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Sticks
2006-Mar-07, 10:51 AM
I was reading through this blog (http://www.badastronomy.com/bablog/2006/03/06/golf-war/)by Phil, and it reminded me of this little thought experiment / idea

Would it be possible to throw an ordinary plastic Frisbee, (or what ever they are made of), from LEO (Shuttle or ISS), in such a trajectory that it would be able to re-enter the atmosphere, without burning up, and eventually land on the planet surface? :think:

(Has anyone thrown one from a high altitude balloon?)

Sticks
2006-Mar-07, 10:53 AM
If you are looking for a why, possibly to look at how atmospheric re-entry can be achieved better with out resorting to ablative shielding or fragile tiles that can be damaged by falling foam.

Jeff Root
2006-Mar-07, 01:04 PM
Would it be possible to throw an ordinary plastic Frisbee,
(or what ever they are made of), from LEO (Shuttle or ISS),
in such a trajectory that it would be able to re-enter the
atmosphere, without burning up, and eventually land on the
planet surface?
How fast can an astronaut throw a Frisbee? On the one hand,
he or she has natural talent, lots of fast-twitch muscles, and
is trained for the task. On the other hand, he/she is wearing
a clumsy spacesuit.

Let's say that our astronaut can throw a Frisbee at
50 kilometres per hour.

Anything in a circular Earth orbit at 250 kilometres altitude
is moving at 27,900 kilometres per hour, and takes 1.5 hours to
go around the Earth. So, when the astronaut throws the Frisbee
straight back along the line of flight, its speed is reduced to
27,850 kilometres per hour, and it falls for the next 0.75 hour,
gaining speed as it falls, until it reaches perigee, halfway
around the globe.

I should do the calculations to determine the speed and altitude
at perigee, but I'm afraid I'm both math-averse and lazy.

I think you can see that a reduction in speed from 27,900 km/hr
to 27,850 km/hr isn't going to reduce the perigee by very much.
A few kilometres. Is that enough to enter the atmosphere from
an initial altitude of 250 kilometres? Almost certainly not.
It might reduce the orbital lifetime of the Frisbee from a year
down to a month, which is a dramatic reduction for such a small
difference in speed (reasonable for an orbit so close to the
atmosphere, and a lightweight object like a Frisbee), but you
aren't going to get re-entry within one orbit.

This page shows speeds, periods, and expected orbital lifetimes
of satellites in circular orbit at various altitudes, and gives
simple formulae for calculating others:

http://www.freemars.org/jeff/speed/index.htm


If you are looking for a why, possibly to look at how
atmospheric re-entry can be achieved better with out resorting
to ablative shielding or fragile tiles that can be damaged by
falling foam.
I don't suppose you can explain what gave you the idea that
throwing a Frisbee could reduce or eliminate the need for heat
shielding.

-- Jeff, in Minneapolis

NEOWatcher
2006-Mar-07, 01:04 PM
Would it be possible to throw an ordinary plastic Frisbee, (or what ever they are made of), from LEO (Shuttle or ISS), in such a trajectory that it would be able to re-enter the atmosphere, without burning up, and eventually land on the planet surface? :think:
Short snide answer...No.:naughty:

Longer more polite answer:
How would it slow down? The heating is the dissipation of the energy of it's speed. And, even if it were thrown backward at 17kmph (thus near zero from earths point of view), then the rate of fall will increase before it reaches sufficient atmosphere to slow it down. Thus; it needs to be slowed down by the atmosphere, therefore heat is released.

Sticks
2006-Mar-07, 01:39 PM
I don't suppose you can explain what gave you the idea that
throwing a Frisbee could reduce or eliminate the need for heat
shielding.

-- Jeff, in Minneapolis

My understanding of the re-entry heating was that it was due to the heating of the gases of the atmosphere that get caught in the pressure wave that builds up in front of the craft. Have a suitablely designed shape, that dissipates the bow-wave, and maybe that would reduce heat on re-entry. A frisbee seemed like a quite good shape to start with.

I did wonder if the spinning action of the frisbee might help, but I suspect not, and in a craft, that is not practical, unless you are one of the bad guys from U.F.O (http://en.wikipedia.org/wiki/UFO_%28TV_series%29)

Oh well scratch another thought experiment :(

NEOWatcher
2006-Mar-07, 01:48 PM
How fast can an astronaut throw a Frisbee?
Apparently faster than I can type. You eloquently beat me to the post.

swansont
2006-Mar-07, 01:54 PM
How would it slow down? The heating is the dissipation of the energy of it's speed. And, even if it were thrown backward at 17kmph (thus near zero from earths point of view), then the rate of fall will increase before it reaches sufficient atmosphere to slow it down. Thus; it needs to be slowed down by the atmosphere, therefore heat is released.

It only needs to slow down if you intend to catch it at the end of its flight, or if the landing has to be "soft." What if that were not required?

NEOWatcher
2006-Mar-07, 02:50 PM
It only needs to slow down if you intend to catch it at the end of its flight, or if the landing has to be "soft." What if that were not required?
A heck of a sonic boom?
Actually, "need" was probably not a good word to use here. "Will" is probably a better candidate.

ToSeek
2006-Mar-07, 02:56 PM
My understanding of the re-entry heating was that it was due to the heating of the gases of the atmosphere that get caught in the pressure wave that builds up in front of the craft. Have a suitablely designed shape, that dissipates the bow-wave, and maybe that would reduce heat on re-entry. A frisbee seemed like a quite good shape to start with.

As NEOWatcher points out, the kinetic energy of the object re-entering has to go somewhere, and the only real possibility is to heat up the atmosphere.

Sticks
2006-Mar-07, 04:58 PM
I did wonder if the angle was just right, and being areodynamic, (unlike a meteoroid), it would gently slow down in the upper atmosphere, so it might survive, but from what you all said, it won't

Oh well as I said before so much for that idea.