Space debris

Mon 18 November 2013

I was recently asked to speak on the radio about space debris. This was prompted by the release of the film Gravity (1), and the re-entry of the GOCE satellite (2).

You can listen to the interview here (Copyright: BBC, broadcast on Radio Scotland, 16 November 2013). At the beginning of the audio clip is the tail-end of a report by my friend Pauline McLean who, unlike me, is a professional. I include it not only because she has a lovely voice, but there's a great wee clip from Mrs Browns' Boys.

Below are some more in-depths thoughts on the subject of space debris, drawing heavily on many excellent sources of information which are referenced below.

What is it?

Space was already fairly well populated with debris by the time the human race put Sputnik into orbit in 1957. This debris was natural, mostly being left over from the formation of the solar system 4.6 billion years ago, with some pieces coming from subsequent collisions and the detritus left behind by comets. This natural debris takes the form of small grains of dust, pebbles, boulders, some being rocky, others containing ice too. The smaller objects are far more numerous than the larger ones.

When one of these objects happens to encounter the Earth, it will likely burn up in the atmosphere due to air friction, leaving only a fleeting streak of light for the human eye to catch. These meteors, also known as shooting stars, are quite frequent: stand under a decent night sky and look up and you'll see one within an hour, unless you're unlucky. Larger objects may create a bright fireball in the sky and fragments of it may reach the ground; this is called a meteorite. Very large objects have the potential to cause serious damage and even loss of life, but that's a subject for another blog post.

In recent decades, natural space debris is being joined by debris that we've put there. Most of our satellites are unlikely to collide with each other because they are of value to us and we have been careful to place them in distinct orbits. So they present no problem. However, in launching these, we have discarded stages of rockets, released smaller objects when stages separate and there's a myriad of tiny objects such as paint flecks, droplets of coolant and ice crystals now orbiting the Earth. Accidents and explosions have occurred, contributing further to the orbital debris.

The larger objects, down to about 5 cm in size, can be tracked and so avoided. However, smaller objects are not only more numerous and difficult to track, but they still present a danger because of the large speeds involved.

A little physics helps make the point. The kinetic energy of an object is the energy associated with motion, and the greater the energy, the greater the damage caused by an impact. The kinetic energy is proportional to mass and the square of the speed: double the mass and you double the kinetic energy; double the speed and the energy quadruples. For this reason, even a fleck of paint can cause serious damage to a satellite.

The Kessler effect

But there's another process which amplifies the space debris problem. When two objects collide they fragment into many more smaller ones. Many objects in low Earth orbit will, in weeks, months or years, experience atmospheric drag and suffer orbital decay which will cause them to burn up, or, if large enough, fall to Earth. But, if the number of objects in low earth orbit exceeds a certain critical value, the frequency of collisions will mean that small objects will be created faster than they are removed. In other words, even if humans don't launch anything else into orbit, the number of objects will continue to increase for decades to come. This is called the Kessler effect. It is worrisome that many recent estimates tell us that we are already at or above the critical number.

A low Earth orbit (LEO) is any orbit with an altitude between 160 km and 2000 km. These include polar orbits which, as the name suggests, take the satellite over both poles. It is LEOs where the space debris problem is now acute. The geostationary orbits (GEOs), in which satellites can remain stationed above a particular point on the Earth's surface, are at a much higher altitude of 35,786 km. Space debris is less of an issue for GEOs just now, but since there is virtually no atmospheric drag at that high altitude, space debris can take centuries or millenia to be naturally removed, and so it's important that LEO space debris mess is not repeated for GEOs.

Notable events

On January 11 2007, the Chinese intentionally destroyed a weather satellite called Fengyun 1C to test an anti-satellite weapon. This immediately created several thousand new pieces of space debris. A little over six years later, a piece of Fengyun 1C hit and severely damaged the Russian BLITS satellite, leaving it spinning and in two or more pieces. (3)

In 2009 the still-operational Iridium 33 satellite collided with the defunct Russian Kosmos 2251 satellite at a speed of approximately 42,000 km/h, which is about 12 km/s. This collision created more than a thousand pieces of 10 cm or larger according NASA estimates. (4)

What can be done?

The US Space Surveillance Network currently tracks about 500,000 pieces of space debris, and of these, more than 20,000 are 5-10 cm or larger in size. There are of course many more smaller ones that cannot be tracked, but that still present a considerable danger. Much of the space debris in orbit can be traced back to the Fengyun 1C satellite and the 2009 satellite collision. (5)

If a tracked piece of debris is predicted to approach the International Space Station (ISS) within less than 50 km, then a debris avoidance maneuver will be performed. Source: (6) Up to the end of 2012, 15 ISS debris avoidance maneuvers were performed. (7)

The other part of the solution is of course to clear up the debris before the worst predictions of the Kessler effect becomes reality. Sending "janitor" spacecraft into orbit to capture debris and bring in to the atmosphere is feasible, but costly. Another alternative, more attractive for smaller objects, is to fire a powerful laser at an object, slightly reduce its speed, which will cause it to lower its orbit with a resulting increase in atmospheric drag hurrying its re-entry into the atmosphere. (8). Even more fanciful still is to use a giant net! (9).