Subtitles section Play video Print subtitles This episode of Real Engineering is brought to you by CuriosityStream, watch over 2,400 documentaries for free for 31 days at curiositystream.com/realengineering. On March 27th 2019, Indian Prime Minister Narendra Modi announced to the world that India had conducted its first successful anti-satellite test. Launching a three-stage missile from Abdul Kalam island on the north-eastern coast of India, with a trajectory taking it over the Bay of Bengal. A trajectory that would eventually lead to it intercepting India's military satellite, Microsat-R, 283 kilometres overhead. The seven hundred and forty-kilogram satellite met its end when the kinetic kill vehicle ploughed through it. Shattering it into hundreds of pieces which proceeded to spread in earth's orbit. The test was universally condemned. A completely unnecessary political posturing move that added significantly to Earth's growing space debris problem. This isn't some far off distant problem that we need to worry about, and the probability of collisions occurring is not only continually rising, but there have already been several collision events that have damaged the international space station and other high-value satellites. We can't blame the current state of affairs entirely on anti-satellite tests like this. Every single time we launch into space we generate some sort of unwanted waste. Solid-fuel rockets deposit aluminium-oxide particles. Explosive bolts fragment into pieces. Even chips of paint can cause issues. In 1983 the Challenger space shuttle was struck by a 0.2 mm chip of paint that managed to gouge this pit out of one of its windows. In fact, in the first 67 Space Shuttle launches 177 impacts were found in the windows. 45 of which were large enough to warrant a replacement window. Post-mission analysis determined all of these impacts we caused by space debris, with 44% being caused by aluminium alloys, 37% by paint chips, 12% by steel, 5% by copper and 2% by titanium.  At fifty thousand dollars a pop, this did not come cheap. Based on these numbers, the Space Shuttle had 67% chance of impact causing significant damage to the windows during their 10 day missions, and these probabilities have only risen over time. In 2007 the likelihood of a collision between any satellite in low earth orbit and a piece of debris over 1 centimeter in size was 17-20% in a single year.  That statistic increased to 25-33% later that year when China tested their own anti-satellite missile. By 2010 the chance of a 1-centimetre piece of debris striking a satellite had increased to 50% a year, after two full-sized satellites Iridium 33, a US communications satellite, and Kosmos 2251, a retired Russian communications satellite, collided at 42,000 kilometres per hour. Obliterating both satellites and producing over 1000 fragments over 10 centimetres in size, and many more too small to be tracked. So the chances of collision are not small by any measure, they are common and it's only a matter of time before another serious incident like this 2009 collision occurs again, so it is prudent that we design satellites to be capable of not only withstanding small impacts but be capable of dodging larger ones. The ISS, for example, is designed to withstand objects up to 1 centimetre in size , and can dodge larger trackable objects 10 centimetres wide. The biggest danger to the occupants of the ISS are objects in between these sizes. That are untrackable but large enough to cause serious damage to the international space station.  When the ISS was being planned, NASA laid out a basic risk management policy. That the probability of any critical component of the ISS being penetrated by space debris would be less than 19% over 10 years.  A critical component is characterized by anything that could potentially lead to a loss of life if it is damaged, and designers are careful to correctly categorise each component on the ISS to ensure this is the case, which often involves performing hypervelocity impact test here on earth. This resulted in things like batteries and ammonia accumulators being categorized as non-critical when they didn't explode during tests, and so received less shielding that other critical components. Space debris is just a fact of everyday life on the ISS that astronauts need to be aware of. To get a better sense of what it's like living with this. I spoke with former ISS commander Chris Hadfield on the phone. When you are onboard a spaceship you have a constant undercurrent awareness of the ever prevalent risk of something hitting your spaceship and causing a leak. Sort of like when you are driving a car, you always know at some point you could have some kind of accident. Yo u know, It's not heavy on your mind. You know it happens. You know the odds are that eventually it will happen for sure, and you just have to find a way to live with it. And so the way we live with it is to understand the risk as accurately as we can. We know the statistics. We know the relatively risk of man made debris versus naturally occurring debris, and we know how the space station is designed to resist it with the multilayer shielding, and then we also have procedures. So let's talk about that shielding first. Shielding the ISS with heavy plates, as tanks do here on earth, is not an option. This is the damage a 13-millimetre spherical bullet will do to a 18-centimetre aluminium plate when travelling at 7 kilometres per second. It prevented penetration and only just managed to prevent a large chunk of spall to break off from the interior surface.  