FAST FACTS ON SPACE DEBRIS

Space debris comes in two types - Natural and Artificial.
• Natural space debris consists of small pieces of cometary and asteroidal material called meteoroids. We see these as meteors when they travel through the Earth's atmosphere.
• Artificial space debris is any non-functional man-made object in space (usually orbiting the Earth).

• Satellites that have reached the end of their life
• Satellites and spacecraft that have failed
• Rocket stages that have launched satellites into space
• Nose cones, payload covers, shrouds, bolts and other launch hardware
• Solid propellant slag
• Space activity cast-aways (accidental or deliberate), eg wrenches, human waste
• Deterioration fragments, eg peeling paint
• Fragments from exploding batteries, fuel tanks (not totally empty), etc
• Fragments from collisions, both accidental and deliberate

At the start of the space age, on October the 4th, 1957, when the last stage of the rocket that launched Sputnik-1, remained in orbit.

The main worry about space debris is possible collision with active or functioning satellites or spacecraft.

Prior to 1957, many astronomers warned that natural space debris (ie meteoroids) would pose such a high collision risk that man's use of space might be severely compromised. Fortunately, this turned out to be not so. The collision probability from meteoroids is very low, although not negligible. The larger the spacecraft and the longer it remains in space, the greater the chance that it will suffer a collision.

At the present time, only a handful of dangerous collisions with artificial space debris are known to have occurred. However, the production of this type of debris is increasing at such a rate that it gives concern for the future.

• Small pieces of space debris (less than 1/10 mm) are prolific enough to cause erosion of optical surfaces. This is like sandblasting, and can ruin telescope mirrors, and decrease the efficiency of solar cells.
• Particles such as paint flakes (under 1 mm) can cause small craters in walls and windows. Almost 100 Space Shuttle windscreens have had to be replaced (as of 2008) due to pits caused by such impacts.

It is believed that any fragment of space debris larger than 1 centimetre will penetrate the walls of existing satellites/spacecraft.

It does seem slightly incredible that a paint flake can cause a crater in a windscreen. However, the reason is the velocity of space debris impacts. A typical impact occurs at a closing velocity of 10 km/sec or 36,000 kilometres per hour!

Such impacts are called hypervelocity impacts to indicate their extreme nature. At these velocities a piece of debris has more kinetic energy (energy due to its motion) than an equal mass of high explosive. An impact with a one kilogram object will thus cause more damage than the explosion of one kilogram of TNT.

• A 2 mm space debris fragment colliding at 10 km/s is like being hit with a cricket ball at 100 km/hour
• A 10 mm fragment at the same speed is like being hit by a large motorbike at 100 km/hour

Velocities of objects in space are determined by the laws of physics and the gravitational field of the body around which the objects orbit.

For the Earth, an object needs to be accelerated to a velocity of around 7 kilometres per second to stay in low Earth orbit (this is why it takes so much rocket fuel to launch a satellite).

Objects are in many different orbits around the Earth, some travelling in the opposite direction to others. With these high orbital velocities it is only natural that collisional velocities will be correspondingly high.

In the case of natural space objects (meteoroids) that orbit around the Sun, these travel at even greater velocities. To remain in its orbit around the Sun, the Earth has to move at 30 km/sec. Other objects that come near the Earth may have relative velocities ranging anywhere from 11 km/sec to 72 km/sec. The average collisional velocity between a meteoroid and a satellite is about 20 km/sec.

As of 2008 the number of pieces of artificial space debris in orbit around the Earth is estimated to be:

Size Range		Number of Fragments
1 - 10 mm		50,000,000
10 - 100 mm		300,000
> 100 mm		12,000

In June 2007 the United Nations General Assembly adopted a set of 7 orbital debris mitigation guidelines for member states (countries) to follow. However, these are legally non-binding under international law.

Most space-faring countries realise that space debris is a problem and have their own programs to try and reduce the creation of more space debris in future space activities. NASA has the most proactive program in this area.

The problem is that these programs do not reduce the amount of debris that is currently in orbit.

There are two ways that satellites and spacecraft may be protected from orbital space debris impacts:
• Computer programs can search for possible collisions between large space debris objects and high value spacecraft. When they detect the likelihood of such a collision, the spacecraft is manuevred (by small thruster rockets) out of harm's way. Such manuevres are now being carried out for large spacecraft such as the International Space Station and the Space Shuttle. However, these operations are expensive and can disturb delicate experiments. Also, not every satellite has the ability to maneuvre.

  Space tracking networks can only track space objects larger than about 100 mm. Since even a 10 mm object can severely damage a satellite it is obvious that collision avoidance will never be 100% effective.

• Debris shields can be designed to provide additional protection for a spacecraft. One obvious way is simply to increase the thickness of the vehicle walls. However, this adds a lot of mass to the craft and makes it a lot more expensive to launch it into space.

