TOP 10 SPACE STORIES of 2015

Astronomers find signs of dark matter close to home, unravel the mystery of a famous supernova, and take a trip to Pluto. by Liz Kruesi

Planetary science drew the most attention in 2015, and for good reason. Mysterious bright spots on the largest asteroid in our solar system puzzled scientists. The spacecraft following a comet as it hurtled toward and then retreated from the Sun continued to make surprising discoveries. And, of course, the year saw the history-making and expectation-shattering observations of Pluto.

But discoveries about celestial objects beyond the solar system deserve attention, too. The center of the Milky Way Galaxy harbors a mysterious glow from dead stars or something even stranger, while astronomy’s most studied stellar explosion is changing before our eyes. Each year, Astronomy ranks the top 10 astronomical discoveries and space stories. Here’s where 2015’s biggest ones fall.

10. The Red Planet under water
The rovers and orbiters
at Mars have uncovered
plenty of evidence that
the planet once had liquid
water on its surface, from etched
river gullies and dried-up shorelines to
minerals that need water to form. But a
new study, some five years in the making,
confirms that the Red Planet hosts
liquid water on its surface today. Since
2010, Lujendra Ojha, from Georgia State
University, and colleagues have used
Mars Reconnaissance Orbiter (MRO)
data to study streaks running down
martian crater walls. They suspected
that the streaks, called “recurring slope
lineae,” which appear to lengthen from
one image to the next, mark flowing
salt water. But they didn’t have proof.
In the new study, published in the
September 28 issue of Nature Geoscience,
Ojha’s team provides the spectral
signature (from MRO) of
salty water at four locations of
recurring slope lineae on the
Red Planet’s surface — confirming
that flowing water is present
today on Mars.
While little water remains today,
scientists know that it must have been
bountiful in the past. A study published
in the April 10 issue of Science analyzed
how much water the planet once had.
Researchers used several Earth-based
telescopes to look at the martian atmosphere
in infrared light. Geronimo
Villanueva of NASA’s Goddard Space
Flight Center and colleagues were looking
for specific colors: one that corresponds
to normal water (H2O) and one
that comes from a heavier form of water
that has an extra neutron (hydrogendeuterium-
oxygen, or HDO).
The scientists mapped the
ratio of these two
types of water
three times over six years (or three martian years) to
compare the water in the atmosphere at
different seasons.
H2O is lighter than HDO and thus
evaporates more easily. So by measuring
the ratio of the two, the researchers
could calculate how much water Mars
has lost over time, and thus how much
water it would have started with.
Villanueva’s team says that 4.5 billion
years ago, some 6 million cubic miles
(23 million cubic kilometers) of water
pooled in a northern ocean covering
nearly 20 percent of the surface. This
martian ocean would have been a bit
larger than Earth’s Atlantic Ocean.
This is more water than many
researchers had expected. “[Mars] was
very likely wet for a longer period of
time than previously thought,” said
co-author Michael Mumma of
NASA in a press statement,
“suggesting the planet
might have been habitable
for longer.”

9. Dark matter hints next door
The invisible and perplexing
material that makes up at least
80 percent of our universe’s mass
keeps leaving clues for astronomers,
but not enough to solve its
identity. While scientists do not
know yet what makes up this
dark matter, one search method
has given tantalizing hints over
this past year.
Scientists believe that when
two dark matter particles collide
they destroy themselves
— a process called annihilation
— and create other familiar
particles. Among this shower
of particles is gamma radiation.
And nearby dwarf galaxies are
an ideal place to look for darkmatter-
produced gamma rays.
“[Dwarf galaxies] are calm,
quiet places; we don’t know any
reason why they should be emitting
high-energy gamma rays on
their own,” says Carnegie Mellon
University’s Alex Geringer-
Sameth, lead scientist of one of
the searches. “Therefore, if you
see some gamma rays coming
from one of these dwarf galaxies,
it is very exciting because it
could be a sign that dark matter
is annihilating within it.”
This past year, a sky survey
uncovered nine dwarf galaxies
within 1 million light-years of
the Milky Way. And one of the
galaxies from this Dark Energy
Survey (DES) was a prime dark
matter target: Reticulum II.
Geringer-Sameth’s team and
another — Dan Hooper and
Tim Linden, both of the
University of Chicago — used
seven years of data from the
Fermi Gamma-ray Space
Telescope to find that this dwarf
galaxy looks a bit brighter than
it should in gamma rays. “We
provide an indication that
something is emitting gamma
rays from the direction of
Reticulum II, and that something
seems to be consistent
with dark matter annihilation,”
says Geringer-Sameth. “While
the signal from Reticulum II is
tantalizing, it would be premature
to conclude it has a dark
matter origin.”
Hooper and Linden calculated
a similar chance that the
signal has dark matter origins.
“You might call that evidence;
you won’t call that a discovery,”
Hooper says of the studies. “We
really need more data to resolve
the issue.” Scientists expect
DES to uncover some 20 more
nearby dwarf galaxies, and
future surveys will find even
more. Scientists will then be
able to compare archived Fermi
gamma-ray data with these
dwarf galaxies to see if they have
a signal similar to Reticulum II’s.

