## 2 Space Debris--A Primer
### The Hazards Posed by Space Debris  
The extent of the damage caused by the impact of a piece of space debris on a functioning spacecraft or satellite depends on how big it is and what it is made of. Depending on the size of the impacting debris particle, it can either cause minor damage like a small hole (if it is the size of a grain of sand, for example) or destroy an entire satellite or spacecraft  
Figure 2: Change in the number of objects in Earth orbit. (Image credit: Orbital Debris Quarterly News, July 2024 Issue.) (if it is the size of a cricket ball, for example). But, of course, catastrophic damage can also come from small collisions--imagine the mirror of the Hubble Space Telescope being struck by something, even something as small as a sand grain, traveling at orbital velocity. The telescope as a whole might survive, but its mirror could be destroyed if the impact were to occur in a critical location. So, in general, while larger pieces cause more damage, the impact of a tiny piece of debris can also render a functioning spacecraft useless, depending on where the impact occurs. Why are impacts by pieces of space debris such a problem? One reason is that orbital debris travels at speeds in the order of 8 km/s, so the closing velocity between a piece of debris and a satellite can be as high as 16 km/s. This energy has to be absorbed by whatever the debris particle hits, which can cause catastrophic failure. A common misconception is that space is big and empty, so why do we even worry about being hit by anything in that vast emptiness? The reality is that debris particle impacts happen all the time. Luckily, nearly all of these impacts are by very small particles, so they are hardly noticed. As a result, the question is not IF a satellite will get hit by a piece of debris, but HOW BAD will the damage be WHEN it does get hit. Spacesuits, for example, can be punctured by a projectile 1 mm in diameter [12]--this makes EVAs a very tricky business! Figure 3 below shows a portion of the International Space Station (ISS), along with several impacts (quite a few, actually) on one of its radiators. So, spacecraft designers work very hard to protect the spacecraft we send into LEO, especially the ones with humans on board, like the ISS. Debris travels slower in GEO (in the order of only several hundred m/sec), so satellites in GEO typically do not have a lot of protection. Satellite protection in LEO is often in the form of shielding that is placed on the exterior of a spacecraft, which is basically there to disperse the energy of an impact when it happens. On occasion, a threatened satellite or spacecraft, such as the ISS, is able to  
Figure 3: Multiple impact sites (red circles) on the International Space Station. (Image Credit: NASA.) move out of the way of a piece of debris that is thought to be on a collision course with it. This \"collision avoidance\" technique is one type of \"active protection\" of our space assets (shielding is referred to as \"passive protection\", since, like a car bumper, it is there \"just in case\" you need it). Figure 4 below shows a summary of the number of times that the ISS has had to implement a collision avoidance maneuver since the mid-1990s--an average of 2-3 times a year. The blue circles indicate the number of potential orbital-path-crossing objects that have come to NASA's attention over that same time period--a number which, overall, is seen in Figure 4 to be steadily increasing. Needless to say, this is a very dangerous trend! Of course, shields protect the working satellites in Earth orbit, but they do nothing to keep defunct objects from falling back down to Earth, which they do from time to time. As a result, space debris is not just a problem for satellites and astronauts--it is also a problem for the people living on Earth. In addition to physical harm to animals and people, hazards posed by debris that falls to Earth include pollution of the atmosphere, airline flight delays or cancellations, and toxic or radioactive debris on the ground, among others.