Page:The evolution of worlds - Lowell.djvu/291

orbital speed of the satellite which shall not be affected by collisions with them is, for the several planets:—

	Miles a Second
Jupiter	3.4
Saturn	2.5
Uranus	1.7
Neptune	1.4

All the satellites but Iapetus have orbital speeds exceeding this, and consequently are retarded also by these particles.

For particles crossing the orbit (2) the mean velocity would be practically parabolic, 1.4, even if the distribution were as

${\frac {1}{r'}},r'$ being the distance from the Sun. The effect would depend upon the angle of approach and in the mean give a greater velocity for the particle than for the satellite within the orbit, a less one without; retarding the satellite in both cases. Thus the total effect of all the particles encountering the large satellites is to retard them and to tend to make them hug their primary.

For retrograde satellites the velocities of impact with inside and outside particles moving direct are respectively:

	Inside	Outside
Jupiter	2.0 + $v$	$v$ + 3.4
Saturn	1.5 + $v$	$v$ + 2.5
Uranus	1.0 + $v$	$v$ + 1.7
Neptune	0.8 + $v$	$v$ + 1.4

In both cases the impact tends to check the satellite.

Comparing with these the velocities of impact for direct satellites in a direct plenum:—