Radial, spokelike features were 
seen crossing the ringlets of the 
B ring at nearly right angles. 
. These features, which scatter 
sunlight in the manner of small 
particles, may be associated 
with electrostatic discharges. 
spherically shaped object. But if the 
sand is orbiting close to an object such 
as Saturn, the grains closer to the object 
will tend to orbit faster, and the sand 
will spread out, first into a ring and 
then into a disk. At some distance from 
Saturn the two effects balance, and a 
pile of sand orbiting there would just 
barely hold itself together under its own 
gravity. 
The Roche limit for Saturn is some- 
where beyond the outer edge of the A 
ring. Inside this radius, a fragmented 
satellite would not re-form, and a cloud 
of dust grains would not aggregate. It is 
significant that the three known plan- 
etary ring systems, those of Jupiter, 
Saturn, and Uranus, are all within the 
Roche limits of their respective planets. 
Outside Saturn’s A ring but close to 
its Roche limit, there are at least five 
small satellites and a thin ring, called 
the F ring. The outer two satellites in 
this group share the same orbit and 
seem to change positions slowly so as to 
avoid collision. Two more of the satel- 
lites seem to “shepherd” the newly dis- 
covered F ring, one orbiting just 
outside this thin filamentary feature 
and one just inside it. Closer to Saturn, 
the fifth of these small satellites orbits 
just outside the A ring and seems to 
push it inward. These satellites and 
rings would be expected to interact and 
eventually collide. Indeed, the two co- 
orbital satellites may be fragments from 
a collision long ago, but their orbits 
have evolved so that the two objects 
now avoid each other. Similar repulsive 
behavior also characterizes the interac- 
tion between the shepherding satellites 
and the nearby A and F rings. All of 
these satellites repel ring material, forc- 
ing it inward or outward, away from 
their orbits. This repulsion seems para- 
doxical, since the force of gravity is at- 
tractive. Such apparent paradoxes, 
however, are common in orbital me- 
chanics; forces tend to have effects that 
are opposite to expectations based on 
everyday experience. For example, a 
force that acts in the direction of mo- 
tion causes an orbiting body to move 
outward, where it orbits more slowly. 
A backward force eventually causes the 
body to orbit faster. 
Thus, near the Roche limit this ten- 
dency of objects to avoid each other 
seems to interfere with the process of 
aggregation or reaggregation of indi- 
vidually orbiting bodies. Farther out, 
the constraints of orbital motion are 
less important than mutual attraction, 
so the aggregating effect predominates. 
In general terms, this is why the regions 
well outside the Roche limits are so 
empty. During the final phase of plan- 
etary formation, bodies there collided 
and stuck to one another, held together 
by their own gravity. The large satel- 
lites that remain in these empty regions 
must have swept up all the original ma- 
terial. The same sweeping process 
seems to describe the formation of the 
planets in our solar system from the 
original gas-dust cloud that was orbit- 
ing the sun during its formation. 
Another curious feature is the radial 
spoke pattern that appears in the mid- 
dle of the B ring, crossing its circular 
ringlets at nearly right angles. Like ev- 
erything else in the rings, the spokes or- 
bit Saturn, but they change shape, 
disappear, and re-form with each orbit. 
They scatter sunlight in a manner char- 
acteristic of small particles, those with 
sizes comparable to the wavelength of 
light. Electrical forces can be important 
for these particles, and so the spokes 
may be associated with the unusual ra- 
dio signals detected by Voyager. The 
available evidence suggests that the 
spokes are related to electrical dis- 
47 
