On Feb 26, 5:42 pm, Tim Little <t...@[EMAIL PROTECTED]
>
wrote:
> On 2008-02-26, IsaacKuo <mech...@[EMAIL PROTECTED]
> wrote:
> > The original concept assumed it's okay to wait decades for the
> > mission to complete, so the required delta-v might not be so
> > daunting.
> If it were not an asteroid crossing Earth's orbit to begin with, then
> there would be a minimum delta-V requirement - almost certainly enough
> for contained waste heat to vaporise it.
The original concept didn't specify the target was Earth, but
that's not really the main issue. The main issue is what the
minimum delta-v requirement would be, and it's not significantly
worse for Earth than any other target. Basically, you want
to nudge a far out object, like a Kuiper belt object. All you
need to do is wipe out the object's 3-5 km/s of orbital
speed, and the Sun's gravity will do the rest. Naturally, you
need to time it well, so it will fall onto the target as it passes
by. It'll take decades for the object to fall onto the target,
but that's compatible with the original scenario.
In principle, at least, a mass driver rocket can be pretty
efficient (also specified in the original scenario). With
a mission delta-v of only 5km/s, an average exhaust
velocity of 3-4km/s is fine. This is within the range of
chemical rockets, which is vaguely limited by the
strength of chemical bonds, which will also be vaguely
the limit of how much you can dump heat into a solid
object without vaporizing it. But since the mass driver
can be pretty efficient, you'll only be contending with
a fraction of that energy. I think it should be possible
to dump the excess heat into the asteroid without
vaporizing it.
However, while this is possible in principle I don't think
it's at all practical. Once the asteroid is heated up,
it's going to naturally glow in infrared unless it's very
well insulated. How well insulated? Well enough that
the heat doesn't leak through the insulation even after
decades.
> Either way, the power requirement for an asteroid of the size being
> discussed would be more like terawatts than megawatts. It would be
> practically impossible to change its orbit enough to hit Earth at all,
> let alone do so stealthily.
By starting off with an object with little orbital speed,
the required energy is minimized. The energy required
scales with the square of the mission delta-v. The
power required is the energy required divided by the
time. With a mission time on the order of many decades,
it's okay if the acceleration burn requires a couple
decades.
So, for example, let's suppose the asteroid is a million
ton Kuiper belt iceball. Roughly 3/4 of the mass is
consumed by the mass launcher rocket, leaving a
250,000 ton payload and a mission delta-v of 4km/s.
The energy required is about 2E15 joules. An
acceleration time of two decades is 630 million
seconds, so the power required is about 3 megawatts.
Of course, this is assuming 100% efficiency. With
an 75% efficient mass launcher system, let's call it
around 4 megawatts. So, for this example it's
megawatts, not terawatts.
Depending on the target, total mission time could be
half a century to a century.
And no, I still don't find this method of attack any
more plausible than in my first reply. Even if we
stealth up the asteroid with a neat semi-magic
reflective sunshade and directional radiator, all
it takes is one active sensor scan during all those
decades to blow the surprise.
Isaac Kuo
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