8. Supernova hunters see quadruple
In November 2014, Patrick Kelly was looking through his
team’s recently collected Hubble Space Telescope images of
galaxy cluster MACS J1149.6+2223 when something stood
out: four stars with exactly the same pattern of light surrounding
one of the cluster’s member galaxies. “I knew it
was a big discovery,” says Kelly, a postdoctoral fellow at the
University of California, Berkeley. He emailed his group
about the find, and they have since confirmed it as a supernova
whose image has been distorted by the cluster galaxy,
which lies along the supernova’s line of sight. Months of
observations have classified this object as a type IIp supernova,
which originated from a massive star.
The distant stellar explosion lies more than halfway across
the observable universe. Its light left the supernova some
9.5 billion years ago. Along its path to Earth, the light encountered
a massive member of the intervening galaxy cluster. The
galaxy warps the fabric of space-time like a bowling ball
warps a trampoline, and so the supernova’s light follows those
curves in space-time, detoured from its path to Hubble.
This “gravitational lensing” causes the light to appear to
come from four different points instead of just one lone
supernova. Norwegian astrophysicist Sjur Refsdal predicted
this type of quadruple-lensed supernova 50 years
ago. The 2014 discovery, published in the March 6, 2015,
issue of Science, has been named Supernova Refsdal after
that scientist.
In his 1964 paper, Refsdal said such a blast could help to
measure the rate our universe is expanding. Because the
explosion’s images show up in four locations, light followed
four different paths to arrive at Hubble. Astronomers can use
each of those paths to map the distribution of normal material
and unseen dark matter in the galaxy cluster. In addition,
those different paths are related to the cosmic expansion rate.
Another spectacle awaits the team. All of those paths also
take a different travel time. After creating a map of MACS
J1149.6+2223, the astronomers realized that the supernova
should have taken a fifth path, too. The light is still traveling
and could appear as early as late 2015, says Kelly.

7. Deciphering a famous supernova
In February 1987, a brilliant new point
of light shone in the southern sky. This
turned out to be the explosive blast marking
the death of a star and earned the name
Supernova 1987A. Lying just 168,000 lightyears
from Earth, it is the closest supernova
to explode since astronomers developed the
tools to study such a blast. And that proximity
makes it a perfect laboratory to watch
how supernovae evolve. Several discoveries
published in 2015 reveal changes to the
blast site and uncover secrets of the explosion
first seen 28 years ago.
SN 1987A is recognized by its ring of
bright nodules, like shining diamonds
along a band. These brilliant spots mark
where the blast’s shock wave is slamming
into previously shed material. While
astronomers had seen the diamonds brightening
for the past 15 years, new observations
show them fading for the first time.
This means the blast’s shock wave is passing
through the ring of material, breaking
it apart. Visible-light observations made
by Stockholm University’s Claes Fransson
and colleagues using the Hubble Space
Telescope show the ring is fading, while
spots outside of the ring are beginning to
light up. They described the observations
in the June 10 issue of The Astrophysical
Journal Letters.
X-ray images from the Chandra X-ray
Observatory also show the ring’s light
changing. David Burrows, who has been
watching SN 1987A evolve for 15 years, says
the blast’s high-energy light is plateauing.
Another 2015 study focused on SN
1987A’s guts.
When a star at least 10 times the Sun’s
mass explodes at the end of its life, the
energies, temperatures, and pressures are
so high that the supernova produces a
range of heavy chemical elements. One of
those is titanium-44 (Ti-44), which is an
unstable radioactive isotope. “The isotope
is produced deep in the core of the explosion,
and its properties — mass, ejection
speeds, and distribution — directly reflect
the physics in the core,” says Steve Boggs of
the University of California, Berkeley.
Like all elements, Ti-44 glows with
specific colors of light, so if scientists look
for those colors, they can learn where that
material is. But none of Ti-44’s colors had
been visible to astronomers until a recent
X-ray telescope, the Nuclear Spectroscopic
Telescope Array (NuSTAR), opened its
eyes and began collecting data.
Boggs and colleagues described in the
May 8 issue of Science their study using
NuSTAR to map Ti-44 in SN 1987A.
The element’s distribution is clumpy and
uneven, implying that the explosion was
off-center. This is the second supernova
remnant the team has been able to probe;
the other is Cassiopeia A. Both explosions
were asymmetrical, Boggs’ team says, which
means now astronomers have to rethink the
theoretical models of these blasts.
Most computer models have assumed a
symmetrical blast, but the new studies prove
something more complex is happening.

