141 DAY IN THE LIFE: DEBRIS AVOIDANCE—NAVIGATING THE OCCASIONALLY UNFRIENDLY SKIES OF LOW-EARTH ORBIT CHAPTER 8 For example, on Space Transportation System (STS)-93, a collision with a paint chip put a 10-mm (0.4-in.) crater into one of the windows of Space Shuttle Discovery, thus leading to its replacement, post mission. The ISS, having been in orbit since 1998, bears the scars of many impacts, including a hole in the edge of a radiator panel on the P6 truss segment (Figure 1), and a shattered portion of a solar array (Figure 2) that was caused by a piece of small debris. ~1 m (~3 ft) Figure 1. Damage to the Trailing Thermal Control Radiator on the P6 truss segment was noticed during a spacewalk in August 2016. Figure 2. Damage to space station solar array by space debris. Without question, the most dramatic event was the collision between the active Iridium 33 communications satellite and the abandoned Kosmos 2251 military communications satellite. The 950-kg (2094-lb) Kosmos and 560-kg (1234-lb) Iridium collided at 42,120 km/h (26,173 miles/h). This collision resulted in the complete destruction of both satellites, and generated more than 1,000 new pieces of space debris that were larger than 10 cm (4 in.) in diameter. The US Department of Defense (DoD) actively tracks (and helps NASA and the ISS avoid) objects as small as 10 cm (4 in.) in low-Earth orbit. Approximately 23,000 objects of this size, other than a few hundred active satellites, are currently in orbit and are classified as orbital debris. Based on ground-based sensors, examination of returned satellite parts, and statistical methods, scientists believe approximately 500,000 objects that are greater than 1 cm (0.4 in.) are in orbit. The population of objects in recent years has increased due to events such as the Iridium/Kosmos collision described above. Hypervelocity Impacts A hypervelocity impact releases a tremendous amount of energy for a given amount of mass, much more so than (for example) a bullet striking a target. Bullets travel on the order of 3500 km/h (2175 miles/h), and typically punch holes in targets. Relative velocities of two objects on a collision course in orbit are roughly 10 times this much, and the collision for the bullet example would involve 100 times as much energy. At this kind of impact velocity, the resulting release of energy is essentially an explosion.