China ASAT test worst single debris event

From Aviation Week

China Asat Test Called Worst Single Debris Event Ever

By Frank Morring, Jr.

Chinese delegates will have some explaining to do in Vienna later this month, when they sit down with representatives of other spacefaring nations to adopt international guidelines designed to mitigate the growing problem of man-made space debris in Earth orbit.

The document drafted by a technical subcommittee of the U.N. Committee on the Peaceful Uses of Outer Space is aimed at preventing the sort of accidental events that have gradually engulfed Earth in a cloud of potentially destructive high-speed debris since the flight of Sputnik 1 kicked off orbital spaceflight a half-century ago. Chinese experts helped draft the document.

But China's Jan. 11 test of a primitive anti-satellite weapon against an aging weather satellite boosted the population of trackable debris by more than 900 objects--an instantaneous 10% increase in the 50-year figure--that threaten all spacecraft flying below about 2,000 km. (1,243 mi.).

"We still await a complete explanation from China as to how this ASAT test squares with its professed desire to seek only peaceful uses of space," says a U.S. State Dept. official, presenting the official U.S. government position on the test.

The test impact over the Chinese launch site at Xichang came with the target--the Feng Yun 1C weather satellite--in polar orbit at an altitude of 537 mi. (AW&ST Jan. 22, p. 24). The satellite and the missile-launched Asat weapon shattered into thousands of pieces that were thrown into a wide range of orbits ranging in altitude from 3,800 km. on the high end down to about 200 km. at the lowest, according to Nicholas Johnson, NASA's chief scientist for orbital debris and a longtime expert in the field.

"This is by far the worst satellite fragmentation in the history of the space age, in the past 50 years," he says.

As of last week, the U.S. Space Surveillance Network (SSN) in Colorado Springs had cataloged 647 of the 900+ items its sensors were tracking. On average, those objects must be at least 10 cm. (3.9 in.) in diameter to be tracked from the ground, although smaller objects can be pinpointed with the two radars at the Haystack Observatory in Tyngsboro, Mass., operated by MIT Lincoln Laboratory for the Defense Dept., and with other systems in the SSN.

Johnson says the measured effects of the Chinese test have followed computer models for debris dispersal very closely. Although some debris that was thrown into a retrograde orbit or down into altitudes where atmospheric drag take effect has reentered the atmosphere, the models predict a debris cloud of some 35,000 objects larger than 1 cm. remains in orbit.

"Many of these debris will be in orbit for 100 years or more because the altitude of the breakup was so high," Johnson says. "Some will come down earlier, but the majority will be up there for a very long time."

The Chinese test came as NASA and its Russian partner on the International Space Station were preparing for a series of three spacewalks in nine days that were scheduled to conclude Feb. 8. Although they present small targets in the vastness of space, astronauts on extravehicular activity (EVA) are particularly vulnerable to space debris. As soon as they had enough information on the test to apply their models, NASA experts ran a risk analysis for the ISS to ensure the station and its spacewalkers--Expedition 14 commander Michael Lopez-Alegria and flight engineer Sunita Williams--wouldn't be exposed to additional risk.

"In general, the risk to the International Space Station did go up immediately after the event, but that risk has gradually declined and now the risk is very close to what it was prior to the test," Johnson said Feb. 5, the day after Lopez-Alegria and Williams completed their second EVA in the series. "The reason is that the debris that was thrown down to where the station is is much shorter-lived."

Although there were incorrect press reports originating in Russia that the station had to maneuver to dodge debris from the test, Johnson says that apparently was based on a reporter's misunderstanding. While the station can maneuver to dodge space junk if controllers know it's coming dangerously near, it hasn't done so since the Chinese test.

The ISS is flying at a particularly low altitude this year--about 220 mi.--so space shuttles can reach it with heavy items like the S3/S4 truss segment due for launch next month, according to William Gerstenmaier, associate administrator for space operations, who discounts the danger to the facility from the Chinese debris field.

"Other spacecraft that are higher will have a more prolonged risk from this event than we would, but from a station standpoint, I think we see risk levels now back to the normal background level," Gerstenmaier says. "The station's pretty well protected from a debris-shielding standpoint [and] we stay low pretty much through this year because [Europe's] Columbus [module] is fairly heavy and so's the [Japanese] Kibo."

Administrator Michael Griffin, who visited Chinese space officials last year as a follow-on to President Bush's meeting with Chinese President Hu Jintao, declined to comment on the possible impact the test will have on any future U.S./Chinese space cooperation. Chinese delegates are expected at a meeting in Kyoto, Japan, early next month where a framework for future space-exploration cooperation is expected to be worked out (AW&ST Dec. 18, 2006, p. 19). Griffin is planning to attend as well.

Christopher Hill, assistant secretary of state for East Asian and Pacific Affairs, raised the issue of the Asat test with Chinese foreign minister He Yafei on Jan. 22. A State Dept. official says He asserted that the Chinese Asat test was not targeted at and does not pose a threat to any other nation's space assets, a position that has been repeated by top People's Liberation Army officers.

"We believe China should respond to calls for a full explanation," the U.S. official says, again presenting the U.S. government view.

Despite He's disclaimer, Johnson says the debris cloud from the test increases the risk to all spacecraft in orbits that pass through it, which number "a few hundred," including satellites in many elliptical orbits, the Hubble Space Telescope, most Earth-observation spacecraft and such low-Earth-orbit communications constellations as Orbcomm, Iridium and Globalstar. Geostationary spacecraft, the Global Positioning System constellation and other spacecraft in orbits above about 2,000 km. are generally unaffected.

Space debris has long been recognized as a problem for space operations, and in October 2002 representatives of 11 space agencies--including China's--adopted mitigation guidelines drafted by the Interagency Space Debris Coordination Committee. Mitigation techniques include depressurizing tanks that might otherwise rupture and avoiding collisions with debris or other objects that would create even more debris.

China's own space assets aren't immune from the threat created by its test. Debris from the Chinese test jeopardizes the three dozen or more operational satellites China has in orbit, and will endanger the crews of future Shenzhou spaceflights as well. In October 2005, China accepted U.S. tracking data on potential debris threats to the Shenzhou 6 spacecraft carrying two Chinese astronauts (AW&ST Oct. 17, 2005, p. 29).