6. Water abounds in the outer solar system
Saturn’s moon Enceladus continues to show why it’s one of the best
bets in the solar system to search for life. Astronomers have suspected
for years that salty water dredged up from a subsurface sea spews
into space out of fissures near the moon’s south pole. But an analysis,
published online September 11 in the journal Icarus, of seven years
of images from NASA’s Cassini spacecraft indicates that Enceladus
has a subsurface global ocean instead of merely a regional sea.
Cornell University planetary scientist Peter Thomas and colleagues
measured a slight wobble in the moon’s rotation. If
Enceladus were solid, its mass would dampen that motion. The
researchers believe, instead, that a liquid water ocean lies between
the moon’s icy surface layer and the rocky interior. They say the
ocean is deeper and the ice shell thinner at the south polar region,
where Cassini has spied some 100 geysers of salt water.
Scientists think that to keep any material in liquid state within
Enceladus’ interior requires the push-and-pull tidal energy from
Saturn. A global ocean is harder to keep warm than a regional sea,
and so this discovery could also indicate that the saturnian satellite
has more tidal energy than originally thought. “If that is correct,”
says team member Carolyn Porco, “and its ocean has been
around a long, long time, then it may mean that any life within it
has had a long time to evolve.”
Some of the material spewing from Enceladus’
underground ocean flows out through the geysers,
flows toward Saturn because of the planet’s gravitational
pull, and then orbits the planet as its E ring.
In the March 12 issue of Nature, Frank Postberg at
the universities of Heidelberg and Stuttgart in
Germany and colleagues described how they used
the Cassini spacecraft to study some of the material
from the E ring. They saw silicon-rich molecules
(called silicates) just a few nanometers wide. When
this type of material is found in space, it almost
always originates from rock being dissolved in
water. But to learn the precise characteristics of that
water-rock interaction, Postberg’s team collaborated
with researchers from Japan to mimic the conditions
needed at Enceladus to produce the sizes and composition of silicate particles they observed. They found the
water needs to be at least 194° F (90° C) and have a pH between 8.5
and 10.5. These characteristics imply hot-spring-heated water; the
only other place where such hydrothermal vents have ever been
seen is on Earth, and these sites host extreme organisms.
The chemical reaction that produces the silicates also creates
molecular hydrogen, and a different instrument on board Cassini
will look for this gas during a late 2015 flight through Enceladus’
plumes. If it detects more molecular hydrogen than expected, it will
confirm hydrothermal activity, says Postberg.
This year, astronomers also found the best evidence so far of
water at yet another location in our solar system: Jupiter’s large
moon Ganymede. NASA’s Galileo spacecraft, which studied the
jovian system in the late 1990s and early 2000s, studied Ganymede’s
magnetic field to learn whether the moon holds a global ocean
under its surface. But the analysis from only 20 minutes of flyby
observations was inconclusive. Fast forward to the past year, when
Joachim Saur of the University of Cologne and his colleagues studied
data from two 7-hour Hubble Space Telescope observations.
Ganymede has an auroral belt in each hemisphere just like
Earth does. Jupiter’s magnetic field also influences these aurorae
and causes them to rock during Jupiter’s 10-hour rotation period.
Saur’s team knew that if Ganymede did not have
an ocean, the aurora belts would change their positions
slightly, tilting about 6°. “However, when a
salty and thus electrically conductive ocean is present,
this ocean counterbalances Jupiter’s magnetic
influence and thus reduces the rocking of the auroras
to only 2°,” says Saur. “We observed Ganymede
with the Hubble Space Telescope for more than
five hours and saw that the aurora barely moved
and rocked by only 2°. This thus confirms the existence
of an ocean.” The researchers think the
ocean lies about 90 miles (150km) below the
moon’s rock-ice crust and is about 60 miles
(100km) thick. This strong evidence of Ganymede’s
ocean continues to increase the number of worlds
in our solar system known to host water.

