RE: starship-design: Massively Distributed Computing for SETI

[Zenon Kulpa <zkulpa@ippt.gov.pl>]

> From owner-starship-design@lists.uoregon.edu Sun Mar 18 18:37:42 2001
> From: "L. Parker" <lparker@cacaphony.net>
> 
[...]
> Without actually sitting down and figuring it out, I would say 
> that Zenon's guess at ten minutes would be the outside limit 
> for retargeting in 3 dimensions. 
?
Sorry, I did not make any guesses concerning the retargeting rate.
I do not know from where you took these ten minutes...


> From owner-starship-design@lists.uoregon.edu Sun Mar 18 20:34:26 2001
> From: KellySt@aol.com
> 
> In a message dated 3/18/01 8:52:13 AM, lparker@cacaphony.net writes:
> 
[...]
> >"Moved visibly" is not the same thing as haven't moved. All stars are in
> >motion, they follow their own orbits about the center of the galaxy 
> >as I am sure you know. Our sun is in one such orbit, any target star 
> >is going to be in another orbit traveling at a different velocity.
> >
> >We cannot simply aim the beam at the star, the star will not be there when
> >the beam arrives. We must aim the beam at where the star will be when the
> >beam gets there. This is a non-trivial task considering that all distances
> >to stars are currently _estimated_.
> 
> I can't see how this wouldn't be a trivial problem? Your not talking 
> a high lateral movement, or any delta-V of the two stars.   
> On the scale of the galaxy the two stars are right on top 
> of one another.  (A couple light years 
> out of a 30-40 THOUSAND light year orbital radius.)
> 
For a starship what is important is not the _relative_ change 
of the star position with respect to its distance from the center 
of the Galaxy, but _absolute_ change of its position
(in light years, say). If the star moves a light year with respect 
to the aim of the starship, the starship must simply travel 
this additional light year (laterally, say) in order to catch it,
no matter how far (or near) the center of the Galaxy is.

[...]
> Tacking, or otherwise manuvering into the  path of the beam 
> is nessisary.  If you fly out of the beam you'll need 
> secoundary motors to manuver back into the beam.  
> You'll need manuvering  engines anyway for in systems work.
> 
Notice however, that the lateral movement of the beam due to
any jitter od the beaming antenna will be rather rapid - in fact,
it may easily exceed the speed of light!
E.g, for a 100 km antenna, its tilt with the speed of 1 mm per second
causes the beam at 1 ly to move laterally with the speed
of 100 000 km/s, i.e., 0.33 c...

Also, helical tacking to follow the orbital path (around the Sun, say)
of the beaming antenna will use lots of additional fuel, 
since it must provide constant acceleration to curve
the path appropriately.

-- Zenon Kulpa