Space debris

distribution of space debris in low earth orbit

Space debris, also called orbital debris, space junk or space waste are man-made objects in Earth orbit that no longer serve any useful purpose. They include defunct satellites, spent rocket stages, released vehicle components, slag from solid rocket boosters, coolant droplets from nuclear reactors, paint flakes, and other objects .

Space debris is seen as a risk to both manned and unmanned spaceflight programs. Objects in orbit travel at very high velocities, and release large amounts of energy when they collide. Large debris (1 cm or more) can severely damage or destroy a satellite on impact.

History and creation of space debris

The history of space debris very closely follows the history of human spaceflight. The oldest piece of debris still in orbit is Vanguard 1, which was launched in 1958. The satellite functioned from March through June of that year before ceasing useful operation, and it is likely to remain in orbit for 200 more years before finally re-entering the atmosphere. Since then hundreds of artificial satellites have been launched into orbit, and many of them remain even after their usefulness has ended.

Defunct satellites aren't the only pieces of debris in orbit, however. Spacecraft from Earth are almost always launched on multiple-stage rockets, and the upper stages of these rockets usually separate from the payload after they have already achieved low Earth orbit (LEO). Vehicles also commonly produce other debris during the deployment phase, including explosive bolts and other components only needed during the launch phase. All of these can remain in orbit, contributing to the debris population.

One of the biggest source of dangerous space debris is explosions. Upper stages of rockets typically contain leftover fuel, and electric shorts or impacts can cause these to explode, creating more debris fragments. About 100 tons of fragments generated during approximately 200 such events are still in orbit.

Chinese anti-satellite weapon test

The largest space debris incident in history was the Chinese anti-satellite weapon (ASAT) test on January 11, 2007. The event created more than 1900 pieces (updated 7/4/07) of trackable debris (approximately golf ball size or larger), estimates of over 1 million pieces 1mm or larger and over 35,000 pieces 1cm or larger. The debris event is more significant than previous ASAT tests in that the debris field is in a higher orbital plane resulting in deorbit times of 35 years and greater. In June, 2007, NASA's Terra environmental spacecraft was the first to be moved in order to prevent impacts.

Russian booster stage explosion

An event of similar magnitude occurred on February 19, 2007, when a Russian Briz-M booster stage exploded in orbit over Australia. The booster had been launched on February 28, 2006, carrying an Arabsat-4A communication satellite but malfunctioned before it could use all of its fuel. The explosion was captured on film by several astronomers, but due to the path of the orbit the debris cloud has been hard to quantify using radar. Although similar in magnitude, the debris field is at a lower altitude than the Chinese ASAT test and much debris re-enters the atmosphere in a relatively short time. As of February 21, 2007, over 1,000 fragments had been identified.

Other debris creation events

Another significant breakup event occurred on 14 February 2007 and was recorded by Celes Trak. This makes three observed events in the first two months of 2007. In 2006, the most breakups occurred since 1993 with eight breakups.

Unusual debris sources

Manned spaceflight is a source of some unique pieces of debris. Astronauts often lose objects during spacewalks, and according to Edward Tufte's book Envisioning Information, space debris objects have included a glove lost by astronaut Ed White on the first American space-walk, a camera Michael Collins lost near the spacecraft Gemini 10, garbage bags, a wrench and a toothbrush. Sunita Williams of STS-116 also lost a camera during an EVA. Most of those unusual objects have re-entered the atmosphere of the Earth within weeks due to the orbits where they were released and their small sizes. Things like these are not major contributors to the space debris environment. Cosmonauts on Russian space stations even disposed of their garbage by ejecting it into space before the problems posed by space debris were realized.

Dangers

Space debris pose a significant problem to both manned and unmanned spacecraft. Objects in orbit around earth travel at very high speeds, and typically collide at relative velocities around 10 kilometers per second. At these speeds even relatively small objects can release large amounts of energy upon impact.

Objects of different sizes set up different challenges for mission planners. Large objects, about 10 cm or larger in diameter, have the largest potential to cause damage to spacecraft, but are also big enough to be easily tracked and are therefore easier to avoid. Very small objects, less than 1 cm in size, cause the least damage individually, but are also by far the most common particles, as they are virtually omnipresent in low-Earth orbits. The biggest danger they present is in causing erosive damage, somewhat like a sandblaster.

Kessler Syndrome

It is also theorized that a sufficient amount of debris in orbit will lead to a chain reaction called the Kessler Syndrome.

The Kessler Syndrome is a scenario, proposed by NASA consultant Donald J. Kessler, in which the volume of space debris in Low Earth Orbit is so high that objects in orbit are frequently struck by debris, creating even more debris and a greater risk of further impacts. The implication of this scenario is that the escalating amount of debris in orbit could eventually render space exploration, and even the use of satellites, too prone to loss to be feasible for many generations.

The Kessler Syndrome is especially insidious because of the "domino effect." Any impact between two objects of sizable mass will create additional shrapnel debris from the force of collision. Each piece of shrapnel now has the potential to cause further damage, creating even more space debris. With a large enough collision (such as one between a space station and a defunct satellite), the amount of cascading debris could be enough to render Low Earth Orbit essentially impassable.

Every satellite, space probe and manned mission has the potential to create space debris. As the number of satellites in orbit grow and old satellites become obsolete, the risk of a cascading Kessler Syndrome becomes greater.

