tag:theconversation.com,2011:/uk/topics/gravity-movie-8400/articlesGravity (movie) – The Conversation2015-11-13T05:27:41Ztag:theconversation.com,2011:article/503472015-11-13T05:27:41Z2015-11-13T05:27:41ZHow to clean up space debris – using game theory<figure><img src="https://images.theconversation.com/files/101592/original/image-20151111-9393-rka6ry.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">ESA/Spacejunk3D, LLC</span></span></figcaption></figure><p>A piece of debris just 10cm in diameter could cause an entire spacecraft <a href="http://www.esa.int/Our_Activities/Operations/Space_Debris/FAQ_Frequently_asked_questions">to disintegrate</a> and it is estimated that there are more than 29,000 objects larger than 10cm in Earth’s orbit. This poses a major risk to the spacecraft to-ing and fro-ing from the International Space Station, not to mention the hundreds of satellites that are now essential to daily lives.</p>
<p>Although there are many organisations that could be seriously affected by space debris, including most governments and many businesses, so far no one has taken any serious action to tackle the problem. But by using the mathematical modelling of game theory, my colleagues and I hope to devise a strategy to encourage these players to act to avoid the kind of disaster that a major space debris collision could cause.</p>
<p>National space agencies and private satellite and communications companies all have an interest in reducing the amount of debris in orbit. If one organisation attempts to remove debris it will benefit everyone operating in space. But because doing so will be complex and very costly, the apparent best option for any one of these players is to wait for somebody else to have a go first. That would give them a cleaner space to operate in without the expense of clearing it up themselves.</p>
<p>The problem, of course, is that if everyone thinks like this, then the amount of debris will just <a href="https://theconversation.com/space-junk-and-the-environment-its-a-very-dark-picture-indeed-2187">keep increasing</a>. Ageing satellites and used rocket launchers are creating new debris all the time, while the total number of fragments goes up every time two pieces collide and break into even smaller pieces. The build-up of space debris in this way could eventually result in a catastrophic cascade of collisions known as the Kessler syndrome. (You may have seen the possible effects of this in the film Gravity.) </p>
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<p>This dilemma of whether to accept the cost of acting or risk disaster by waiting is the kind of strategic problem studied by game theory. A situation like the space debris problem, where players act just for their own benefit instead of taking group interests into account, is referred to in game theory as the “<a href="https://www.sciencemag.org/content/162/3859/1243.full">tragedy of the commons</a>”. As a result, a shared resource (in this case, space and low-earth orbit) is over-used by all individuals and is no longer useful to anyone, leading to higher costs for everyone involved.</p>
<p><a href="https://theconversation.com/economic-theories-that-have-changed-us-game-theory-43633">Game theory</a> comes from economics and studies the interactions and strategic decision-making of several entities. These entities can be individuals, organisations, governments and even intelligent or <a href="http://www.computerworld.com/article/2575099/business-intelligence/the-rise-of-intelligent-agents--automated-conversion-of-data-to-information.html">automated computer programs (“agents”).</a> </p>
<p>In computer science, techniques from game theory are popular in research into artificial intelligence and multi-agent systems because they can help design software to analyse strategic situations and take good decisions without human supervision. For example, it can help design an agent to take part in an auction for you, instantly bidding or negotiating for commodities to get the best possible deal.</p>
<p>In game theory, strategic situations are classically modelled as a game featuring several players that each have a choice of several actions. They choose which action to take based on their own preferences and the behaviour of their opponents. The outcome for each player then depends on the choices of everyone in the game. A famous example is the <a href="https://theconversation.com/how-understanding-the-prisoners-dilemma-can-help-bridge-liberal-and-conservative-differences-46166">prisoner’s dilemma</a> game, in which two criminals receive different sentences depending on whether they cooperate with the authorities and give evidence against their accomplice.</p>
