Proteus, the second largest moon of Neptune next to Triton, was
discovered by the Voyager 2 spacecraft and Stephen P. Synnott in
1989 thirty three years after the discovery of a significantly
smaller moon, Nereid. It was missed by ground telescope observations
for a couple of reasons. One, it orbits so close to Neptune, that
the reflective glare of the planet itself obscured it. Second,
Proteus is the darkest object in our solar system. Reflecting only
about 6% of the sunlight that strikes it, it is as dark as Saturn’s
dark moon, Phoebe.
It has a pro-grade orbit; this means that it orbits Neptune in the
same direction as the planet’s rotation. It takes Proteus 27 Earth
hours to orbit Neptune, indicating its close equatorial trajectory.
Its irregular shape appears to be unaltered geologically. No other
moon in the solar system is as large as Proteus while still having
an irregular, non-spherical shape. Astronomers say that it is as
large as it can possibly be without its own gravitational pull
forcing it into a spherical shape. It is heavily cratered and photos
of the moon give it the appearance of the top of a head of
cauliflower. Its craters are thought to have been formed by impact
of asteroids or comets.
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