Space Debris

Space debris comes in two varieties: natural and artificial. Natural space debris consists of the meteoroids that Earth encounters on its orbital journey around the Sun. These meteoroids themselves are in orbits around the Sun, and they vary in size from a few meters in diameter to tiny specks of material smaller than a grain of sand. It is estimated that around 100 metric tonnes of such material rains down into Earth's atmosphere on a daily basis, although there is controversy about this estimate. Fortunately, most of it burns up harmlessly in the atmosphere, producing the fleeting flash of light that is associated with a shooting star. The vast majority of such meteoroids are at the tiny end of the size spectrum. However, they can still have significant energy, due to speeds on the order of a few tens of kilometers per second. Fortunately, the chances of an orbiting spacecraft encountering a large meteor (say a few centimeters across) are negligible, which is just as well as the consequences would be catastrophic. For example, the energy of a 2.5-cm (1-inch) rocky meteoroid traveling at 20 km/sec (12.4 miles/sec) is about the same as a 20-ton truck traveling at 110 km per hour (70 mph), and such a projectile would make short work of an orbiting satellite! But the probabilities are such that the main issue with natural debris is the peppering of the spacecraft's surfaces by high-velocity dust particles, producing a general degradation of thermal blanketed and painted surfaces.

The threat to satellites posed by artificial debris, on the other hand, is much greater, as once the debris size reaches a millimeter and above, the artificial debris objects in orbit begin to outnumber the natural debris objects. And this trend continues, so that the chances of an impact with a 10-cm (4-inch) chunk of artificial debris is much greater than the odds of encountering a meteoroid of similar size.

Artificial debris, as the name implies, are useless lumps of man-made material in space that have ended up in orbit as a by-product of launching spacecraft. Various agencies have proposed formal definitions of artificial space debris. The United Nations Committee on the Peaceful Uses of Outer Space states, "Space debris is defined as all man-made objects, including fragments and elements thereof, in Earth orbit or re-entering the atmosphere, that are non-functional.'' Space turns out to be just another arena of human activity that is being steadily polluted. However, the junk we leave in space is generally more dangerous than our terrestrial garbage as it is moving about at high speeds, posing a potentially lethal hazard to people and spacecraft in near-Earth orbits. Since the dawn of the space age in October 1957, when the first satellite Sputnik 1 was launched, there have been a total of 27,000 catalogued objects launched, with about 9000 catalogued objects still currently in orbit (at the time of this writing). Indeed, the upper stage of the launch vehicle that put Sputnik 1 into orbit also entered orbit, and became the first item of artificial space debris with a mass of about 4 metric tonnes. A catalogued object is any object large enough to be routinely tracked by a number of ground-based sensors to allow its orbit to be determined. Once the object's orbit is known, its details are placed in the U.S. Space Command catalogue, with its own unique catalogue number. The sensors used to do this job are mostly large radars, which once comprised the ballistic missile early warning system used during the Cold War. The sensitivity of these sensors is such that any object larger than about 10 cm (4 inches) in LEO and larger than around 1 m (3 feet) in geostationary Earth orbit (GEO) are tracked and catalogued.

Of the 9000 current catalogued objects, about 5% are operational spacecraft, but the majority have no useful function. Of this majority, many are large derelict upper stages of launch vehicles, which have accompanied their spacecraft payload into orbit. Others are smaller objects that are released into space during the process of launching spacecraft. About 40% of the total are fragments resulting from the accidental, explosive breakup of upper stages or spacecraft in orbit. In many cases the unintentional mixing of leftover propellant and oxidizer in a launcher upper stage has produced an explosive event that has torn the upper stage apart, producing several hundred new catalogued objects overnight!

A simple subtraction—27,000 minus 9000—gives us about 18,000 objects that have either flown away into interplanetary space or have fallen from orbit and reentered Earth's atmosphere. Most of these objects have indeed come back through the atmosphere, burning up harmlessly, although there are some famous instances when large pieces of spacecraft have reached the ground (such as the unwelcome arrival of parts of a nuclear reactor from Cosmos 954 over Canada in 1978, and Skylab's reentry over Australia in 1979). Figure 6.9 shows the number of catalogued objects in LEO over time from 1957 to 2001. There are about another 1000 or so objects in other orbital regions, making up our total number of approximately 9000. Figure 6.9 shows an almost continuously rising trend. However, interestingly we can see that this trend is interrupted, particularly in the early 1980s, the early 1990s, and around the year 2000, when the curve dips or flattens out. These periods correspond to times when the Sun's activity was at a maximum, producing atmospheric heating, with a resulting increase in atmospheric density (see The Effects of Earth's Atmosphere, above). This in turn produced a rise in drag on orbiting satellites, increasing the numbers of reentries into the atmosphere. So we can say that solar maximum is a time to put on the hard hats!

