Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!watmath!clyde!burl!ulysses!bellcore!decvax!decwrl!glacier!oliveb!hplabs!qantel!lll-lcc!lll-crg!mordor!ut-sally!utastro!ethan
From: ethan@utastro.UUCP (Ethan Vishniac)
Newsgroups: net.space,net.astro
Subject: detecting alien spacecraft
Message-ID: <512@utastro.UUCP>
Date: Fri, 14-Mar-86 17:44:24 EST
Article-I.D.: utastro.512
Posted: Fri Mar 14 17:44:24 1986
Date-Received: Mon, 17-Mar-86 03:33:34 EST
Organization: U. Texas, Astronomy, Austin, TX
Lines: 36
Keywords: anti-matter, gamma ray observatory
Xref: watmath net.space:6482 net.astro:1480

[Suddenly a bug rang out!!]

   I have in my hand a preprint from someone in my own department (Michael
Harris).  On the assumption that he would like the publicity and because
it might start a good discussion in the group I am posting the abstract
of the paper and a brief synopsis of its contents.

       On the Detectability of Antimatter Propulsion Spacecraft

     It is shown that the NASA Gamma Ray Observatory will be able
to detect large interstellar spacecraft at distances up to 300pc by
the gamma ray emission from the propulsion system alone.  The distance
limit is set by the possibility of  recognizing such objects by their proper
motions.

In this paper Michael Harris points out that alien spacecraft using an
antimatter drive will have large proper motions and be quite conspicuous.
Assuming that the velocity is close to the speed of light and that GRO
will have a lifetime of 10 years then the two instruments with the
best angular resolution can see motion at distances of 100 pc (EGRET)
and 300 pc (BATSE).  This assume purely tangential velocity.  
Stellar motions are not in excess of 10 arc seconds per year or 2 arc minutes
over the lifetime of the GRO.  In contrast the angular resolution of the
instruments is 1.6 degrees and 0.5 degrees.  We see that we are angular
resolution limited and that a 15 fold increase in distance might be possible
before confusion sets in.  Moreover the large proper motions are due to
nearby stars and they are not effective sources of gamma rays.  The luminosity
of a given spacecraft is obviously a matter of conjecture.  However, reasonable
numbers would make them easily detectable.

-- 
"Ma, I've been to another      Ethan Vishniac
 planet!"                      {charm,ut-sally,ut-ngp,noao}!utastro!ethan
                               ethan@astro.UTEXAS.EDU
                               Department of Astronomy
                               University of Texas