--- Lúcio de Souza Coelho <[EMAIL PROTECTED]> wrote: > On 6/15/07, Tom McCabe <[EMAIL PROTECTED]> > wrote: > >How exactly do you control a megaton-size hunk of > >metal flying through the air at 10,000+ m/s? > > Clarifying this point on speed, in my view the > asteroid would not hit > Earth directly. Instead it would first make > aerobrake maneuvers to > enter Earth orbit and then to de-orbit.
I refer you to my earlier explanation of drag- no decent-sized asteroid could possibly get slowed down enough by the atmosphere to make a stable Earth orbit in one pass, so it's just going to skip out into space. It'd be an excellent choice for large-scale fireworks, though. > So, as far > as I can see, its > speed would be less-than orbital - <7,900 m/s . > Still, a very high > speed, granted. > > > The reason most metallic meteors > > survive is precisely because they are small- the > drag > > deceleration exerted on an object is roughly > inversely > > proportional to its size, because inertia goes up > with > > r^3 while surface area (and hence drag) only goes > up > > with r^2. > (...) > > Okay, that's a sound argument. In that case, the > alternatives seems to be: > > (a) Cutting small shunks of a big metallic asteroid > and then making > controlled crashes of those. (This kind of reduces > to the drop capsule > method, but in a cruder - and perhaps cheaper - > version.) Okay, this I can buy. > (b) Or, for starts, target only tiny asteroids a few > meters in length > - the so called "meteoroids". Indeed I remember a > project for moving > meteoroids with solar sails > (http://alglobus.net/NASAwork/papers/AsterAnts/paper.html > - though in > the case they were meant for processing in Earth > orbit.) I wonder > though if the economics of scale won't favor a less > "distributed" > model... This could work if it's economical to find and send a mission to these meteoroids. > As materials science progresses, perhaps there will > be an alternative > (c): making the mass-to-surface ratio of a larger > asteroid go down > artificially, by instanlling an inflatable, > heat--and-friction > resistant "shield" around the asteroid. An "asteroid > parachute", so to > speak. But that seems far more speculative than (a) > and (b)... Unless this material is made out of very, very thick diamond nanofiber or something of similar strength and rigidity, it's just going to get ripped to shreds by the forces of deceleration. At 10,000 m/s (initial aerobrake speed), even a small force of 1,000,000 N means that this parachute is dissipating 10 GW. By the typical standards of re-entry, that's enough force to aerobrake a ~200-ton asteroid, which would only be a few meters across. > In any case, I think that the belief that asteroid > mining is of use > only for space construction due to energetics simply > does not hold. > There are ways around that - even if they are not so > cool like > crashing asteroids a hundred meters wide. :) > > ----- > This list is sponsored by AGIRI: > http://www.agiri.org/email > To unsubscribe or change your options, please go to: > http://v2.listbox.com/member/?&; > ____________________________________________________________________________________ We won't tell. Get more on shows you hate to love (and love to hate): Yahoo! TV's Guilty Pleasures list. http://tv.yahoo.com/collections/265 ----- This list is sponsored by AGIRI: http://www.agiri.org/email To unsubscribe or change your options, please go to: http://v2.listbox.com/member/?member_id=4007604&user_secret=7d7fb4d8
