The United States has neutralised its damaged spy satellite over which control had been lost. Dag Wallström, a specialist in aerodynamics and guided weapons at FOI, explains how it might have been done.

“It is a question of hitting something the size of a small bus at a height of 200 km. The satellite is travelling at over 7 km a second, weighs two and half tons and it is to be hit by an object weighing 10 kg going in the opposite direction. At that speed a hit will be sufficient. There is no need for any explosive. The satellite is travelling on a ballistic trajectory and they have a good idea of what that trajectory is. The missile warhead is equipped with an IR seeker which can ‘see’ the satellite in the cold environment of space.”

“The missile compares the angle between its own course and the line of sight to the satellite. This angle needs to be kept constant. This method is known as line-of-sight control and was probably the method used. The warhead is probably equipped with small thruster rockets to provide any necessary course correction during the final interception phase. It is not all that difficult to hit a non-manoeuvring target that is following a ballistic trajectory,” explains Dag Wallström.

The satellite was hit by a Standard Missile 3 (SM-3) which is used against ballistic missiles and is a development of the air defence missile most commonly used by the US Navy. SM-3 is the Navy’s equivalent to the US Army’s Patriot missile which was used against Iraq’s SCUD missiles during the first Iraq war.

In the current United States plan for a missile shield, Patriot and SM-3 missiles are envisaged as the final barrier closest to the earth’s surface.

The top secret satellite is believed to have been a type of surveillance satellite. It was orbiting at relatively low altitude, just above the earth’s atmosphere. It was equipped with thruster rockets to adjust its orbit and to control its descent into the sea on its final journey.