Frogwatch wrote:
OK, here is my attempt to revive things.
I recently did a simple calculation concerning lightsails and how fast
they could go. I used a graphene film coated with 100 nm of Au as my
sail. I was surprised that no matter how big you make the lightsail,
it cannot exceed solar escape speed using only solar photons. Of
course, the closer you start to the sun the faster you can go but
still cannot exceed suns escape velocity.
I do not have that calc in front of me now but does this seem right to
others? My conclusion is that your lightsail has to be initially
accelerated to escape speed before deploying


If you neglect relativistic effects and dimming of the star over time then,
no matter how small the light pressure is, as long as it is there you can
always escape from any star orbit no matter how massive the star is. Of
course, it might take a long time. To see this, imagine the following flight
plan: At the perigee of the orbit, you put your lightsail at a 45 degree
angle for one minute, this gives you an epsilon increase in speed. Put the
light sail aligned with the light flux and wait one orbital period, you get
no acceleration during that time but you will come back to the same point,
and have the same light pressure, so you once again can put your lightsail
at a 45 degree for on minute and gain exactly the same epsilon increase
in orbital speed. Repeat N times with N large enough to have N*epsilon
to be equal to escape velocity. On your way out you can keep some acceleration
from the light sail to have a little extra push.

That is a ridiculous flight plan, it would take a lot of time to escape that
way. But it does show that you can reach escape velocity.


Alain Fournier