This week, a spacewalking cosmonaut will tee up a golf ball just outside the international space station and let loose with a publicity-generating drive – a shot that has already generated a debate over the dangers posed by orbital debris. But if you think one foam-rubber “golf ball” is a cause for concern, how about a concert grand piano?
That’s roughly the size and weight of the piece of equipment NASA plans to throw over the space station’s side next spring — if they can figure out which way it’ll go once it’s cast away.
The 1,430-pound (650-kilogram) unit is called the EAS, or Early Ammonia Servicer. It was installed in mid-2001 as an emergency reserve for the station’s coolant system. Once the mature thermal control system is activated next month during Discovery's STS-116 mission to the space station, the EAS becomes surplus space hardware.
NASA had originally planned to bring the EAS back to Earth inside a returning space shuttle — but the subsequent tightening of shuttle cargo manifests in the face of a hard “stop flying” date of 2010 eliminated that option. Keeping the apparatus (and its potential hazardous liquids) attached is also not a safe option. So NASA developed a tentative plan to jettison it manually during the STS-118 mission next July, and now are considering moving that up to the STS-117 mission in March.
Just push it?
NASA orbital analysts originally thought that the safest way to do so was to push it away from the station manually in the direction opposite the station’s motion. This worked fine last spring for SuitSat, a worn-out spacesuit that was hooked up with amateur-radio equipment to become an experimental satellite. SuitSat fell behind and below the space station, then slipped into a lower but faster orbit, and passed safely beneath. Month by month, SuitSat continued to fall, never threatening a collision with the station.
The problem with EAS, which makes it different from every other object deliberately or accidentally separated from any previous human-occupied space station, isn’t just that it’s heavy. It's also dense. It is, in fact, denser than the space station, which gives it a ballistic number — the degree to which anobject resists air drag, measured in mass per cross-sectional area — up to twice as high, depending on which side of the EAS faces “into the wind” of the thin atmospheric drag at that altitude.
Instead of dropping into a lower, faster orbit, the EAS will hold its momentum better than the station, and the station will drop faster and pull ahead. So far, so good — but the station needs to maintain its perpetually decaying altitude through repeated reboosts. Keeping track of the derelict EAS looming in its higher orbit could significantly complicate navigation at Mission Control in Houston.
Practicing ‘safe separation’
Meanwhile, just the physical process of casting EAS loose has been a challenge. Spacesuited astronauts have been practicing “safe separation” for more than a year in test facilities at NASA’s Johnson Space Center. For practice, they have used a frictionless air-bearing table, where the astronauts as well as the objects they're manipulating ride on cushions of air that provide no resistance to horizontal motion.
From this experience, a plan has emerged. In space, the shover-astronaut will stand on a platform attached at the end of the station’s robot arm, while another astronaut undoes the EAS’s restraint bolts. Once it’s free, the astronaut on the robot arm is supposed to use his or her hands to push the unit gently toward the station's back end. Too forceful, and the safety relief hinge of the arm’s platform will fold away; too gentle, and the EAS might not clear the station's structures on its way out.
Specialists had calculated that a 2-inch-per-second (5 cm/sec) push and a departure angle within 30 degrees to the station's orbit ought to be adequate for near-term safety. Since astronauts have regularly achieved rates up to four times faster with much tighter angular precision, getting rid of the unit no longer poses a clearance challenge. But the problem of keeping the unit at a safe distance has grown more serious as analysts realized how the EAS would behave after it’s jettisoned.
‘Honey buckets’ keep falling on our heads
Deliberate disposal of garbage from manned space stations in the 1970s and 1980s was the rule, not the exception — and experience indicated it was safe. If the trash dumps departed in a safe direction, air drag effects made them descend into lower orbits. Conversion of potential energy (altitude) into kinetic energy (speed) made them speed up and pull ahead of their former homes, but they never would get high enough to threaten recontact.
The Russians even installed a trash airlock for the regular disposal of food containers, dirty clothing, small items of failed equipment, and sealed fecal filters from their space toilet. When military trackers at the North American Aerospace Defense Command, or NORAD, detected new subsatellites drifting away from Soviet space stations, they would catalog the blips and humorously refer to them as "honey buckets" (an old, old euphemism, most recently an aviation term for the spartan latrine in the back end of an aircraft).
Even on Mir in the mid-1990s, space littering was the accepted practice. NASA astronaut Shannon Lucid, who spent half a year aboard the station in 1996-1997, recalls being astonished to see her Russian shipmates “Yuri and Yuri” (as she affectionately referred to them – Yuri Usachev and Yuri Onufrienko, who both later commanded expeditions on the international space station) gray-taping together masses of surplus packaging foam a few days before any scheduled spacewalk. Once in the airlock, they shoved the 3- to 4-foot-long (meter-long) conglomerations out the hatch and tossed them overboard.
The past and future of space junk
These activities, aside from giving earthside radar operators more blips to catalog, did no real harm. The reason is that the air drag, even at the 220-mile (350-kilometer) average altitude of the space stations, was enough to inexorably rob this litter of its orbital energy, causing them to slip lower and lower, and burn up within months. Even large, heavy pieces of Russian station-related hardware that were left in orbit could be safely tracked during the several years it took them to slip from space.
The same has been the case with the typical pieces of junk that have come off the international space station. However, the availability of earthbound cargo space in the shuttle (which brings it all the way home) and to a lesser extent in the forward holds of departing Progress and Soyuz vehicles (which are jettisoned to burn up in the atmosphere) has discouraged the policy of throwing stuff out from the station itself.
The space junk problem posed by the piano-sized EAS is just the first of its kind, and it won't be the last. In years to come, when the shuttle stops servicing the station and other vehicles come on line, none will have the down-cargo capacity of the Columbia-class winged orbiters. Big pieces of the station, perhaps replaced by large structures sent up on new launch vehicles and space tugs, will have to be disposed of along the same path that the EAS will soon pioneer.
It’s a harbinger of big space junk problems in the next decade — so getting it right this first time may be critical to planning for the international space station's future.