The Kessler Syndrome presents a unique problem to human space travel. Space debris is very difficult to deal with directly, as the small size and high velocities of most debris would make retrieval and disposal impractically difficult. Given thousands of years, most debris in Low Earth Orbit would eventually succumb to air resistance in the rarefied atmosphere and plunge to the Earth. If magnetically susceptible, the debris could fall in a few decades due to the drag of the Earth's magnetic field.

To minimize the chances of damage to other vehicles, designers of a new vehicle or satellite are frequently required to demonstrate that they can be safely disposed of at the end of its life, for example by use of a controlled atmospheric reentry system or a boost into a graveyard orbit.

Fortunately at the most commonly used Low Earth Orbits residual air drag helps keep the zones clear. Altitudes under 300 miles will be swept clear in a matter of months.

References

1. D.J. Kessler (1991). "Collisional Cascading: The Limits of Population Growth in Low Earth Orbit". Advances in Space Research 11: 63.

Significant debris impact events

The first verified collision with catalogued space debris occurred in 1996, tearing off a boom from the French satellite Cerise.

Lottie Williams is on record as the first and only (as of September 2006) person ever to be hit by space debris created by humans. While walking in a park in Tulsa, Oklahoma, on January 22, 1997 at 3:30 am, she noticed a light in the sky that she said looked like a meteor. Minutes later, Williams was hit in the shoulder by a 6-inch blackened metal object that was later confirmed to be part of the fuel tank of a Delta II rocket which had launched a U.S. Air Force satellite in 1996. Ms. Williams was not injured.

Space debris avoidance maneuvers

The first official Space Shuttle collision avoidance maneuver was during STS-48 in September 1991. A 7-second reaction control system burn was performed to avoid debris from Kosmos satellite 955.

Mitigation

In order to mitigate the generation of additional space debris, a number of measures have been proposed: The passivation of spent upper stages by the release of residual fuels is aimed at decreasing the risk of on-orbit explosions that could generate thousands of additional debris objects.

Taking satellites out of orbit at the end of their operational life would also be an effective mitigation measure. This could be facilitated with a "terminator tether," an electrodynamic tether that is rolled out, and slows down the spacecraft. In cases when a direct (and controlled) de-orbit would require too much fuel the satellite can also be brought to an orbit where atmospheric drag would cause it to de-orbit after some years. Such a maneuver was successfully performed with the French Spot-1 satellite at the end of 2003. It will re-enter in approximately 15 years.

In orbital altitudes where it would not be economically feasible to de-orbit a satellite, like in the geostationary ring they are brought to a graveyard orbit where no operational satellites are present.

Proposals have been made for ways to "sweep" space debris back into Earth's atmosphere, including automated tugs, laser brooms to vaporize or nudge particles into rapidly-decaying orbits, or huge aerogel blobs to absorb impacting junk and eventually fall out of orbit with them trapped inside. However, currently most effort is being devoted to prevention of collisions by keeping track of larger debris, and prevention of more debris.

Other ideas include the gathering of larger objects into an orbital "junk yard", where they could be used as resources should future needs arise, while keeping them out of the way.

Some spacecraft, like the International Space Station, are now armored to mitigate damage with this hazard. Astronauts on EVAs are also vulnerable.

Tracking, Measuring and Modelling

distribution of space debris in geostationary orbit

Because the most useful satellite orbits are LEO (low-earth orbit) and GEO (geostationary orbit) these regions of space now have the greatest concentration of debris.

The U.S. Strategic Command is maintaining a catalogue currently containing about 10,000 objects, also to prevent misinterpretation as hostile missiles. Observation data gathered by a number of ground based radar facilities and telescopes as well as by a space based telescope is used to maintain this catalogue. Nevertheless, the majority of debris objects remain unobserved. There are more than 600,000 objects larger than 1 cm in orbit (according to the ESA Meteoroid and Space Debris Terrestrial Environment Reference, the MASTER-2005 model).

Other sources of knowledge on the actual space debris environment include measurement campaigns by the ESA Space Debris Telescope, TIRA, Goldstone radar, Haystack radar, and the Cobra Dane phased array radar. The data gathered during these campaigns is used to validate models of the debris environment like ESA-MASTER. Such models are the only means of assessing the impact risk caused by space debris as only larger objects can be regularly tracked.

Returned space debris hardware is also a valuable source of information on the (submillimetre) space debris environment. The LDEF satellite deployed by STS-41-C Challenger and retrieved by STS-32 Columbia spent 68 months in orbit. The close examination of its surfaces allowed the analysis of the directional distribution and the composition of debris flux. The EURECA satellite deployed by STS-46 Atlantis in 1992 and retrieved by STS-57 Endeavour in 1993 could provide additional insight.

The solar arrays of the Hubble Space Telescope returned during missions STS-61 Endeavour and STS-109 Columbia are an important source of information on the debris environment. The impact craters found on the surface were counted and classified by ESA to provide another means for validating debris environment models.

External links

• NASA Orbital Debris Program Office
• Space-Track - The Source for Space Surveillance Data
• "What is Orbital Debris?" from the Center for Orbital and Reentry Debris Studies at The Aerospace Corporation
• Intro to mathematical modeling of space debris flux
• Leonard David, "The Clutter Above," Bulletin of the Atomic Scientists, July/August 2005.
• SOCRATES: A free daily service predicting close encounters on orbit between satellites and the thousands of rocket bodies and other pieces of debris orbiting Earth.
• A summary of current space debris by type and orbit.