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One tonne satellite falling to Earth
http://www.x404.co.uk/forum/viewtopic.php?f=19&t=20545		 Page 1 of 1
Author:  l3v1ck [ Sun Nov 10, 2013 7:58 am ]
Post subject:  One tonne satellite falling to Earth
Quote:
A one-tonne satellite is falling out of control and is likely to crash into the Earth sometime during the weekend, scientists have warned.

The Gravity Field and Steady-State Ocean Circulation Explorer was launched in March 2009, to study changes in sea level, ocean circulation and the planet's gravitational field.

It has now run out of fuel and is spiralling back to Earth.

The European Space Agency says it cannot predict exactly where, or when, but that it is highly unlikely to cause any casualties.

News CLICKY
So if they don't have a clue where it'll land, they're just relying on pure luck for it not to hurt anybody.
Author:  jonbwfc [ Sun Nov 10, 2013 10:30 am ]
Post subject:  Re: One tonne satellite faling to Earth
Well, the odds are in their favour insofar as the vast majority of the earth's surface doesn't actually have any people on it at any given time. However a one tonne weight falling from even low orbit is going to make a fair old dent if it hits dry land anywhere. Or possibly a wave big enough to be slightly concerned about if it hits just off an inhabited coastline.
Author:  davrosG5 [ Sun Nov 10, 2013 12:42 pm ]
Post subject:  Re: One tonne satellite faling to Earth
Assuming any of it survives re-entry I doubt very much that any of the pieces will still be weighing 1 tonne.
Author:  jonbwfc [ Sun Nov 10, 2013 1:06 pm ]
Post subject:  Re: One tonne satellite faling to Earth
davrosG5 wrote:
Assuming any of it survives re-entry I doubt very much that any of the pieces will still be weighing 1 tonne.

Depends on the design. A one tonne cube coming down from orbit will pretty much still be a whopping great lump when it hits the ground/sea. A more irregular shape will ablate more pieces away.

The satellite in question looks like this...

Image

Assuming the wings come off (or have been remotely detached) there won't be much of them left. The central tubular section would probably come down in one piece though, or at least in a couple of pieces close together, which can have an equally damaging effect. The biggest chunk that hit the ground from the Russian meteor last year was a half a tonne. Wouldn't surprise me if it was roughly as big a lump as that. The satellite will have a fraction of the energy (because the whole meteor was much much bigger and the meteor was going much faster when it hit the atmosphere and started decelerating) but that chunk took out windows 55 miles away.

Let's do a back of a beermat calculation. Escape velocity is roughly 11Kmps at ground level. 11Kps = 11,000 m/s (obv). I'm going to assume an object coming down from orbit will be doing a tenth of that when it hits, having lost 90% of it's velocity through air resistance. Say the bit that's left weighs 300Kg, so it's lost also 2/3 of it's mass as it dropped. Total KE = 300,000*(1100*1100)= 363GJ of energy when it hits. Let's also assume the earth it hits absorbs about 90% of the energy safely, so we say 5GJ of energy 'leaks out' as an explosion. 5GJ is roughly the equivalent of just over a tonne of TNT exploding. That's going to give a lot of people a bad day if they're under it. And, frankly, I'm erring on the cautious side - the actual impact energy will probably bigger than that.

It's hard to say exactly, obviously, but two things seem fairly obvious

1) If this thing was no worry at all, they wouldn't have bothered telling anyone. Burned out satellites de-orbit regularly without anybody bothering to mention it to the press. The fact this one did get mentioned means there's some risk involved.

2) If it comes down over a populated area, the people nearby will know about it.

The statement is correct that the risk is small but as I said that's a function of the fact that there's a very high probability of it coming down in place there are very few or no people in danger from it doing so. it doesn't mean that it's not going to be destructive to wherever it does hit.
Author:  paulzolo [ Sun Nov 10, 2013 7:17 pm ]
Post subject:  Re: One tonne satellite falling to Earth
Interestingly, the BBC was giving the weight of the thing in stones - that's Imperial weights. I have no idea why.
Author:  jonbwfc [ Sun Nov 10, 2013 7:40 pm ]
Post subject:  Re: One tonne satellite falling to Earth
Gah, I'm an idiot. KE =0.5*m*v*v. I forgot the half.

So a 300Kg chunk crashing from orbit having shed 90% of it's velocity on the way and with a 10% 'energy to explosion' ratio would in fact be the equivalent of half a tonne of TNT.

Actually, I don't think I'd want to be anywhere near it when it hit, even with that reduction.
Author:  jonbwfc [ Sun Nov 10, 2013 7:41 pm ]
Post subject:  Re: One tonne satellite falling to Earth
paulzolo wrote:
Interestingly, the BBC was giving the weight of the thing in stones - that's Imperial weights. I have no idea why.

I'm surprised they didn't give in percentage of the weight of a blue whale.
Author:  pcernie [ Sun Nov 10, 2013 7:49 pm ]
Post subject:  Re: One tonne satellite falling to Earth
jonbwfc wrote:
paulzolo wrote:
Interestingly, the BBC was giving the weight of the thing in stones - that's Imperial weights. I have no idea why.

I'm surprised they didn't give in percentage of the weight of a blue whale.


Image
Author:  hifidelity2 [ Mon Nov 11, 2013 10:28 am ]
Post subject:  Re: One tonne satellite falling to Earth
paulzolo wrote:
Interestingly, the BBC was giving the weight of the thing in stones - that's Imperial weights. I have no idea why.

because we dont want any of these new fangled metric units here please :D
Author:  jonbwfc [ Mon Nov 11, 2013 11:35 am ]
Post subject:  Re: One tonne satellite falling to Earth
It's down without hurting anyone

Debris landed in the far South Atlantic. That page doesn't say so, but a radio report I heard this morning said roughly 25% of the satellite made it to sea level. Big splash, but nobody there to see it.

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