According to press reports, a US reconnaissance satellite is expected to make an uncontrolled reentry after communication and control with the satellite was lost.

The uncontrolled nature of the reentry means that while the satellite is expected to reenter the atmosphere in the next month or so, it can’t be said for sure where the remains of the satellite will land. A large enough chunk of satellite landing in a populated area could cause damage to property or even injuries or fatalities. The greatest probability, however, is that it will hit an ocean area but this may still present a hazard to shipping or aircraft.

Knowing what sort of satellite chunks to expect is really about knowing the construction of the satellite itself. Unfortunately, the satellite in question is highly classified so the best we can do is speculate.

There’s  at least a couple of types of spy satellite up there, radar survey and optical. The US optical reconnaissance are often said to resemble the hubble space telescope (pointing down not up!). Referred to as ‘Keyhole’ or ‘Misty’ by the general public, this series of optical satellites is said to carry a mirror 2-3 meters in diameter which would probably be somewhat heavy and thick and I would guess liable to survive reentry.

Then there’s the radar satellites which I can’t think of having any single massive component although in the interests of ’stealth’ (if such a thing is possible for satellites) they may forgo highly-observable solar arrays and instead use RTGs. An RTG may release some radioactive fuel upon reentry but it would likely be released over such a wide area that it would be barely detectable above background. In the 1970s, an RTG on the Apollo 13 LEM survived reentry from lunar-return velocities and the subsequent impact without releasing any radioactive material. The biggest risk to your health from a Satellite RTG it would seem, would be if it hit you on the head.

Having looked around the web at this story, there is some discussion among amateur satellite tracking enthusiasts that this may be NROL-21 / USA 193, a satellite launched in December 2006 which failed to deploy correctly. Due to the low orbit, it is speculated that this is a radar reconnaissance rather than an optical satellite.

Gordon Johndroe, a spokesman for the National Security Council declined to comment as to whether the satellite may be ’shot down’. The USA has successfully tested ASAT weapons in the past but could be very hesitant to use them for a few reasons I can think of:

• It may contravene the Outer Space Treaty
• The debris from a ‘kill’ would spread out and become harder to track while also presenting a hazard to other satellites / spacecraft.
• The net result of a ‘kill’ would be that you still had the same amount of mass going in roughly the same direction at the same speed but it would all be coming down in different places (depending on the drag coefficient of each fragment)

I’ll be interested to see whether we get any more news on this reentry. It really could be the case here that ‘no news is good news’