starship-design: still some hope

[Steve VanDevender <stevev@efn.org>]

Ken Wharton writes:
 > Let's not give up on this light-deceleration quite yet.
 > 
 > Sure, light pressure can't do it alone, but there's more than just light.
 > There's also the solar wind and gravity.  Here's a way we could maybe use
 > all three:
 > 
 > The sail doesn't come in straight toward the sun, but rather in a tight
 > parabolic orbit around it.  We'd have to figure out how close we could get,
 > but the closer the better.  The details of the star probably matter quite
 > a bit as well.

The critical notion here is that you have to be able to decelerate to
below your periapsis escape velocity before you reach periapsis with the
target star.  Most likely the orbit of the ship will be hyperbolic
(characteristic of velocity higher than the escape velocity at
periapsis) without intervention from the other factors of thrust from
light and solar wind.  Given that the periapsis escape velocity will be
pretty low compared to the velocity of the ship as it approaches the
system, I don't think the problem of orbital capture by the target star
is significantly different than the problem of braking to a complete
stop relative to the target star, since the difference between the
velocities is only a few tens of miles per second.