查看完整版本: 《nature》关于美国击落报废的间谍卫星的新闻

《nature》关于美国击落报废的间谍卫星的新闻

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News

US missile slams into runaway satellite
Military '80-90%' certain that toxic fuel tank was successfully broken up.

Geoff Brumfiel

The missile carried no explosives, but relied on force of impact to break apart the satellite.US DODThe US Pentagon has successfully fired a missile into an out-of-control US reconnaissance satellite, it announced today. The National Reconnaissance Office lost contact with the satellite shortly after it launched in December 2006, and Pentagon officials feared that the unused, toxic fuel could pose a health risk should the fuel tank survive the satellite's imminent fall back to Earth.

At 03:26 GMT on 21 February, the Navy cruiser USS Lake Erie fired a modified Standard Missile 3 (SM-3), which intercepted the satellite as it passed 247 kilometres over the Pacific Ocean. Twenty-four minutes later, the Joint Space Operations Center in Vandenberg, California, reported an impact. "We are very confident that we hit the satellite," says General James Cartwright, who is head of Strategic Command, which co-ordinated the launch.

The military does not yet know for sure whether the impact, which carried no explosives but rather relied on the force of collision to break apart the satellite, managed to destroy the 450-kilogram tank of hydrazine fuel. But a fireball was seen at the point of impact, and spectroscopic observations from a military plane indicate that a hydrazine vapour cloud was formed in the orbital space near the collision. Cartwright says that he is "80–90% sure" that the tank was breached. He adds that further analysis, which should be ready in 24–48 hours, will be needed to confirm the hit.


The US military watched the collision happen.US DODThe hydrazine, which is used for propulsion in space, is thought to have frozen solid, and would probably survive re-entry if the tank was intact. Pentagon officials feared that this could cause a toxic plume over a populated area. "The issue here was the hydrazine," Cartwright says.

Outside experts say that the chance of the hydrazine actually injuring people, if the tank survived, was very small. An analysis on Arms Control Wonk, a popular defence blog, calculated that the chances of the satellite hitting an urban area were about 2 in 10,000.

Debris cloud
The pieces of debris created by the impact seem to be quite small, says Cartwright. "Thus far we haven't really catalogued anything bigger than a football." Most of this debris will probably burn up as it re-enters Earth's atmosphere.

The Pentagon says that most of the debris from the 2,250-kilogram satellite will re-enter in the next 24-48 hours. A few pieces have already re-entered over the Atlantic and Pacific Oceans, says Cartwright. And some amateur astronomers gathered in Prince George, British Columbia, to watch a lunar eclipse spotted around two-dozen debris trails moving across the night sky, according to an e-mail being circulated amongst astronomers and arms control experts.

The United States has pledged to help deal with any pieces that manage to strike the ground in other countries, although Cartwright says that there is no indication that this has yet happened. Additional debris will re-enter over the next month.


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Good intentions?
In the wake of the shoot-down, China — which destroyed a satellite of its own in a missile test in 2007 — asked the United States to provide more data about the strike. Such information is necessary "so that relevant countries can take precautions", says foreign minister Liu Jianchao. China's Communist Party paper, The People's Daily, accused Washington of "desperately trying to explain away" the military purpose behind the test. China was roundly criticized by many for the apparent military intent of its test last year.

Cartwright denies that the impact had anything to do with proving that the United States has an anti-satellite capability. "We did that in the 1980s," he says, referring to an earlier Pentagon test. "There is no reason to go back and reprove what we have already done."

"It has been a huge technical success," says Jonathan McDowell, an astronomer at the Harvard–Smithsonian Centre for Astrophysics in Cambridge, Massachusetts. But, he adds, the test could prompt other countries to carry out testing of their own that could produce dangerous orbital debris. "Will we be saying that this was [just] the beginning?" he asks. "Only time will tell

News

Experts suspicious of 'splatellite' plan
The US government's decision to shoot down its errant spy satellite has met with concern.

Geoff Brumfiel

The US navy plans to shoot down an errant satellite with an SM-3 missile.US NAVY PHOTOSA plan by the US government to shoot down an out-of-control spy satellite has been described as a cynical tit-for-tat move in response to China doing the same last year. Scientists and arms-control experts fear that the operation will create damaging debris and weaken international efforts to ban space weaponry.

On 14 February, officials from the Pentagon, White House and NASA announced plans to use a ship-based missile to strike the satellite as it passes roughly 240 kilometres overhead. The satellite, which belongs to the National Reconnaissance Office in Virginia, dropped out of control after its launch in December 2006, and would re-enter Earth’s atmosphere around early March if no action were taken.

