Tossing Out Trash From the Space Station Takes More Planning Than You’d Think

Job one is to make sure it doesn’t come back and hit you at 17,000 mph.

International Space Station
Standing on one end of a big robotic arm, Clay Anderson settled months of debate within NASA on July 23, 2007, when he manually shoved a large ammonia tank safely away from the International Space Station. This manual-jettison maneuver became standard procedure.

Astronaut Clay Anderson leaned back, his feet fixed in the portable foot restraint attached to the end of the International Space Station’s robot arm. In his gloved hands he held onto a pair of handles mounted on the side of a refrigerator-size object called the Early Ammonia Servicer (EAS). Clay leaned farther backwards and then rotated his body forward to stand upright, releasing the EAS as he stood up in the foot restraint. “Jettison!” he shouted as the EAS tumbled away from his outstretched arms.

Before Anderson’s July 2007 EVA, the ISS program had disposed of trash by loading it aboard space shuttle orbiters for return to Earth. But the loss of Columbia in 2003 had grounded the shuttle fleet, requiring us to develop another solution. As technical lead for space environments in the ISS Program Integration Office, I proposed an official jettison policy, which would allow astronauts to push space junk into a decaying orbit where it would burn up during reentry.

Working closely with Nick Johnson, one of the world’s foremost authorities on orbital debris, I documented the requirements for any jettison operations from the ISS. Flight dynamics experts developed the requirements governing orbital separation and tracking of jettisoned objects, and EVA operations specialists outlined the capabilities of astronauts to manually perform jettisons. The U.S. State Department even weighed in with a requirement that any jettisoned object must pose less than a one-in-10,000 chance of harming anyone on the ground.

Jettisoning trash from a spacecraft is no mere stroll to a dumpster. First and foremost, you have to make sure that whatever you throw away doesn’t come back and hit you—a frightening possibility in the weird realm of orbital mechanics.

Our idea was to have EVA astronauts manually push jettisoned items away in the direction opposite the station’s orbit. Analysis showed that a surprisingly small retrograde change in velocity was required: only about 1 to 1.5 inches per second would ensure no recontact. The drag of the jettisoned object would be greater than that of the ISS, further ensuring that the jettisoned object would keep moving behind and below the ISS until it eventually burned up in the atmosphere. In the case of the EAS, however, we scheduled a thruster burn to raise the ISS orbit after jettison just to make sure. Safety engineers insisted that we define a jettison “cone” to account for any directional errors, so that even if an object were at the edge of the cone, it would still fly away safely. Simple trigonometry led to the conclusion that pushing an object away at two inches per second (a rate easily achievable by an EVA astronaut) within a 30-degree cone centered on a line directly opposite the direction that the ISS was traveling as it orbited the Earth would be enough.

We successfully jettisoned a few objects and received little attention. This changed in early 2006, when safety personnel remembered that the Early Ammonia Servicer was nearing the end of its design life.

The EAS had been carried up by the space shuttle Discovery on STS-105 in August 2001. It was essentially an ammonia reservoir that could be used to top off the space station’s cooling system in the event that leakage occurred. The ISS cooling system proved to be leak-free, and so we found ourselves with a full EAS at the end of its five-year design life. The engineers who’d designed it could not certify that the structure would remain intact after five years. There was a risk, albeit a very small one, that it might simply break off the station. Furthermore, the shuttle program refused to carry the fully-loaded EAS tank back to Earth, citing potential danger to both the flight and ground crews.

And so we found ourselves in a pickle: It wasn’t safe to return the EAS to the ground, nor was it safe to leave it on board. I suggested we jettison the EAS during an upcoming EVA. The safety team hated the idea: They didn’t believe a single astronaut would be physically capable of shoving a big tank full of toxic ammonia away from the ISS, and they were also skeptical of the orbital mechanics calculations that had indicated this method was safe. But my managers backed me, and I began my long campaign to convince their bosses—all the way up to NASA Administrator Michael D. Griffin—that jettisoning the EAS was the right call.

Despite all our analyses and simulations, it remained to be proven that an EVA astronaut in a foot restraint at the end of the ISS’s robotic arm could push the massive EAS with enough velocity to ensure separation. Discussions with experienced astronauts, and tests of a full-scale EAS mock-up on a special “air bearing floor”—picture a glorified air hockey table­—at the Johnson Space Center gave us plenty of confidence in our plan. Still, one colleague told me “we’ll jettison EAS over my dead body.” Finally, less than a month prior to the launch of space shuttle Atlantis carrying Anderson up as the flight engineer for ISS Expedition 15, I presented my case at Kennedy Space Center. My plan to jettison the EAS was approved.

Two months later, I stood in the robotics support room in Mission Control. At last the months of hard work were coming to an end. I even had a draft email written to my colleague who had said all this would happen over his dead body. (Subject line: “Just checking on you.”) But as the EAS moved slowly away into its decaying orbit, to my horror it appeared to be heading straight for one of the station’s huge, flimsy solar arrays! My 27-year career at NASA flashed before my eyes. Fortunately it was just a trick of the camera’s orientation, and the EAS easily cleared the ISS structure. A short time later the ISS crew performed a small separation burn for added insurance. Mission accomplished!

I pulled up that email and let my finger hover over the “send” key. But my better angels prevailed, and I deleted it instead. Less than 16 months later, the EAS re-entered the atmosphere. Parts of it remained intact enough to splash into the vast ocean between Australia and New Zealand, harming no one on the ground or on the sea.

Mike Engle retired from NASA after 38 years, having served as a space shuttle flight controller and a chief engineer for the astronaut office, among many other roles.

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This story is a selection from the February/March issue of Air & Space magazine

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