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Path: utzoo!watmath!wateng!broehl
From: broehl@wateng.UUCP (Bernie Roehl)
Newsgroups: net.space
Subject: Re: Money to burn
Message-ID: <2437@wateng.UUCP>
Date: Wed, 29-May-85 10:51:23 EDT
Article-I.D.: wateng.2437
Posted: Wed May 29 10:51:23 1985
Date-Received: Thu, 30-May-85 02:45:21 EDT
References: <1925@mordor.UUCP>
Reply-To: broehl@wateng.UUCP (Bernie Roehl)
Organization: U of Waterloo, Ontario
Lines: 54

Hmm...

  To get first data back in ~100 years, and given that 40 of those years
is the time it would take the data to make the trip back, you have 60 years
to get to a star 20 ly away.  Thus, you must be travelling at .3c, average.
  However, the speed in the target system must be reduced to 0.05c; this is
a non-trivial task!

  Doing the whole thing by 1990 for less than $1B is, to say the least,
extremely challenging.

  Personally, I don't think it's possible.  However...

  The most promising form of propulsion is probably the use of ground-based
lasers.  You would use one shuttle flight to put the probe itself in orbit,
along with a *very* long-lasting nuclear power source and self-repairing
on-board systems (don't laugh; serious research has been done in this area).
The second shuttle flight would carry a modified Centaur stage and a *huge*
solar sail.

  You mate the two in earth orbit, and use the Centaur to take the thing on
a trajectory past some massive body (e.g. Jupiter) to give it more of a kick
(or, more accurately, redirect its velocity vector to a more useful direction).
One possibility is to use Jupiter to cancel the craft's orbital velocity
relative to the Sun, and use the Sun's gravity for even more of a slingshot.

  All this maneuvering would take a number of years, but the end result would
be a craft leaving the solar system with a reasonable velocity (still nowhere
near c of course, put respectable nevertheless).

  By the time all of this is done, we may very well have powerful enough
ground-based (or space-based?) lasers that we can start using them to propel
the craft further.  The craft would unfurl its sail, and we would focus the
laser on it, providing thrust.  Since lasers produce a highly collimated
beam, there is no real 1/(r squared) problem.

  In theory, you can eventually get the thing moving at a pretty good clip;
after all, you get a small but continuous acceleration for ~60 years.
(remember of course that a ground-based laser would not be able to "see" the
probe continuosly, but it wouldn't be that many years before space-based
lasers would supplant the ground-based one(s)).

  The big problem is slowing the probe down when it arrives.  The best way
is to have a laser on the destination end, but in this case that seems a
little unlikely.  Other than that, no solution seems immediately obvious.

  Of course, I would hope that in the 100 years it would take to get data
back from the probe, we will have reached the point where the probe itself
is little more than an historical curiousity.  Personally, I want to be
there when the probe arrives...

-- 
        -Bernie Roehl    (University of Waterloo)
	...decvax!watmath!wateng!broehl