Our panel of experts considers the extent and impact of the increasing dangers of space junk.
The growing problem of space debris – the millions of pieces of old satellites, used rocket launchers and other man-made junk currently orbiting the Earth – has become increasingly well-known in recent years. The danger it poses to our satellite and manned space infrastructure is a hot topic for scientists and engineers. And it’s now come to the attention of the wider public thanks to the Oscar-winning film Gravity, in which the destruction of a satellite starts a seemingly unstoppable cascade of debris collisions known as the Kessler effect.
Numerous ways of removing old space debris have been put forward in recent years but none have so far been tested in space. For the latest of our reader Q&As, we put your questions on the extent of the problem and the viability of the proposed solutions to a panel of experts, including:
- Robin Biesbroek, leading the current e.Deorbit study for ESA on how to remove space debris;
- Nicholas Johnson, NASA chief scientist for orbital debris (recently retired);
- Dr. Jaime Reed, head of R&D for earth observation, navigation, and science at Airbus Defence and Space;.
How much of a threat does space debris pose to our satellite infrastructure and manned space missions?
Robin Biesbroek: It poses a threat in the sense that by now we get weekly collision warnings for very large satellites at ESA. We then have to analyse the threat and take actions if the probability of collision is high. Ten years ago we would need only one collision avoidance manoeuvre (CAM) every two years; now we need three per year for the large satellites. In the future the number of CAMs per year will increase and it costs time and money and may even interrupt the services that the satellite needs to provide. Furthermore, it may become hazardous for astronauts. We have already seen cases where astronauts on board the International Space Station needed to shelter against possible debris impact, and in the future this will happen more often.
Jaime Reed: The main worry is that large objects that are not controlled (e.g. dead satellites) collide, releasing a huge cloud of small objects that cannot be tracked but have enough energy to easily destroy operational satellites. Although the problem beyond Earth’s orbit is much lower (the volume is greater and there have been fewer missions), these missions must usually pass through the polluted region, which still poses a risk.
Nicholas Johnson: Even large, dedicated shields typically can only protect against debris 1 cm and smaller. The largest single threat to the operation of the International Space Station is orbital debris. However, space is still a big place, and only a very few operational spacecraft have been seriously affected by orbital debris. Orbital debris is not an operational issue beyond Earth orbit, but we are already seeking to limit its presence in orbits about the Moon and Mars.
How is the issue of space debris affecting the design of new satellites, both in terms of deorbiting them but in also in terms of greater protection from debris damage?
RB: All ESA satellites must have a propulsion system that still works at the satellite’s end of life and has enough propellant to lower the orbit in such a way that re-entry occurs within 25 years. For very large satellites we will need to do a controlled re-entry, which requires a highly reliable propulsion system with more propellant, an accurate pointing system and adequate ground-coverage. At the moment not many platforms have this capability so we need to develop new platforms. The second issue is the protection from space debris. This means again more propellant to account for collision avoidance manoeuvres, but for example also larger solar panels to take into account that over the years some cells of the panels won’t provide power anymore due to holes caused by space debris impacts.
JR: Future satellites will be designed to be more robust to being hit by small pieces of debris. Because the relative speed can be around 14km/s even tiny particles can cause damage to sensitive items like solar arrays. Therefore by adding shielding to some of these items, to absorb the impact energy, the problem can be mitigated.
NJ: Complex, dedicated orbital debris shields are normally not required for robotic satellites, although spacecraft design features and additional sheets of multi-layer insulation can increase mission survivability. On the other hand, tailor-made orbital debris shields can be necessary for some piloted spacecraft.
What are the key challenges that make pulling old debris out of orbit so difficult?
NJ: Great expense and energy were expended to place spacecraft and launch vehicle stages into orbit. Removing them requires the same. The challenge is to devise a concept of operations which is technically feasible, affordable, and practical. Derelict spacecraft and rocket bodies are often tumbling and might contain hazardous materials or have become fragile. Discrete capture of small debris would be difficult due to their great numbers and widely disperse orbits.
JR: For large objects the challenge is to catch the object without damaging it (i.e. causing new debris), and then control it whilst applying a force to push or pull it into the atmosphere. The objects might also start off spinning and therefore they need to be stabilized first. Since these objects can be several tonnes, the effect is like standing on ice and trying to capture and control a large family car or truck spinning on the ice. This all requires a very versatile propulsion and guidance system.
What do you think is the best solution proposed so far for removing old debris that doesn’t have built-in de-orbiting capability?
RB: At the moment it looks like catching very large debris with either a net system, harpoon or robot arm, and then de-orbit it into the atmosphere, is the preferred solution. At ESA we hope to select the preferred solution by summer this year.
