The current LEO environment contains about 3,200 intact objects, making active space debris removal a priority.
The European Space Agency (ESA) reveals that over the last 60 years, more than 5,250 launches have resulted in some 42,000 tracked objects in orbit, of which about 23,000 remain in space and are regularly tracked by the US Space Surveillance Network and maintained in their catalogue, which covers objects larger than about 5-10 cm in low-Earth orbit (LEO) and 30 cm to 1m at geostationary (GEO) altitudes. Only a small fraction – about 1200 – are intact, operational satellites today, meaning much of the rest is active space debris. The current LEO environment contains about 3,200 intact objects
‘This large amount of space hardware has a total mass of more than 7,500 tonnes,’ ESA adds, going on to explain that over 290 in-orbit fragmentation events have been recorded since 1961, and that these fragmentation events ‘are assumed to have generated a population of objects larger than 1cm numbering on the order of 750,000. The sporadic flux from naturally occurring meteoroids may only prevail over that from human-made debris objects near sizes of 0.1-1mm.’
‘With today’s annual launch rates of nearly 100, and with future breakups continuing to occur at average historic rates of four to five per year, the number of debris objects in space will steadily increase,’ the agency concludes, adding: “As a consequence of the rising debris object count, the probability for catastrophic collisions will also grow progressively; doubling the number of objects will increase the collision risk by approximately four times. As the debris population grows, more collisions will occur.
‘In a ‘business-as-usual’ scenario, such collisions will start prevailing over the now-dominating explosions within a few decades from now. Ultimately, collision fragments will collide with collision fragments, until the entire population is reduced to subcritical sizes. This self-sustained process, which is particularly critical for the LEO region, is known as the ‘Kessler syndrome’. It must be avoided by the timely application of mitigation and remediation measures on an international scale.’
Professor Johann-Dietrich Wörner’s thoughts on space debris
In February, SciTech Europa met with the director general of the European Space Agency, Professor Johann-Dietrich Wörner, where he discussed, amongst other things, the issue of space debris. Talking about what he termed ‘security in space’ he said “this concerns the safety of assets such as satellites, and one of the biggest threats they face is from space debris.”
He added: “We need to have stricter regulations to act against future space debris and to ensure that missions to the moon don’t pollute that environment, too.”
The current Low Earth Orbit (LEO) environment contains several thousand intact objects and ESA, as a space technology and operations agency, has identified active removal technologies as a strategic goal. Active Debris Removal (ADR) is necessary to stabilise the growth of active space debris.
However, Wörner informed SciTech Europa, there is little-to-no business case for missions to remove active space debris so far and, as such, the public sector has a special responsibility in this field. As such, for the ESA DG, it is important to focus on preventing future missions from contributing to the amount of debris already in the space environment.
He said: “It is my personal desire to see every future spacecraft fitted with an independent second system which can be used should the satellite’s main system fail. This could be used to de-orbit the craft at the end of its life, and this is a solution which could be realised with a reasonable amount of investment.”
Studies performed with long-term evolution models like DELTA have shown that a ‘business as usual’ scenario will lead to a progressive, uncontrolled increase of object numbers in LEO, with collisions becoming the primary debris source. As such, Wörner’s idea is all the more important. Indeed, according to ESA, even in a future scenario in which no further objects are added to the space environment (no launches, no debris release, no explosions), the results of simulations have shown that the number of debris objects would continue to grow even under these idealised conditions – under which a collision rate of once every 10 years can be assumed.
For Wörner, there is a further option: “There are some 4,500 satellites in orbit right now, and only 1,500 of them are functional. The other 3,000 could be recycled in some way so that we can make further use of their valuable components.”
The RemoveDEBRIS mission
Since SciTech Europa spoke to the ESA DG, several interesting developments have taken place in the field of active space debris removal. For instance, in April SpaceX launched new technology to the International Space Station (ISS), with the RemoveDEBRIS mission to test different approaches to removing debris from the Earth’s orbit, while an international science package will study powerful lightning from space – Atmosphere-Space Interactions Monitor (ASIM).
UK-built satellite RemoveDEBRIS is designed to test different approaches to removing debris in space from the Earth’s orbit. Led by the University of Surrey and built by the world’s leading small satellite manufacturer Surrey Satellite Technology Limited with technology designed by Airbus, the RemoveDEBRIS mission will be used to tackle the problem of space junk by attempting to capture simulated active space debris using a net and harpoon, whilst also testing advanced cameras and radar systems. Once these experiments are completed, it will unfurl a drag sail to bring itself and the debris out of orbit, where it will burn up as it re-enters the Earth’s atmosphere.
The harpoon and deployable target experiment will see a deployable target extends outwards from the platform which is used as a target for the harpoon. The harpoon and the deployable target form the harpoon target assembly (HTA). The distance for harpoon firing is 1:5m on a 10 x 10cm target.
The harpoon is designed with a flip-out locking mechanism that prevents the tether from pulling out of the target. As for net and harpoon demonstrations, success will be assessed by the images collected by the two supervision cameras.
UK Science Minister Sam Gyimah said at the time of the launch: “Space debris is one of the key challenges we face and it’s great to see a British university and some of our innovative space companies leading the way on the search for solutions. It’s also a fantastic example of the unique expertise found in the UK’s growing space sector and the value that it adds to international projects.”
Professor Guglielmo Aglietti, director of the Surrey Space Centre at the University of Surrey, said: “It is important to remember that a few significant collisions have already happened. Therefore, to maintain the safety of current and future space assets, the issue of the control and reduction of the space debris has to be addressed.
“We believe the technologies we will be demonstrating with RemoveDEBRIS could provide feasible answers to the space junk problem.” In June, the RemoveDEBRIS spacecraft was deployed from the ISS.
Efficiency of active space debris removal
ESA believes that the active removal of space debris can be more efficient in terms of the number of collisions prevented versus objects removed ‘when the following principles are applied for the selection of removal targets, which can be used to generate a criticality index and the according list:
- The selected objects should have a high mass (they have the largest environmental impact in case of collision);
- Should have high collision probabilities (e.g. they should be in densely populated regions and have a large cross-sectional area); and
- Should be in high altitudes (where the orbital lifetime of the resulting fragments is long).
The long term
Long-term environment simulations can be used to analyse orbital regions that are hotspots for collisions. The most densely populated region in LEO is around 800–1000km altitude at high inclinations. The collision hotspots can be ranked by the number of collisions predicted to occur under a ‘business as usual scenario.’
The space agency also states that its own internal studies have shown that continuous removal actions starting in 2060 will only have 75% of the beneficial effect compared to an immediate start, meaning that the importance of the successful demonstration of the technology on board RemoveDEBRIS cannot be understated due to the way in which it will be able to inform ESA’s CleanSpace initiative, which is looking at the required technology developments, including advanced image processing, complex guidance, navigation and control and innovative robotics to capture debris.
This article will appear in SciTech Europa Quarterly issue 28, which will be published in September, 2018.