RE: starship-design: Massively Distributed Computing for SETI

["L. Parker" <lparker@cacaphony.net>]

> Given the relative positions of the stars haven't moved
> visibly in thousands
> of years.  We can aim a fixed vector.  The only problem is
> earth orbit around
> the sun.  If beam array isn't a ring around the sun, I.E. if
> its around a
> point in orbit, the ship would have to follow the beam in a
> helical course
> around a direct vector from the sun, toward the target star.
>
> No serious math needed, or really possible.  We can't aim the
> beam because we
> can't know where the ship is to aim it at.

Actually, there is more to it than that.

Lets start with the relative motion first.

"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_.

Then you must add for the motion of the beam array in its orbit about Sol.
As was stated by Kelly this induces a helical component, and if we are in
orbit about Earth or some other planet, it induces another helical
component. Although it is possible for the ship to correct its course for
helical movement of the beam source, this involves tacking the sails to
maintain a steady course on a continuous basis and involves an element of
risk if we lose the beam entirely and it doesn't solve the other problem.

That is actually the easiest problem to solve. The second problem involves
Doppler drift. As the ship gains velocity, it begins to experience Doppler
drift or "red shift". Unfortunately, the sail is composed of a material
designed to reflect a particular wavelength of radiation. It may also
reflect other wavelengths, but not with the same efficiency. Therefore the
beam transmitter must be capable of tuning the output across a range of
frequencies to keep the energy received by the sail at a constant frequency.

This means we must know the exact speed, course and distance of the sail at
all times, constantly update the targeting data for the sail, the beam
source, and the sail's destination, all in four dimensions, one of which has
at least one harmonic component if not two to compute a continually changing
target solution and frequency solution for the beam.

Any way we approach this, the solution is going to be compute intensive, and
it involves many of the same types of calculations being done by the SETI
team, which was why I found the article so interesting.

Lee