The US military has issued a warning notice barring flights above a large area of the northern Pacific for two and a half hours early on Thursday morning. The stricken spy satellite marked for destruction by US warships will pass over the taped-off area just at this time, indicating that the first shot will take place then.

The NOTAM (NOTice To AirMen) warning reads:

02/062 (A0038/08) - AIRSPACE CARF NR. 90 ON EVELYN STATIONARY RESERVATION WITHIN AN AREA BNDD BY 3145N 17012W 2824N 16642W 2352N 16317W 1909N 16129W 1241N 16129W 1239N 16532W 1842N 17057W 2031N 17230W 2703N 17206W SFC-UNL. 21 FEB 02:30 2008 UNTIL 21 FEB 05:00 2008. CREATED: 18 FEB 12:51 2008

A "CARF" (Central Altitude Reservation Function) designation indicates a NOTAM intended to keep commercial and private flights clear of military operations, and SFC-UNL means the height band of this warning zone reaches from the surface to "unlimited" altitude - in other words all the way into space. The UTC time referred to is the same as UK time, so the zone exists from 0230 to 0500 on Thursday morning for British readers.

The latitudes and longitudes can be plotted with the crippled spy sat's ground track overlaid, which has been done by satellite watcher Ted Molczan in handy pdf form here. Those running Google Earth can get a better look using this kmz file, compiled by Molczan's fellow sky-watcher Alan Clegg from the pdf.

As will be evident, the barred area is a cool 1,400 miles long and nearly 700 miles wide at the surface, giving the US Navy plenty of elbow room to fire their interceptor missiles up into the descending spacecraft's path.

Reports have it that three US Aegis air-defence warships, the cruiser Lake Erie and the destroyers Decatur and Russell, will be waiting for the satellite west of Hawaii. Each ship carries a specially modified Standard SM-3 interceptor, originally intended for defence against lower-flying ballistic missile warheads. The three interceptors are on separate ships in case of a technical issue with the Aegis radar and fire-control system.

As it passes over the firing area, the satellite will be approximately 3,000 miles and ten minutes out from the western coast of Canada, the next land it will pass over. The satellite has much more mass than the soaring "exo-atmospheric kill vehicle" it will smack into, so this gives some idea of the onward track the wreckage might follow in the event of a hit.

The Pentagon believes most of the resulting debris from a successful shot will burn up soon afterwards, and almost all should be gone within "two orbits". Boosters and other gubbins from the interceptors will presumably fall within the ocean NOTAM area.

The firing area seems to have been chosen so as to minimise the chances of debris falling anywhere other than in the ocean or North America, which could lend credence to the idea that the intercept is primarily aimed at safeguarding the satellite's technology. ®