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From: (Gordon D. Pusch)
Subject: Re: Could intersteller debris make space travel between other stars 
	not   possible?
Date: 12 Dec 2000 21:44:22 -0600

> quisp wrote:
>> Could intersteller debris make space travel between other stars not
>> possible?  Haven't I heard correctly that beyond the solar system there is a
>> large amount of debris or space dust?  It would seem to make sense due to
>> the fact that there are no large objects to suck up the debris.  If there
>> was large amounts of debris between solar systems it seems like that would
>> make us a prisoner to our own solar system.

One can set bounds on the amount of dust by noting how far away we can still
see stars without their light being absorbed. Similar bounds may be set for
the amount of interstellar gas. Calculations by Robert Forward using the
current estimates on the gas density suggest that the oncoming interstellar
gas will pose a serious radiation hazard as viewed from the spaceship frame
for relative velocities in excess of ~0.25 * c.

The dust hazard is harder to quantify, but to put it in some perspective,
a one nanogram dust particle impacting at ~0.25c relative will carry about
2.7 kilojoules of energy, which is comparable to the detonation of about
12 grams of TNT. However, the actual effect of impacting such a dust grain
is hard to predict, since not all of that energy will be absorbed at the
point of impact; instead, it will will appear as an intense shower of
moderate-energy (~32 MeV/nucleon) radiation that will take a fair thickness
of matter to stop.

Current proposals for shielding interstellar spacecraft include lasers and
magnetic fields to ionize and divert the gas, sandwiches consisting of
thousands of layers of thin metal foil as Julian Bordas <>
wrote, or clouds of liquid metal droplets sprayed ``ahead'' of the spacecraft
and recollected as the ship's acceleration causes it to ``catch up'' with
them (which in addition to serving as shielding would also provide radiators
for the ship's engines).

Julian Bordas <> replies:

> Unlikely.  A C Clarke wrote a novel I forget the name, Where the
> spaceship had an ablative nose made of water ice.  If there are large
> enough amount to cause a problem then the ship will need to be designed
> to counter this problem. A many layered foam AL foil sandwich is one
> option.

The book/story you are referring to is ``The Songs of Distant Earth;''
however, such a massive, solid shield no longer appears to be favored
by anyone. (Clarke was able to ignore the huge mass-penalty the shield
represented because he was hypothesizing that the ships were driven by
tapping enormous amounts of ``free'' energy from the vacuum...)

-- Gordon D. Pusch

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