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<img alt="" src="https://images.theconversation.com/files/101593/original/image-20151111-9362-1vfud8l.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/101593/original/image-20151111-9362-1vfud8l.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=333&fit=crop&dpr=1 600w, https://images.theconversation.com/files/101593/original/image-20151111-9362-1vfud8l.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=333&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/101593/original/image-20151111-9362-1vfud8l.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=333&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/101593/original/image-20151111-9362-1vfud8l.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=419&fit=crop&dpr=1 754w, https://images.theconversation.com/files/101593/original/image-20151111-9362-1vfud8l.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=419&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/101593/original/image-20151111-9362-1vfud8l.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=419&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption">Catch the satellite.</span>
<span class="attribution"><span class="source">ESA</span></span>
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<p>Classical game theory tells you how to act in situations like the prisoner’s dilemma to achieve the best outcome. One the most important elements of the theory is the <a href="https://theconversation.com/the-legacy-of-john-nash-and-his-equilibrium-theory-42343">Nash equilibrium concept</a>. This means that players are assumed to be perfectly logical and behave rationally. Interestingly, it seems that when players take the most rational decisions it does not always lead to cooperative behaviour or the best outcomes.</p>
<p>For our space debris dilemma, more recent versions of game theory, such as <a href="http://opim.wharton.upenn.edu/%7Esok/papers/r/graham-romp/romp-chapter3.pdf">dynamic game theory</a> and <a href="https://www.cs.cornell.edu/home/kleinber/networks-book/networks-book-ch07.pdf">evolutionary game theory</a>, are particularly useful because they can deal with changing circumstances. For instance, evolutionary game theory assumes that the players aren’t fully rational but are also socially and biologically conditioned. This provides a better way of describing the behaviour of social human beings or, on a bigger scale, multinational organisations.</p>
<h2>Space game</h2>
<p>We aim to create a realistic computer model of debris removal situations that can be used to perform an analysis using game theory. This should be able to explain the different ways entities involved in space debris build-up behave. For example, it could predict the amount of effort each entity would be willing to invest on clean-up given the immediate and long-term risk to their space assets such as satellites.</p>
<p>This will then enable us to better understand how different debris removal strategies might work and determine the best ones for different players to take. For example, each player could commit to removing one piece of debris each year, or a number of pieces proportionate to the number of new satellites the player launches. Game theory can basically tell us whether we can expect such strategies to result from the self-interested interaction between the parties involved.</p>
<p>The final result should be a mechanism to “steer” the situation and create incentives to encourage the self-interested players to take actions that won’t lead to the tragedy of the commons. For example, internationally agreed taxes or fines could make removing or preventing the growth of space debris in the immediate best interests of certain players. Without such action, the mess we’re creating in orbit is only likely to get worse.</p><img src="https://counter.theconversation.com/content/50347/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Karl Tuyls receives funding from ESA. </span></em></p>We need to find a way to break through the potentially disastrous stalemate wherever everyone waits for someone else to clear up the junk in orbit.Karl Tuyls, Professor of computer science, University of LiverpoolLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/231622014-02-17T13:53:56Z2014-02-17T13:53:56ZBaffled Baftas don’t know how to be British<figure><img src="https://images.theconversation.com/files/41689/original/yd78vgbn-1392631107.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Not-so-Brit awards.</span> <span class="attribution"><span class="source">Joel Ryan/PA</span></span></figcaption></figure><p>Britishness seemed to be main motif in the BBC’s broadcast of the 2014 <a href="https://www.bafta.org/">Baftas</a>. When host Stephen Fry mentioned the event was the highlight of the British film calendar, he backtracked and asked, “Is there such a thing as a British film calendar?”</p>
<p>He did well to ask, because the constellation of stars he took great trouble to show off – Leonardo Di Caprio, Brad Pitt, Angelina Jolie, Cate Blanchett, Bradley Cooper, Amy Adams, Christian Bale, Tom Hanks – is no different to what we’d expect to see at the Oscars. </p>
<p>At the Oscars, one wouldn’t have had to rely on Twitter to learn that Brad Pitt and Angelina wore matching Yves Saint Laurent tuxedos, Lily Allen was in Vivienne Westwood, Amy Adams wore Victoria Beckham and Cate Blanchett wore Alexander McQueen; there would have been a whole series of programmes right up to the start of the broadcast breathlessly recounting every aspect in great detail, and using the very latest technological developments to broadcast every stitch to an eager public, garnering worldwide unpaid publicity for the giant fashion houses. But as Oprah Winfrey said before the British show started, “This is not about glitz and glamour.”</p>
