Near Space Environment

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Near Space Environment

█ CECILIA COLOME

The near-Earth environment is far from empty. In addition to the natural meteoroid material, solar wind plasma, and cosmic rays, the space above Earth's atmosphere contains several hundreds of satellites and thousands of tons of space debris. Space debris orbiting the Earth consists of mostly non-functional man-made objects, many of which are fragments of satellites or rockets and residues from launches. On February 1, 2003, the space shuttle Columbia tragically ended its 16-day mission during its re-entry into Earth's atmosphere. One of the first questions that the scientific community investigated was whether Columbia had been struck by space debris.

There are about 600 active satellites orbiting the Earth. They are used for communication, remote sensing for weather, land surveys, national security, navigation, and support for scientific missions. These satellites are located in only a few orbital regions, mostly in the semisynchronous orbit, or the low Earth orbit (LEO) and the geosynchronous orbit (GEO). It is also in these regions where most of the space debris is located. The space debris around Earth not only poses risks to active satellites, but also to space missions and astronomical observations. Space debris is a major source of light pollution in wide-field imaging of astronomical objects.

One of the main problems associated with space debris is its duration, or lifetime. In contrast to meteoroids, which either burn in Earth's atmosphere or cross the near-Earth region to continue their travel through the solar system, space debris potentially can remain in orbit for millions of years. There are three issues of crucial importance in regards to space debris, namely how it can be cleaned up, how to avoid debris collisions with active spacecraft, and how to minimize the generation of more debris.

As early as the 1970s NASA began to investigate the feasibility of forcing space junk into the Earth's atmosphere, where remnants not destroyed by re-entry would fall to the ground. The central idea of this project, known as Orion, was to focus a high-powered laser beam into individual debris fragments, causing their outer layers to vaporize, and creating a thrust that would deflect their orbits. The research for the Orion project demonstrated that the clean-up would be extremely expensive, mainly because of the high power required by the laser and the high cost of the adaptive optics necessary to focus energy into small objects at great distances from the ground. This idea still might serve for the future, when technology may be able to equip satellites with the high-powered lasers and enable them to "sweep" space debris into the Earth's atmosphere.

There are two major risks from objects reentering the Earth's atmosphere. First, if they are too large to evaporate completely during re-entry, they could cause damage on the ground. Second, if the falling debris contains radioactive material, the atmosphere or ground could be contaminated. Currently, roughly 50 nuclear devices orbit the Earth, carrying a total of 1,300 kg (1.3 tons) of radioactive material. There have been at least two confirmed nuclear mishaps from space. In 1964, the orbit of an American satellite decayed into the Earth's atmosphere, releasing radioactive radiation over the Indian Ocean, and in 1978, a Soviet satellite lost its orbit and crashed in northern Canada, dispersing more than 30 kg (66.1 pounds) of enriched uranium. Nuclear reactors were very popular in space because they provide large energy sources in very small and lightweight volumes. All theses devices were built and launched prior to 1988, and since then, nuclear reactors have not been incorporated into satellites.

As the density of the Earth's atmosphere decreases with altitude, objects in LEO experience more air friction than objects at higher altitudes. Over time, the orbits of non-functional objects decay to lower altitudes. The re-entry of large objects, with cross-sections of one square meter or more is significant; about one object re-enters daily, and some of them have survived the heat produced by re-entry air friction. Two notorious examples of debris re-entry are from the tanks belonging to a Delta rocket. In 1997, one of the tanks landed near a house, not far from a busy highway in Texas; a second tank from the Delta rocket landed in South Africa near Cape Town in 2000. To this date, there has been only one reported incident of a human being struck by space debris: in 1997, a woman in Tulsa, Oklahoma, was hit on the shoulder by a 6-inch piece of metal, and fortunately, it did not lead to any serious injury.

Both meteoroids and space debris pose a serious hazard to spacecraft and astronauts. The vast majority of meteoroids are small dust particles with typical sizes of tenths of a millimeter. Although they are small, due to their high speeds, up to 70 km/s, they represent a hazard in space. Current satellites are well shielded to withstand meteoroid impacts. Nevertheless, meteoroid collisions on spacecraft can be devastating for their operations. During a collision of a meteoroid, it evaporates partially or completely, and it may cause the evaporation of a small area of the external material on the spacecraft. The result is a plasma of electrons and ions. These particles are capable of inducing high electric currents on spacecraft, interfering with their basic control operations.

