March 30, 2012
Space debris and the risks to satellites
One definition of the word space is empty – but the space above the Earth is anything but empty. So is there room up there for any number of satellites to orbit the world safely? No, not according to our experts, who recently had to move one of our satellites to avoid a collision.
Distribution of catalogued objects in space.
Satellites in orbit can all be affected by debris and space weather caused by the Sun. Currently collisions or near-collisions assessed to be 1 in 5 year occurrence, but the more items there are in space the higher the risk.
EUMETSAT is one of a number of European agencies working together to investigate setting up a European Space Situational Awareness system. SSA systems can detect hazards that could threaten critical space and ground infrastructure.
These risks could be from:
Europe currently has five weather satellites orbiting the Earth, but there are thousands of other types of satellites, including those needed for Defence and telecommunications. Currently the warnings EUMETSAT receives come from the US Air Force, which tracks everything larger than 10 cm² — anything smaller can’t be picked up on radar but can still be damaging.
The greatest risk is to satellites in Low Earth Orbit, where there are around 13,000 operating satellites and around four million pounds of space debris — everything from small screws and bolts to big chunks of spacecraft. All of this has the potential to hit and severely damage, or even destroy, a satellite.
In 2011 EUMETSAT had around 30 warnings of possible conjunctions (collisions) with debris objects and had to undertake a Collision Avoidance Manoeuvre in May to move Metop-A away from debris from the Cosmos-2251 satellite. This month, on 2 March, we had to move Metop-A again, because of debris from the Iridium-22 satellite — which broke up in 2009 after colliding with the Cosmos satellite. Analysis of the trajectory of both the satellite and the debris showed the relative velocity between Metop and the piece of Iridium was around 12.026 km/s. This risk was classed as 1:2610 – anything higher than 1:10,000 is a high risk — so the satellite was moved. The actual manoeuvre was small — using only around 0.071 kg of fuel — and Metop-A products for the Space Environment Monitor (SEM) and Global Ozone Monitoring Experiment (GOME) instruments were unavailable for a short period.
Impacts of a collision
Although moving a satellite obviously has impacts, they are minimal compared to the costs and impacts of having a satellite badly damaged or destroyed. At a minimum it would take more than a year to replace the satellite in orbit. The main impact from the loss of Metop would be on weather forecasting. The Metop satellite makes a considerable contribution to Numerical Weather Prediction (NWP) and NWP underpins the forecasts of the National Meteorological Services. Satellite data are particularly valuable in the Southern Hemisphere, where there are few conventional observations. The loss of Metop would also impact ocean and climate monitoring. Accurate climate monitoring needs a constant stream of historic data – a break of a year could have a significant effect on trend analysis.
In the future the risk of a loss of operations will be mitigated by having dual Metop satellites. Metop-B is due to be launched on 23 May. However, this implies another operational satellite exposed to conjunction risks, potentially doubling the number of avoidance manoeuvres to be executed. Another form of mitigation is removing redundant satellites from Low Earth Orbit by moving them into re-entry orbit. This planned for Metop-A, once Metop-C has been launched.
At a seminar last year representatives from major stakeholders — including EUMETSAT, the European Space Agency, the European Commission, the European Defence Agency and national governments — agreed that Europe needs to investigate having its own SSA capability. EUMETSAT is now liaising with ESA to help determine the requirements. ESA is leading the programme, which, if agreed, could see a European SSA become operational in 2013/14.
Mike Williams, Head of Control Centre Division, sums it up: “We are interested in having as much conjunction data as possible. The more data we have, the greater risk analysis we can do. We have a process in place — MIAMI (Manual In-plane Avoidance Manoeuvre Insertion) — which means the impact of moving a satellite can be minimal, especially versus the nominal risk. The loss of a satellite would be a disaster.”
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