Re: Poynting-Robertson Effect

[Leigh Palmer <palmer@sfu.ca>]

> The Poynting Robertson effect actually _decelerates_ dust grains and
> smallish meteoroids (10^-3 cm up to about 1 cm in diameter) in solar orbit.

I'll get around to answering this question sometime soon, but the statement
that the Poynting-Robertson effect "decelerates" objects in near circular
orbits is sufficiently incorrect that a quick response is called for.

In physics I know of no use for the term "deceleration". If it means
"slowing down" as it does in common parlance, then this statement is
clearly incorrect. As a particle spirals inward toward the Sun it speeds
up; it does not slow down! That may seem perverse since the radiant force
is clearly doing negative work on the particle. As the particle's kinetic
energy increases, however, its gravitational potential energy decreases
twice as fast, so overall the negative work does result in reduced energy.

> It originates because in the rest frame of the orbiting object sunlight
> actually arrives with a small component of its Poynting vector anti-parallel
> to the meteoroid's velocity vector.

The meteoroid has a velocity vector in its own rest frame?

Leigh