CHAPTER 8 DAY IN THE LIFE: DEBRIS AVOIDANCE—NAVIGATING THE OCCASIONALLY UNFRIENDLY SKIES OF LOW-EARTH ORBIT 142 Orbital Debris—Conjunctions and Relative Velocities Although debris comes from many sources, most travel at a very high speed relative to the ISS, due to orbital mechanics. This chapter will examine a common debris source: spent rockets. Many communications satellites operate at a high altitude (37,000 km [22,991 miles]) that causes them to orbit at the same rate in which the Earth rotates. This process is termed geosynchronous. As a satellite travels to that altitude when first launched, a rocket stage is often used and then expended with an orbit that has a high point of many thousands of kilometers and a low point of a few hundred kilometers above the Earth. This particular elongated orbit is called a geosynchronous transfer orbit, and an orbit of this shape is more generally called an elliptical orbit. Oftentimes, rocket bodies left in these transfer orbits later explode or otherwise disintegrate into debris that travels in roughly the same elliptical orbit. Over time, atmospheric drag at the low point of the orbit gradually drops the high point of the orbit until the debris reenters the Earth’s atmosphere. The process can take decades or even centuries, depending on how much drag the object creates. Figure 3 shows an example of a piece of debris that is in an elliptical orbit and at a different orientation than that of the ISS. ISS Debris Conjunction Figure 3. Conjunction with an object in an elliptical orbit. The ISS travels in a circular orbit at a lower altitude while the debris travels in an elliptical orbit. The lower portion of the debris’ orbit can cross the plane of the ISS. In this elliptical orbit, an object travels quickly when closer to the Earth (for the transfer orbit described above, 35,600 km/h [22,000 miles/h]) and slower when far away from the Earth (for the transfer orbit, 5,700 km/h [3600 miles/h]). In this orbit, it takes 10.5 hours for the object to complete an orbit of the Earth. In comparison, the ISS orbits in a nearly circular orbit of 400 km (249 miles) and stays at about 400 km (249 miles) above the Earth as it travels. The velocity is constant at 28,000 km/h (17,300 miles/h), and it takes the ISS 90 minutes to go around the Earth. The ISS and any given piece of debris will probably never cross paths. With 23,000 large objects being tracked in orbit, however, the ISS typically has a close approach every few days. For the example shown in Figure 3, a close approach would involve the ISS traveling 28,000 km/h (17,300 miles/h), and the debris traveling 35,000 km/h (22,000 miles/h) in a different direction, due to the angle between the orbits. This close approach is called a conjunction. A great deal of effort goes into assessing the risk from this conjunction and protecting the ISS from a catastrophic collision. Protecting the International Space Station from Space Debris The ISS has been shielded for smaller pieces of orbital debris (up to 1 cm [0.4 in.]) and is the most heavily shielded spacecraft ever flown. The shielding generally consists of a metal outer bumper offset from the inner pressure shell, which is also known as a Whipple Shield (see Chapter 3). When debris strikes the outer bumper, the debris vaporizes and dissipates the kinetic energy of the space junk, pitting the bumper but leaving the inner pressure shell intact. The portions of the ISS most likely to incur a strike are the portions that face forward into the direction of flight when traveling in the normal local vertical/local horizontal attitude. For this reason, the shielding is the highest on the US Segment, which faces into the normal flight direction of the ISS.