New Scientist Article on Solar Sails

(Gordon D. Pusch) wrote in message ...
(JimM) writes (in response to "Geoffrey A. Landis"
):

The energy transfer to a solar sail can be accounted for from the
Doppler shift of reflected photons; even when the reflectivity is 100%,
a photon looses energy when reflecting from a moving sail. This effect
exactly corresponds to the energy increase of the sail. No sophisticated
physics is needed to analyze this effect, it is a problem suitable
for a homework assignment for a college undergraduate.

I had not thought of it exaxtly that way when I saw the item in New
Scientist.

But is seemed to me that the assumption of a 100% effective mirror
could not be correct, so that some light striking the mirror must turn
to heat. Thus the 2nd Law of Thermodynalics is preserved (I see the
2nd law as being the key to proper understanding of this) and so the
light sail functions.

Nope, sorry --- it's a simple matter of Newtoniam mechnaics and
conservation of momentum: If the photon reflects off the sail,
it's momentum changes, and the sail must therefore experience an equal
and opposite change of momentum in recoil from the photon's reflection.

The 2nd Law and thermodynamics plays no part in the simple Newtonian recoil
of the sail, and indeed absorbtion would actually _decrease_ the sail's
performance, since if the photon is absorbed, it only transfers =HALF=
as much momentum to the sail as if it is reflected back in the opposite
direction from whence it came --- and when the heat energy is re-radiated
as infrared photons, they are emitted in a RANDOM direction, resulting in
no _AVERAGE_ change in the sail's momentum --- so absorbing a photon is a
_LOSS_, not a gain.

The 2nd Law of Thermodynamics is in many ways the most fundamental law
of physics--indeed, of all science. No physical process can be
properly described without reference to it. If you do not know this
you are not much of a physist and should consider some other line of
work.

The 2nd Law says, among other things, that no machine (more generally,
no time-wise phyisical process) can ever be 100% efficient. A solar
sail is a machine. Ergo a solar sail (or indeed any mirror) cannot be
100% efficient.

The sail is "recoiled" * from sunlight, but the process is not 100%
efficient, and the light energy lost takes the form of heat. If
efficiency were too low, the sail would melt or evaporate.

In the light of above your description of what happens is nonsense. Go
back to Physics 101.

JimM

* As you quaintly put it. Where I come from I'd certainly talk in
terms of Newton's First Law of Motion -- "action and reaction are
equal and opposite" -- but that would only translate to a 100%
effective process in the case of entirely inelastic materials, which
is a theoretical construct that can never exist in the real world,
like absolute zero for example.