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.
Common occurrence
"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
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.
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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