The strike is necessary to prevent the dispersal of around 450 kilograms of hazardous hydrazine thruster fuel onboard, according to James Jeffrey, assistant to the president and deputy national security adviser. If the fuel survived re-entry, it could be dispersed over an area of roughly 20,000 square metres, although “the likelihood of the satellite falling in a populated area is small,” he says. “Nevertheless, if the satellite did fall in a populated area, there was the possibility of death or injury to human beings.” The Pentagon denies that the shoot-down is to protect classified technologies on the satellite.

But scientists familiar with both satellite re-entry and the US missile defence system question the decision. The chances that the tank, which is 1 metre in diameter, will survive and strike land are extremely small, says Geoffrey Forden, a physicist at the Massachusetts Institute of Technology in Cambridge. “Most likely it will land in the ocean,” he says. The reasons given for the plan “don’t sound too credible to me”, he adds. “I think they’re doing it mainly to tell the Chinese that we can blow up a satellite too,” says Jonathan McDowell, an astronomer at the Harvard–Smithsonian Center for Astrophysics in Cambridge, Massachusetts. “This gives the US cover to carry out a test.”

The firing will probably take place in the coming weeks, although not before the return of the space shuttle Atlantis, which is expected back from the International Space Station on 20 February. David Wright, a physicist at the Union of Concerned Scientists, a non-profit organization in Cambridge, Massachusetts, says that he believes the station could be vulnerable to debris. But NASA administrator Michael Griffin says he is “very comfortable” with the decision.

Hitting the satellite could have serious consequences. In January last year, when China used an interceptor to destroy one of its own, obsolete weather satellites, the test littered more than 100,000 debris fragments throughout low-Earth orbit. Much of this hazardous debris will remain there for decades, posing a risk to other satellites.

The errant US satellite is at a much lower orbit than the Chinese one was, and therefore debris would be shorter lived and less likely to cross the path of other spacecraft. But it is also at least 2.5 times larger than the Chinese one, so it will create more debris. Furthermore, the cloud could behave unpredictably, says Wright.


The projected interception point between the orbit of the out-of-control National Reconnaissance Office satellite (red) and the ship-launched Standard Missile-3 (green). This can also be seen in Google Earth . Credit: G. FordenGoogle EarthThe government plans to destroy the satellite using a ship-launched Standard Missile 3, or SM-3. The missile is designed to use a kinetic kill vehicle to ram incoming ballistic missiles, destroying them before they damage US targets. It is a smaller and slower device than the ground-based interceptors located at Fort Greeley in Alaska and Vandenberg Air Force Base in California. But it is better at intercepting targets, according to Forden. “They have had quite a bit of success with the SM-3,” he says.

Travelling at 3–4 kilometres per second, the device would smash into the 2,250-kilogram satellite, which itself will be moving at roughly 8 kilometres per second. “At these speeds it is like setting off a huge amount of high explosive at the satellite,” Wright says. Even without carrying explosives, the energy of the collision could boost fragments of the satellite into a higher orbit, creating hazards for other craft. “It sounds like a bad idea to me,” he says.

The announcement came just two days after Russia proposed a treaty, backed by China, to ban the use of space weapons — including those used to destroy a country’s own satellite — at an international conference on disarmament in Geneva, Switzerland.

The proposed US shoot-down would have far-reaching diplomatic implications. “If you do this,” says McDowell, “you have converted your missile-defence system into a missile-defence and anti-satellite system.”

“It would reinforce people’s sense of the United States as being irresponsible,” says Rebecca Johnson, executive director of the Acronym Institute for Disarmament Diplomacy in London. The United States has blocked a ban on space weapons for more than a decade on the grounds that it would interfere with its right to develop a missile-defence programme. Using that system to destroy an orbiting satellite would probably anger countries such as Russia.

McDowell says he thinks the shoot-down, following in the wake of China’s test last year, will dramatically weaken already floundering efforts for a ban on space weapons. That in turn could be hazardous for satellites everywhere. “Just because the Chinese were idiots, doesn’t mean that we have to be bigger idiots,” he says.

Additional reporting by Rachel Courtland.

Updated:
The US Military is restricting airspace in a section of the Pacific from 02:30 to 05:00 Hawaii Time on the morning of Thursday 21 February. Since the satellite is expected to pass through this area during that time, observers suspect this might be when the satellite shot will take place.