At 13 centimetres a 1 metre squared aluminium plate like this would weigh about 338 kilograms. When the ISS started construction in 1998 the per kilo cost to launch to the International Space Stations orbit using the Space Shuttle was about 93,400 dollars.  Placing a 1 metre squared shield like this at a cost of 31.5 million dollars. This is an extremely inefficient use of material, and the ISS uses something much more elegant called a Whipple shield. The Whipple shield uses the debris' own velocity to stop it. At 4 kilometres per second and higher, the energies involved are so immense that the projectile itself breaks apart and vaporises on impact.  Whipple shields take advantage of this by creating a shield that is composed of several thin sheets of armour separated by space. So when a meteoroid or debris does strike it, it first breaks up into thousands of smaller superheated fragments. Thereby spreading the energy of the impact over a larger area for the following shield layer. The European Space Agency conducted tests of their kevlar Whipple shields which protect their ATV vehicle. They did this by shooting a 7.5 mm diameter aluminium bullet at 7 kilometres per second, which tore straight through the kevlar shield, but left only a scorch mark on the 3-millimetre aluminium wall behind it.  These kind of impacts occur fairly frequently and as Chris Hadfield told me during the call: If you just sit quietly by the wall of the space station and wait a while, you can hear things hit your ship. And that's kind of an interesting thing. It doesn't happen too often and sometimes all you are hearing is the vehicle cooling and heating in the sun, so you are hearing the natural popping of metal expanding or contracting, but occasionally you hear just like the sound of a small bullet or high speed stone banging into the thin aluminum hull of your ship So astronauts are occasionally reminded of the space debris problem, and have to be careful not to cut their suits while on space walks on sharp impact edges. While this is a highly effective form of shielding that minimises the weight of shielding needed, it is only effective for smaller debris. Larger particles would tear right through this shield, and for those circumstances, the ISS and other satellites literally have to dodge the incoming shrapnel. Ground-based radar like the Haystack Radar are the main source of spatial data we have on space debris. It is an X-band radar system that simple stares at selected points in space and waits for debris to pass through its radar beam.  This gives us size, speed and direction information, which is fed into a database that allows NASA and other space agencies around the world to predict potential collisions. When a collision is predicted maneuvres can be planned to allow the international space station to dodge it, but these maneuvres come with a cost, and mission control needs to assess if the risk is worth that cost. They start by drawing an imaginary box around the international space station. 50 kilometres squared and point seven five kilometres deep. This acts as an exclusion zone and any tracked debris that passes through it will send an alert to mission control. From there careful risk analysis begins. If there is a one in ten thousand to one in one hundred thousand chance of collision the ISS receives a yellow warning. Which means flight controllers must perform avoidance maneuvres if they do not interfere with mission objectives. This can be as simple as interfering with microgravity experiments to forcing the Soyuz to miss a launch window. If there is a greater than one in ten thousand risk , then a red warning is received and the international space station must take action. Control momentum gyros can be used to alter the stations orientation, while thrusters on the Zvezda module or on docked vehicles can be used to provide the necessary acceleration. Boosting to a higher orbit requires expensive propellant, but the ISS already needs to perform reboosts every few months to maintain its orbit, so these dodging maneuvers will just alter the scheduling of these already needed boost burns. These exclusion zones exist for all satellites in NASA's database and on March 29th 2012, Julie McEnery (hup the Irish) the project scientist for the Fermi Gamma-Ray Space Telescope received an automated email alert. Informing her of a predicted incursion between Fermi and Cosmos 1805, a retired Russian spy satellite, where the two would pass within 700 feet of each other. The decision on what to do with this information was left to her, and the lessons learned from the previous satellite collision were not lost on her. In order to ensure they did not collide Fermi would need to rotate away from its view of the sky to point its thrusters in the direction of travel. It then performed a 1-second burn that would