Space Traffic Control: 2nd June 2017
Dr Stuart Eves (SSTL)
Space now forms a critical part of national and international infrastructure, with satellites providing vital weather information, communications and navigational data. This infrastructure is threatened by two main hazards: space weather (i.e. solar activity) and space debris in low Earth orbit (LEO).
When the Sun is more active it emits more UV and X-rays, which heat the Earth’s atmosphere and cause it to expand. Satellites in LEO may require boosting to correct their orbits, but the increased drag can help to reduce the amount of space debris by causing it to re-enter the atmosphere and burn up. Coronal mass ejections (CME) affect spacecraft directly and also disrupt the ionosphere, causing difficulties in communication and control from the ground. Solar activity is unpredictable. There have been very large CMEs in the past: the Carrington event in 1859 affected the US telegraph system and caused fires, while data from tree-rings and ice-cores suggests an even larger event occurred in 714 AD. There is also evidence that we are nearing the start of another “Maunder minimum” of low solar activity, when its beneficial effect on reducing space debris would be lost.
A satellite (the Solar Monitoring Mission) at the L1 Lagrange point between Earth and Sun monitors solar activity and measures the properties of CMEs headed towards Earth. A proposed additional satellite at the L5 point would provide advance warning of active areas on the Sun before they rotated towards Earth. CMEs with the same magnetic polarity as the Earth’s field are repelled and cause relatively little trouble; those of opposite polarity can potentially be disastrous.
The first pieces of space debris were parts of Sputnik 1 in 1957. There are now more than 16000 objects in LEO. The deliberate destruction of a Chinese weather satellite in 2007 added about 2500 pieces, and a collision between an Iridium satellite and a Russian Cosmos satellite added another 1000. There are currently more than 23000 objects 10 cm or larger in size that are being tracked.
Most debris is in a belt between 600 and 1200 km above the Earth, with another belt at around 1450 km. The density of debris is close to the threshold of the “Kessler Effect”, in which collisions lead to a runaway increase in numbers of pieces of debris. The problem requires international cooperation to resolve. This is already happening: there are agreements that satellites which have completed their missions and rocket boosters will be deorbited to avoid them adding to the problem. What we don’t know is how collisions between objects moving at orbital velocities (7.5 km per second) work: do they tend to produce many small pieces of debris or fewer large ones?
The key requirements are more accurate tracking and the capability to detect smaller objects, down to around 1 cm in size. Possibilities include tracking by the WISE infra-red telescope that has been used to search for near-Earth asteroids, or by dedicated satellites. Ground-based laser radar, the European ELT (when it’s not being used for proper astronomy) or the Square Kilometre Array (SKA) could also be used. The SKA is very sensitive, and scattering of radio signals from sources over the horizon could be used to track debris, although to the SKA astronomers it would just be noise.
Mitigating or removing space debris is a more difficult problem. There are many ideas, including the use of lasers either from the ground or from satellites, deployable nets and grappling with a retrieval satellite. Moving the debris to a “space graveyard” where it can be processed is also an option; there are even suggestions that debris could be converted into rocket propellant.
Notes and summary by Chris Hooker.