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February 15th, 2005, 02:56 PM
Hey there,

I'm working on a story and it involves doing strange things to asteroids, like hollowing them out. Rather than actually going to asteroid school myself, I thought I'd ask if anyone out there might know some actual science about such matters who'd let me pick their brains a bit.



February 15th, 2005, 05:04 PM
Well, I've got a bachelor's in physics, I've worked for a orbital sciences company (but my speciality is now radars) but more importantly, my husband has master's degrees in both astrophysics and aerospace engineering and has worked extensively on orbits mechanics. As such I guess I wouldn't be your science "guy," but maybe I'm married to one? :rolleyes: If your questions are nifty enough I might be able to talk him into helping. :D

Another thing: do a search on these threads for hillbilly tutorial; you'll find all the math behind some orbital determinations there. For what that's worth. ;)

February 15th, 2005, 05:59 PM
Hey Yorkwood,

I read an article, actually quite a while ago, that outlined how one could take a metal based asteriod, bury a compressed air container in the middle of it, then use either lazers or some kind of huge magnifying lense to melt the asteriod as it rotates, when the hot part reaches the middle, -POOF-, you have a perfectly formed metal sphere. :cool:

It hink it might have been in Analog, or something like that.

Of course it would be a bit more complicated than that, and lately I've read that scientists think asteriods are not usually a solid body, they are collections of rocks and dust loosely held together by gravity.


February 15th, 2005, 06:09 PM
Oh, I remember something on this - parabolic mirrors are used to melt a hole down the axis of a nickle-iron asteroid, then the hole is packed full of asteroidal ice and a cap is installed to seal it. Then several parabolic mirrors to heat the asteroid so it becomes molten, melting the ice into water and then steam that's suppose to inflate the molten asteroid like a ballon. When it reaches the right size, explosive bolts blow the cap off and the steam rushes out. The nickle-iron cools and you've got yourself a sphere-shaped rock that's got a thick enough skin to keep in atmosphere and radiation out.

February 15th, 2005, 11:38 PM
You may also want to read the book 'Hammer of God' by Arthur C Clarke and other such works that relate to asteroids. I found this book quite enlightening when it came to asteroids, meteroids, and comets.

February 16th, 2005, 01:56 AM
Oh, I remember something on this . . .

Ex, you are a human encyclopedia. I remembered vaguely, but you knew details. (Applause)


February 16th, 2005, 04:05 AM
Thank you for your wonderful replies. This is some nice list. Missus and her husband might be right up me alley, and everyone else has been so kind, (and if you think you know this alias you play poker at pokerroom.com and I was bluffing). But there is interest, so, thank you, I will proceed.

Ok. Here's a taste of what I need: Given virtually unlimited power sources (a gee-whizz aneurtonic fusion thingie, details do not matter much was invented in Feb 2005 and we are now in 2088):

it is decided to turn an asteroid into a mini inside-out planet. Total surface area, say montanana-like 100,000 sq miles interior oblong spheriod (pick one) 45 miles diameter by 800 or thereabouts, made out of an asteroid.

Questions relate to: spin required to provide, say .5g apparent gravity internally, air patterns assuming full of one atmosphere normal air at interior surface, that sort of thing. Some question regarding the enery necessary to melt such an object, how long it would take at a given rate of whatever and various other fiddly engineering details.

Cheers, and thanks in advance. Email me if you need to use two syllable words ;-)


February 16th, 2005, 12:04 PM
I'll see what I can do for you, & get back with you tomorrow. Cheers!

One question: size range of asteroid. Are we talking going for one of the 5 biggest in the system, or something more middling sized (even the biggest ones are still pretty small, BTW). You're not going to get 800 miles out of any of them, I'm afraid. Ceres, the largest in the system, has a diameter a little over 600 miles. Or are you looking just for 45 mile diameter roughly?

February 16th, 2005, 05:16 PM
So let's assume that you can hollow out some sort of asteriod into a rough cylinder of the dimensions mentioned: 1280 km x 72 km. (1.6 km / mi) A sphere, or spheroid, in my opinion, wouldn't be as practical of a shape (for what I'm presuming is your intended purpose) as you would have an inner surface "gravity" that varies.

Regardless of whether an asteroid of such dimensions actually exists, I don't think it would be too difficult to push multiple asteroids together and fuse them somehow (given your virtually unlimited power sources).

Centripetal acceleration is just a_c = v^2/r. So if you want to figure out the velocity people would have to be travelling on the inner surface it would just be v = sqrt(a_c * r). For a_c = 0.5g,
v = sqrt(0.5 * 9.8 m/s^2 * 32000 m)
= 396 m/s
= 891 mph

Now with respect to air, the thing would have to be air-tight, because you won't have gravity to hold it in. Being air tight, there would be some kind of natural pressure, but I suspect that as a side effect of the angular acceleration there will be a pressure gradient inside the cylinder. Also (and maybe more importantly) there might be a centrifuge effect.

We know that air pressure varies with height here on earth by:
P = Po*exp(-mgh/kT).

I thought it would be interesting (alright a good way of procrastinating) to derive this for the inside of a rotating cylinder.
- If we consider a slab of air of area A and infinitessimal thickness dh, the "gravitational force" experienced by this air is simply m*a_c.
- m is found by density (rho)*volume (Adh).
- The pressure that this slab then exerts is then F/A or
dP = dF/A = -rho*A*dh*a_c/A (you can cancel the A's out at this point)
- a_c = v^2/r (as above)
- annoyingly, v is dependent on r, so lets write things in terms of the angular velocity (v = w*r)
- Further, let's say that your 32 km radius is denoted R, hence r = R-h
- then a_c = (w*(R-h))^2/(R-h)
= w*(R-h)
- put this back into the above equation,
dP = -rho*dh*w*(R-h)
- have a conniption(sp?) because rho is also dependent on pressure
(rho = mP/kT)
- This sets us up to solve the differential equation:
dP/P = -(m*w/(k*T))*(R-h)*dh
- integrate both sides
ln P = -(m*w/(k*T))*(R*h - h^2/2) + c
- initial condition: P = P_o at h = 0

This gives us:
P(h) = P_o * exp(-(m*w/(k*T))*(R*h - h^2/2))
R = radius of cylinder
h = height from surface of cylinder (measured inwards)
P_o = 1 atm
m = mass of one air molecule
w = angular velocity
k = Boltzmann's constand
T = temperature

Interestingly, there is a dependence on m - which is different for the different gasses. So you may have to play with the concentration of nitrogen, oxygen and carbon dioxide, to make sure that you're inhabitants are breathing properly.

I'm curious to see how this compares with what others get.

February 16th, 2005, 08:07 PM
*Greg attempts to read choppy's post, get's to the third line... gets confused... blinks a few times... leaves thread, (and is reminded why he doesn't write science fiction).