. Iris is fourth from the right.
Geology Iris is an S-type asteroid. The surface is bright and is probably a mixture of
nickel-
iron metals and
magnesium- and iron-
silicates. Its spectrum is similar to that of
L and
LL chondrites with corrections for
space weathering, so it may be an important contributor of these
meteorites. Planetary dynamics also indicates that it should be a significant source of meteorites. Among the
S-type asteroids, Iris ranks fifth in
mean diameter after
Eunomia,
Juno,
Amphitrite and
Herculina. Its shape is consistent with an oblate spheroid with a large equatorial excavation, suggesting it is a remnant planetesimal. No collisional family can be associated with Iris, likely because the excavating impact occurred early in the history of the Solar System, and the debris has since dispersed. But at rare oppositions near perihelion Iris can reach a magnitude of +6.7 (last time on 31 October 2017, reaching a magnitude of +6.9), so that on much of each hemisphere, the sun does not set during summer, and does not rise during winter. On an airless body this gives rise to very large temperature differences. ==Observations==