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The recently Oscar-nominated film Gravity underscored the very real danger of space debris from old and existing satellites and space missions in the low earth orbit. Reducing the number of satellite missions will not be enough, and there are a number of initiatives underway to have space junk removed.
Disaster from space debris, as epitomised in the film Gravity. Picture courtesy of Warner Bros
Two thirds of all known artificial objects are within the relatively diminutive Low Earth Orbit (LEO), creating a highly congested environment. The level of congestion is particularly high around the poles, where during routine operations, approximately ten objects every week come within two kilometres of a polar orbit satellite. In recent years the European Space Agency has performed three collision avoidance manoeuvres per year. The vast majority of items in space however are not manoeuvrable.
The critical density limit within the LEO has already been surpassed, meaning that even if there are no further satellites introduced into the LEO, the level of space debris will continue to rise. If space systems continue to be launched at the current rate, it will not be long before we experience a cascading Kessler syndrome, where a single collision triggers a chain reaction of devastating effect. In order to maintain a sustainable space environment, reducing the number of satellites we send up will not be enough.
Only 7 per cent of the artificial objects which are regularly monitored from the ground are active satellites, with the remainder being items of space debris. While it is fragments between one and ten centimetres which pose the major immediate threat to satellites, it is the larger items which should be removed from space first. This is because as they breakup – through collisions and spontaneous explosions – they release large quantities of hazardous fragments.
In order to stabilise the situation, and to allow space flight to continue in the future without restriction, we need to be removing five to ten objects from space every year. Steps need to be undertaken to mitigate the possibility of the Active Debris Removal (ADR) from colliding with the target. In order to do this, the target needs to be tracked and its motional attitude understood using systems such as the TIRA satellite tracking radar in Germany. As the ADR system approaches the target it would then use a relativistic tracking mode, using either radar or lidar. The next stage of the procedure is to manoeuvre the target into a lower orbit.
Here are some initiatives being conceived, or already underway:
1. Use a Grapple
One idea – as being researched by the Swiss Space Center’s CleanSpace One project – is to use robotic grappling technologies. CleanSpace One is a 30-40 kilogram CubeSat heavily based on Commercial off the Shelf (COTS) technologies. It is predicted that the launch will take place in 2017-18, where it will be ejected from the launch vehicle at an altitude of 700 kilometres. The CleanSpace One will then approach SwissCube-1 at 28,000 kilometres per hour, manoeuvring into the same orbital path. CleanSpace One will then enter its rendezvous phase before grappling the target. CleanSat One will then transport SwissCube-1 back into the Earth’s atmosphere where, due to the speed of re-entry, the two satellites will disintegrate.
2. Use Nets
The Japan Aerospace Exploration Agency (JAXA) has teamed up with Nitto Seimo – a fishing net manufacturer – in order to design a net based solution for clearing up the debris. It is not yet sure when the system will be launched, but Nitto Seimo aims to complete the system in two years. The net would be fixed to a piece of space debris using a satellite’s robotic arm. The tether would then be detached along with the tip of the arm. As the net orbits Earth, it would become charged with electivity. Interactions with the Earth’s magnetic field would then gradually draw the net towards the Earth.
3. Attach Rockets to Large Pieces of Debris
Another method which has been proposed is attaching small solid state rocket motors to large pieces of space debris, and using these to stabilise the object, manoeuvring it into a lower orbit where there is less risk of a collision. Also, at these lower orbits air resistance is greater; therefore the satellites orbit will decay faster than it would have done otherwise.
- 4. Use a Large Solar Sail
Surrey Space Centre is currently developing a solar sail based de-orbiting technology named CubeSail. Following the year long solar sailing demonstration phase – where they aim to display the propulsive effects of solar radiation – they will begin de-orbiting the satellite using drag augmentation. The aerodynamic drag against the twenty-five metre square sail will then cause a rapid decay of the satellite orbit.
5. Use Ground Lasers to Ease Debris Into the Right Orbit
It is however still important to consider possible methods of protecting satellites against the medium sized pieces. The CleanSpace project – no relation to the CleanSpace One project – is a three year project which was started in June 2011. The project is run by a consortium, combining the expertise of nine partners, while being let by CILAS. As detailed previously, removal of large pieces of space debris is the priority. While the previous solutions all focus on removing space debris using space vehicles, this project aims to remove debris using a ground based solution. The objective of the project is to define a global architecture which includes surveillance, identification and tracking. An innovative ground-based laser would then be focused into the debris, creating a small thrust by ablating its surface. This will have the effect of altering the trajectory of the fragment, forcing it into a lower Earth orbit.
Conclusion: Clean up and Reduce
In conclusion, there are many solutions currently being looked at which would help in removing some of the space debris from orbit as it is too late to solve the issue by solely removing space debris. Other solutions, such as improving space situational awareness and limiting the number of new satellites launched, needs to be applied.
It is however a vital stage in protecting the space domain for future generations and the next step in the process, now that the technology has started to be developed, is to try and find agreement regarding how to divide the cost of the cleanup operation and to develop a set of protocols by which to remove items from space.