separate the two satellites crossing temporally. These thrusters had not been tested before, as they were designed to take the satellite out of orbit at the end of its life, and so there was significant anxiety within the team that they could malfunction and end Fermi's mission prematurely. Thankfully that did not happen, and Fermi continues to give us valuable information about the Universe to this day. These issues are only going to grow as human activities in space grows, and it's time we began thinking more seriously about how we manage our cosmic neighbourhood. Mainstream media tends to incredibly alarmist about this issue. “So all of that is debris that you are looking at there, so your concern for debris is well placed. We maybe putting so much debris is space that we will close ourselves off from space travel because of the dangers it would take to get through our own garbage heap. Space debris, obviously does not look like this, the vast majority of it is too small to see. Occasionally we have massive debris, like the upper stage of Apollo 12s Saturn V rocket, which is still in orbit and expected to return to earth in the next couple of decades, but this material is easy to dodge. After all, space is a big place. We are not going to be trapped on the planet, we are not going to lose all technology related to satellites. Even now, just a few weeks after India's anti-satellite test, a decent amount of the debris will have drifted back to earth. We can take Operation Burnt Frost, the US's own anti-satellite test in 2008, as proof of this. There were multiple similarities between this and India's own test, with similar orbits and altitudes. Data from the Combined Space Operations Centre shows that the majority of the debris from this test had fallen back to earth within 2 months, while other pieces that managed to be ejected into higher orbits eventually return to earth about 2 years later  This was just the larger trackable debris. Smaller debris decays faster as it has a larger area to mass ratio, making them more sensitive to atmospheric drag. Even in orbit, molecules do exist that collide with satellites and debris, causing them to lose slow down and lose altitude. This isn't a reason to ignore the problem. If the problem continues to grow as our space activities grow, the potential loss in money from damaging collisions and the potential chain reaction this could cause in a busy earth orbit is going to motivate efforts to fix the problem. It's not just the cost of the satellite that will motivate efforts. Entire economies have been built upon the services they provide. International treaties dictating space operations need to be updated to mitigate the issue, ensuring any satellite placed in Earth orbit will be required to be capable of bringing itself out of orbit and be capable of dodging debris when needed. As we saw earlier with Fermi this is a feature of some satellites, but not all. Some satellites simply become giant bullets when they retire. This cannot be allowed to continue. This alone will not be enough to ensure space debris is kept to a reasonable level, and active clearance may be needed if trends continue. Launching more objects into orbit to solve the problem seems to me to be a very expensive and ineffective way to deal with the problem. Someone will have to fund it, which will be difficult as most companies want to add things to space not remove them. Not only that, but it will also add to the space debris problem with the natural bi-products of launches, while only being capable of taking down large objects. A more promising technology will involve using high power lasers that will be powerful enough to ablate material from the object, which will provide thrust to slow its orbit and thus increase its rate of decay. There are issues with a technology like this, as it could be used as an anti-satellite weapon which would not sit well with other space faring nations. To overcome that it would need to be a joint venture between all space faring nations.  Just as the ISS became an international effort to unite mankind, cleaning up our cosmic neighbourhood can also become a uniting problem. International cooperation and rivalries have long been the driving force for advancement in the field of aerospace. World War 1 and 2 advanced aviation at a unprecedented rate, and it's conclusion led to an international race to the moon. If you want to learn more about this troubled birth of aerospace. I highly recommend watching this documentary titled “Pioneers in Aviation” on curiositystream. It will take you from the early years of the Wright Brothers to the foundation of the Boeing and Douglas Aircraft Companies through the difficult years of the great depression and the rapid advancement during the world wars, culminating in the space race. You can watch all 3 hours of this 3 part series for free, by signing up to curiositystream using the code realengineering, or using the link the description. This will give you a month of completely free access to over 2,400 documentaries and nonfiction titles from some of the world's best filmmakers. After that first free month you can continue your access for just two dollars ninety nine a month.