<p>But what are the Baftas about? What are they for? You might think to honour, celebrate and promote British cinema. But a different story emerges when one looks at the <a href="http://awards.bafta.org/award/2014/film">nominees</a>. Bruce Dern, Leonardo Di Caprio and Tom Hanks, for example, were contenders for Best Actor. </p>
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<img alt="" src="https://images.theconversation.com/files/41692/original/yj789dmr-1392631540.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/41692/original/yj789dmr-1392631540.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=399&fit=crop&dpr=1 600w, https://images.theconversation.com/files/41692/original/yj789dmr-1392631540.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=399&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/41692/original/yj789dmr-1392631540.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=399&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/41692/original/yj789dmr-1392631540.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=501&fit=crop&dpr=1 754w, https://images.theconversation.com/files/41692/original/yj789dmr-1392631540.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=501&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/41692/original/yj789dmr-1392631540.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=501&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption">Alfonso Cuarón and David Heyman with the Outstanding British Film award.</span>
<span class="attribution"><span class="source">Ian West/PA</span></span>
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<p>Indeed, when the first award of the evening was announced, and <a href="http://www.theguardian.com/film/2013/nov/07/gravity-review">Gravity</a> won Outstanding British Film, the <a href="http://www.digitalspy.co.uk/movies/news/a551609/baftas-2014-gravity-best-british-film-win-sparks-online-debate.html">twittersphere went into a frenzy of speculation</a> as to what was British about it. A friend of mine – who works for an independent British distributor – insisted “It is NOT a British film. It is NOT a British film. It is NOT a British Film.” Three times. Just in case we didn’t get the message.</p>
<p>Of course, there’s no need to get too purist about these things. The nominations don’t necessarily reflect a particular definition of British cinema, one which would probably run something along the lines of: films predominantly financed in Britain, about British stories, with a predominantly British cast and crew (<a href="http://www.theguardian.com/film/2013/nov/03/philomena-review-steve-coogan-judi-dench">Philomena</a> and <a href="http://www.bfi.org.uk/news-opinion/sight-sound-magazine/reviews-recommendations/film-week-selfish-giant">The Selfish Giant</a> would be unproblematic examples). But they do reflect British film culture: the films celebrated are the films that have entertained, delighted and informed us here, be they British or not. </p>
<p>And later in the show, when Alfonso Cuarón returned to the stage to collect his award for Best Director, and had presumably been made aware of the brouhaha over Gravity’s win for Best British Film he said:</p>
<blockquote>
<p>I consider myself part of the British Film Industry. I’ve lived here for 13 years and made about half my films here. I guess I make a good case for the curbing of immigration.</p>
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<figcaption><span class="caption">Outstanding British Film 2014.</span></figcaption>
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<p>Yet, at the end of his speech, the cinematic culture Cuarón feels a part of was made clear when he thanked Guillermo del Toro and Alejandro Iñárritu, Mexican compadres and current colleagues in the higher reaches of global cinema. “I wouldn’t order breakfast before consulting them first,” he said.</p>
<p>A concern with Britishness and its articulation continued as a recurring motif. Earlier in the show, after Stephen Fry introduced her as a “ghastly piece of shrieking, stinking offal”, Emma Thomson replied, “Is it me or being British that means being referred to as a stinking piece of offal make me feel so much better about myself?” </p>
<p>The finale of the evening was when Prince William, in his role as president of Bafta, introduced Jeremy Irons to bring out the pomp and ceremony and recount the highlights of Helen Mirren’s career. Accepting the award for her Fellowship of the Baftas, Mirren first thanked her old teacher, Alice Welding, who recently died at the age of 102, for having inspired her to desire to live in a world of literature and poetry. She finished off her <a href="https://www.youtube.com/watch?v=01pOFqFtmyw">acceptance speech</a> with a dazzling oration that invoked both acting and Albion, the “We are such stuff as dreams are made on” speech from The Tempest.</p>
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<span class="caption">Best Actor winner Chiwetel Ejiofor.</span>