The distribution of catalogued objects in LEO with orbit height is shown in Figure 6.10. We see that there are few objects at low altitude, say, less than 500 km. This is because the drag perturbations on debris are relatively large (due to the higher atmospheric density at low altitudes), sweeping the debris

Figure 6.9: Numbers of objects greater than 10 cm (4 inches) in size in low Earth orbit (LEO) over time, from Sputnik 1 (October 1957) to 2001. (Figure compiled from data supplied courtesy of Dr. Hugh Lewis, University of Southampton, UK.)
Figure 6.10: The current distribution of objects in LEO, greater than 10 cm (4 inches) in size, plotted against height. (Figure compiled from data supplied courtesy of Dr. Hugh Lewis, University of Southampton, UK.)

into Earth's atmosphere. Also, peaks in debris density occur in orbital regions where there are lots of spacecraft and where the atmospheric density is too low for drag sweeping to be effective in removing objects from orbit. A good example of this are the peaks in debris density at altitudes of around 800 to 1000 km (500 to 620 miles), where there are a large number of Earth observation satellites, and where the atmosphere is too tenuous for drag to be effective in removing the resulting junk caused by operating these spacecraft. Figure 6.10 also gives us an idea of the average spacing between large (greater than about 10 cm in size) objects currently in LEO.

The vertical axis suggests that the peak in debris spatial density is around 2 x 10"8 objects per cubic kilometer. A simple calculation reveals the significance of this obscure statistic. If the objects were distributed evenly, then each one would have its own volume of space in which to wander around, equivalent to a cube about 370 km (230 miles) across. On the one hand, it seems like a huge amount of space for the chunk of debris to get lost in. A cube of this size contains about 50 million cubic kilometers! On the other hand, traveling at typical LEO speed, the object can traverse this space in less than a minute. The bottom line is that space debris is not evenly spaced out in orbit, and debris does come together now and again. However, the low spatial density tells us that this does not happen often; indeed, at the time of this writing, only three collisions between catalogued objects have been verified. The first of these occurred in 1996 between a small French satellite called Cerise and a fragment of an old Ariane launch vehicle.

People who operate shiny, expensive spacecraft in LEO are understandably protective of their investment in orbit, and debris is an obvious threat to their spacecraft's mission. Clearly, if the spacecraft were to be hit by a large chunk of space debris, the impact would be catastrophic, given that relative speeds in orbit are typically on the order of 10 km/sec (6.2 mile/sec). Remember, however, that all of the large objects in orbit are catalogued and their orbits are known. So, using computer simulation, the spacecraft's operators are able to keep an eye on all 8000 or so LEO objects in the catalogue to see if any of them are predicted to make a close approach to their valued asset. This is done routinely in operations rooms around the world. If an uncomfortably close encounter is predicted, the spacecraft's orbit is changed to reduce the threat. This type of maneuver has been performed many times by manned shuttles in orbit, as well as by numerous unmanned spacecraft in LEO.

In addition to these large objects in orbit, there are huge numbers of smaller debris objects in near-earth orbits. At the small end of the size spectrum, it is estimated that there are tens of millions of objects in the 1mm to 1-cm size range. Many of these result from explosive breakups in orbit, but they also have more benign origins, associated with the degradation of spacecraft surfaces exposed to the space environment. The effects of solar ultraviolet radiation and atomic oxygen erosion, combined with repeated thermal cycling on each orbit (due to the spacecraft being exposed to the heat of direct sunlight followed by extreme cold when in Earth's shadow), causes paints and thermal blankets to become brittle. Over time, flakes of material peel off, leaving a wake of small debris particles around the spacecraft as it orbits the Earth. This does not pose any threat to the spacecraft itself, other than the gradual process of general deterioration. But if these small particles are encountered by spacecraft in other orbits, they can impinge on them at high speeds, typically 10 km/sec (6.2 miles/sec). One well-known consequence of this is the frequent need to replace space shuttle windows, due to damage caused by paint flake cratering.

The space debris that poses perhaps the greatest threat to spacecraft is the intermediate-sized objects in the range of 1 to 10 cm, of which it is estimated that there are a few hundreds of thousands in orbit. This is because they are generally too small for their orbits to be determined (and thus they are not in the catalogue), but they are large enough to deliver a lethal blow to an operational spacecraft. Obviously, if you cannot predict when they are coming, then you cannot perform orbit change maneuvers to avoid potential impacts.

Despite the large number of objects in this size range, there is an awful lot of space in LEO, so thankfully the chances of an impact with this size of object is still very small. For objects smaller than about 2 cm (about 1 inch) in size, however, it is possible to adopt a different protection strategy— shielding. This has been used extensively to protect the International Space Station (ISS) from debris impact, and may be used more widely in future unmanned spacecraft in orbital regions where the debris impact threat is considered significant. How can a shield be devised to stop a 2-cm chunk of aluminium traveling at speeds that make a high-velocity bullet look harmless? The idea for such a shield was first proposed by Fred Whipple in 1946, and the simplest form of the Whipple shield is shown in Figure 6.11. The construction is straightforward; a bumper shield plate is fixed to the

Figure 6.11: Encounter with a debris projectile (a) without a Whipple shield and (b) with a shield.

spacecraft's surface, such that there is a gap between them. The idea is that an impacting debris object is disrupted and vaporized by its encounter with the outer bumper plate, producing a spray of smaller debris particles that impact on the inner spacecraft surface. Since the energy of this spray is spread over a wider area of the spacecraft's surface, the chances of penetration are reduced. The effectiveness of the shield can be improved by introducing multiple bumper shield plates, and playing tunes on the separation distances between them. Generally, however, spacecraft designers would prefer not to implement such shields, as they introduce complexity and mass into the design, with the potential to increase mission costs.

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