The dimensions of space debris cover a wide size range, from tiny dust particles to large non-functional rockets. Some of the main sources of space debris have been explosions of rockets. The collision avoidance with the larger (>10 cm, or >4 in) debris population is performed by tracking methods from the ground, either by radar or by optical measurements. Meteoroids are generally small, too small to be tracked reliably. Their potential collisions with spacecraft are taken into account in the shield design of spacecraft, and because they cannot be tracked, their collisions with active spacecraft can be treated only statistically. Ground-based radars are mostly used to monitor the space debris in LEO, while optical observations are used to track objects in GEO. Both methods have their own advantages and limitations. Radar measurements are not affected by weather nor day-night conditions, but because of their narrow bandwidths they cannot detect small objects at great distances. Optical tracking of space debris through telescopes requires the objects to be illuminated by sunlight against a dark sky. In LEO, objects can be observed for only a few hours, but for objects in GEO, this method can be used during an entire night. Several countries are currently using radar and optical methods for tracking and making catalogues of space debris. Among them are England, France, Germany, Japan, Russia, Spain, and the United States.

The Haystack Auxiliary and Goldstone radars in the United States have provided ample data on the debris population with sizes smaller than 30 cm. The international collective effort has provided almost 9,000 catalogued large (>10 cm, or > 4 in) objects. These catalogues are essential to avoid catastrophic collisions with active spacecraft. Three catalogues are updated regularly, one by the United States Space Command Satellite, one by the Russian Space Surveillance, and the other by the Information System Characterizating Objects in Space of the European Space Agency (ESA).

Explosions of spacecraft are considered to be the main source of large fragments of space debris. Part of the Pegasus rocket exploded in 1996, two years after its launch, creating 700 fragments large enough to be catalogued. The explosion of the Chinese Long March 4 rocket created more than 300 large fragments. At least three reported maneuvers of satellites have been performed in order to avoid collisions with space debris: both the European Remote Sensing Satellite (ERS-1) and the Satellite pourl'observation de la Terre (SPOT–2) in 1997, and the Inter national Space Station (ISS) in 1999. A severe space accident occurred in 1996 when the French CERISE spacecraft was hit by a catalogued object, thought to be a fragment of the Ariane rocket's upper stage.

In order to gain better data on the space debris population, in 1984 the space shuttle Challenger deployed NASA's Long Duration Exposure Facility (LDEF). Its retrieval was scheduled for 1986, but due to the loss of the space shuttle Challenger it was postponed until 1990, when it was retrieved by Columbia. The LDEF orbited the Earth for almost 6 years, providing data on the near-Earth space environment, and returned to Earth covered by more than 30,000 craters. The LDEF was a large cylinder weighing more than 20,000 lbs, one of the heaviest objects deployed by any space shuttle. It contained 86 trays on its periphery where 57 experiments were carried out. These experiments were designed by NASA, the Department of Defense, universities and private companies, and were aimed for meteoroid and space debris studies, radiation surveys, and infrared video surveys. A major challenge in the study of the trays on LDEF was to distinguish between craters created by meteoroid impacts and those due to collisions with space debris, was accomplished by extensive chemical analysis. The data collected by LDEF had a major impact on the design of spacecraft after 1990. Most of the design changes involved the substitution of materials that deteriorate in space, such as Teflon, Kapton, Dracon, Mylar, and polymeric films. For example, the design of the radiator of the International Space Station was changed from Teflon to a ceramic paint. In general, ceramic materials are better survivors of erosion due to bombardment of atomic oxygen and UV radiation.

One peculiar kind of potential space debris are tethers. Tethers are chains or ropes that connect astronauts to their spacecraft while working in space. They are also used as links between components on spacecraft. Tethers are a potential source of debris if they are discarded from spacecraft, but they also might help in the reduction of space debris. As a tether crosses the Earth's magnetic field lines, it becomes an electric generator. This energy source can be used not only to deploy spacecraft, but also to create the necessary thrust for lowering the altitude of non-functional objects. NASA has developed a unique experiment for future uses of tethers in space, The Propulsive Small Expendable Deployer System (ProSEDS), a thin wire 5 km long connected to a 10 km non-conductive rope. ProSEDS was scheduled for launch in 2003, and remains a high-priority for launch payload.

All effective clean-up procedures of space debris are still in experimental phases, although great advances have been made in slowing the increase of space debris with time. Spacecraft are now covered with longer lasting paints and their protective covers are much less affected by erosion by small meteoroids, particle bombardment, and UV radiation. Newer satellites are becoming increasingly smaller. This also reduces the probability of more generation of space debris, because the smaller the object is, the lower the probability of experiencing collisions.