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 together2.5 times more spacewalks than had previously occurred since the beginning of space flight.

NASA

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 mobileit can be rolled and pitched and yawed, or even moved ("translated," in NASA parlance) in three dimensions to avoid a potential collision with debris.

Lee Hutchinson

Ars Senior Science Editor John Timmer wrote back in May about the complex process behind moving unmanned satellites around in orbitspecifically, 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?

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 roomor "Mission Control" as it's more commonly known. His station name is TOPOTrajectory 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."

Lee Hutchinson

"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 yearsthat 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?"

Continued here:

How NASA steers the International Space Station around space junk