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| Weather may delay satellite shot | Weather may delay satellite shot |
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| The US might have to delay shooting down a defunct spy satellite on Thursday due to bad weather. | The US might have to delay shooting down a defunct spy satellite on Thursday due to bad weather. |
| Waves in the Pacific are too big for US warships to get into a correct position to fire a missile at the spacecraft. | Waves in the Pacific are too big for US warships to get into a correct position to fire a missile at the spacecraft. |
| The US Navy has been on standby to destroy the "spysat" before it enters Earth's atmosphere. | The US Navy has been on standby to destroy the "spysat" before it enters Earth's atmosphere. |
| The satellite, known as USA 193, stopped communicating and lost control a few hours after it was launched on 14 December 2006. | The satellite, known as USA 193, stopped communicating and lost control a few hours after it was launched on 14 December 2006. |
| Officials say the shootdown was approved amid concerns that toxic hydrazine fuel aboard the satellite could harm or kill humans if inhaled. | Officials say the shootdown was approved amid concerns that toxic hydrazine fuel aboard the satellite could harm or kill humans if inhaled. |
| Once the space shuttle Atlantis had returned safely to Earth, the Pentagon announced the opening of a window of opportunity for it to try to shoot down USA 193. | |
| The military plans to destroy the satellite with an SM-3 missile fired from a Navy cruiser parked on the western side of Hawaii. | The military plans to destroy the satellite with an SM-3 missile fired from a Navy cruiser parked on the western side of Hawaii. |
| The US government has issued notices to aviators and mariners to remain clear of a section of the Pacific Ocean from 0230 to 0500 GMT on Thursday. | The US government has issued notices to aviators and mariners to remain clear of a section of the Pacific Ocean from 0230 to 0500 GMT on Thursday. |
| Precision strike | Precision strike |
| The US intends to intercept the 2,300kg (5,000lbs) spacecraft - believed to be a radar imaging reconnaissance satellite - when it is at an altitude of 240km above the ground. | The US intends to intercept the 2,300kg (5,000lbs) spacecraft - believed to be a radar imaging reconnaissance satellite - when it is at an altitude of 240km above the ground. |
| Hitting USA 193 just at the edge of the Earth's atmosphere minimizes the amount of debris that would remain in space. | Hitting USA 193 just at the edge of the Earth's atmosphere minimizes the amount of debris that would remain in space. |
| BROKEN SATELLITE Owner: National Reconnaissance OfficeMission: ClassifiedLaunched: Dec 2006Weight: 2,300 kg (5,000lbs)1,134kg (2,500lbs) could survive re-entryCarrying hydrazine thruster fuel But the missile, launched from a Navy cruiser, will have to do better than just striking the bus-sized spacecraft. | BROKEN SATELLITE Owner: National Reconnaissance OfficeMission: ClassifiedLaunched: Dec 2006Weight: 2,300 kg (5,000lbs)1,134kg (2,500lbs) could survive re-entryCarrying hydrazine thruster fuel But the missile, launched from a Navy cruiser, will have to do better than just striking the bus-sized spacecraft. |
| It needs to pierce the satellite's spherical fuel tank, which contains more than 450kg (1,000lbs) of toxic hydrazine, which would otherwise be expected to survive re-entry. | It needs to pierce the satellite's spherical fuel tank, which contains more than 450kg (1,000lbs) of toxic hydrazine, which would otherwise be expected to survive re-entry. |
| US General James Cartwright, vice-chairman of the joint chiefs of staff, said that if allowed to plummet to Earth, its fuel tank would probably survive re-entry, allowing hydrazine to disperse over a danger zone roughly the size of two football fields. | US General James Cartwright, vice-chairman of the joint chiefs of staff, said that if allowed to plummet to Earth, its fuel tank would probably survive re-entry, allowing hydrazine to disperse over a danger zone roughly the size of two football fields. |
| He added that exposure to the thruster fuel would have similar effects to inhaling chlorine or ammonia - a burning sensation in the lungs and, if too close and too much, then possibly death. | He added that exposure to the thruster fuel would have similar effects to inhaling chlorine or ammonia - a burning sensation in the lungs and, if too close and too much, then possibly death. |
| Window of opportunity | Window of opportunity |
| By rupturing the one metre-wide tank, the military hopes to disperse as much hydrazine as possible in space before the National Reconaissance Office (NRO) "bird" falls to Earth. | By rupturing the one metre-wide tank, the military hopes to disperse as much hydrazine as possible in space before the National Reconaissance Office (NRO) "bird" falls to Earth. |
