Shooting down satellites?

[CDarwin]

The US is planning to shoot down it's failing spy satellite.

 

Is that really as stupid as it sounds? Because it looks to me like the U.S. military is set to turn a single, compact object that probably wouldn't do any harm into a cloud of debris that could potentially do a lot of harm, particularly to other satellites and to US stances on the demilitarization of space and criticisms of China's satellite killer.

 

Physics isn't my forte, though, so maybe I'm misinterpreting.

[D H]

A large, intact satellite could fall all the way to the Earth and cause a lot of damage if it hit in the wrong place. It could also give away a lot of secrets if it it gets in the wrong hands.

 

A bunch of fragments will simply burn up in the upper atmosphere. Acceleration due to friction is proportional to surface area and inversely proportional to mass. Those little chunks collectively have a lot more surface area than does the intact satellite.

[Mag]

I havet read the article, but I suppose it depends how they shoot it down.

If they just explode it, yeah, I imagine there will be plenty of debris just floating around in space.

but if they can explode it and manage to push all the debris into earth, then it will disintegrate.

[D H]

I havet read the article, but I suppose it depends how they shoot it down.

The title is a bit of a misnomer. They are not really going to "shoot the satellite down". They are going to hit it with a missile that will blow up on contact (or at least in proximity). The satellite is already on the way down. Hitting it with a missile will not change that. It is currently moving about 17,600 miles/hour. Blowing it up will not change that by much (some pieces will get an extra hundred miles per hour so, others a reduction by a hundred miles per hour or so).

 

Blowing this satellite up will accomplish at least four ends (if they can hit it):

• It will nearly eliminate the likelihood of hazardous chemicals (i.e. hydrazine) from reaching the ground.
  
• It will most likely render sensitive avionics and sensors useless if the pieces fall into the wrong hands.
  
• It will drastically reduce the likelihood of anything reaching the ground. Debris from a satellite is most likely to hit the ground if the satellite remains intact through at least part of the entry. If the satellite finally breaks up in the dense lower atmosphere the fragments will quickly slow down and fall. An upper atmosphere breakup means the fragments are subject to very intense heating as air drag is roughly inversely proportional to fragment size.
  
• Last but certainly not least, it gives the military a chance to play with their fancy ASAT toys.

[iNow]

As DH mentioned, it's not about the most efficient way to bring it back to earth, it's about destroying the information it contains since we can't control where it lands and who sees said information.

[D H]

Don't forget that last item I mentioned: Giving the military a chance to play with their fancy ASAT toys.

[iNow]

Indeed. Dusty toys that don't get used kinda suck.

[CDarwin]

Might this have any political implications, though?

[MrMongoose]

What has this got to do with militarisation? I didn't realise that it's an act of war to dissemble your own equipment.

[swansont]

What has this got to do with militarisation? I didn't realise that it's an act of war to dissemble your own equipment.

 

It's about having the ability. If you can do this to your own satellites, you can do it to someone else's.

[CDarwin]

What has this got to do with militarisation? I didn't realise that it's an act of war to dissemble your own equipment.

 

It wouldn't be an act of war, but it would be shooting a missile into space to blow up a satellite. If it raised my eyebrows, it might raise someone else's, or erode the United States' credibility in criticizing China for it's anti-satellite missile tests.

 

I don't really know. Space politics isn't a particular area of expertise.

[DrDNA]

That thing is coming down regardless of what anyone tries to do. It is not being brought down intentionally.

The story is that it contains 1000 lbs or so of hydrazine (the same amount it went up with) because they have not been able to fire the control rockets. Besides big chunks of metal that could give a major headache, there is a fear that it will break apart over, and rain toxic hydrazine down on, a major inhabited area.

If they shoot it too soon => a bunch of dangerous orbiting space junk.

If the shoot too late => atmospheric interference and probably can't hit it.

[YT2095]

pity it can`t be rescued and taken to the ISS, that amount of Hydrazine would last quite a while, and since for the most part it`s already Up there, seems a waste just to destroy it and all the other bits on there like solar arrays etc...

[CaptainPanic]

The story is that it contains 1000 lbs or so of hydrazine (the same amount it went up with) because they have not been able to fire the control rockets. Besides big chunks of metal that could give a major headache, there is a fear that it will break apart over, and rain toxic hydrazine down on, a major inhabited area.

 

Ok, so my understanding of hydrazine (N2H4) is that it is an unstable explosive with a flash point of 52 deg C.

 

My understanding of things falling back to earth is that they heat up... heat up enough to become shooting stars.

 

Now, really, I should believe that this hydrazine is actually gonna rain down on me? Come on! It will react, and form hydrogen and nitrogen. The hydrogen will also burn up and form water. It's not the toxicity of their hydrazine that the US military is afraid of. (And if such things really concerned them, they wouldn't be putting nuclear reactors in orbit).

