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INTRODUCTION
Space debris, also known as orbital 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 capability 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.
Most satellites, the International Space Station, the Space Shuttle, and the Hubble Space Telescope are all in Low Earth Orbit (commonly called "LEO"). This orbit is almost identical to our previous baseball orbiting example, except that it is high enough to miss all the mountains and also high enough that atmospheric drag won't bring it right back home again.
Every satellite, space probe and manned mission has the potential to create space debris. Any impact between two objects of sizeable mass can spall off shrapnel debris from the force of collision. Each piece of shrapnel 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 unusable.
3.1.1 Advantages of LEO:
Low Earth Orbit is used for things that we want to visit often with the Space Shuttle, like the Hubble Space Telescope and the International Space Station. This is convenient for installing new instruments, fixing things that are broken, and inspecting damage. It is also about the only way we can have people go up, do experiments, and return in a relatively short time.
3.1.2 Disadvantage of LEO:
The first is that there is still some atmospheric drag. Even though the amount of atmosphere is far too little to breath, there is enough to place a small amount of drag on the satellite or other object. As a result, over time these objects slow down and their orbits slowly decay. Simply put, the satellite or spacecraft slows down and this allows the influence of gravity to pull the object towards the Earth.
3.2 MEDIUM EARTH ORBIT ( MEO):
MEO (Medium Earth Orbit, stockings orbits). Are satellites moving on orbits close moderately of about 20000 km. Its use is intended for mobiles communications, navigation (GPS), measurements of space experiments and effective use of the electromagnetic spectrum.
A medium earth orbit (MEO) satellite is one with an orbit within the range from a few hundred miles to a few thousand miles above the earth's surface. Satellites of this type orbit higher than low earth orbit (LEO) satellites, but lower than geostationary satellites.
The orbital periods of MEO satellites range from about two to 12 hours. Some MEO satellites orbit in near perfect circles, and therefore have constant altitude and travel at a constant speed. Other MEO satellites revolve in elongated orbits. The perigee (lowest altitude) of an elliptical-orbit satellite is much less than apogee (greatest altitude). The orbital speed is much greater near perigee than near apogee. As seen from a point on the surface, a satellite in an elongated orbit crosses the sky in just a few minutes when it is near perigee, as compared to several hours when it is near apogee. Elliptical-orbit satellites are easiest to access near apogee, because the earth-based antenna orientation does not have to be changed often, and the satellite is above the horizon for a fairly long time.
A fleet of several MEO satellites, with orbits properly coordinated, can provide global wireless communication coverage. Because MEO satellites are closer to the earth than geostationary satellites, earth-based transmitters with relatively low power and modest-sized antennas can access the system.
Because MEO satellites orbit at higher altitudes than LEO satellites, the useful footprint (coverage area on the earth's surface) is greater for each satellite. Thus a global-coverage fleet of MEO satellites can have fewer members than a global-coverage fleet of LEO satellites.
3.3 GEOSTATIONARY ORBITS:
As the height of a satellite increases, so the time for the satellite to orbit increases. At a height of 35790 km, it takes 24 hours for the satellite to orbit. This type of orbit is known as a geosynchronous orbit, i.e. it is synchronized with the Earth.
One particular form of geosynchronous orbit is known as a geostationary orbit. In this type of orbit the satellite rotates in the same direction as the rotation of the Earth and has an approximate 24 hour period. This means that it revolves at the same angular velocity as the Earth and in the same direction and therefore remains in the same position relative to the Earth.
GEO satellites provide the kind of continuous monitoring necessary for intensive data analysis. By orbiting the equatorial plane of the Earth at a speed matching the Earth's rotation, these satellites can continuously stay above one position on the Earth's surface. Because they stay above a fixed spot on the surface, they provide a constant vigil for the atmospheric "triggers" for severe weather conditions such as tornadoes, flash floods, hail storms, and hurricanes. When these conditions develop these GEO satellites are able to monitor storm development and track their movements.
3.3.1 Applications of Geostationary Satellites:
Geostationary satellites have modernized and transformed worldwide communications, television broadcasting, and meteorological and weather forecasting. They also have a number of significant defense and intelligence applications.
3.4 HIGHLY ELLIPTICAL ORBIT (HEO):
HEO (Highly Elliptical Orbit, highly elliptical orbits). These satellites do not follow a circular orbit, but its orbit is elliptical. This implies that much greater distances reached at the point furthest from the orbit. They are often used to map the surface of the Earth, as they can detect a wide angle of Earth's surface. The perigee about 500 km and apogee of 50000 km, your orbit is tilted, the period varies from 8 to 24 hours, used in communications and space surveillance and very sensitive to the asymmetry of the Earth (the orbit is stabilized if i=63.435°).
Remember Kepler's second law: an object in orbit about Earth moves much faster when it is close to Earth than when it is farther away. Perigee is the closest point and apogee is the farthest (for Earth - for the Sun we say aphelion and perihelion). If the orbit is very elliptical, the satellite will spend most of its time near apogee (the furthest point in its orbit) where it moves very slowly. Thus it can be above home base most of the time, taking a break once each orbit to speed around the other side
APPROACHES TO ACTIVE DEBRIS REMOVAL
Various approaches to remove debris from space have been proposed, and some seem more technologically feasible than others. Techniques range from attaching tethers, solar sails, or solid rocket motors to debris objects, to active capture via nets followed by removal to other orbits. Of these techniques, one seems particularly feasible and is selected for use in the study presented herein.
4.1 ELECTRO-DYNAMIC TETHERS:
In general, a tether is a long cable (up to 100 km or longer) that connects two or more spacecraft or scientific packages. Tethers in space can be used for variety
of applications such as power generation, propulsion, remote atmosphere sensing, and momentum transfer for orbital maneuvers, microgravity experimentation, and artificial gravity generation. Electro-dynamic tethers are conducting wires that can be either insulated or bare, and that makes use of an ambient field to induce a voltage drop across its length.
Electro-dynamic tether moves in the Earth’s magnetic field and is surrounded by ionospheric plasma. The solar arrays generate an electric current that is driven through the long conductor. The magnetic field induces a Lorentz force on the conductor that is proportional to length, current, and local strength and direction of the magnetic field. Electrons are collected from the plasma near one end of the bare conductor, and are ejected by an electron emitter at the other end.
The use of Electro-Dynamic Tethers (EDTs) takes advantage of the effect of placing a conductive element in the Earth’s magnetic field. The object to be de-orbited is connected via a tether to a de-orbiting element, and both ends have a means of providing electrical contact to the ambient ionospheric plasma. The interaction of the conducting tether moving at orbital speeds induces current flow along the tether, causing a Lorentz force due to the interaction between the tether
and the Earth’s magnetic field; this causes an acceleration on the object to which the tether is attached. Figure shows a notional EDT system and the resulting force on the spacecraft to which it is attached.