Re: Sunshine

"Jeff Root" <[EMAIL PROTECTED]> wrote in message
news:[EMAIL PROTECTED]
> My understanding is that solar wind has a greater effect on
> atoms, molecules, and small dust particles than light pressure
> has, but light pressure has a greater effect than solar wind
> on larger particles and objects.  Why the difference?
>

In order for light pressure to have a significant effect, a particle has to
be large enough for a light wave to interact with it.  Generally this
transition occurs when the particle has a circumference of about one
wavelength (very approximately).  A particle a lot smaller would not
interact with the light to a significant degree.  A typical atomic size is
of the order of 1 Angstrom unit; the wavelength of most of the sunlight is
between 2000 and 10000 Angstroms.  So atoms are not much affected by light
pressure (but are affected to some extent), while tiny dust grains of micron
size are affected a lot.

> How do light pressure and the pressure exerted by solar wind
> compare at different distances from the Sun?

Without checking the details, I believe they both fall off with the inverse
square of distance from the Sun, so the ratio stays about the same.  Light
pressure falls off exactly as 1/r^2, but the solar wind may not be perfectly
isotropic, and may be stronger in the equatorial directions.

>
> How large does an asteroid (iron or dense chondrite) have to
> be in order to fall into the Sun rather than be completely
> vaporized by the Sun's heat and blown away by solar wind?

I'm not sure what you are asking here.  Are you asking about the
Poynting-Robertson effect?  Or are you asking what happens when something
falls straight into the Sun?  Consider the latter.

Imagine something heading straight for the Sun.  You need to figure in the
vaporisation temperature of iron or rock (say, for guesswork, these are
about 1500-2000K) and calculate what distance from the Sun something has to
be to reach that temperature.  A small object (mm size) would vaporise
pretty fast and be blown away when it got to about half of Mercury's
distance; a big rock (km size) would not evaporate fast enough because it
takes time for the radiative heating to reach the interior, so it would
ablate as it fell, and part of it would probably reach the photosphere in
the form of a solid or liquid (but not stay that way for long).  You need
use the heat capacity of the material and integrate the heating effect with
the accelerating speed and increasing heating.  Not trivial, but not
impossible to do.

Specify your question more clearly, then it may be possible to answer.  This
could be a good question for an astronomy exam (honours level).

-- 
Mike Dworetsky

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