Orbiting about 250 miles (400-ish km) above our heads is one of the most complex and expensive engineering projects that the human race has ever put together: the International Space Station (ISS). The station masses around 450 tons (400 metric tons) and is a bit larger than an American football field. Its assembly required dozens and dozens of launches by Russia and the US (including 37 space shuttle flights), and it took astronauts and cosmonauts 155 spacewalks to get the whole thing bolted together—2.5 times more spacewalks than had previously occurred since the beginning of space flight.

The ISS has taken 13 years and as much as $150 billion to build and fly; to call it valuable real estate is an understatement. As we Americans are relaxing for the Fourth of July and drinking beers or lighting off fireworks, high above our heads, six human beings are working in space. But the station isn't just sitting up there, static and unmoving. The ISS' orbit decays due to atmospheric drag at the rate of about two kilometers per year; it must periodically be boosted in order to maintain its height. Moreover, the entire massive structure is mobile—it can be rolled and pitched and yawed, or even moved ("translated," in NASA parlance) in three dimensions to avoid a potential collision with debris.

Ars Senior Science Editor John Timmer wrote back in May about the complex process behind moving unmanned satellites around in orbit—specifically, what it took to move NASA's Fermi Gamma-ray Space Telescope out of the way of some debris in its orbital path. But the ISS isn't an unmanned satellite; its mass is much larger. More importantly, it has six living, breathing human beings on board. How does one move 400 tons of fragile space station when there's an asteroid or something bearing down on it?

Ground control to Major Tom

To find out how to throttle-jockey the ISS around in orbit, I took a drive over to NASA's Johnson Space Center and met up with Josh Parris, a NASA ISS flight controller. Parris is one of the people tasked with manning a console in the ISS flight control room—or "Mission Control" as it's more commonly known. His station name is TOPO—Trajectory Operations Officer. As has been the case since the earliest days of manned space flight, the ISS flight controllers are all highly skilled individuals; Parris and his coworkers have all undergone years of specialized training to reach the point where they are trusted with "sitting a console."

"TOPO is in charge of maintaining the knowledge of where the space station and visiting vehicles are, where they're going to be, and to make sure they don't get hit by anything," he explained. There aren't a lot of operational satellites at the ISS' normal flying altitude of about 400 km, but there is a fair amount of debris circling the earth at about the same height. There have been hundreds of potential "conjunctions" in the last couple of years—that is, warnings by ground-based radar sources about potential collisions between the station and some debris. In 2013 alone, there have been 67 potential conjunction notifications.

"What exactly makes up the debris?" I asked Parris. "Is it from the Chinese blowing up satellites?" "That's a big chunk of it," he confirmed. "Also, the collision between the old Russian Kosmos satellite and the Iridium satellite is a source of a lot of the debris we see. And that's just the stuff that's made it down to our orbit; there's plenty of debris still above us, just waiting to come down."

"Who tracks these things?" I asked. "Is there a big computer map like you see in the movies with fancy graphs and stuff?"

"USSTRATCOM, at Vandenberg Air Force Base," he replied. "They maintain a catalog of all pieces of debris—all objects in space—and routinely, three times a day, they screen the ISS trajectory versus that catalog. They're the ones who notify us if there's a close approach."

A "close approach"? That sounds scary. I asked Parris to elaborate, and he explained that there is an imaginary "pizza box"-shaped perimeter around the International Space Station. This perimeter extends two kilometers above and below the station and 25 kilometers "cross-track" and "down-track" (track here refers to the orbital path the station traces). If a piece of debris is expected to come anywhere within that box, USSTRATCOM notifies NASA.

The TOPO controller keeps track of everything inside that pizza box-shaped perimeter, and they compute the probability of collision for all of the objects that they're tracking. TOPO assigns cautionary thresholds to each object depending on how likely a collision is. Any object with between a 1-in-10,000 and 1-in-100,000 chance of colliding with the station meets the "yellow" threshold. Flight rules say that the station must be moved out of the way in response to a yellow threshold object unless such a move results in a mission impact—"Like, if we do the burn, we're going to miss an opportunity to launch a Soyuz, for instance," explains Parris. "Do we delay the Soyuz, or do we do the maneuver?" A "red" threshold is assigned to any collision with a likelihood of between 1 (in other words, absolutely certain) and 1-in-10,000. Flight rules are more strict for maneuvers in response to red threshold objects: the station is always moved for a red threshold object, regardless of mission impact, unless a maneuver represents more risk than not maneuvering (for example, if there's a piece of equipment that's damaged on the ISS and a maneuver would exacerbate that damage).

Thrusters and gyroscopes

The ISS, for all its size and apparent fragility, is actually pretty agile. It has four gyroscopes, called Control Momentum Gyros, or CMGs, which allow it to change its attitude. These gyros fall under the responsibility of the ADCO (Attitude Determination and Control Officer) flight controller, with whom TOPO often works when figuring out how to handle incoming debris.

Additionally, the station has several sets of thrusters that allow it to rotate and translate. The Zvezda service module is equipped with thrusters, and there are thrusters on docked vehicles like the Progress resupply craft and the ESA ATV that can also be employed. Space shuttles could also be used when they were still operational. For a typical debris avoidance maneuver, the station will be subjected to delta V of between 0.5 and 1 meter per second.

One of the parameters TOPO keeps track of is the station's mass, since the precise amount of thrust required to generate the required delta V varies depending on the mass that must be moved. The ISS' mass varies primarily when vehicles dock and undock. The level of delta V generated during a typical avoidance maneuver isn't enough to disrupt the routine of the crew—they're aware of when the maneuvers are happening, but it's all controlled from the ground and they don't have to do anything special. "They're not up there with joysticks zooming the station around, are they?" I asked. "No," laughed Parris. "It's all commanded from the ground."

Listing image by Lee Hutchinson