During more than seven years of operations by the International Space Station (ISS), approximately three dozen pieces of debris were released and subsequently cataloged by the U.S. Space Surveillance Network (SSN).
Although some of the items were separated from the ISS accidentally, some were intentionally cast-off. Once the object has safely cleared ISS, it is possible, in some cases, for the object to return to the vicinity of ISS and to pose a later collision risk. Most objects will decay faster than ISS, and, therefore, will quickly fall below the altitude of the Earth orbiting structure.
An official ISS "Jettison Policy" has been developed and is going through an approval process, crafted to ensure that decisions to deliberately release objects in the future from the ISS are based upon a complete appraisal of the benefits and risks to the space station, other resident space objects, and we folks down here on the Earth.
Although debris of all sizes are of interest, an emphasis of the new ISS jettison policy is on dealing with objects that might pose the greatest risk to the station itself, its stable of logistics vehicles--such as the space shuttle, Soyuz, Progress, Europe's Automated Transfer Vehicle, and Japan's H-2 Transfer Vehicle--as well as other operational satellites.
"The release of debris from space stations has been a common occurrence for more than 30 years," advised Nicholas Johnson, NASA Chief Scientist for Orbital Debris at the NASA Johnson Space Center in Houston, Texas.
"The generation of debris onboard a space station is a natural consequence, but its accumulation can present a direct or indirect hazard to the crew as well as result in reduced productivity," Johnson reported at the 36th Committee on Space Research (COSPAR) Scientific Assembly, held July 16-23 in Beijing, China.
On one hand, it is normally preferable to dispose of debris via a logistics vehicle--like cramming refuse into an automated Russian Progress vessel that is then propelled into Earth's atmosphere for destructive disposal.
However, the size of some debris could prevent its transfer to another vehicle or the nature of the debris could pose a health hazard to the space station crew. "Hence, the jettison of debris into space, in special cases, can be the most viable option," Johnson explained.
Work on a jettison policy for ISS began at NASA's Johnson Space Center, kick-started informally several years ago. Talk was spurred by two factors: recognition that refuse was already mounting up within the space station, creating difficulties for the crew; and significant hardware attached outside the ISS needed to be removed in the future.
"Following the loss of space shuttle Columbia in February 2003, and the immediate cessation of shuttle flights, the debris situation onboard ISS worsened without the opportunity to routinely remove tons of debris via the space transportation system," Johnson pointed out.
Rate of release
Taking a look at previous space stations circling Earth, Johnson made these observations:
- During three months of manned operations with the Soviet
Salyut 4 space station in 1975, more than a dozen debris from the
orbital complex were detected and cataloged by the U.S. Space Surveillance
6, the first long-duration Soviet Union space station housed crews for
four years and in the process produced more than 100 new debris.
7 was responsible for twice as many debris in a similar time interval
as Salyut 6.
- The former Soviet Union's Mir space station, which supported crews over a span of 14 years, created more than 300 cataloged debris events, but its rate of release was significantly lower than its predecessor.
Turning to the U.S. Skylab space station that orbited Earth in the 1970s--a program that saw a trio of visitations by individual three-person crews--Johnson told SPACE.com: "The Skylab mission, including all three crew expeditions, had a total of only 22 debris identified and cataloged by the U.S. Space Surveillance Network. Many of these were associated with the launch and deployment of Skylab before the arrival of the first crew."
Objects, small and large
In the era of the ISS and spacewalks around the complex, both small and large objects, are often thrown off the station for convenience--although tools on occasion accidentally slip away.
Such was the case in December 1998 when a slidewire carrier and a worksite interface were lost by members of the STS-88 crew while spacewalking for ISS. These objects were large enough to be tracked and cataloged. Three other objects were also released by STS-88 spacewalkers, one inadvertently (an insulation blanket) and two by design (antenna spools).
The sizes of debris cast off into space can vary dramatically. Some are off-the-radar screen, untrackable flotsam. On the opposite end, Johnson said, there's the 33-foot (10 meters) in diameter KRT-10 antenna that was cosmonaut-kicked into space from the rear of Salyut 6 when it failed to eject automatically.
Most space walks do not involve the jettison of large objects from ISS, but they often result in small objects being released, Johnson reported. For instance, during spacewalk operations performed by the ISS Expedition 10 crew in January 2005, a total of 20 objects were released: 16 electrical caps and four covers. These small items were not tracked and cataloged by U.S. surveillance tracking gear. All probably reentered rapidly, he explained.
One of the most unusual objects jettisoned from ISS was a four-year-old Russian Orlan M spacesuit, Johnson pointed out. Instead of loading the used suit into a Progress vehicle for disposal, the ISS crew equipped it with a transmitter and dubbed it Radio Skaf.
This "Suitsat" was released in early February of this year, and isn't expected to reenter until early to mid-September.
Restricted to special occasions
Johnson emphasized in his COSPAR paper that the release of debris from space stations--and stemming from human space operations in general--has been commonplace for more than four decades.
"Due to the relatively low altitude of such activities, all debris are relatively short-lived and have no long-term effect on the near-Earth space environment," Johnson noted.
Recognizing the occasional need to jettison items, Johnson concluded, the ISS program has been developing a formal jettison policy. Logistics vehicles, he noted, will remain the primary means of removing refuse and non-functional items from the ISS.
"The jettison of debris will be restricted to special occasions dictated by safety and/or perational needs. Moreover, the approval process for release of objects from ISS will be comprehensive and will emphasize the safety of the crew, ISS, visiting vehicles, other space objects, and people on Earth," Johnson reported.
Formal adoption of the ISS jettison policy, Johnson stated in his COSPAR paper, is anticipated in the very near future.
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