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The ISS is a big target for space junk. This diagram shows which areas of the ISS are most vulnerable to impacts from orbital debris. Click to enlarge.


Artist's rendition of space junk surrounding the Earth. Click to enlarge.
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Space Junk and ISS: A Threatening Problem
By Leonard David
Senior Space Writer
posted: 03:30 pm ET
07 January 2002

iss_junk_020107

BOULDER, COLORADO - Space is hazardous on its own. Nature provides generous doses of vacuum, radiation, micrometeoroids, and extreme conditions of heat and cold.

But one class of danger is human-made. Busted up bits of satellites, rocket bodies, and other high-velocity leftovers can ruin your day. Losing an automated spacecraft to a head-on collision with space refuse is one thing.

A greater worry is that the International Space Station (ISS) and its crew might encounter an incoming fragment of space flotsam. That was just the concern last November when a Russian satellite broke apart, spreading hundreds of fragments at an altitude just above the ISS.

So alarmed was NASA about the breakup of the Russian satellite, space debris experts went into scramble mode to ensure that it was okay to launch space shuttle Endeavour on its STS-108 mission to the orbiting complex.

A host of new safety issues were flagged due to the Russian satellite breakup.

Possible threats to spacewalking astronauts were weighed. Because a space shuttle is less protected from debris impacts contrasted to the partly shielded ISS, assessments were also made regarding problems that Endeavour could face during independent flight throughout its STS-108 trek.

A U.S. Space Surveillance Network of sensors that keeps a vigil on satellites and space flotsam worked with NASA to plot out the newly created debris cloud caused by the Russian spacecraft.

Once thoroughly reviewed, risks were judged to be within NASA guidelines. STS-108 was sent on its way.

This behind-the-scenes saga, however, underscores the menace posed by space debris - a problem likely to linger for many years to come.

High-altitude headache

Last November's high-altitude headache involved Russia's Cosmos 2367. Lofted in December 1999, the "Cosmos" label doubles for the spacecraft's true duties: It was an Electronic Ocean Reconnaissance Satellite -- or EORSAT -- built to keep an eye on the naval movements of other nations.

Cosmos 2367 had been the latest of the Cosmos 699 series of three-metric-ton spacecraft that started space operations in 1974.

For unknown reasons, the spacecraft broke up on November 21. In an orbit just 19 miles (30 kilometers) above the ISS, the Russian craft busted up into over 300 large and small pieces. Most fragments were concentrated in a 124-mile (200 kilometer) to 310-mile (500 kilometer) altitude, while other bits were tossed into orbits that reached 1000 kilometer altitude.

Approximately 40 percent of the debris were immediately thrown into orbits that crossed the orbit of ISS.

"Once the debris are found and routinely tracked by the Space Surveillance Network (SNN), the ISS collision avoidance scheme should handle potential close encounters," said Nicholas Johnson, chief scientist and program manager for NASA's Orbital Debris Program Office at the Johnson Space Center in Houston, Texas.

"Of course, debris too small to be seen by the SSN can inflict serious damage to the ISS. The shuttle is more vulnerable to debris than ISS since it does not have the extensive shielding of the station," Johnson told SPACE.com.

ISS reboost

Even at the close of last year's STS-108 flight, another space debris concern came to light.

In mid-December, flight controllers made slight changes to Endeavour's departure from the station. The added time permitted a small jet firing by the shuttle to boost the station's future path away from a spent Russian rocket upper stage set a drift since the 1970s.

Mission Control had been given the word the old stage could pass within three miles (5 kilometers) of the station. That's too close for comfort.

With shuttle Endeavour's reboost, coupled with three larger reboosts it had done earlier, the station was raised a total of more than nine statute miles (14 kilometers) by Endeavour.

Thanks to the shuttle engine pulses, the ISS was then predicted to pass more than 40 miles (64 kilometers) away from the debris.

This old rocket stage

In regards to space debris, there was double trouble near the end of 2001.

Two major satellite breakups, the worst in 20 months, occurred just four weeks apart in the fourth quarter of 2001, both producing in excess of 300 large fragments, Johnson reports.

Along with Cosmos 2367, a fourth stage of India's Polar Satellite Launch vehicle exploded December 19. That blast created more than 300 chunks of debris. Some of that wreckage is already penetrating ISS altitudes. The spent booster stage had been drifting through space for only two months, and was utilized to orbit Indian, German, and Belgium satellites.

The reason for the Indian upper stage breakup is under investigation, Johnson explains. A potential cause, he adds, is residual hypergolic propellants within the rocket body. "The release of all residual propellants and compressed fluids at the end of launch vehicle stage and spacecraft operations is highly recommended by U.S. Government agencies, as well as many spacefaring nations around the world."

This process, called "passivation", vents leftover fuels, fluids and gasses into space. Doing so prevents mixes of volatile chemicals that, in a sense, can turn a used rocket stage into a time bomb. Over the years, time after time, rocket stages, including those used by the United States, Europe, as well as Russia, have unexpectedly exploded and scattered chunks of junk through space.

About three weeks after the Cosmos 2367 breakup, Russia's 12-year old Molniya 3-35 spacecraft also fragmented. On December 14, while passing over the Southern Hemisphere, the communications satellite broke into two dozen separate pieces, with most falling into the Earth's atmosphere immediately.

The final fragmentation for 2001 appears to be a 10-year-old Ariane 4 upper stage. It pitched into orbit an Intelsat 601 spacecraft back in 1991. That Ariane rocket component generated several pieces of debris.

"The clear message here is that passivation is essential for curtailing the growth of the Earth orbital debris population," Johnson explains.

Explosive past

Dealing with space debris has a long past. According to a newly issued report on the history of on-orbit satellite fragmentations, prepared by Phillip D. Anz-Meador of Viking Science & Technology, Inc. for NASA, the first serious satellite fragmentation took place in June 1961.

That event involved an Ablestar rocket stage that had dropped off a trio of satellites -- Transit 4A, Injun, and Solrad 3 -- into orbit.

Since the 1960s, space debris and the ability to operate within the finite region of space around the Earth has been a topic of growing worry.

Last decade alone, 55 in-space explosions occurred, as well as the first recorded unintentional collision between space objects. The rate of satellite breakups increased notably in the 1970s, and has continued through the 1990s and into the new millenium at an average pace of approximately 5.5 fragmentations per year, the Anz-Meador report notes.

"Large, manned space stations and large constellations [of satellites] of the late 1990s and beyond demand a better understanding of the hazards of the dynamic Earth satellite population," the report cautions.

 

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