CHAPTER 8 DAY IN THE LIFE: DEBRIS AVOIDANCE—NAVIGATING THE OCCASIONALLY UNFRIENDLY SKIES OF LOW-EARTH ORBIT 140 The potentially destructive nature of space debris. This photo (from a ground test) shows the damage done to a solid block of aluminum by a small 7-g (0.2-oz) projectile traveling at 7 km/s (4.3 miles/s). The low-Earth orbit environment in which the International Space Station (ISS) flies is, compared to anything on the Earth, a very empty place. But it is not completely empty. The detritus of more than 50 years of human activity in space encircles the Earth as a cloud of orbital debris— a nearly invisible threat to every satellite in orbit, including the ISS. Orbital Debris—A Serious Threat to all Spacecraft Similar to the way the ocean floors across the globe are the final resting place for shipwrecks from thousands of years of human seafaring, the remnants of more than 50 years of human activity in space has left bits and pieces of hardware that continue to orbit the Earth. This debris (popularly known as “space junk”) consists primarily of dead satellites, expended stages from rocket launches, and fragmentation from collisions, explosions, or other breakups of these initially large pieces of hardware—sometimes decades after their mission has ended. The size of the junk ranges from multi-ton satellites and rocket stages to small-piece parts of satellites such as nuts and bolts, and even paint chips. These objects all orbit the Earth at up to 28,000 km/h (17,500 miles/h) in various orbits, meaning that any encounter between them and an operational satellite such as the ISS will usually be at extremely high velocities and would result in a hypervelocity-impact collision. The effects of a collision on a satellite can range from minor to catastrophic, depending on the velocity and especially the size of the impacting object. Many instances of damage have occurred from collisions between operational spacecraft and space debris.