total number of space debris objects in Earth orbit is on the order of:

• 20 000 - for sizes larger than 10 cm
• 600 000 - for sizes larger than 1 cm
• more than 300 million - for sizes larger than 1 mm

Any of these objects can cause harm to an operational spacecraft, where a collision with a 10-cm object would entail a catastrophic fragmentation, a 1-cm object will most likely disable a spacecraft and penetrate the ISS shields, and a 1-mm object could destroy sub-systems on board a spacecraft.

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. that are larger than 10 cm) result from about 200 in-orbit explosions in the course of space history.

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

In the most probable scenario, explosion 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 objects are ground to sub-critical 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 and 1400 km.

This run-away scenario is called the Kessler syndrome -it was first postulated by NASA's Don Kessler in 1978.

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

One must distinguish between debris-related risks on orbit and risks due to re-entries. 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 sub-system of an operational spacecraft.

Large space debris objects (e.g. spacecraft, rocket bodies or fragments thereof) that re-enter 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 in day-to-day life.

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

Giotto in 1985, with Whipple shield at bottom

The International Space Station has debris shields deployed around the manned 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.

For debris objects that are large enough to be contained in the US Space Surveillance catalogue, one can predict their orbits and compare them with the ISS orbit to determine close approaches. Assuming that the determination accuracy of both orbits is known, then a given fly-by distance can be translated into an in-orbit collision risk.

If such a risk exceeds the ISS threshold level, then the Space Station performs an avoidance manoeuvre. The ISS performed seven of these manoeuvres by mid-2008.

Q11: How do unmanned spacecraft protect themselves against space debris?

The most probable impacts are due to small space debris. One can efficiently increase the protection of an unmanned spacecraft by moving sensitive equipment away from the most probable impact direction, and/or by covering sensitive parts of the spacecraft with protective fabric layers (the 'bullet-proof vest' approach).

Such measures can significantly increase the survivability of a spacecraft against debris up to 1 mm in size.

Q11: What measures are already being take to avoid space debris - and which ones need to be taken in the future?

ESA Space Debris Mitigation guidelines, April 2008

Spacecraft operators are currently focusing their efforts on controlling the space debris environment. The ultimate goal is to prevent a collisional cascading process from setting in over the next few decades. Initial steps aim at reducing the generation of hazardous debris by avoiding in-orbit explosions or collisions with operational spacecraft, and by removing spacecraft from densely populated altitude regions at the end of their mission. These measures can stabilise the environment in the short term.

In the long term, the existing in-orbit mass, which fuels the collisional cascading process, must be removed. This can be most efficiently done by (costly) space debris remediation activities that actively remove old spacecraft and rocket stages, in which most of the mass is concentrated.

Long-term environment projections indicate that this is a mandatory step to maintain the space debris at a safe level for future space operations.

Q12: Are there any international agreements covering space debris mitigation?

There is an international consensus on the necessary space debris mitigation measures. The most prominent international body where such measures are discussed and elaborated is the Inter-Agency Space Debris Coordination Committee (IADC).

IADC has produced a set of mitigation guidelines, which also served as input to a set of seven Space Debris Mitigation Guidelines adopted by the United Nations Committee on the Peaceful Uses of Outer Space:

1. Limit debris release during normal operations.
2. Minimise the potential for break-ups during operational phases.
3. Limit the probability of accidental collisions.
4. Avoid intentional destruction and other harmful activities.
5. Minimise the potential for post-mission break-ups resulting from stored energy.
6. Limit the long-term presence of spacecraft and launch vehicle orbital stages in the low-Earth orbit region after the end of their mission.
7. Limit the long-term interference of spacecraft and launch vehicle orbital stages with the geosynchronous region after their end of mission.

What could be the next steps?

The agreed-upon space debris mitigation measures must be followed by space debris environment remediation measures. Such measures are presently being discussed at the International Academy of Astronautics (IAA) and in an ad-hoc international working group on the 'sustained use of outer space'.

More information

Dr Heiner Klinkrad
Head of Space Debris Office
ESA/ESOC
Robert-Bosch-Str. 5
64293 Darmstadt, Germany

Tel: +49-6151-90-2295
Fax: +49-6151-90-2625
email: Heiner.Klinkrad [@] esa.int