How to Dodge a Space Bullet in Three (Not So Easy) Steps

Avoiding superfast space trash is part of life on the International Space Station.

The International Space Station is in constant danger of super-fast space debris.
National Journal
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Alex Brown
April 3, 2014, 1 a.m.

Get­ting to space isn’t easy. Dodging bul­lets once you get there is even harder.

Thanks to care­less­ness and satel­lite col­li­sions, Earth’s at­mo­sphere is littered with a half-mil­lion or so pieces of debris. And they’re all trav­el­ing 17,500 miles per hour — roughly 10 times the speed of a bul­let. Even a golf ball at that speed could take out a satel­lite sys­tem.

The dangers of space debris are not lost on NASA, par­tic­u­larly as it at­tempts to pro­tect the In­ter­na­tion­al Space Sta­tion and the as­tro­nauts who live in­side it. In fact, just this month NASA had to move the In­ter­na­tion­al Space Sta­tion to avoid a po­ten­tial col­li­sion. Here’s how NASA does it:

Step 1. Track the debris.

This ac­tu­ally isn’t NASA’s job. Most debris track­ing is left to the Air Force. With its Space Sur­veil­lance Net­work, the Air Force fol­lows roughly 15,000 pieces of whizz­ing space trash. The net­work is made up of 30 radar and tele­scope fix­tures that keep a run­ning satel­lite cata­log.

Each day, the Air Force con­ducts roughly 400,000 spot checks to con­firm ob­jects are in the or­bit they’re pre­dicted to be. How im­port­ant is it to know where debris is? Ac­cord­ing to NASA, even paint flecks trav­el­ing at or­bit­al speeds can dam­age a space­ship.

Track­ing has be­come even more dif­fi­cult — and cru­cial — since last year. Se­quest­ra­tion forced the Air Force to shut down the Space Fence, a series of radars that had ac­coun­ted for 40 per­cent of the net­work’s ob­ser­va­tions.

The Space Fence had served as a trip wire, pass­ively watch­ing for new activ­ity rather than act­ively track­ing known ob­jects. Without it, it be­comes harder to see col­li­sions than can lead to lar­ger debris fields. A new, bet­ter-equipped Space Fence is sched­uled to be de­ployed in 2018. For now, NASA main­tains that cur­rent track­ing is up to the chal­lenge of keep­ing its ships safe.

Step 2. As­sess the risks.

Every eight hours, track­ers check the or­bit path of the space sta­tion against known debris that might cause danger. If ob­jects will pass with­in a cer­tain prox­im­ity, they alert NASA. In the first half of 2013, for ex­ample, 67 such warn­ings were sent.

Once warned, NASA starts crunch­ing the num­bers. Sci­ent­ists try to de­term­ine if the ob­ject will pass with­in a so-called “pizza box.” That’s the area about a mile above and be­low the space sta­tion, ex­tend­ing about 15 miles to each side.

If debris might enter that area, NASA and its Rus­si­an part­ners start to come up with a plan. If col­li­sion risk is at least 1 in 100,000, the space sta­tion will make a pre­cau­tion­ary move un­less it in­ter­feres with its mis­sion. If the risk reaches 1 in 10,000, a move be­comes man­dat­ory.

Plan­ning a move re­quires about 30 hours, NASA says. Move­ments gen­er­ally take place sev­er­al hours be­fore debris gets close.

Step 3. Get out of the way.

This can be achieved in sev­er­al ways. Gyro­scopes on board the ISS can re­ori­ent where the sta­tion faces with­in its or­bit. Thrusters can move it in­to a dif­fer­ent or­bit. In ad­di­tion, NASA can use the pro­pel­lant sys­tems of space­craft at­tached to the sta­tion to change its loc­a­tion.

To cal­cu­late the prop­er thrust, NASA keeps pre­cise track of the sta­tion’s mass, de­pend­ing on which vehicles are docked. Be­fore a man­euver, NASA does a double check with debris track­ers to make sure the move­ment won’t in­ter­sect ISS with a sep­ar­ate piece of space trash.

In the rare case debris is dis­covered at the last second, mis­sion con­trol moves in­to the Pre­de­ter­mined Debris Avoid­ance Man­euver, which moves ISS to a dif­fer­ent loc­a­tion at a half-meter per second. If that’s not an op­tion, the crew takes shel­ter in the Soy­uz es­cape craft.

Move­ments are con­duc­ted from the ground; as­tro­nauts don’t hit a throttle to re­lo­cate their home. In fact, dur­ing the debris man­euver earli­er this month, the crew was asleep as ground en­gin­eers moved them to safety.

NASA says there wasn’t likely to be an im­pact, but the sev­en-minute man­euver con­duc­ted March 16 provided a “healthy mar­gin of clear­ance” from the debris of a Rus­si­an satel­lite launched in 1979.

En­gin­eers used thrusters from Rus­sia’s Pro­gress 54 space­craft that was docked with the ISS, rais­ing the sta­tion’s alti­tude by about half a mile. The debris passed safely with plenty of room to spare, NASA said.

What next? Back to track­ing, wait­ing for the next space bul­let with as­tro­nauts in its sights — and get­ting out of the way be­fore it’s too late.


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