what does the term "forward scattering" mean?

In reference to comet brightness estimates? I've seen this term several
times in articles about the comet C/2006 P1 (McNaught) which will soon
reach perihelion, but I have no idea what it means.

wrote:

In reference to comet brightness estimates? I've seen this term several
times in articles about the comet C/2006 P1 (McNaught) which will soon
reach perihelion, but I have no idea what it means.

It can be difficult to predict the brightness of a comet
because much of the light comes from sunlight
scattered off dust and molecules in the coma and
the tail. Even if we knew the exact distribution of
the material -- which we don't -- we still would have
trouble because the interaction of light with small
dust particles can be complicated. When light strikes
a large object, like a baseball, it bounces back in the
the direction whence it came (for the most part).
But when light strikes an object which is roughly the
same size as the light's wavelength, the results can
vary strongly: under some conditions light bounces
backwards, but under others, much of the light is
scattered _forward_, in its original direction of
motion. Physicists call this "forward scattering."

I'd be surprised if the typical prediction of
a comet's apparent brightness turns out to be
within a factor of four or five, due to all the
complications. If someone has the numbers,
though, I'd be happy to be proved incorrect.

Michael Richmond

Allison Kirkpatrick wrote:
In reference to comet brightness estimates? I've seen this term several
times in articles about the comet C/2006 P1 (McNaught) which will soon
reach perihelion, but I have no idea what it means.

I think that it roughly means scattering in which most of the scattered
light continues moving in roughly the same direction it originally was.

Thus, for instance, from here on Earth, Saturn's B ring looks especially
bright, whereas the C (crepe) ring looks very faint. However, I seem to
recall that from behind the planet (where we on Earth cannot observe
from, of course), the opposite is true: the B ring is faint/dark, while
the C ring is bright. This indicates that there is relatively more
forward scattering with the C ring, and more back scattering with the B
ring.

Forward and backward scattering are not necessarily complementary, since
light may also be absorbed (later to be re-emitted, usually at a longer
wavelength). In this context, my guess is that the significance of
forward scattering has to do with what kind of particles are doing the
scattering.

--
Brian Tung
The Astronomy Corner at http://astro.isi.edu/
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wrote:
In reference to comet brightness estimates? I've seen this term several
times in articles about the comet C/2006 P1 (McNaught) which will soon
reach perihelion, but I have no idea what it means.

For an example, imagine a flashlight (UK: torch) on a foggy night. As
you point the beam away from you, the fog appears lit up in the beam.
The light you see is "back scatter." Hand the flashlight to someone
else. As they shine the light past you, the light you see is "side
scatter." Have them point the light towards you (but not directly at
you). The fog that is lit up (not the direct, unimpeded light from the
flashlight itself) is forward scatter. The three will not be at the
same intensity even at the same distance (and, under some
circumstances, may not even be the same color).

The same kind of thing exists with comet dust, except that fog is far
more predictable.

Austin

Thanks to everyone who has replied.

So I would assume that this "forward scattering" from a comet is a
relatively rare situation (since I never hear about it), and is
pertinent with this particular object because it is (or is going to be)
at a position approximately along a line from the sun to Earth? The
three bodies must be at least approximately aligned in order for
forward scattering to make any significant difference in the comet's
apparent brightness, correct?