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back to article Duck and cover: ROSAT is the next re-entry

Space-watchers are looking skywards again as another large piece of space junk approaches re-entry. The 2,400 kg German ROSAT is due to re-enter sometime between October 21 and October 24, according to DLR, the German space agency. It is also predicted to be a “dangerous” re-entry, with the DLR saying that 30 individual pieces …

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Boffin

Dangerous

>>This latest fall is given a 1:2,000 chance that some fragment may hit someone on Earth

>>(which puts it at somewhat more dangerous than the 1:3,200 chance estimated for UARS

>>on its re-entry).

But to calculate the chance of it hitting YOU you need to divide by the number of people on Earth. Or all those living between 53N and 53S anyway - if you're worried, head for Norway.

There's demonstrably more chance of being eaten by a grue.

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Stop

Stay out!

No! Don't come here.

There are, like, polar bears roaming the streets, mad men handing out Nobel Peace Prizes, oil under the sea and...and...vikings! Lots of dangerous vikings.

http://www.theregister.co.uk/2010/02/10/street_view_frogmen/

Xenophobia? What is that then?

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Selfish

Also, grues don't exist, the satellite does.

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Bronze badge
Joke

"the satellite’s mirror as likely to be the source of the largest fragments"

So at least the girls (and vain men), will be able to check their lippy before it hits them.

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Alien

Does anyone remember the T.V. series Dead Like Me?

A comedy series in which a girl (Ellen Muth) is hit by space debris (the toilet seat from Mir) and becomes a Grim Reaper.

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Holmes

I guess the aphorism writers got it right:

what goes up must come down... sooner or later.

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Any statisticians on their lunchbreak yet?

Slow day at work here, so I got to thinking, how do they calculate these odds? 1:2000 odds that a fragment it might hit someone...

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Silver badge

Aggregate odds I think

I would guess it's something like:

Odds of coming down over a populated area e.g. 1 in 10

x Odds of hitting someone if a piece comes down in a populated area, e.g. if it comes down over a square kilometre with 5,000 inhabitants and we treat a piece entering the same square metre as a person a 'hit', then each piece has a 5,000 in 1,000,000 (1 in 200) chance of hitting.

So that's each piece got a 1/10 chance of having a 1/200 chance = 1/10 * 1/200 = 1/2000. (And with 30 pieces that's 30 1 in 2000 chances.)

Of course this example is just made up numbers, but then gain so is their estimate - if they don't know when it's going to come back to Earth (2 days+ of uncertainty) then they don't know where it's going to come down, hence the population of the theoretical impact area is uncertain at best. think of two scenarios for touchdown, one over the ocean, another over a capital city - wildly different odds of striking a person.

The important thing is that if they calculate this risk the same way every time, they can tell which impact scenarios are (relatively) more or less dangerous even if the numbers they are using turn out to be wrong.

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Presumably they can estimate how big an area would be affected by the impact, as a percentage of the total area where it might land. Multiply that by the population of the area.

The word "fragment" is slightly misleading, it might be 50Kg or more travelling at 28,000 kmph at god knows what temperature. Wouldn't actually have to hit you to kill you.

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Boffin

Maths

To a first approximation: Probability of someone being hit equals (area of one human being) times (human population of planet) divided by (surface area of planet Earth) times (number of fragments of the satellite expected to reach ground)

One should really use the population and area of that part of the planet over which the satellite flies. Since the excluded part of the planet is mostly polar, and fewer people live near to the poles, it'll raise the probability somewhat. above the simple calculation above.

I'll leave the choice of human area as an exercise for the reader. Plan (standing), Plan (sitting), or front elevation? Or an average of them? Then you may want to think about the considerable ground-relative horizontal velocity component of the fragments, and the probability of a random human being in the shelter of a wall, tree, mountain, or other object able to bring the fragment to a stand-still before it hits them.

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Boffin

A 50kg object enterring Earth's atmosphere at speed, after a while in space will actually be at a temperature prettyy close to absolute zero. The leading edge of such an object may well be heated to incadescence, but the bulk will be at about 3 Kelvin, in much the same way that most meteorites, when they 'land', are very cold indeed.

So, yes, it would have to hit you to kill you, or hit something that then hits you. It isn't going to make anything around it burst into flames, unless, of course, it's filled with falmmable rocket fuel*.

*Which it isn't

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Really?

I'm assuming 50Kg is the weight of the piece which actually hits the ground (maybe I misread the article).

At 8000m/s isn't that around 16GJ of kinetic energy? Surely that would harm anyone within a certain distance of the impact, whether it hits them or not?

