Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!henry
From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: net.space
Subject: Re: Money to burn
Message-ID: <5643@utzoo.UUCP>
Date: Thu, 30-May-85 14:04:47 EDT
Article-I.D.: utzoo.5643
Posted: Thu May 30 14:04:47 1985
Date-Received: Thu, 30-May-85 14:04:47 EDT
References: <1925@mordor.UUCP>, <2437@wateng.UUCP>
Organization: U of Toronto Zoology
Lines: 18

> ...  Since lasers produce a highly collimated
> beam, there is no real 1/(r squared) problem.

Alas, not so.  Lasers do not eliminate the inverse-square problem, they
merely postpone it.  Keeping a beam focused on a lightsail over
interstellar distances is still a major problem, although solutions are
known (albeit ones that involve very large structures, which would have
to be space-based).

Note also that producing any noticeable amount of thrust with a lightsail
requires tremendous laser power output.  Don't be misled by thinking that
it only has to match sunlight; sunlight is (speaking *very* roughly) a
kilowatt per square meter.  When the sail gets large, the necessary laser
power gets huge.  Note also that lasers are inefficient, maybe 15-20%
at best.
-- 
				Henry Spencer @ U of Toronto Zoology
				{allegra,ihnp4,linus,decvax}!utzoo!henry