 

It's just a matter of preventing military equipment falling into the wrong hands. The rest of the story is to make it look like the US are doing a good thing (after they criticized the Chinese for doing the same to a weather satellite).

 

And I also don't believe that the US military is concerned about space debris. They don't seem to do long-term thinking... or any kind of thinking at all.

[MrMongoose]

I understand why people are concerned, but it seems as ridiculous as not developing a chemical which could potentially kill cancer because it could potentially be used to kill Chinamen.

[gcol]

YAWN...... What's all the fuss?

 

The chinese did it to show they could, and generated a lot of space debris.

The Americans will do it, to show they can, and will leave a lot of debris, too.

Next the Russians will do it, for the same reasons and same result.

 

Same old, same old, arms race and sabre rattling.

 

The real news story will be if they miss, if a highly secret chunk falls into commie hands, or damages a third world subsistence farmer's opium crop.

 

Wake me up if that happens.

[Pangloss]

On a somewhat related note, did anyone catch the Nova ep this past week about the American MOL and Soviet Almaz projects? That's the first time I'd heard that Almaz stations were equipped with a 23mm cannon. Apparently this sort of thing has been going on longer than we thought.

[D H]

The problem is that the vehicle has been dead in space for quite some time. The vehicle is very, very cold. The hydrazine fuel is almost certainly frozen.

 

This is the cornerstone of the military's justification for getting a chance to playing with their fancy ASAT toys. A spherical tank containing a lot of very cold material makes a very good reentry vehicle in the sense the odds are quite good it will hit the ground with a lot of the contents still inside. It depends a lot on where in the atmosphere the satellite breaks up. We don't know where that breakup will happen.

 

BTW, it is unclear whether the "hydrazine" on the vehicle is plain old vanilla hydrazine (N2H4) or one of the slightly less reactive derivatives of hydrazine formed by replacing one (MMH) or two (UDMH) of the hydrogen with a methyl group. All three compounds are used as propellant and all three are colloquially called "hydrazine".

[Pangloss]

Doh, now I'm gonna have to move this back to General Physics! (lol)

[D H]

The military was not initially concern about the reentry. There initial response: "Yawn. It will burn up in the atmosphere."

 

Apparently it was NASA that told the military about the heightened risk resulting from the vehicle being in space unpowered and unheated for a long time:

From http://www.msnbc.msn.com/id/23172469/:

NASA Administrator Michael Griffin sketched out a different scenario, however, during Thursday's news conference with Gen. James Cartwright, the vice chairman of the Joint Chiefs of Staff. Griffin said NASA experts calculated that the hydrazine was frozen solid due to the satellite’s yearlong drift through the cold of space.

The tank, with its half-ton ice core of hydrazine, would thus become one of the most perfect re-entry vehicles ever to fall back to Earth.

 

Griffin explained that the contents of the tank could turn to slush during the fall, but would very likely survive and leak toxic gas over the crash site. Another expert told msnbc.com privately that

the solid ice would provide structural support against the 20 to 25 G’s of deceleration experienced by the satellite during re-entry.

[Phi for All]

I understand why people are concerned, but it seems as ridiculous as not developing a chemical which could potentially kill cancer because it could potentially be used to kill Chinamen.

Are you saying the Chinese are like a cancer?

[Mr Skeptic]

If the satellite were allowed to reach a low enough orbit, wouldn't there be enough atmosphere to stop all the debris? It is my understanding that many satellites are in a sufficiently low orbit as to need boosting every once in a while so they don't fall down. In fact, if the satellite is falling, then there is sufficient atmosphere to slow it down, so I don't think there will be much problem of space debris. Now, if the satellite was in a high orbit, that would be a different story.

[DrDNA]

Ok, so my understanding of hydrazine (N2H4) is that it is an unstable explosive with a flash point of 52 deg C.

 

My understanding of things falling back to earth is that they heat up... heat up enough to become shooting stars.

 

Now, really, I should believe that this hydrazine is actually gonna rain down on me? Come on! It will react, and form hydrogen and nitrogen. The hydrogen will also burn up and form water. It's not the toxicity of their hydrazine that the US military is afraid of. .

Yes. I said "the story is" because it is not my story and (in my op) may not be entirely factual

 

That said, I do recall reading at the time that a significant amount of hydrazine contaminated the debris that made its way to the ground from the Columbia incident.

 

Are you advocating doing nothing?

 

(And if such things really concerned them, they wouldn't be putting nuclear reactors in orbit)

 

That is one of the first questions that came to my mind as well. This should pretty much show beyond a resonable doubt to any sane person that we don't need nuclear anything in space....at least in the immediate future for sure.

 

It's just a matter of preventing military equipment falling into the wrong hands.

I agree that this it is likely that this is a major factor in the decision, but I am not convinced that it is the only one.

The rest of the story is to make it look like the US are doing a good thing (after they criticized the Chinese for doing the same to a weather satellite).