| Officials expect that over 50% of the debris will fall to Earth within the first 15 hours after the strike - or within its first two revolutions of Earth. | Officials expect that over 50% of the debris will fall to Earth within the first 15 hours after the strike - or within its first two revolutions of Earth. |
| General Cartwright (r) will advise on when to take the shotGen Cartwright, and Gen Kevin Chilton, head of Strategic Command, will advise US defence secretary Robert Gates on exactly when to launch the missile. | General Cartwright (r) will advise on when to take the shotGen Cartwright, and Gen Kevin Chilton, head of Strategic Command, will advise US defence secretary Robert Gates on exactly when to launch the missile. |
| The mission could go forward on any day until 29 February, when the satellite is projected to have re-entered the atmosphere, making it virtually impossible to attempt to hit with a missile. | The mission could go forward on any day until 29 February, when the satellite is projected to have re-entered the atmosphere, making it virtually impossible to attempt to hit with a missile. |
| Left to its own devices, about half of the spacecraft would be expected to survive the blazing descent through the atmosphere, scattering debris over several hundred kilometres. If struck with the missile, about 25% of USA 193 is likely to survive the fall to Earth. | Left to its own devices, about half of the spacecraft would be expected to survive the blazing descent through the atmosphere, scattering debris over several hundred kilometres. If struck with the missile, about 25% of USA 193 is likely to survive the fall to Earth. |
| Russian concerns | Russian concerns |
| But Russia's defence ministry has effectively branded the US operation a cover for testing an anti-satellite weapon. | But Russia's defence ministry has effectively branded the US operation a cover for testing an anti-satellite weapon. |
| "Essentially, speculations about the danger of the satellite hide preparations for the classical testing of an anti-satellite weapon," a statement reported by Itar-Tass news agency said. | "Essentially, speculations about the danger of the satellite hide preparations for the classical testing of an anti-satellite weapon," a statement reported by Itar-Tass news agency said. |
| USA 193 lost control a few hours after launch on a Delta II rocketThe Russian defence ministry argued that various countries' spacecraft had crashed to Earth in the past, with many using toxic fuel on board, but that this had never before merited "extraordinary measures". | USA 193 lost control a few hours after launch on a Delta II rocketThe Russian defence ministry argued that various countries' spacecraft had crashed to Earth in the past, with many using toxic fuel on board, but that this had never before merited "extraordinary measures". |
| Last year, China carried out a test using a ground-based ballistic missile to destroy a satellite in space, prompting international alarm and fears of a space arms race. | Last year, China carried out a test using a ground-based ballistic missile to destroy a satellite in space, prompting international alarm and fears of a space arms race. |
| On Tuesday, a US State Department spokesman stressed that the action was meant to protect people from the hazardous fuel and was not a weapons test. | On Tuesday, a US State Department spokesman stressed that the action was meant to protect people from the hazardous fuel and was not a weapons test. |
| Gen Cartwright has also denied claims that the main aim of the operation was to destroy secret components on USA 193. | Gen Cartwright has also denied claims that the main aim of the operation was to destroy secret components on USA 193. |
| He explained that classified parts would be burned up in the atmosphere and, in any case, that would not be a reason for shooting down the satellite. | He explained that classified parts would be burned up in the atmosphere and, in any case, that would not be a reason for shooting down the satellite. |
| SATELLITE DESTRUCTION PLAN 1 SM-3 missile launched from a US Navy cruiser in Pacific Ocean2 The three-stage missile heads for collision location, where the relative "closing" speed will be 10km/s (22,000mph)3 Satellite reaches desired altitude of 240km (130 nautical miles), close to edge of atmosphere4 Missile makes contact and breaks fuel tank, freeing hydrazine into space5 Debris scattered over hundreds of kilometres | SATELLITE DESTRUCTION PLAN 1 SM-3 missile launched from a US Navy cruiser in Pacific Ocean2 The three-stage missile heads for collision location, where the relative "closing" speed will be 10km/s (22,000mph)3 Satellite reaches desired altitude of 240km (130 nautical miles), close to edge of atmosphere4 Missile makes contact and breaks fuel tank, freeing hydrazine into space5 Debris scattered over hundreds of kilometres |
| Paul.Rincon-INTERNET@bbc.co.uk | Paul.Rincon-INTERNET@bbc.co.uk |