Hi folks,

As Arild and Ray suggested, I've tacked the following into the
glossary (just ignore the assorted control codes...) and yes,
it ought to have been there before. Personally, until I started
playing around with determining orbits of irregular natural satellites,
I'd never heard of the Hill radius.

As you'll see, the outer planets have even larger Hill spheres
than Jupiter; their greater distance from the sun more than
compensates for their lesser masses.

Incidentally, in theory, retrograde satellites could be held onto
at still greater distances than this. In practice, all the known
satellites in the solar system, both retrograde and prograde,
orbit well inside the Hill radii.

-- Bill
--------------------------------

The ^Hill's sphere^ radius is a rough estimate of the distance
within which a satellite can remain in orbit around a planet.
It's usually given in the "more info" section for each planet.
The following table gives the Hill's sphere radius for each planet:
!
Rplanet ^km^ ^AU^
Mercury 94.4 230000 0.0015
Venus 167.1 1011000 0.0068
Earth 234.9 1497000 0.010
Mars 319.8 1084000 0.0073
Jupiter 740 51800000 0.35
Saturn 1100 63800000 0.43
Uranus 2700 68310000 0.46
Neptune 4700 115620000 0.78
Pluto 5100 5783000 0.039
!
The Hill's sphere radius can be computed as

Rh = R * cube_root( Mplanet / 3Msun)

where Rh = Hill radius; R = planet/sun distance; Mplanet = mass
of the planet; and Msun = mass of the sun.