JR: We have proposed a method involving rendezvousing with the debris (a bit like the shuttle rendezvousing with the international space station), capturing the target with a harpoon and tether, and then dragging the object back into the atmosphere to burn up. The advantage of this approach is that this can be tested on the ground and be largely based on existing technologies. Also the harpoon can be launched from a large distance away, which avoids the possibility of a collision between the chaser spacecraft and the target.
How feasible would it be to somehow sweep up large amounts of debris? One reader suggested creating a foam blanket that could be manufactured in orbit and then act like a debris sponge. Is it more likely that we will have to target individual objects?
NJ: The average relative velocity in low Earth orbit (LEO) is about 10 km/s. Only the smallest (and least hazardous) debris could be captured with a blanket or similar retarding device.
JR: The issue with collecting small targets is that they are difficult to track with radar and relatively spaced apart. The risk of collision is currently small, which means that most satellites do not get significantly damaged over their lifetime at present, but this does mean that a sweeper would have to be huge to collect a significant mass of small debris.
Would it be possible and/or useful to push larger pieces of debris into deep-space orbit rather than rather than trying to de-orbit them so they burn up in the atmosphere? How might we go about doing this?
JR: Yes and in fact this is planned for some future missions. It is also common practice for telecoms satellites in geosynchronous orbit. For older debris this might be possible but in general the objects need to be taken from 800km altitude to 2000km altitude (above all the operational satellites) and this takes more fuel than coming back into the atmosphere (about 100km).
RB: This option requires more propellant than to de-orbit the debris, but may have other advantages such as using smaller rocket engines with lower loads on the structure. For non-LEO satellites, for example for satellites that are located far away from Earth, but still bound to its gravity such as ESA’s GAIA or Herschel-Plack missions, we don’t need much propellant to leave Earth, so for those kind of missions it is certainly an option.
What plans are in place other than good housekeeping to prevent the Kessler effect?
NJ: Without an effective debris removal program, the so-called Kessler effect will lead to increasing amounts of debris in Earth orbit. However, the rate of increase is very slow, even over the next century. [Unlike in the film Gravity, where the effect becomes catastrophic within minutes]. Hence, we still have time to continue our research and development efforts to identify a practical debris removal process. For impacts by small debris, new debris shield technologies are being developed with emphasis on greater effectiveness with lower mass shielding.
RB: Even if we stop launching new satellites, the Kessler effect does not stop: space debris will increase due to collisions. Several independent studies at ESA, NASA and institutes have indicated that only by removing at least several large space debris objects from densely populated orbits per year, can we stop the Kessler syndrome.
All of the current debris has been inserted into orbit during the current polarity of the magnosphere. As its polarity is known to reverse occasionally, what will be the reversing effect upon this debris?
NJ: Most debris, whether small or large, does not acquire a charge that is significant enough to alter its orbital motion in a meaningful manner.
RB: This is difficult to predict. I don’t see a big effect on the orbit, however it may have an effect on the attitude of space debris. In the worst case it could spin up the debris, making it more difficult to capture.
What international agreements are there for dealing with this problem and how effective do you think they are? What else could the international community do to tackle the issue?
NJ: For large, intact objects, the United Nations Outer Space Treaty is rather clear: the member state responsible for the vehicle is the only one who can remove or authorize the removal of the vehicle. However, the international community recognizes that orbital debris is a global issue that requires a global response. Once an effective debris removal capability is developed, operations are likely to be financed and conducted under an international agreement.
RB: Agencies like NASA and ESA have their own codes of conduct, for example the requirement to remove a satellite from the LEO protected zone within 25 years. International treaties deal more with ownership of space debris, but ISO standards can actually mandate you to perform a controlled de-orbit. None of these are binding though. International space law will need to find a solution at some point, when more and more satellites start colliding and commercial services fall out because of it, or when there are fatalities.
What is the current status of programmes or missions aiming to deal with this issue of old debris and what needs to happen to ensure action is taken?
RB: We started the Clean Space initiative over three years ago and are now developing several technologies dealing with space debris mitigation and remediation. In 2013 we looked with different European companies at possible business plans for space debris removal, and this year we started the design of the e.Deorbit mission. After we have selected the capture technique this summer, we will do a detailed design that we will then propose to European ministers at the end of next year. It is then up to Europe to decide if e.Deorbit will be funded to be developed and built.
JR: Airbus DS and the University of Surrey are working on a small demonstrator mission called REMOVEDEBRIS which aims to demonstrate key technologies such as net and harpoon capture techniques. This mission should fly in 2016. Switzerland is also working on a small demonstrator mission called CLEANSPACE, which aims to demonstrate capture using a small claw mechanism.