<span class="attribution"><span class="source">Joel Ryan/AP</span></span>
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<p>It was a rather theatrical and very British end to a Baftas that saw <a href="http://www.theguardian.com/film/2014/jan/12/12-years-a-slave-review">12 Years a Slave</a>, a film which had Channel Four funding, a British director and a large British cast, win Best Film, but Gravity, with its American money and cast and its Mexican director, win Best British film. Chiwetel Ejiofor, black and British, won Best Actor. </p>
<p>Oh, and <a href="http://www.telegraph.co.uk/culture/golden-globes/10072554/The-Great-Beauty-review.html">The Great Beauty</a>, the winner of Best Foreign Film, didn’t even make it to the broadcast. It was put in the little “These awards were handed out earlier” addendum after the end of the main programme. So the Britishness of these Baftas seems to be defined by placing America at the centre, various articulations of Britishness on the margins and in the “specialised” categories – such as the “<a href="http://www.theguardian.com/film/2014/feb/16/baftas-2014-peter-greenaway">Michael Balcon Outstanding Contribution to British Cinema</a>”. And the rest of the world? Utterly out of the picture.</p><img src="https://counter.theconversation.com/content/23162/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>José Arroyo does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Britishness seemed to be main motif in the BBC’s broadcast of the 2014 Baftas. When host Stephen Fry mentioned the event was the highlight of the British film calendar, he backtracked and asked, “Is there…José Arroyo, Lecturer, Department of Film and Television Studies, University of WarwickLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/203842014-01-31T14:48:43Z2014-01-31T14:48:43ZCleaning up space debris with sailing satellites<figure><img src="https://images.theconversation.com/files/38038/original/nb7vkw7r-1387305576.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Lost in space!</span> <span class="attribution"><span class="source">ESA</span></span></figcaption></figure><p>Since the birth of space flight in 1957, the number of man-made objects orbiting the Earth has grown every year. There are now more than 15,000 such objects larger than 10cm, at least those that we know of. Even very small particles can pose a risk to spacecraft, because of the high relative velocities at which they travel. Not only can space debris affect critical equipment such as communications satellites, but it can also endanger manned space flights. </p>
<p>A dramatic illustration of the dangers of space debris is given in the film “Gravity”. It may have taken some <a href="http://ideas.time.com/2013/10/02/astronaut-gravity-gets-me-down/">artistic license with science</a> to craft a good story, but its main premise is plausible. What Gravity showed was the worst case scenario, known as the Kessler syndrome, where a collision between two objects generates a cloud of smaller debris, which triggers a chain reaction of further catastrophic collisions, thereby rapidly increasing the amount of debris. This could make the low Earth orbit unusable for spacecrafts.</p>
<p>Most of those are useless fragments of once-useful objects, which were created by explosions, collisions or missile tests. For instance, an accidental collision between the Iridium-33 and Kosmos-2251 satellites in 2009 caused them to shatter into 2,200 (recorded) fragments. Smaller space debris is much harder to track, but NASA estimates that up to 500,000 objects larger than 1cm, and 135 million particles over 1mm in size may now be orbiting the Earth.</p>
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<span class="caption">Catalogued objects in Low Earth Orbit (LEO) larger than 10 cm.</span>
<span class="attribution"><span class="source">NASA</span></span>
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<p>Space debris is becoming a serious issue, and many space agencies have started working on solutions. One approach being taken by JAXA, Japan’s space agency, is to use a magnetically charged 700m-wide net made from aluminium and steel wires. If used at the right height it will attract floating space debris to it. When enough has been caught, the system can be ordered to fall out of its orbit back to Earth. During that process the debris, along with the net, will burn up as it enters Earth’s dense atmosphere. JAXA will be doing a test launch of the system next month.</p>
<p>The other approach is to remove existing inactive satellites from orbit. A prime target for this experiment would be the European ENVISAT satellite which stopped functioning in 2012 and now drifts uncontrolled in orbit. At an altitude of 800km and with mass of more than 8,000kg, the ENVISAT satellite would take more than 150 years to deorbit – that is, drop out of its orbit – naturally. </p>
<p>Throughout that time the satellite would be at risk of colliding with other objects and generating further debris. A more sustainable solution is to remove future satellites from orbit after they have served their purpose, thereby mitigating the growth of the amount of space debris. This is why international guidelines have been proposed which will restrict post-mission deorbiting time to 25 years for all new satellites.</p>
