CHAPTER 8 DAY IN THE LIFE: DEBRIS AVOIDANCE—NAVIGATING THE OCCASIONALLY UNFRIENDLY SKIES OF LOW-EARTH ORBIT 152 A Close Call Captain Daniel C. Burbank, Expedition 30 Orbital debris penetrating the hull of our spacecraft is one of the “Big 3” threats that astronauts and flight directors worry about the most—along with fire and a toxic atmosphere. Orbital debris is a particularly insidious threat because there are hundreds of thousands of chunks of debris traveling around the Earth at enormous velocity, and most are too small to track with radar. Because of this, we essentially fly spacecraft in low-Earth orbit—in what pilots might euphemistically refer to as the “big sky, little airplane” theory of operation—where the likelihood of running into something is statistically extremely low. The difference between flying airplanes and flying spaceships is that things move MUCH faster in space than they do in the air, meaning the kinetic energy is exponentially higher. Something traveling in low- Earth orbit at a typical 28,000 km/hr (17,500 miles/hr) might travel at 100 times the speed of an airplane flying at 282 km/hr (175 miles/hr), but its energy (per given kilogram of mass) is 10,000 times greater. Although the odds are low that we will hit anything, the consequences of doing so are therefore potentially catastrophic. The call came up from Houston late in the evening on Friday, March 23, 2012, that JSpOC was tracking a late-notice conjunction—space jargon for a short- notice potential impact from orbital debris. The TCA was early the next morning. This hunk of debris was left over from a 2009 collision between an out-of-service Russian Kosmos satellite and an Iridium communication satellite. It was “draggy” (not very dense), thereby making its trajectory difficult to predict and track, which further limited the prediction accuracy. The way conjunctions usually work is that they are identified well in advance and, even if they start out red (high risk and/or high uncertainty), they gradually become yellow (moderate risk) and then green (essentially no risk) as the trajectories are refined. Even when they don’t turn green, the ground usually has plenty of time to plan an ISS or Progress engine burn, called a DAM, and nudge the space station away from the impending impact (this was before the Russian and American programs were able to implement the quicker PDAM process). In this case, the uncertainty stayed high and the exceptionally late notice Much of the debris from these events is actually above the altitude of the ISS. Figure 10 shows the density (number) of tracked objects present in a cubic kilometer of space at a given altitude. As is shown in Figure 10, the highest density of debris is at approximately 800 km (500 miles) altitude, and was the result of the Iridium and Fengyun events. Figure 10. Low-Earth orbit debris density (from http://www.unoosa.org/pdf/pres/stsc2011/tech-31.pdf, NASA report). The y-axis displays the density of objects in terms of number of objects per cubic kilometer while the x-axis shows the altitude. The collision between the Iridium and Cosmos satellite lead to a peak of about 5.5 x 10-8 particles per cubic kilometer at an altitude around 800 km (500 miles). Although the bulk of this debris is above the ISS orbit, it will descend over time due to atmospheric drag, and the number of conjunctions with the space station will increase. For these reasons, the flight control team continues to refine its tools available to assess and protect against threats from orbital debris.