Shooting down satellites?

[Mr Skeptic]

What's that? friction making things speed up? I don't think so. If anything, the satellite would loose kinetic energy from friction, and exchange potential energy for kinetic energy. The smaller orbit would require the faster speeds. Nothing paradoxial here, things speed up when they fall down.

[D H]

What's that? friction making things speed up? I don't think so. If anything, the satellite would loose kinetic energy from friction, and exchange potential energy for kinetic energy. The smaller orbit would require the faster speeds. Nothing paradoxial here, things speed up when they fall down.

The upper reaches of the atmosphere are very tenuous. A vehicle in low Earth orbit in the tenuous part of the atmosphere never loses kinetic energy. In those upper reaches, atmospheric drag is best viewed as a perturbative effect. I will call the position/velocity phase state vector of a vehicle the vehicles "state". Atmospheric drag does not effect the near future state nearly as much as it effects the state half an orbit later. The vehicle gains kinetic energy via exchange with potential energy faster than it loses kinetic energy via friction -- that is, until it reaches some critical atmospheric density. The altitude at which the atmosphere has critical density is called "entry interface". Below entry interface, friction does slow the vehicle down. The Shuttle and Martian landers rely on this. They do not carry anywhere near enough fuel to completely cancel their orbital velocity.

[ydoaPs]

I'm confused. What's the big deal about hydrazine? Last time I checked, N₂H₄+O₂->N₂+2H₂O. Wouldn't all the hydrazine be nitrogen and water by the time re-entry is done?

[Cap'n Refsmmat]

Haven't you read the thread? The hydrazine is likely frozen in a ball of ice and will easily survive re-entry.

 

I'm not exactly sure why some people worry about debris -- surely if it's going to re-enter, breaking it into bits will leave us loads of small bits that will enter the atmosphere in another month or two. That's not really a long-term problem, except if pieces get blasted into higher orbits.

[Realitycheck]

Unless they miss.

[D H]

Unless they miss.

As far as the satellite is concerned, missing will just put the military in the same position it would have been in had they just let it fall. The military even uses this as one of the justifications for trying to hit it.

 

On the other hand, if they miss, the military itself will have a lot of egg on their face. From http://dsc.discovery.com/news/2008/02/15/spy-satellite-challenge.html,

"If they can't hit this, they can't hit anything," said John Pike, a Washington, DC-based military policy analyst with GlobalSecurity.org.

 

A number of respected space experts think the hydrazine tank rationale is merely a ruse. The real reason is that the military wants an excuse to play with the fancy ASAT toys and send a message to China and North Korea. More at http://blog.wired.com/defense/2008/02/fishy-rationale.html.

[Pangloss]

I always listen very closely to anything John Pike has to say.

[ydoaPs]

A number of respected space experts think the hydrazine tank rationale is merely a ruse.

 

Like I said. The hydrazine will be undergoing evaporation due the the heat of re-entry AND it will be reacting with the air. I'd have to see some calculations before I just blindly accept that enough hydrazine will survive re-entry to pose any sort of problem.

[Skye]

Well it seems to have worked.

 

http://ap.google.com/article/ALeqM5h7aoM2ii3QVBCAV8m2HtJSuPxPNwD8UUMS5G2

[Pangloss]

The Lehrer News Hour had an MIT physics professor on last night talking about how it couldn't possibly work and then going on to talk about how it would have a negative impact on foreign relations. I hope he's very embarassed today. It was a classic example of subverting science to advance political agenda. He had a perfectly reasonable point on the political impact, but he was apparently quite wrong about the science.

[D H]

Like I said. The hydrazine will be undergoing evaporation due the the heat of re-entry AND it will be reacting with the air. I'd have to see some calculations before I just blindly accept that enough hydrazine will survive re-entry to pose any sort of problem.