5. Ceres takes center stage
Since March 6, NASA’s Dawn spacecraft has been in orbit around Ceres,
the largest object in the asteroid belt lying between Mars and Jupiter. For
a full recap of the spacecraft’s adventures and discoveries, see “Dawn mission
reveals dwarf planet Ceres” (p. 44). Dawn will continue its studies
until June 2016. Ceres is the second asteroid Dawn has orbited; the first was
Vesta, between July 2011 and September 2012.
Ceres’ pockmarked surface is riddled with craters like those seen at
Saturn’s icy moons. “The features are pretty consistent with an ice-rich
crust,” said Dawn planetary geologist Paul Schenk of the Lunar and
Planetary Institute in Houston in a press statement. The spacecraft has
mapped the heights of surface features like craters and mountains.
Bright spots on the dwarf planet’s surface also have mystified planetary
scientists. These reflective regions first came into view at the beginning of
2015 and have since resolved into a multitude of spots. They sit within
Ceres’ northern Occator Crater, which spans 57 miles (92km) and is
2.5 miles (4km) deep. Researchers at first believed they were ices or salts,
but bad luck repeatedly stymied their efforts to gain spectra of the mysterious
spots. Based on the reduced reflectivity of the spots, however, the consensus
is turning to salt.
In August, Dawn had reached its penultimate orbit, circling Ceres
from 910 miles (1,470km) out. A few months later, the spacecraft will have
transitioned to its final science
orbit, at just 230 miles (375km)
above the surface.
In addition to mapping the surface
and measuring the heights of
the mountains and craters on Ceres,
Dawn is working to learn about the
composition of materials on the
asteroid’s surface. The spacecraft
also is measuring how different
locations on Ceres pull with more or
less gravity. The answers will let scientists
map the world’s gravity and
learn how the dwarf planet’s rocky
interior is distributed.

4. Youngest cluster of galaxies seen
The process of forming clusters of galaxies is
not one that astronomers can watch in real time
because it takes billions of years. Instead, they
look for galaxy clusters at different stages in their
development. Because light travels at a constant
speed, the light collected from more distant
objects means scientists are seeing those objects
further back in time. In 2015, astronomers reported
they had found the youngest cluster yet, still in
an early stage of formation.
To find this protocluster, Joseph F. Hennawi of
the Max Planck Institute for Astronomy in
Heidelberg and colleagues searched for the
extremely bright centers of galaxies hosting
actively feeding supermassive black holes. These
quasars, as they are known, are used in two ways:
first, as markers for large galaxies, and second, as
flashlights to see through nearby gas clouds. Such
gas clouds glow because they absorb the active
galaxy’s light and then re-emit it. The researchers
were looking for a specific color of light that energized
hydrogen throws out, called Lyman alpha.
They spied four active galaxies near to one
another on the sky. When they studied their light
in more detail, they saw all four lie the same distance
from Earth and the light from these objects
has been traveling for 10.6 billion years. No one
had ever seen, nor expected to find, four quasars
in the same gravitationally bound group, so this
discovery was a surprise.
The team also saw these galaxies embedded in
an enormous cloud of hydrogen. The conglomeration
existed when the universe was just about
3.2 billion years old, and the gas clump stretches
about 1 million light-years across. “It’s 100 percent
clear that it’s a protocluster,” says team member J.
Xavier Prochaska of the University of California,
Santa Cruz. “It’s a structure that will evolve into
something like [the] Virgo [Cluster] today.”

