Sky & Telescope - December 2019 - 21

Pluto still does. Triton is the largest
retrograde moon in the solar system.
Its arrival doomed most of Neptune's other moons. Triton probably
smacked into some, tossed others into
AVERAGE
DISTANCE FROM
the planet, and ejected still others
THE SUN
altogether. One moon that managed
4.5 billion km
to hang on was Nereid, but its orbit
(30.1 a.u.)
became stretched out due to the newYEAR
comer's gravity. Nereid now has the
165 Earth years
most elliptical orbit of any moon in the
ESTIMATED DAY
solar system, with an orbital eccentric16 hours 7 minutes
ity of a whopping 75%.
AXIAL TILT
Triton suffered, too. Its initial orbit
28.3°
around Neptune was elongated, but
MASS
Neptune's tides forced its path to
17.15 Earths
become circular, internally scorchRADIUS
ing the moon and melting all its ice
3.9 Earth radii
and maybe even its rock. Volcanic
eruptions must have spewed out lots
of gas, wrapping the moon in a thick
atmosphere. Today, Triton's atmosphere resembles Pluto's,
tenuous and full of nitrogen. Geysers send additional gas
into the air.
Triton is about as close to Neptune as the Moon is to
Earth. Inside Triton's orbit lie seven other satellites, all on circular paths, all but one found by Voyager. The largest, and the
second largest orbiting Neptune, is Proteus. Voyager images
revealed a battered world with a huge crater named Pharos.
The crater might be a scar from an impact that created the
small moon Hippocamp, which Showalter spotted in 2013 in
an orbit that lies near Proteus.
Beyond the orbits of Triton and Nereid, ﬁve other irregular
satellites pursue elliptical and inclined orbits around Neptune. The most distant, Neso, ventures farther from its planet
than does any other moon in the solar system. At its extreme,
Neso skirts 72 million kilometers from its master - nearly
half the distance between the Sun and Earth. So distant is
Neso that it takes 26 years to orbit Neptune once, nearly as
long as Saturn takes to circle the Sun. Yet Neptune can retain
this remote retrograde moon because the planet itself is so far
from the Sun's gravity.
In 2003 Scott Sheppard (Carnegie Institution for Science)
and his team discovered Psamathe, a retrograde moon whose
orbit overlaps Neso's. He speculates that the two moons
might once have been a single object that split apart when a
large comet hit it.
"Almost all these moons are just points of light" through
a telescope, he adds. "So we really don't know much about
them."

Return to the Ice Giants?
All these moons and the planets they
orbit provide rich targets for future
spacecraft. An orbiter like the ones
Jupiter and Saturn have received would
MEAN DENSITY
(WATER = 1.00)
scrutinize these systems for years. To
1.64
reach the planets quickly, a spacecraft
KNOWN MOONS
must swing by Jupiter, whose gravity
14
would ﬂing the craft outward. The
RINGS
next launch opportunities to Uranus
6
and Neptune occur around 2030, leadDISCOVERED
ing to encounters around 2040.
September 23,
No such missions are yet funded,
1846
but planetary scientists in both the
United States and Europe are drawing up plans. After all, every
planet from Mercury to Saturn
has received at least one orbiting
spacecraft, and we now know
that planets the size of Uranus
and Neptune abound throughout
the galaxy (S&T: Sept. 2019, p. 16).
Orbiters to the two nearest examples would divine clues to
their interiors, compositions, and atmospheres. Moreover,
such spacecraft would ﬂy past moon after fascinating moon
and could discover additional satellites as well.
Uranus in particular presents a timely opportunity. By bad
luck, Voyager ﬂew past when one of the planet's poles pointed
nearly sunward, which meant that, even as Uranus and its
regular satellites turned, one side of each world stayed hidden
in darkness. In contrast, sunlight will illuminate them in
full at the next equinox, in 2050, so a spacecraft still in orbit
then could see all of Uranus and its regular moons.
The challenges of reaching these distant worlds are great,
but the scientiﬁc rewards are likely to be far greater.

NEPT UNE PL AYING CA RD: TERRI DUBÉ / S&T, BACKG ROUND IM AG E:
A LLIES IN TER ACTIV E / SHU T TERSTOCK .CO M, NEPT UNE IM AG E:
N ASA / JPL; HIPPOCA MP: N ASA / ESA / M. SHOWA LTER (SE TI
INSTIT U TE )

NEPTUNE STATS

u MOON SHARD Little Hippocamp (about 18 km across) appears
around Neptune in this Hubble Space Telescope composite. Also visible
are four other moons and a couple of rings. The black bar prevents the
planet's light from swamping the image.

¢ Ever since childhood KEN CROSWELL has been especially
intrigued by distant and mysterious Uranus, Neptune, and
Pluto. He is the author of eight books, including Planet Quest
and Ten Worlds.

Rings
Hippocamp
Thalassa

Neptune

Galatea
Despina
Larissa

Rings

sk yandtele scope.com * D EC E M B E R 2 019

http://www.SHUTTERSTOCK.COM http://www.skyandtelescope.com

Sky & Telescope - December 2019

Table of Contents for the Digital Edition of Sky & Telescope - December 2019