NASA did do those calculations. The hydrazine was frozen solid in a spherical titanium fuel tank. It takes a lot of force to break a spherical titanium fuel tank. The fuel tank has to be very strong because of the huge pressure difference between the near vacuum of space and the operational contents of the tank. Because of the inherent strength and the spherical shape of the tank, the fuel tank was very likely to have survived the re-entry intact. Moreover, the frozen block of hydrazine in the tank would have added a lot of structural integrity to the tank.

 

The hydrazine was frozen solid. It would have taken a lot of heat to raise the frozen block to the melting point and then over more heat (a lot of heat) to melt that much hydrazine. If the vehicle stayed intact through a significant part of the reentry, not enough of the frictional heating would have reached the tank to even come close to melting all that hydrazine. The tank was not only very likely to have survived the re-entry intact, it would have done so loaded with slushy hydrazine. The fuel lines most likely would have been severed. Some hydrazine would have escaped during entry, but by sublimation rather than evaporation. On the ground, we would have had a tank with openings full of hydrazine slush. The tank would have eventually heated up, releasing that hydrazine at ground level.

 

In short, the hydrazine-based rationale for breaking up the satellite with a missile was a plausible one. Whether that or having a chance to play with their fancy ASAT toys was the real reason, I don't know.

[iNow]

Could have been a not-so-gentle reminder to China that we can still one-up them from time to time.

[Pangloss]

You could be right, and I don't know that that's such a bad thing, but I think there is no reason to really doubt what's being reported about the situation.

 

The thing that really irks me are these claims that the US is heating up a space arms race. Like this quote from a Reuters article today:

 

"It does not take an arms race to mess up space," said Michael Krepon, co-founder of the Henry L. Stimson Center, a nonpartisan public policy group in Washington. "All it takes is a few destructive anti-satellite tests."

 

But China's ASAT program was pursued when the United States didn't HAVE an ASAT program. The US has been scrambling to cobble together a patchwork system that was designed for a different purpose entirely. Sure, that's an arms race, but the United States can hardly be held responsible for it, and regardless of whether the US participates in it, at least one other country is going to develop their program anyway.

 

The overall political situation is almost comical. If a killer asteroid were heading for the Earth right now and the only thing that could stop it from wiping out a major city was an SM3 missile, SOMEBODY would find some reason to criticize the US for using it.

 

More ridiculous quotes from the media:

 

The action was similar to China's unwise anti-satellite test in January 2007

 

(source) No it wasn't! The China test was "unwise" because the satellite it shot "down" was 500 miles up, and the debris created by that test may be cluttering up orbital space for years. The result of THIS action was exactly the opposite -- it made something, however small the odds, SAFER.

 

John Barry in Newsweek raises this ridiculous tripe:

 

"USA 193, weighing around 5,000 pounds, is the size of a school bus. But . . . [t]hree-quarters of the earth's surface is water. Ninety-five percent is uninhabited. Suppose USA 193's debris were to cover a few square miles, which is a plausible estimate. The earth's surface is 197 million square miles -- all but one-20th of which is uninhabited.

 

Uh, if the US has the means to easily rectify a dangerous threat, shouldn't it use those means? How much does anyone want to bet that if we HADN'T shot down this thing, and the toxic mass were dumped on a school house in Europe, that we would have had to listen to ENDLESS roasting of the Bush administration over its failure to act?

 

The only quote that I thought was really on-point and relevent today was this one from Ivan Oelrich at the Federation of American Scientists:

 

"To put this in some perspective, the US produces 36,000,000 pounds of hydrazine every year. The world 130,000,000 pounds. This is transported around the country in trucks and on trains. At any given moment FAR more hydrazine is being shipped on the country's highways, through towns and cities and inhabited areas, than the amount on this satellite. (And far more dangerous materials, like chlorine.) So I do not buy the public safety argument. If the administration were concerned about public safety, they would take the millions of dollars spent on this intercept and spend it on traffic lights at a dangerous intersection or on vaccines for children."

 

I think that's a reasonable point, not hype or exaggeration. Most of the rest of what I'm hearing right now, though, is just jumping on the ABB bandwagon.