On Sep 25, 2:31*pm, Frogwatch wrote:
On Sep 25, 2:13*pm, Frogwatch wrote:



On Sep 25, 1:34*pm, "nom=de=plume" wrote:

"Wayne.B" wrote in message

.. .

On Fri, 25 Sep 2009 08:08:57 -0400, Tom Francis - SWSports
wrote:

He estimated that a single sail, six miles across, could
boost an object the size of our moon out of orbit and complete one
grav of acceleration in ten years.

Not to be argumentative, but since the moon already has a surface area
far larger than 6 miles across, why isn't it being boosted out of
orbit as we speak ?

Reflectiveness? Gravity effect of the Earth? No idea... interesting
question.

For that matter, why isn't the Earth being boosted?

--
Nom=de=Plume

Both (earth and moon) are being "boosted" by solar pressure,
effectively reducing the gravity of the sun by a tiny tiny amount.
Put your small payload in circular orbit about the sun WITHOUT the
sail and it goes happily about the sun. *Unfurl your sail and suddenly
the orbit is perturbed a tiny amount due to the force on the sail.
Near the sun, assume the force due to sunlight is constant (it really
drops as 1/r^2) so your change in velocity of sail is then: *v=t*F/m
where t is time your sail is unfurled, m is mass of sail + payload and
F is force due to sunlight. *After some time, furl your sail and you
settle into a new orbit.

Whether or not you escape from the sun and then go on to build up more
speed depends on the ratio of the force of the sun to the force due to
gravity and as both of these vary as 1/r^2, then this depends on the
size of the sail. *Basically, at any distance you need a sail large
enough to outweigh gravity in order to escape from the sun, otherwise
you just change your orbit but stay in orbit about the sun.

More "Waaaaay too much information":
It turns out that a perfectly reflective (shiny) surface gets twice
the boost from sunlight that a perfectly absorbing (black) surface
does simply due to conservation of momentum.

Nothing worse than a physicist with time on his hands.