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INTRODUCTION
Space debris, also known as rbital debris, space junk, and space waste, is the collection of defunct objects in orbit around Earth. This includes everything from spent rocket stages, old satellites, fragments from disintegration, erosion, and collisions. Since orbits overlap with new spacecraft, debris may collide with operational spacecraft.
Since the number of satellites in Earth orbit is steadily increasing, space debris, if left unchecked, will eventually pose a serious hazard to near-Earth space activities, and so effective measures to mitigate it are becoming urgent. Equipping new satellites with an end- of-life de-orbit and orbital lifetime reduction capabil- ity could be an effective future means of reducing the amount of debris by reducing the probability of collisions between objects, while using spacecraft to actively remove debris objects and to retrieve failed satellites are possible measures to address existing space debris.
Most space debris is less than 1cm (0.39in) including dust from solid rocket motors, surface degradation products such as paint flakes, and coolant released by nuclear power satellites. Impacts of these particles cause erosive damage similar to sand blasting.
The risk of satellites being hit by debris is increasing at an alarming rate. The solar panels present in the satellites are very delicate. So even very small size debris could be a cause for the malfunctioning of the panel, which in turn may interrupt the efficiency of the data transfer.
In communication systems the satellites usually are grouped into networks. If a satellite is being hit by big debris then there is every possibility of it losing its ability to function properly. This may break the communication network leading to large amount of financial and material loss for a certain amount of time until a replacement is made.
The most space debris created by a spacecraft's destruction was due to the upper stage of a Pegasus rocket launched in 1994. Its explosion in 1996 generated a cloud of some 300,000 fragments bigger than 4 mm and 700 among them were big enough to be catalogued. This explosion alone doubled the Hubble Space Telescope collision risk. To prove this we have found a ¾ inch hole in the Hubble.
Currently about 19,000 pieces of debris larger than 5 cm are tracked, with another 300,000 pieces smaller than 1 cm below 2000 km altitude. For comparison, ISS orbits in the 300–400 km range and both the 2009 collision and
2007 antisat test event occurred at between 800–900 km.
SPACE DEBRIS
2.1 DEFINITION:
Satellites have become an integral part of the human society but they unfortunately leave behind an undesirable by-product called space debris. Orbital space debris is any man-made object orbiting around earth which no longer serves a useful function. Non-functional spacecrafts, abandoned launch vehicle stages mission related objects and fragments from breakups are all considered orbital space debris. Since the last decade there are growing concerns that artificial orbital debris generated by space activities is degrading the near earth space environment. Recent statistical data shows that 70% of the catalogued objects in Earth orbit, larger than 1 cm size, are in low earth orbit (LEO). Figure 1 shows the distribution of LEO debris. The increasing threat posed by space debris to active satellite demands high attention. Collisions and explosions will proliferate the debris population drastically thereby degrading the space environment further.
The lifetime of all orbital debris depends on their size and altitude. In LEO, an object below 400 km will deorbit within a few months because of atmospheric drag and gravitational force, whereas, objects above 600 km may stay in the orbit for tens of years. As the LEO is a limited resource, it is very important to explore the various space debris mitigation techniques and suitable measures are to be taken to solve the space debris problem.
Three categories of space debris, depending on their size:
1. Category I (<1cm) - They can make significant damage to vulnerable parts of a satellite.
2. Category II (1-10cm) - They tend to seriously damage or destroy a satellite in a collision.
3. Category III (>10cm) – They may completely destroy a satellite in a collision and can be tracked easily.
SPACE DEBRIS EVENTS AND ITS ENVIRONMENT
There has been a steady growth of space debris since the launch of Sputnik in
1957, with jumps following two of the largest debris creating events in history: the
2007 Chinese anti-satellite (ASAT) test and the 2009 Iridium-Cosmos collision.
The first of these events occurred on January 11, 2007, when China intentionally destroyed its Fengyun-1C satellite while testing its newly developed ground-based ASAT system. It was the largest debris-creating event in history, producing at least 150,000 pieces of debris larger than one centimeter (NASA
2008, 3). The resulting debris has spread into nearpolar orbits ranging in altitude from 200 to 4,000 kilometers. Roughly 80 percent of this debris is expected to stay in orbit for at least the next one hundred years and threatens to impact operating satellites (CelesTrak 2009). The test illustrates how a single unilateral
action in space can create long-term implications for all space-faring nations and users of satellite services.
