OK, the Navy is going to shoot down an errant satellite, because it has a tank full of fuel. But how does shooting it (the missile just causes an impact, it does not carry a warhead) into a zillion pieces help? The fuel is still going to spatter to Earth, albeit possibly it in a more distributed pattern.
You’ve seen this item in GD and MPSIMS but this is a factual question!
Yeah, *that’s *why they’re doing it. 
If they rupture the tank before reentry, that thousand pounds of hydrazine (BP 114°C) will be vaporized. It’ll never make it to the lower atmosphere, so that worst that’ll happen is the destruction of a few thousand pounds of ozone in the stratosphere.
Presumably, the hydrazine will ignite when the warhead blows up the bird. However, if they weren’t to shoot the bird, the odds of there being a lot of hydrazine when the thing hits the ground are pretty slim. SOP for any compressed gas is to have a venting mechanism when the temperature of the storage tank get’s too hot to keep it from rupturing. So, given that the bird will probably heat up to around 3000F during the descent, the fuel will vent, thus causing the thing to tumble around and heat up even more, and fragment pretty heavily.
The only reason they’re doing this, however, is that they’re worried that the electronics will fall into the wrong hands.
Personally, I don’t care. I think the idea that we are going to shoot it down is fricking cool, and they should put it on PPV. I would buy it…
On Earth, sure, but would they put a pressure valve on a spacecraft? If the thing got hot, the venting could screw up the orbit. Or something.
FAS Strategic Security Blog:
U.S. Plans Test of Anti-Satellite Interceptor Against Failed Intelligence Satellite
Spaceflight Now:
U.S. plans to fire missile at falling spy satellite
And back to FAS:
[list=a][li]There is definitely a pressure relief on the hydrazine tank to prevent accidential bursting. The tank is doubtless insulated, and venting is usually designed to minimize any effects on orbit.[/li][li]“Shooting down” a satellite will actually result in a lot of debris scattered in orbit. Admittedly, they’re doing this in very low orbit, so the majority of the debris will fall into the atmosphere in a few weeks, but this is the same sort of thing the Chinese were excoriated for doing last year, although the satellite they hit resulted in longer-duration debris.[/li][li]Anybody who thinks that this is being done because of the negligable hazard that the satellite re-entry presents to people on the ground is totally missing the ulterior motives, i.e. to assure destruction of any sensitive technology on the bird and to use this as an excuse, despite efforts to prevent the militarization of space, to demonstrate that the United States has an effective ASAT (and by extension, late mid-course and terminal phase ABM) capability with the Aegis system/Stanard Missile 3/LEAP.[/list][/li]
Stranger
Would increasing the total surface area by breaking it into multiple pieces ensure that a higher portion of it would burn up upon reentry?
Nope. Look at Columbia, where there was a large number of debris which were only lightly singed. They even managed to recover a videotape from one of the astronauts camcorders.
Not really. Initial “impact” with the atmosphere will result in thermal and aerodynamic loads that will break up the more fragile structure of a satellite into fragments, and once they get above a certain L/D (“lift over drag ratio”) they’re pretty much going to float down at a subsonic terminal speed that is much lower than what would be necessary to to cause significant heating from ram pressure. The fragments themselves could pose a hazard, but this is true in any flydown scenerio. Normally when objects are deliberately de-orbited they are directed toward a fly-down zone that will most likely place debris in mid-ocean, but from an orbital descent it is hard to assure that the debris pattern will be within a small zone. So far, no one (outside of television series) has been struck by a de-orbiting toliet seat, but sooner or later it may happen.
The reason that RVs like the Space Shuttle Orbiter require high temperature thermal protection is because they’re dense and because they have “sharp” leading edges which tend to concentrate pressure and therefore heat. Blunt body re-entry vehicles like the truncated-cone Gemini or Apollo, the cone-sphere Discoverer RV, or the biconic AMaRV/DC-X tend to experience much less heating by creating a shock wave stand off layer where air flow stagnates, so that the thermal protection never experiences the maximum heating. Heat shields on Gemini and Apollo are described as “ablative” but in fact see only very moderate amounts of ablation, to the point that the Gemini VIII capsule was re-used for an (unmanned) test for the abortive Blue Gemini program without refurbishment.
However, I find it highly unlikely that a propellant tank carrying ~1,000 lbs of MMH is going to withstand re-entry, and hitting the satellite with a KV isn’t going to create a very predictable fly-down, though the kinetic energy involved may be enough to distribute components widely enough to prevent acquiring enough jigsaw pieces to put back together. But it also makes a very effective demonstration of what is possible, without having to run a deliberate test.
Stranger
However, Columbia never met up with a large chunk of metal traveling very quickly in the opposite direction. There’ll be a lot of energy released in an inelastic collision between interceptor and satellite.
How much?
From Wired
9.8 - 7.8 = 2, so SM-3 is travelling at 2 km/sec at intercept.
Before the collision:
m1v1 = 2300 X 7800 = 17940000
m2v2 =20 X 2000 = 40000
momentum = 17980000 constant
E1 = 69966000000 joules
E2 = 40000000 joules
Etotal = 70006000000
After collision
2320 X vfinal = 17980000
vfinal = 7750 meters/second
Efinal = 1/2(2320 X 7750 X 7750) = 69672500000
E lost as heat = 333500000 joules
33.5 million joules will be lost as heat.
To put this in perspective
Dynamite releases 4.3X10^9 joules per ton, so the collision will release as much energy as 155 pounds of dynamite.
A 2008 Ford F-150 SuperCrew, has a curb weight of about 5500 pounds, close to the combined weight of satellite and interceptor.
How big are the pieces likely to be if you detonate 155 pounds of dynamite in the cab of an F-150? The engine block might survive, but I’d guess that most of the truck would be reduced to itty bitty bits.
Small nitpick: it will be traveling at -2 km/s in the direction of the satellite’s travel. It will also have a vertical component and possibly a cross-range component, and those components will sum (as vectors) to something near the burnout velocity. The SM-3 burns out ascending and immediately begins slowing down due to gravity, but after apogee it begins to recover that speed. Your order of magnitude is right on, but you may be off by as much as 25% by assuming a 1-D collision.
There are two launch windows for hitting the satellite from a given position, firing along a given azimuth: the early opportunity involves firing the SM-3 so that it reaches apogee and is descending when it hits the target; the late opportunity involves firing the SM-3 so that it’s still coasting upwards after burnout.
Given the wailing and gnashing of teeth that took place after the Chinese intercept, I’m guessing we’ll set our intercept up so that the interceptor comes down from above the satellite, so debris is accelerated downward and we can better predict the splash pattern.
…and that’s why you don’t need to put an explosive warhead on a missile defense interceptor. 
We don’t go to calculate with the numbers we want, we go to calculate with the numbers we have. 