This came up on another discussion forum I'm on, so I did a little Q&A to tackle some of the comments I've seen in the media. Here it is, with apologies if it goes over ground others around here have already covered.
What are they aiming at? The satellite in question is USA 193, a satellite launched a few months ago as a covert project of the National Reconnaissance Office - in other words, a spy satellite. It apparently failed soon after launch, stranding it in a low orbit which atmospheric drag is now pulling lower and lower, to the point that left to itself it will re-enter in about a month's time.
Is the satellite a ten-ton monster? The figure of 'ten tons' seems to be being bandied around rather a lot. This is probably because the phrase 'spy satellite' tends to be associated with the NRO's line of massive high-resolution optical reconnaissance satellites descended from the KH9 'Big Bird' of the early 1970s. These are allegedly around about the size of Hubble, and have been launched by heavyweight boosters such as Titan III and IV, which have the capability to put 12 to 20 tons into orbit.
USA 193, by contrast, was launched by a Delta II, specifically the 7920 variant, a standard medium-weight launcher that can put little more than 3 tons into a polar orbit. Whilst hardly small, USA-193 is a medium-weight satellite no larger than many other science or Earth-resources satellites.
Does it pose a hazard to the ground? To begin with, the likelihood of debris from an uncontrolled re-entry falling in a populated area is low. Something like three-quarters of the area under this satellites orbital path is water, and much of the rest is thinly inhabited. However, equally it encompasses the vast bulk of the Earth's populated land area, so the chances of it landing near a populated region can't be disregarded. But then again, the space shuttle Columbia broke up during re-entry over a densely-populated swathe of Texas, but caused only limited damage and no injury to anyone on the ground.
If it does come down near a built-up area, what is likely to survive? Two sorts of material tend to reach the ground from uncontrolled re-entry: dense, heavy items and, perversely, very light ones. The former reach the ground because they are dense enough to punch through the atmosphere and solid enough to resist melting, whilst the latter have little enough kinetic energy in comparison with their surface area that they are slowed down without overheating and fall relatively slowly to ground. As it happens, empty fuel tanks are such an item, and quite a few have survived satellite re-entry in the past.
So, is this the satellite's fuel tank a hazard? The justification for the proposed shootdown is indeed that USA 193 has a fuel tank on board; the concern is not so much that the tank itself might reach the ground intact, but that it might spill its load of hydrazine fuel when doing so. Now, I've known people in the space business who've worked with hydrazine, and they will tell you that it is indeed a spectacularly nasty substance: toxic, inflammable, carcinogenic and smelly to boot. I was sceptical at first that there was any chance of the tank reaching the ground with any hydrazine aboard, as it would certainly rupture during re-entry. However, the suggestion from the US is that the hydrazine has frozen, in which case I can see that it might not all leak from even if the tank loses containment. That it would freeze is quite possible if USA 193 has indeed been out of control for several months. Satellite thermal control is a difficult problem and without careful measures such as carefully maintaining the angle at which the it faces the Sun and using active heating or cooling systems, satellites usually overheat or get far too cold.
But does this justify a shootdown? Frankly, I'm sceptical. Plenty of other satellites have failed in low orbit with a full fuel load and have burned up without too much of a problem. My suspicion, like that of many others, is that this is a convenient excuse for the US Dept of Defense to demonstrate, following the Chinese anti-satellite test last year, that it too is capable of intercepting space vehicles. The other suggestion, that the plan is to deny the prospect of sensitive components from reaching the ground intact, is possible but I suspect less likely.
Can you actually shoot a satellite down? Not as such. At present, USA 193 is in a very low (and getting lower through atmospheric drag) circular orbit. The weapon that would be used to attack it - an updated version of the US Navy's long-serving Standard surface-to-air missile - does not have a warhead as such, but rather has a guided final stage that flies into the target at a closing speed of around 10 km/s. At that speed, the kinetic energy of the collision is equivalent to the interceptor's own weight in explosive, so the interceptor and much of the satellite would be vaporised in a powerful explosion. The surviving fragments of the satellite would end up in new orbits all of which passed through the point of interception but which were perturbed to some extent from the original circular orbit. Now, given how close the original orbit is to the atmosphere, it's easy to see that any significant deviation from it is likely to be an orbit that dips into the atmosphere at some point. In other words, most of the debris is likely to end up in orbits that will hit the atmosphere within a single pass around the Earth, although a fraction of the remains - those parts that were flung directly forward from the impact - will end up in elliptical orbits with a low point at the original orbit height but a new, higher apogee (high point). Even so, these will burn up within a few months at most as their perigee (low point) will be low enough for atmospheric drag to pose a significant effect.
Is there a risk to other satellites? A small one. As noted above, some debris is likely to be flung into short-lived orbits above the original one, and there is a chance that these might cross the paths of operational satellites. However, the risk is small; bear in mind that the single biggest target in low orbit is the Space Station, and the DoD and NASA will have had to assess the risk to it as minimal to be proceeding with this plan. The Chinese test, which was carried out against a satellite in much higher orbit, probably produced much more in the way of potentially hazardous space debris than this shootdown would.
Will it work? The US anti-missile system has had a very chequered record in tests, but to be fair shooting at a minibus-sized satellite in a known orbit is likely to be much easier than aiming at an oil-drum-sized warhead launched only a few minutes previously. If the Americans are planning this as a bit of sabre-rattling, then I'd assume that they are fairly confident that it's going to work.