"At approximately 10:26 p.m. EST (Feb 21, 03:30 UTC) today, a U.S. Navy AEGIS warship, the USS Lake Erie (CG-70), fired a single modified tactical Standard Missile-3 (SM-3) hitting the satellite approximately 247 kilometers (133 nautical miles) over the Pacific Ocean as it traveled in space at more than 17,000 mph. USS Decatur (DDG-73) and USS Russell (DDG-59) were also part of the task force.

"Due to the relatively low altitude of the satellite at the time of the engagement, debris will begin to re-enter the Earth's atmosphere immediately. Nearly all of the debris will burn up on reentry within 24-48 hours and the remaining debris should re-enter within 40 days".

In visualising the event, it is best to look at the collision as if sitting stationary nearby. The interceptor approached the collision point from some direction, probably in front and with a speed around 2 km/s relative to the impact point. USA-193 then came along at orbital speed - ie 7 kilometres metres per second (17,000 miles per hour in another measuring system).

The challenge that the mission faced was to get the interceptor and USA-193 into the same place at the same time, with better than one metre precision, and about one thousandth of a second to play with. A Pentagon press briefing February 20 gave the operating margins of the missile for this attempt when the launch window was described as 'tens of seconds' in duration - ie around one minute. It is an indication of the manoeuvrability when set against the targetting requirement.

Using the Fengyun interception as a reference, the evidence is that the USA-193 should not be treated as the sort of ballistic collision that we saw in school science experiments. With such a closing speed, the catastrophic disruption of USA-193 and its interceptor produced an expanding ellipsoidal (egg-shaped) debris cloud still having something similar to USA-193's original orbital characterisics. The long axis of the 'egg' is pointing towards the Earth.

Within the cloud is a collection of trajectories near to the original inclination but spreading either side, and with a range of apogees and perigees due to the vertical component of the spreading.

Fragments leaving the cloud with the lowest orbital velocity hit the upper atmosphere fairly soon after the collision. It is a little like Soyuz or Shuttle having fired a retro-rocket. It was reasonable to assume that the first re-entries of fragments occured about one quarter to one half orbit after the fragmentation.

This was borne out when a group of amateur astronomers assembled at the Prince George Astronomical Observatory in western Canada to observe the total lunar eclipse. They reported numerous trails across the sky fanning outwards. The report is reminiscent of the Mir re-entry when observers in Fiji saw a collection of trails across the sky.

The choice of the interception orbit ground track was a very good one. Optical observation of the fragments in orbit was possible immediately after the collision and the initial re-entries were visible. Additionally, the next three circuits of the Earth passed over very little land.

A Notice to Airmen (NOTAM) issued soon after noon, 2008 Feb 18 described a restriction over a defined area of the Pacific Ocean near Maui. A copy of the NOTAM was posted to the 'Seesat-l' discussion group by Ted Molczan. It ran 02:00-05:30 on Feb 21 UTC. and revealed the area where a missile is likely to be fired.

The plot below shows the track of USA-193 for the interception when it went directly through the area defined in the NOTAM at around 03:30 UTC - consistent with the missile launch at 03:26 with the SM-3 missile taking 4-5 minutes to reach orbital altitude.

USA-193's general direction of travel was eastward so the first landfall after the interception was Canada where the re-entry observation of fragments was reported. The Prince George site is marked (PGAO).

Where the ground track is shown as yellow, USA-193 was illuminated by the Sun. The blue segment represents the eclipsed part of the orbit. The orbit height is to scale with the Earth and the 'fence' around the Intercept Zone is 400 km high.

The map below shows the ground tracks of the next three circuits of the Earth, showing how little of the inhabited areas the fragments crossed. The major land masses are Canada and Australia, both sparsely populated. If the Pentagon's hope, expressed in a press briefing February 14, of half the fragments re-entering within two revolutions is realised then very little land is at risk.

The track is based on an element set published through 'Seesat-l' on Feb 17, again by Ted Molczan. USA-193's orbital period is just below 90 minutes. The green line around USA-193 represents the satellite's horizon.

It is best to start with what this area was not.

The area laid out in the NOTAM was not designed to catch post-collision fragments from the satellite and intercepting warhead because there were none heading in that direction. The SM-3 missile is three-stage, so the area covers the splashdown of stages 1 and 2.

Three missile carrying ships were deployed some distance from each other. There were also support and tracking ships to accomodate in the zone.

The area is also elongated east-west. This may be to allow for the fact that the ground track of USA-193 moves a little to the west each day so several days' launch attempts were encompassed in one definition.

All of these factors came together to determine the shape of the area. Further NOTAMs and their maritime equivalents were issued for the same area, covering similar times on February 22 and 23 but February 21 was the priority day because of the advantage offered by the lunar eclipse.

Regarding optical tracking, this map shows the position of USA-193's debris cloud at the moment it entered eclipse. The Prince George Observatory is nearly directly beneath. At the eclipse point, the elevation as seen from there was about 25 degrees above the horizon towards the west.

The best positions for optical tracking were on the night side of the terminator between the intercept point and eclipse. That way, the sky background was be as dark as possible. It means that seaborne and airborne tracking stations had the best optical view immediately after the event.

One fortuitous thing is that, although it was Full Moon, a total lunar eclipse was near its mid-point at the time of the interception, thereby taking away one potential interfering factor. This was probably a major consideration in choosing the intercept time and date along with the avoidance of land masses along the ground track for a few hours after the event.

Choice of when and where was a compromise based on a 'safe' area to launch a missile and the orbit of USA-193. The interception could have been set for a time when USA-193 was passing over the area in a southbound direction. On February 21, it occur at around 12:55 UTC.

There are some disadvantages in the southbound option. The interception would have happened in the Earth's shadow so optical tracking close to the event would not be possible, though infrared observation would still have been a possibility, subject to interference from moonlight. Also, the next few Earth circuits passed over significant population centres. They included populated parts of Africa, the Middle East, southern Russia, several central Asian states, the Peoples' Republic of China, and Europe.

The event would also have occured in the glare of a high-elevation full Moon so any infrared tracking of the disintegration event might have prove difficult.

I am indebted to Sebastian Stoff's excellent tracking program 'Orbitron' for producing the maps on this page, and to the versatility of Google Earth. Also thanks to Ted Molczan and others for information forthcoming through 'Seesat-l'.