  The zeta ring is the innermost component of the Uranian ring system. It is of
scientific interest because its morphology changed significantly between the
Voyager 2 encounter in 1986 and subsequent Earth-based observations around
2007. It is also of practical interest because some Uranus mission concepts
have the spacecraft pass through the inner flank of this ring. Recent
re-examinations of the Voyager 2 images have revealed additional information
about this ring that provide a more complete picture of the ring's radial
brightness profile and phase function. These data reveal that this ring's
brightness varies with phase angle in a manner similar to other tenuous rings,
consistent with it being composed primarily of sub-millimeter-sized particles.
The total cross section of particles within this ring can also be estimated
from these data, but translating that number into the actual risk to a
spacecraft flying through this region depends on a number of model-dependent
parameters. Fortunately, comparisons with Saturn's G and D rings allows the
zeta-ring's particle number density to be compared with regions previously
encountered by the Voyager and Cassini spacecraft. Finally, these data indicate
that the observed changes in the zeta-ring's structure between 1986 and 2007
are primarily due to a substantial increase in the amount of dust at distances
between 38,000 km and 40,000 km from Uranus' center.