3. A surprise glow at the galaxy’s center
When astronomers have a
new telescope that can resolve
types of light never seen before,
they can usually expect a surprise.
And that’s exactly what
the Nuclear Spectroscopic
Telescope Array (NuSTAR)
uncovered when it collected
a million seconds worth of
high-energy X-ray light from
the center of the Milky Way.
Astronomers found a diffuse
glow, but they can’t pin down
what’s causing it.
Kerstin Perez was using
NuSTAR data to study the glowing
material around a neutron
star lying in the galactic center.
But she couldn’t get rid of a
pervasive signal in the central
13 light-years by 26 light-years.
Once she convinced herself and
her colleagues that this signal
truly exists, they went to work to
figure out what it could be.
NuSTAR doesn’t just take
pictures; it also spreads the
light out in a spectrum, collecting
information about the
intensity of light at each individual
color to make it easier
to analyze. To figure out what
creates the haze the researchers
saw, they considered types
of objects that would give
a similar light pattern, says
Perez. “And then you think,
how many of those objects
would you have to have in
order to make up how bright
we see it.” This analysis led
the NuSTAR team to four possibilities,
which they described
in an April 30 Nature article.
Three of the possibilities
are stellar remnants stealing
gas from a companion. As this
material piles up, it ignites and
glows in X-rays. The idea is that
there are so many of these pairs
that NuSTAR can’t separate
them from one another, so they
appear as a haze.
One of these types of
corpses could be thousands of
white dwarf stars, each 90 percent
of the Sun’s mass. Another
could be about a thousand
black holes and neutron stars
— the dense leftover cores of
once massive stars. And the
third option is some thousand
millisecond pulsars, which are
neutron stars that have had so
much material dumped onto
them by their companions that
their rotation rates have sped
up dramatically. The problem is
that astronomers have no idea
how so many of these objects
— whatever they might be —
could exist in a small region in
the galactic center.
The fourth possibility is
that as material falls toward the
supermassive black hole at the
center of the Milky Way, some
of it gets shot out at high speed.
This streaming material could
be interacting with nearby
clouds of gas, causing them to
glow. But the hazy glow that
NuSTAR sees doesn’t look oriented
in the right way for this
explanation.
While scientists with
NuSTAR hope that upcoming
telescopic observations can help
narrow down which of these
possibilities is responsible for
this emission, they don’t expect
to learn the answer soon.

2. Europe’s visit to a comet
The European Space Agency’s Rosetta spacecraft
has been watching how Comet 67P/Churyumov-
Gerasimenko changes as it passes through its
closest approach to the Sun and then hurtles away.
The history-making mission has revealed many
cometary secrets.
Ever since Rosetta beamed back its first images
of Comet 67P, scientists have wondered what made
its unexpected double-lobed “rubber duck” shape.
Now, they have an answer. According to a paper
published October 15 in Nature, two separate
objects collided to form the comet. To reach this
conclusion, the researchers measured how regions
were sloped, looked at the orientations of features
on the surface, and calculated the local gravity
across the surface.
Rosetta also has returned thousands of images
of Comet 67P. It has photographed boulders balancing
on just a small part of their surfaces, piles
of rubble that seem to have come from falling
rocks, and jets of gas spewing from pits dozens of
feet across possibly created by sinkholes. The
spacecraft also has spied about 120 bright areas
several feet wide on the comet’s surface, and scientists
say these are most likely patches of water ice
reflecting sunlight.
After analyzing data of one water-ice patch on
the comet’s “neck,” scientists say the area seems to
appear and disappear with the comet’s 12-hour
rotation. They think that as the region feels direct
sunlight, ice on the surface and just an inch (a few
centimeters) below are heated and turn directly to
gas — a process called sublimation. The sunlight
also warms the layers of ground beneath the
region, and so further-buried ice makes its way as
gas to the surface. As the patch rotates into darkness,
the surface cools again and the just-risen gas
turns to ice. The scientists, who reported this
water cycle in the September 24 issue of Nature,
say the process repeats each cometary day.
Rosetta’s refrigerator-sized Philae lander had
also studied the comet’s surface, even though the
sequence of events to land this spacecraft didn’t
go as planned. After dropping from Rosetta on
November 12, 2014, and bouncing several times
before finally tumbling to rest, Philae stayed alert
for just around 60 hours before falling into hibernation.
Because of its unplanned bounces, the
lander was able to compare two different sites on
the comet’s surface. The first landing site appears
to have a soft dusty material about 8 inches (20cm)
thick covering a much harder material, possibly
icy or crystalline in nature. Philae’s final resting
spot, however, lacks that dusty coating.
At the first landing location, the craft “smelled”
16 organic compounds, including four never
before detected on a comet. Another instrument
detected several gases at the same location, like
water vapor, carbon monoxide, and formaldehyde.
Comets are expected to be pristine relics from the
early solar system, but Comet 67P has more complex
chemistry than expected, and some of the
molecules discovered on the comet’s surface are
important for biology.
After hibernating for seven months, Philae
surprised everyone when it woke up again June
13. Over the next few weeks, Philae and Earth
had spotty conversations, with the last command
sent and received July 9. Scientists have no way to
know whether Philae still sits atop Comet 67P, or
whether it has been pushed off by actively spewing
jets of gas.
Rosetta will continue watching Comet 67P
through September 2016, at which point mission
scientists will most likely try to land the spacecraft
on the comet for a last look.

