Read our interview with Dr Space Junk Alice Gorman<\/a><\/strong><\/em><\/p> The International Space Station<\/a> and China\u2019s Tiangong space station also occupy low-Earth orbit, both at an altitude around 400km.<\/p> There are plenty of objects like artificial satellites and bits of spacecraft that have fallen out of use and remain in orbit around Earth. These are collectively known as space junk.<\/p> Satellites can remain in position long after they have stopped functioning and the process of launching them is also a decidedly messy affair: spent rocket stages and separation bolts also contribute to the debris circling around Earth.<\/p> All of this space junk is orbiting Earth at speeds of up to 28,000km\/h and as it continues to accumulate it poses an increasing risk of collision.<\/p> For example, in the 20 years of its operation, the ISS has had to shift around 30 times to avoid being hit by orbital debris.<\/p> But the problem isn\u2019t just the risk of a collision, because a satellite hit by debris can shatter into thousands of fragments, any of which can go on to collide with other satellites and so on.<\/p> This could potentially trigger a runaway cascade of satellite destruction \u2013 a chain reaction of collisions \u2013 until a whole region of orbital space is so full of hazardous debris that it\u2019s rendered unusable or impassable for decades.<\/p> This possibility is known as the Kessler Syndrome, and will be familiar to anyone who\u2019s watched the film Gravity <\/i>(one of our pick of the best space and sci fi movies<\/a>).<\/p>![\"The](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2020\/11\/ISS-orbiting-Earth-ef8a841.jpg\" "\\"\\"\/")