<p>Most satellites designed today take will take longer to deorbit, and new technical solutions are necessary to meet the guidelines. This is where Surrey Space Centre (SSC) working with the European Space Agency (ESA) have developed a <a href="http://www.surrey.ac.uk/features/surrey-space-centre-pioneers-%E2%80%98gossamer-sail%E2%80%99-deorbiting-satellites">Gossamer Sail for Satellite Deorbiting</a>. The idea is to attach a large and very light, or gossamer, sail to a satellite, which can be deployed after its mission is over. </p>
<p>The lower Earth orbit has some atmosphere, which enables the large sail to generate enough aerodynamic drag to slow down and deorbit the satellite more rapidly. Unlike existing deorbiting systems based on chemical or electrical propulsion, the gossamer sail system is relatively simple and does not require propellant or electrical power throughout its deorbiting phase.</p>
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<span class="caption">A gossamer sail deorbiting system deployed from a host satellite. The 5×5 m square sail consists of four quadrants which are supported and tensioned by four diagonal deployable masts.</span>
</figcaption>
</figure>
<p>The gossamer deorbiting system is designed to automatically orient the sail in the direction where maximum drag can be achieved, ensuring quicker deorbiting. Furthermore, the sail is made reflective, which allows it to make use of the solar radiation pressure to manoeuvre; solar sailing, so to speak. This enables the satellite to be lowered to an orbit where the aerodynamic drag takes over, allowing the satellites to be placed in higher orbits and still meet the deorbiting requirements.</p>
<p>Developing and testing the SSC gossamer deorbiting sail was quite an engineering challenge. The 5×5m sail and the four deployable masts that support it have to be packaged inside a space measuring approximately 10×10×20cm. To achieve this, the sail is made of an ultra-thin membrane and the special carbon-fibre masts can be coiled up tightly (much like a tape measure).</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/36934/original/hc6s8k87-1386164464.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/36934/original/hc6s8k87-1386164464.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/36934/original/hc6s8k87-1386164464.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=341&fit=crop&dpr=1 600w, https://images.theconversation.com/files/36934/original/hc6s8k87-1386164464.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=341&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/36934/original/hc6s8k87-1386164464.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=341&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/36934/original/hc6s8k87-1386164464.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=429&fit=crop&dpr=1 754w, https://images.theconversation.com/files/36934/original/hc6s8k87-1386164464.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=429&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/36934/original/hc6s8k87-1386164464.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=429&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Deployment sequence of the gossamer sail, as tested at the Surrey Space Centre.</span>
</figcaption>
</figure>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/36691/original/ys4gpb5f-1385997999.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/36691/original/ys4gpb5f-1385997999.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/36691/original/ys4gpb5f-1385997999.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/36691/original/ys4gpb5f-1385997999.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/36691/original/ys4gpb5f-1385997999.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/36691/original/ys4gpb5f-1385997999.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/36691/original/ys4gpb5f-1385997999.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/36691/original/ys4gpb5f-1385997999.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The gossamer sail after a deployment test at the Surrey Space Centre.</span>
</figcaption>
</figure>
<p>The SSC gossamer sail is expected to be tested within the next year. After the technology has been successfully demonstrated in space, the system can then be fitted to much larger satellites as an end-of-life deorbiting system. This will provide satellite operators with a means to meet the 25-year deorbiting guidelines, which in turn will help safeguard the possibility of space flight for future generations.</p><img src="https://counter.theconversation.com/content/20384/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>This research was in part funded by the European Space Agency (ESA).</span></em></p>Since the birth of space flight in 1957, the number of man-made objects orbiting the Earth has grown every year. There are now more than 15,000 such objects larger than 10cm, at least those that we know…Mark Schenk, Research Fellow, University of SurreyLicensed as Creative Commons – attribution, no derivatives.