When Voyager 2 flew past Neptune in 1989, it revealed
six moons circling the ice giant that astronomers hadn’t
seen from Earth. But at least one more didn’t come out to
play with the spacecraft. Instead, it remained hidden until
Mark Showalter of the SETI Institute pored over more than
150 archived images of Neptune taken by the Hubble Space
Telescope between 2004 and 2009. The new moon was so
difficult to spot, in fact, that it required combining several
images together to artificially extend their exposure times,
then searching for the object based on where it should be,
given Newton’s laws of motion, to confirm its presence.
The trick worked. Showalter — who also helped discover
Pluto’s moons Kerberos and Nix — and his colleagues
announced the new moon, dubbed Hippocamp, in a paper
published February 20 in Nature. “We weren’t expecting new
moons,” Showalter says of the effort. “It was quite a thrill.”
Originally designated S/2004 N1, the tiny moon is just under
11 miles (17 kilometers) across. That makes it Neptune’s small-
est. Showalter and his team believe, based on its orbit and size,
that Hippocamp is a chip off the old block — or rather, off
another neptunian moon: Proteus. Discovered by Voyager 2,
Proteus is one of the ice giant’s larger satellites, with a diameter
of about 260 miles (420 km). It also sports a large impact crater
marring most of its surface — and Hippocamp may be a piece
knocked off by that impact.Although Hubble spotted the moon, it can’t see the world
in enough detail to reveal much more than its presence. “It’s
a completely unresolved dot,” Showalter says. “We don’t have
telescopes to resolve it, and that’s what you would need to do
to figure out its surface.”
A Neptune orbiter mission could change that, he says.
Although none is currently planned, the ice giant and its
moons remain high on the list of places that planetary scientists
want to send robotic explorers, so our first up-close look at
Hippocamp may come within a few decades.2019 marked the 50th
anniversary of the first crewed
Moon landing. But Apollo astro-
nauts only set foot on the lunar
nearside. At 10:26 A.M. Beijing
time January 3 (10:26 P. M. Eastern
Standard Time January 2), the
farside got a little closer when
China’s Chang’e-4 lander suc-
cessfully became the first craft to
soft land there. Later the same
day, the lander deployed its
refrigerator-sized rover, Yutu-2.
Chang’e-4 touched down in
Von Kármán Crater, within theMoon’s South Pole-Aitken Basin.
In February, the International
Astronomical Union’s Working
Group for Planetary System
Nomenclature approved the
name Statio Tianhe for the craft’s
landing site. Chang’e-4 and its
rover study the lunar farside’s
geology during the daytime,
which lasts about two weeks.
During the two-week-long night,
the pair power down until the
Sun rises again.
The lander’s miniature bio-
sphere experiment wowedthe world in mid-January with
photos of the first plants to
grow on the Moon: cotton
seeds that sprouted and grew
for about a week before suc-
cumbing to nighttime tempera-
tures of roughly –290 degrees
Fahrenheit (–180 degrees
Celsius). By late June, the Yutu-2
rover had far exceeded its
expected three-Earth-month
lifetime and driven more than
700 feet (213 meters) from the
lander. The mission’s initial
results include discovering10
Hippocamp joins
the neptunian family
Chang’e-4 brings the Moon’s farside nearer
8
Neptune’s tiny moon Hippocamp is so difficult to spot that astronomers had to
combine several Hubble Space Telescope photos together, as they’ve done here,
just to discover it. Neptune, which lies at the center of the image, was blotted
out to better see the faint features around it, including its rings and moons.
Hubble also took the color inset of the planet. NASA, ESA, AND M. SHOWALTER (SETI INSTITUTE)This panorama of the
farside of the Moon,
taken by China’s
Chang’e-4, shows its
landing site, now
named Statio Tianhe, in
the South Pole-Aitken
Basin. Also visible is the
mission’s Yutu-2 rover,
which has traveled
more than 700 feet
(213 m) from the lander.
CNSAHippocamp
Galatea
Despina
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