New article on mass-drivers

From: Dan Wylie-Sears

> I would think that you could burn up an a fair amount of stuff in the
> atmosphere before it gets to be a problem, even compared to the amount
> that causes trouble when it enters the atmosphere in one lump. Have
> you found or done anything quantitative to say how much a threat could
> be reduced by having part of it burn in the atmosphere,

I haven't, but the conclusion I report is the one drawn by the experts who have.

> or how much of
> the debris might miss the earth entirely if an asteroid were
> pulverized a plausible length of time before impact?

Of course that would entirely depend on how we define "plausible length of time". Sure, if one could pulverize an asteroid a number of years before impact, such that many of its pieces wound up missing the Earth entirely, then that's obviously an improvement on the situation. But that's different from the Hollywood presentation which I was urging readers to not take seriously.

> Is the Nature
> article available online?

Not as far as I know.

> I certainly favor using materials already in space. But as phrased,
> it doesn't follow: it neglects the possibility that non-rocket launch
> systems (mass drivers, skyhooks, or whatever else) might make it
> feasible to launch materials for the first settlements from the
> surface of the earth.

The studies I referred to considered it against the rules to consider any system outside of what was currently technologically possible. A skyhook would rely on a material currently not producible in industrial quantities. I suppose an Earth-based mass driver wouldn't necessarily have to be based on currently unavailable building materials, but, in having to launch out of a much steeper gravity well, and through a thick atmosphere, would have to be much larger, longer, and more powerful than what O'Neill proposed. (And hence, many multiples more expensive.) Even the celestial mechanics would be more complicated and limiting than for the lunar example cited.

If staying within the boundaries of current technologies, or those easily extrapolated from current ones, ET materials are more or less the only economically feasible option for any space construction massing into the gigatons.

Regards,

Mike Combs