  Specially designed shields take advantage of the fact that two thin walls separated by a space are more resistant to debris penetration than a single thicker wall. This type of design is called a Whipple shield after the astronomer Fred Whipple who came up with the idea in the 1950's. The outer wall absorbs a lot of the debris energy so that the inner wall is not punctured. This type of shield and modifications to this design are currently installed on the International Space Station. However, once again this does not offer 100% protection.

Although the probability of collision with a piece of orbital space debris is currently very low, it will not remain so forever, even if no more debris is put into orbit.

The reason for this is that collisions between objects in space will slowly but surely increase the number of dangerous debris fragments. These in turn will produce more debris and so on in a chain reaction. In a few hundred years, the amount of debris will be so great that space operations will be severely limited.

In 1989 science fiction writer Frederick Pohl predicted a similar situation in his novel "Homegoing". Set many years in the future, mankind had generated so much space debris that he was confined to the surface of the Earth.

A number of solutions have been proposed to this problem:
• Objects in low altitude orbits (below about 500 km) are affected by atmospheric drag. This lowers their orbit until they re-enter the atmosphere and are thus naturally removed from orbit. The lower the orbit the faster it decays.
• Space "tugs" could be employed to "catch" large space debris objects and either lower their altitudes for natural decay, or bring them back to Earth.
• Giant "sponge" like objects could be deployed to "catch" or "soak up" small debris pieces. After a time, the sponge would be removed from orbit.
• Attach tails or tethers to large pieces of space debris to increase the drag they experience and lower their orbits.
• Use large ground based lasers to "push" small pieces of debris into lower orbits.

All of these schemes will be very expensive and use technology that is still to be developed. The laser scheme looks as if it might be the most promising. However, political problems might outweigh technical problems.

There is currently no agency involved in active removal of existing space debris from orbit.

However, an Australian firm based in Canberra (EOS) has been given a Federal Government research grant to investigate laser removal of space debris.

Most pieces of space debris will burn up as they enter the Earth's atmosphere. This ablation process starts around a height of 100 km and is usually complete by the time the object has descended to about 20 km.

Very heavy or refractory pieces (and occasionally very light pieces) may not burn up completely, and some part of the object may make it down to the ground. However, even these objects have lost most of their "space" velocity and hit the ground at no more than 100 metres per second.

It has been estimated that one piece of space debris may make it down to the Earth's surface almost every day. Most of these are very small and most will fall into the ocean or an unpopulated region of the Earth's surface. Very few are ever recovered, much to the disappointment of reentry scientists.

The probability of being hit by a piece of space debris is extremely low and a lot less than the probability of your being hit by a car while crossing a road.

Occasionally a large reentering derelict satellite may contain hazardous cargo. Two cases of radioactive debris have actually made it to the ground (these were from radioactive power sources). Such reentries are known well in advance (although the exact point of landfall can never be known precisely), and trained teams can be sent to secure the site and ensure no danger to any people in the area. Australia has a contingency plan set up to deal with such an event.

If a satellite has maneuvring capability and still has remaining fuel at the end of its life, it can be set up so that it reenters over a large area of ocean. NASA carries out a reentry survivability analysis for all large spacecraft, and if this indicates that a significant fraction of the object may survive the reentry process, they will attempt to control the reentry of such objects into a safe area.

If a large object is non-controllable, it may be possible to destroy the object through collision with an antisatellite missile launched from the ground. Such a collision produces many thousands of pieces of space debris, each of which will burn up when it reenters the atmosphere. However, as long as this is done at a sufficiently low altitude (below 300 km), the additional space debris produced will all decay from orbit (through atmospheric drag) within a month or two. However, the cost of such an operation is very expensive, and it could be argued whether this approach is worthwhile, considering the very low probability of damage or injury.

By "decay" we mean a reduction in orbital height due to atmospheric drag.

The decay lifetime of a space object depends on its altitude, the level of solar activity, and its mass to cross-sectional area.

Objects with a large mass to area ratio will remain in orbit longer as they are less affected by drag.

High solar activity increases the density of the atmosphere in low Earth orbits and reduces satellites' decay lifetimes.

On the average a satellite in an initial 300 km high orbit will have a decay lifetime of only a few months. One in a 500 km orbit has a lifetime of around 10 years, and one at 1000 km altitude will stay in orbit for thousands of years.

Some astronomers have been worried about orbital space debris proliferation because of two concerns:
• Increased numbers of medium to large scale space debris will cause light trails across astronomical images, decreasing their scientific and asthetic value. The trails also may confuse automatic computer analysis of large numbers of images.
• Extremely large numbers of very small objects (fractions of a millimetre) are expected to increase the background or ambient night-time sky glow, limiting the extent to which astronomers can see faint objects (eg very distant galaxies).

Australian Space Academy

For more detailed information on various aspects of this topic see the space debris section of this web site.

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