STORIES TO WATCH FOR IN 2016
• The European Space
Agency’s LISA Pathfinder,
a mission to test
the technologies needed
for a full-scale gravitational
wave observatory,
will begin to return
results.
• The Japan Aerospace
Exploration Agency will
launch Astro-H to study
the high-energy universe.
• NASA will launch its
Origins, Spectral Interpretation,
Resource
Identification, Security,
Regolith Explorer
(OSIRIS-Rex) asteroid
sample-return mission.
• Astronomers will begin
closing other telescopes
on Hawaii’s Mauna Kea
in order to make way for
the Thirty Meter Telescope
slated to begin
operations there in the
early 2020s.
• Juno will arrive at Jupiter
to peer through the
giant world’s thick
clouds.
• Advanced Laser Interferometer
Gravitationalwave
Observatory (LIGO)
will return data on gravitational
waves.

1. Pluto and its moons revealed
When NASA’s New Horizons spacecraft
flew by Pluto, Earth watched and celebrated.
“The target didn’t disappoint,” says
Principal Investigator S. Alan Stern. “It’s
absolutely stunning.” And even though the
science collection lasted just months, the
New Horizons mission had been decades
in the making. NASA chose the mission in
2001, the spacecraft launched in 2006, and
it reached Pluto on July 14, 2015.
Seeing the pixelated blobs of Pluto and its
largest moon, Charon, evolve into complex
worlds through the eye of New Horizons
was rewarding, satisfying, and awesome,
says Stern. That’s because everything about
Pluto surprised scientists. They expected a
frozen, cratered, and long-dead world with
an equally old-looking system of moons.
Instead, Pluto’s surface is young, with
smooth frozen plains, icy mountains as high
as the U.S. Rockies, topography that resemble
dunes, a glacial lake, and ice that has
recently flowed around other features in the
same way that glaciers move on Earth’s surface.
The scientists estimate that the
uncratered swaths of terrain are 100 million
years old, while other regions are billions of
years in age.
Pluto’s varied surface with such youthful
areas means that something internal must
be warming it to make it pliable. And while
all the objects in our planetary system would
have been warm shortly after the solar system
formed 4.5 billion years ago, scientists
didn’t think such a small object could stay
warm all these years. “We expect small planets
to typically run out of energy a lot sooner
than the big planets. It’s like a small cup of
coffee cools off faster than a bucket of coffee,”
says Stern. But what New Horizons has
revealed about Pluto, he adds, changes the
expectations of planetary geology.
Scientists have also created a map of
methane ice distribution, and this material
seems to prefer a region of young terrain
that scientists have informally named
“Sputnik Planum.” Outside of this area,
methane is still present and congregates
on crater rims and brighter regions but
avoids crater centers and darker regions for
unknown reasons.
The up-close photos of Pluto have also let
scientists precisely measure the width of the
dwarf planet: 1,473 miles (2,370km). This
secures Pluto as the largest known object
orbiting beyond Neptune.
After New Horizons flew by Pluto, it
looked back and watched the dwarf planet
eclipse the Sun. This alignment let scientists
study Pluto’s atmosphere as sunlight filtered
through it. Above the surface lie distinct
haze layers that extend to about 80 miles
(130km) out, several times farther than
researchers expected. And New Horizons
detected wisps of a nitrogen-rich atmosphere
1,000 miles (1,600km) out.
While Pluto has been the main focus,
Charon also has shown surprises. It too
has a varied surface, with some regions
void of impact craters. Cliffs stretch hundreds
of miles across the surface, indicating
the crust has fractured. A deep canyon,
4 to 6 miles (6 to 10 km) deep, also scours
Charon’s surface.
New Horizons snapped photos of Pluto’s
four smaller moons as well: Nix, Hydra,
Styx, and Kerberos. While Charon is
751 miles (1,208 km) across, each of these
four is just a few dozen miles wide.
Most of New Horizons’ data is still on
board the spacecraft and will be downloaded
piece by piece over the next several
months. Researchers will pore over the additional
data in the next few years, learning
more every day about Pluto and its moons.
Even though humans saved this dwarf
system for last in our exploration of the
solar system, just the first views exceeded
and upended expectations and have given
researchers a treasure-trove of new science.