Duringmore than seven years of operations by the International SpaceStation (ISS), approximately three dozen pieces of debris were released andsubsequently cataloged by the U.S. Space Surveillance Network (SSN).
Althoughsome of the items were separated from the ISS accidentally,some were intentionally cast-off. Once the object has safely cleared ISS, it ispossible, in some cases, for the object to return to the vicinity of ISS and topose a later collision risk. Most objects will decay faster than ISS, and,therefore, will quickly fall below the altitude of the Earth orbitingstructure.
Anofficial ISS "JettisonPolicy" has been developed and is going through an approval process, craftedto ensure that decisions to deliberately release objects in the future from theISS are based upon a complete appraisal of the benefits and risks to the spacestation, other resident space objects, and we folks down here on the Earth.
Althoughdebris of all sizes are of interest, an emphasis of the new ISS jettison policyis on dealing with objects that might pose the greatest risk to the stationitself, its stable of logistics vehicles--such as the space shuttle, Soyuz,Progress,Europe's AutomatedTransfer Vehicle, and Japan's H-2Transfer Vehicle--as well as other operational satellites.
"Therelease of debris from space stations has been a common occurrence for morethan 30 years," advised Nicholas Johnson, NASA Chief Scientist for OrbitalDebris at the NASA Johnson Space Center in Houston, Texas.
"Thegeneration of debris onboard a space station is a natural consequence, but itsaccumulation can present a direct or indirect hazard to the crew as well asresult in reduced productivity," Johnson reported at the 36thCommittee on Space Research (COSPAR) Scientific Assembly, held July 16-23 inBeijing, China.
Onone hand, it is normally preferable to dispose of debris via a logisticsvehicle--like cramming refuse into an automated Russian Progress vessel that is thenpropelled into Earth's atmosphere for destructive disposal.
However, the size of some debris could prevent its transfer to another vehicleor the nature of the debris could pose a health hazard to the space stationcrew. "Hence, the jettison of debris into space, in special cases, can be themost viable option," Johnson explained.
Workon a jettison policy for ISS began at NASA's Johnson Space Center, kick-startedinformally several years ago. Talk was spurred by two factors: recognition thatrefuse was already mounting up within the space station, creating difficultiesfor the crew; and significant hardware attached outside the ISS needed to beremoved in the future.
"Followingthe loss of space shuttleColumbia in February 2003, and the immediate cessation of shuttle flights,the debris situation onboard ISS worsened without the opportunity to routinelyremove tons of debris via the space transportation system," Johnson pointedout.
Rate of release
Takinga look at previous space stations circling Earth, Johnson made theseobservations:
- 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 Network.
- Salyut 6, the first long-duration Soviet Union space station housed crews for four years and in the process produced more than 100 new debris.
- Salyut 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.
Turningto the U.S. Skylab spacestation that orbited Earth in the 1970s--a program that saw a trio ofvisitations by individual three-person crews--Johnson told SPACE.com: "TheSkylab mission, including all three crew expeditions, had a total of only 22debris identified and cataloged by the U.S. Space SurveillanceNetwork. Many of these were associated with the launch and deployment ofSkylab before the arrival of the first crew."
Objects, small and large
Inthe era of the ISS and spacewalks around the complex, both small and largeobjects, are often thrown off the station for convenience--although tools onoccasion accidentally slip away.
Suchwas the case in December 1998 when a slidewire carrier and a worksite interfacewere lost by members of the STS-88 crew while spacewalking for ISS. Theseobjects were large enough to be tracked and cataloged. Three other objects werealso released by STS-88 spacewalkers, one inadvertently (an insulation blanket)and two by design (antenna spools).
Thesizes of debris cast off into space can vary dramatically. Some areoff-the-radar screen, untrackable flotsam. On the opposite end, Johnson said,there's the 33-foot (10 meters) in diameter KRT-10 antenna that wascosmonaut-kicked into space from the rear of Salyut 6 when it failed to ejectautomatically.
Mostspace walks do not involve the jettison of large objects from ISS, but theyoften result in small objects being released, Johnson reported. For instance,during spacewalkoperations performed by the ISS Expedition 10 crew in January 2005,a total of 20 objects were released: 16 electrical caps and four covers. Thesesmall items were not tracked and cataloged by U.S. surveillance tracking gear.All probably reentered rapidly, he explained.
Oneof the most unusual objects jettisoned from ISS was a four-year-old RussianOrlan M spacesuit, Johnson pointed out. Instead of loading the used suitinto a Progress vehicle for disposal, the ISS crew equippedit with a transmitter and dubbed it Radio Skaf.
This"Suitsat" was releasedin early February of this year, and isn't expected to reenter until early tomid-September.
Restricted to special occasions
Johnsonemphasized in his COSPAR paper that the release of debris from spacestations--and stemming from human space operations in general--has beencommonplace for more than four decades.
"Dueto the relatively low altitude of such activities, all debris are relativelyshort-lived and have no long-term effect on the near-Earth space environment,"Johnson noted.
Recognizingthe occasional need to jettison items, Johnson concluded, the ISS program hasbeen developing a formal jettison policy. Logistics vehicles, he noted, willremain the primary means of removing refuse and non-functional items from theISS.
"Thejettison of debris will be restricted to special occasions dictated by safetyand/or perational needs. Moreover, the approval process for release of objects fromISS will be comprehensive and will emphasize the safety of the crew, ISS,visiting vehicles, other space objects, and people on Earth," Johnson reported.
Formaladoption of the ISS jettison policy, Johnson stated in his COSPAR paper, isanticipated in the very near future.
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