If that is the case, the size of a person becomes irrelevant, what matters is the probability that someone is within the danger area.

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New Measurement unit

According to space.com spacecraft dimensions are measured in 'school beses'

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Coat

This is an outrage !

6.5 whatsis ?

Oh, you mean 1,572.4 jubs. Now I can imagine it :)

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Thumb Up

Meteorites

I just checked the maths - found (to my surprise) that the 1 in 2000 chance was a perfectly reasonable estimate. (The chances of you personally getting hit is of course about seven billion times smaller).

Which begs the question: given the large number of meteorites that reach the earth's surface every year, why is it so rare for anyone to be struck by one? (Maybe, we spend most of our time under roofs, and most meteors reaching the ground are small enough and slow enough not to penetrate a roof). Anyone know for sure?

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Boffin

Most meteorites are much smaller than man-made sattellites.

My mother's partner has a nice example of a (quite large) meteorite, as a coffee-table ornament. It is metallic (so pretty much a mixture of iron and nickel) with a few small rocky inclusions, is about a foot across, and weighs in the order of 20kg. Most meteorites are smaller than this (in the centimetre range), and most meteors never become meteorites, breaking up in the atmosphere.

There have been cases in recent years of meteorites hitting houses and cars, and IIRC one of someone's dog being hit by one, and apparently, there have been human deaths. Some interesting falls:

http://www.icq.eps.harvard.edu/meteorites.html

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Headmaster

Begs the question?

I don't think you know what that expression means.

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Alien

What goes up... must come down

... unless it has an anti-gravity wave generator fitted and switched on..

Mines the one with the key to own personal 'IAC' in the pocket

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Anonymous Coward

To Lehrer got it right, about the Germans attitude to re-entry

"Once the rockets are up, who cares where they come down

That's not my department," says Wernher von Braun"

If you don't know the source: http://www.lyricsfreak.com/t/tom+lehrer/wernher+von+braun_20138402.html

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Alien

This is surely a cover story....

"The agency highlights the satellite’s mirror as likely to be the source of the largest fragments, because of its heat resistance"

Sounds to me like Jor-el is looking for someone!!:

http://www.google.co.uk/imgres?q=phantom+zone&start=236&hl=en&biw=1273&bih=604&tbm=isch&tbnid=K4txHz7EdFgRCM:&imgrefurl=http://www.capedwonder.com/superman-the-movie-gallery-act-one/&docid=e_CakawFi-THgM&imgurl=http://www.capedwonder.com/images/picture-folder/images/Donner-years/krypton/CW-STM-Phantom-Zone-01.jpg&w=1920&h=796&ei=gGKdTrGYB4Ll0QH16f2VCQ&zoom=1&chk=sbg

I wouldn't want to be General Zod right now.

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Thumb Up

Rise of the Machines?

I guess not in this case.

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Pirate

Due to come down on October 21st?

I wonder if it'll hit Harold Camping?

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Mushroom

Meteorites

Darn it, Loyal Commenter, I'm jealous. My meteorite is only about 10 pounds. Dad found it out in his cow pasture while he was digging a hole for a fence post. Sadly, the Iron component has mostly rusted by now, but the Nickel portion is still rather shiny. Looks to have been quite hot at some point, since you can see the melted/flowed portions. It's not something I would have wanted to have hit me on the head!

Dave

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There is a great deal of misunderstanding of how the odds are calculated by NASA. The figure of "1 in 2000" means that for every *** 2000 such reentry events *** one person on Earth may be harmed.

That is reflected in the frequency of injuries caused by meteorites striking Earth. In the last one hundred years there is only one documented case of a person being struck by a meteorite. Since there are approximately 500 to 1000 meteorites per year that exceed 10 kilos mass striking Earth it is clear that the odds of being struck are minuscule.

Further, objects less than a few hundreds of kilos mass strike the Earth at no more than a few hundred kilometres per hour due to atmospheric drag.

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Incidentally, the mirror isn't anything similar to an ordinary telescope mirror. It is entirely metal and is made of concentric very slightly tapered cones nested together, It is called a "grazing incidence mirror" which reflects X-rays when they strike at a very low angle of incidence.

Objects in Earth orbit are not at 3 Kelvin since they are illuminated by direct sunlight at least half the time. That is why they are usually covered by IR reflecting gold coated mylar mirror material. Without the IR reflecting insulation they would overheat. In a vacuum there is only one way to reject excess heat and that is by radiation. Convection and conductance do not work. The problem this close to a star is keeping the object (satellite) cool enough.

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