will england :: humor : What happens when. . .

Date: Thu, 30 Jul 1998 15:01:57 GMT
From: Peter Morrison
Subject: Re: Hangin's too good for 'im! Burnin's too good for 'im!

Steve VanDevender said on 29 Jul 1998
16:56:26 -0700:

(First Revision)

> A mere large asteroid skipping through the atmosphere at a lazy
> 20 km/s can easily cook the strip of ground underneath it. A 50
> kg luser traveling at 0.5 c has excess energy of about 15% of its
> mass, or 7-8 kg * c^2. A hydrogen bomb is less than 0.01 kg *
> c^2 of energy. Realistically the luser missing the planet would
> probably only interact with around 1000 km of atmosphere, but
> even in that tiny period of time if the luser sheds a tiny
> fraction of its energy the results would be, umm, spectacular --
> shedding only 1/1000 of its energy would equate to a high-yield
> H-bomb detonation. I suspect that it could lose a very
> substantial amount of its energy since a 1000 km high cylinder of
> air with the luser's cross-section would have more than enough
> mass to completely ablate the luser. At best you'd get a
> tremendous ball of ultra-hot plasma zooming off into space,
> peeling a noticeable amount of atmosphere off with it. This
> plasma would be energetic enough to really seriously heat a strip
> of ground underneath it with a glow reaching up into hard X-rays.
> I suspect you'd also get a sonic boom you could measure with 2
> digits on the Richter scale.


(Second Revision, with accurate figures)


Date: 31 Jul 1998 01:28:29 -0700
From: Steve VanDevender
Subject: Re: Hangin's too good for 'im! Burnin's too good for 'im!

Fortunately other folk have already addressed that logarithmic is
not at all like asymptotic. Brief research of my own indicates
that a Richter scale measurement of N results fromA seismic waves
of amplitude K * 10^N, so magnitudes above 10 are possible and
meaningful. Of course at some point you have seismic waves with
an amplitude in kilometers implying big chunks of the Earth's
crust are flying into orbit.

I found the one figure I was missing earlier to do a more
sophisticated analysis of our relativistic luser's impact.
The "standard" density of air is 1.225 kg/m^3 (sea level air
pressure at 15 deg C).

So we can do an order-of-magnitude calculation of just how much
the luser decelerates. I'll be pretty conservative because I can
still scare the shit out of people with the predicted effects.

As the luser approaches the Earth it looks a bit brighter and
bluer than usual. Not much time to stare at it, though. We'll
assume that the luser wears a nice sturdy spacesuit that keeps
the interplanetary medium slamming into it at 0.5 c from ruining
the fun to come.

Soon the luser encounters the fringes of the Earth's atmosphere,
and gets HOT. Damn hot. Even really thin gas at 0.5 c carries a
lot of energy. Enough to slow the luser down appreciably,
unfortunately well over the 12 g figure we were pondering for
getting him up to speed. Within a millisecond or so the luser
starts hitting even denser gas. Acceleration increases.

Now, let's consider that fairly conservative estimate I was
talking about. Say that in his grazing trajectory the luser
passes through the equivalent of a mere 10 km of sea-level
atmosphere. Let's also say the luser has an aerodymanic
cross-section of a mere 0.2 m^2. The luser will sweep up a
column of air with a volume of 2000 m^3, or a mass of 2450 kg.
This takes less than a millisecond (it does take quite a bit more
than 10 km of travel to sweep up the mass, since at first it's
thinner air at higher altitude). That's not enough time for any
aerodynamic effects to make any difference; our 50 kg luser picks
up well over a metric ton of air and keeps it because the air
can't move aside fast enough. Note that's a lot more mass of air
than of luser. The luser decelerates from 0.5 c down to a much
smaller fraction of c in a millisecond, an average deceleration
of _billions_ of gs. We now have a thin pancake of degenerate
matter consisting of 50 kg of luser and 2450 kg of air, carrying
7 kg * c^2 of excess energy, a few kilometers off the ground.
For our luser, the LART is over; the LARTing of everyone
underneath is just beginning.

Under that kind of pressure the mass will spread out a bit
sideways, which will sweep up more air, producing more
deceleration, ensuring that this big ball of fun won't run off
before everyone gets to meet it. A few billion g of deceleration
may very well create enough pressure to induce nuclear fusion in
the pancake, although that really can't add more than some 1 kg *
c^2 of energy to the total, as if we needed any more.

The upshot is that pretty much all the luser's kinetic energy
will get dumped into a very impressive piece of plasma as hot as
a supernova core, which shoots back out into space over the next
few milliseconds while still dumping a lot of energy into the
atmosphere. The shockwave from the passage is going to be
_impressive_. It's an airburst equivalent to a few thousand
hydrogen bombs. The radiation coming from the expanding plasma
ball should fry most of the hemisphere it's visible from as it
recedes. It would be a good day for 2,000,000 sunblock.

Steve VanDevender "I ride the big iron"

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