The second major space-debris creating event was the accidental collision between an active Iridium satellite and a defunct Russian military satellite on February 10, 2009. The collision created two debris clouds holding more than
200,000 pieces of debris larger than one centimeter at similar altitudes to those of the 2007 Chinese ASAT test (Johnson 2009b). It was the first time two intact satellites accidentally crashed in orbit, challenging the ―Big Sky Theory‖.
Currently, the highest spatial densities of space debris are in near-polar orbits with altitudes of 800 to 1,000 kilometers. These are known as ―critical orbits‖ because they are most likely to reach the point where the production rate of new debris owing to collisions exceeds that of natural removal resulting from atmospheric drag. They exist because several large fragmentation events have occurred in these regions, such as the two described above, and because debris lifetimes can last up to decades at these altitudes.
2.3 SPACE SURVEILLANCE NETWORK (SSN):
The United States Space Surveillance Network detects, tracks, catalogs and identifies artificial objects orbiting Earth, i.e. active/inactive satellites, spent rocket bodies, or fragmentation debris. The system is the responsibility of the Joint Functional Component Command for Space, part of the United States Strategic Command (USSTRATCOM). Space surveillance accomplishes the following:
1. Predict when and where a decaying space object will re-enter the Earth's atmosphere;
2. Prevent a returning space object, which to radar looks like a missile, from triggering a false alarm in missile-attack warning sensors of the U.S. and other countries;
3. Chart the present position of space objects and plot their anticipated orbital paths;
4. Detect new man-made objects in space;
5. Correctly map objects travelling in the earth's orbit;
6. Produce a running catalog of man-made space objects;
7. Determine which country owns a re-entering space object;
8. Inform NASA whether or not objects may interfere with satellites and
International Space Station orbits.
The following table shows the estimated amount of debris objects by their
size:
TYPES OF ORBITS
Since the launch of the first satellite in 1957 humans have been placing an increasing number of objects in orbit around the Earth. This trend has accelerated in recent years thanks to the increase in number of states which have the capability to launch satellites and the recognition of the many socioeconomic and national security benefits that can be derived from space. There are currently close to 1000 active satellites on orbit, operated by dozens of state and international organizations. More importantly, each satellite that is placed into orbit is accompanied by one or more pieces of non-functional objects, known as space debris. More than 20,000 pieces of space debris larger than 10 cm are regularly tracked in Earth orbit, and scientific research shows that there are roughly 500,000 additional pieces between 1 and 10 cm in size that are not regularly tracked. Although the average amount of space debris per cubic kilometer is small, it is concentrated in the regions of Earth orbit that are most heavily utilized…and thus poses a significant hazard to operational spacecraft.
The artificial satellites are classified for the size (large >1000 kg, medium size
500 –1000kg, small (minisatellites 100-500 kg, microsatellites 10-100 kg, nanosatellites 1-10 kg, picosatellites 0,1-1 kg and femtosatellites <100 g)); for the applications (exploration, communications, navigation and observation); for the character (military, civil and dual); and for the orbital height (LEO, MEO, HEO, GEO).
3.1 LOW EARTH ORBIT ( LEO )
LEO (Low Earth Orbit, which means low orbits). Orbiting the Earth at a distance between 500 and 2000 km of and its speed allows them to fly around the world in 2 hours approximately, with a velocity between 20000 and 25000 km/h. They are used to provide geological data on the movement of Earth's plates, remote sensing, spatial investigation, metereology, vigilance and the phone
industry satellite. Allow the determination of space debris and the utilization of the electromagnetic spectrum.