 

For the record, I believe that most would agree that certain equipment is best kept out of the wrong hands.

 

And my understanding is that the the Chinese did NOT do the same thing.

1. The Chinese incident was an intentional weapons test. This one probably is not.

2. In the Chinese incident, they apparently didn't care how much space debry they generated nor any other consequence.

 

I'd be careful making off the cuff comparions between the US and China. In many cases, there is none.

 

And I also don't believe that the US military is concerned about space debris. They don't seem to do long-term thinking... or any kind of thinking at all.

 

I can absolutely guarantee you the US military does indeed care about space debris. It has a VERY severe impact on military satellites and military operations.

[D H]

If the satellite were allowed to reach a low enough orbit, wouldn't there be enough atmosphere to stop all the debris? It is my understanding that many satellites are in a sufficiently low orbit as to need boosting every once in a while so they don't fall down. In fact, if the satellite is falling, then there is sufficient atmosphere to slow it down, so I don't think there will be much problem of space debris. Now, if the satellite was in a high orbit, that would be a different story.

 

Satellite reentry 101.

 

First things first. Satellites in orbit well above the Earth's atmosphere will essentially stay in those orbits forever. While our atmosphere can sustain life up to only about 10 kilometers or so (Mt. Everest is at the limit; most climbers need to carry oxygen), our atmosphere extends much higher than that. The atmosphere rotates more-or-less at the Earth's rotation rate; it is not in orbit. Vehicles in low Earth orbit go 7.87 kph (17,600 mph); about 16 times faster than the atmosphere they are flying through. The difference in velocity means they encounter a stiff breeze. Paradoxically, this makes the speed up. A satellite in a decaying orbit suffers a reduction in total energy but gains kinetic energy.

 

Except near the end of its decay, the decay of a satellites orbit is quite gradual. Even the International Space Station, which has a large surface area-to-mass ratio, falls about 100 meters per day. Another paradoxical effect of orbit decay is that it tends to circularize the orbit. A vehicle in a non-circular orbit will encounter greater drag near perigee than at apogee. Velocity changes at perigee change the apogee, and velocity changes at apogee change the perigee. The orbit quickly becomes essentially circular (but with an ever decreasing altitude).

 

Atmospheric density in the upper atmosphere is roughly exponential function of altitude. This means that near the decay is anything but gradual once the satellite starts plunging into the thicker parts of the atmosphere. The end is very quick, as we recently saw with the Columbia disaster.

 

Vehicles like the Shuttle are designed to stand up to reentry. Most satellites are not. Those satellites not designed to stand up to the intense heating and intense acceleration of entry break up. Some junk might (and has) reached the ground. All satellites in low Earth orbit will eventually fall to the Earth. To avoid damage, satellites are supposed to carry enough fuel to do a planned reentry. The planned reentry makes them fall harmlessly into the ocean.

 

The vehicle in question is a dead vehicle. It can't perform a deorbit burn.

[DrDNA]

Satellite reentry 101.

 

First things first. Satellites in orbit well above the Earth's atmosphere will essentially stay in those orbits forever. While our atmosphere can sustain life up to only about 10 kilometers or so (Mt. Everest is at the limit; most climbers need to carry oxygen), our atmosphere extends much higher than that. The atmosphere rotates more-or-less at the Earth's rotation rate; it is not in orbit. Vehicles in low Earth orbit go 7.87 kph (17,600 mph); about 16 times faster than the atmosphere they are flying through. The difference in velocity means they encounter a stiff breeze. Paradoxically, this makes the speed up. A satellite in a decaying orbit suffers a reduction in total energy but gains kinetic energy.

 

Except near the end of its decay, the decay of a satellites orbit is quite gradual. Even the International Space Station, which has a large surface area-to-mass ratio, falls about 100 meters per day. Another paradoxical effect of orbit decay is that it tends to circularize the orbit. A vehicle in a non-circular orbit will encounter greater drag near perigee than at apogee. Velocity changes at perigee change the apogee, and velocity changes at apogee change the perigee. The orbit quickly becomes essentially circular (but with an ever decreasing altitude).

 

Atmospheric density in the upper atmosphere is roughly exponential function of altitude. This means that near the decay is anything but gradual once the satellite starts plunging into the thicker parts of the atmosphere. The end is very quick, as we recently saw with the Columbia disaster.

 

Vehicles like the Shuttle are designed to stand up to reentry. Most satellites are not. Those satellites not designed to stand up to the intense heating and intense acceleration of entry break up. Some junk might (and has) reached the ground. All satellites in low Earth orbit will eventually fall to the Earth. To avoid damage, satellites are supposed to carry enough fuel to do a planned reentry. The planned reentry makes them fall harmlessly into the ocean.

 

The vehicle in question is a dead vehicle. It can't perform a deorbit burn.

 

 

I didn't know a lot of that. Thanks for the tutorial!