Hyperion,
the largest of Saturn's irregular moons.
It tumbles through the void 1 and a half
million km from its parent planet,
spinning with a chaotic motion that
makes its future orientation almost
impossible to predict. Unlike many of
Saturn's other 292 confirmed moons, it
isn't smooth and round, but jagged,
porous, and irregular, like a giant
frozen cosmic sponge. It's a world that
defies almost every conventional rule of
planetary science. But why?
Voyager found hints of a violent past, a
great crater visible from afar. But it
wasn't until Cassini explored Hyperion
in 2005 that it all became clear.
Complex gravitational interactions,
ancient proto moon collisions, and an
unusual composition make this arguably
the strangest place in the Saturn
system. Oh, and did I mention it has the
power to zap you with electricity from
2,000 km away? Yes, you did hear me
right.
I'm Alex Mccoan and you're watching
Astramm. Join me today as we venture a
billion km from Earth to this
extraordinary moon, exploring the
violent history of its birth, the maths
behind its chaotic motion, and the
incredible discovery that Hyperion can
dish out electric shocks in space.
To understand Hyperion, we first must
look back to the mid9th century, a
golden age for observational astronomy
when our understanding of the geography
of the solar system skyrocketed thanks
to an explosion of tech and precision
instruments in the 1840s.
With teams all around the world making
new discoveries, finding Hyperion was
actually not a singular event, but a
two-way transatlantic tie. On the
evening of the 16th of September, 1848
at the Harvard College Observatory, the
American Father and Son team of William
and George Bond spotted a faint object
that did not appear on any existing
charts of the Saturnian satellites.
William Cranchbond was a wellrespected
astronomer. Whilst he had a lack of
formal education, he worked his way up
to become the first director of the
Harvard Observatory. He was utilizing
its new 15-in equatorial refractor, one
of the most powerful instruments in the
world at the time when he and his son
first spotted Hyperion.
It was just two nights later, thousands
of kilometers across the Atlantic, that
William Lassel, a wealthy British brewer
who had turned his fortunes towards the
stars, made the same discovery using a
24in speculum reflector of his own
design. Thankfully, the resulting
dispute was less of a conflict and more
of a celebration of human ingenuity.
Because the observations were so close
in time, the astronomical community
eventually settled on a rare triple
credit discovery honoring both Bonds and
Lassel together. It was soon named
following a suggestion by John Herschel
who argued that Saturn's moon should be
named after the Titans, the siblings of
Kronos in Greek mythology. Hyperion, the
Titan of watchfulness and observation,
was a fitting choice for a moon that, as
would be later discovered, never looks
the same way twice. In fact, this moon
was unusual from the start. It was the
first nonsperoid moon to be discovered,
and as we learned more, things only got
stranger.
Hyperion is a world that refuses to be
categorized. It's one of the largest
bodies in the solar system that's highly
irregular in shape. A status it shares
only with Neptune's moons Proteius and
Nerid.
In the hierarchy of Saturn's moons is
the eighth largest by radius, but
curiously only the ninth most massive. A
discrepancy that hints at its unusual
internal structure. And as we discovered
thanks to Voyager data in the 80s, it
also rotates chaotically.
In fact, while we often think of moons
as spherical mini planets, Hyperion is
anything but this. It's a battered
elongated mass that has been compared to
everything from a potato to a burger to
a deflated football. But why exactly is
it so strange?
One of the most curious things about
Hyperion is its density. At
approximately 0.54 gram per cubic cm,
this moon is only about half as dense as
liquid water. Even compared to pure
water ice, which has a density of
roughly 0.92 g per cm, Hyperion is an
outlier. This tells us that it cannot be
a solid monolithic block of ice and
rock. Instead, it is likely a pile of
rubble. If you were to stand on it, the
ground beneath your feet would be a
fragile honeycomb latis of ancient water
ice and organic dust. In fact, data from
the Cassini mission suggests that more
than 40% of Hyperion's interior is just
empty space.
This high porousity is likely a relic of
its formation. Unlike larger moons that
underwent differentiation, a process
where internal heat melts the interior
and allows heavy materials to sink to
the core, Hyperion likely never stayed
warm enough for long enough to compact
its structure. It instead remains a
primitive collection of debris,
potentially even the reaced remains of a
much larger proto Hyperion measuring
perhaps 1,000 km across. A massive
collision likely shattered this world,
leaving Hyperion as the largest
remaining piece. The rest of the debris
might have rained down on Titan or been
ground into the dust that now forms the
dunes of that orange world. More on
Titan later. But this internal structure
of Hyperion isn't just a window into the
past. It also impacts on the moon today.
Not only does it give the moon an
extremely low escape velocity, in fact,
a professional baseball player could
nearly throw a ball into orbit from
Hyperion's surface. But the moon acts as
a shock absorber, and this explains
Hyperion's rather unusual appearance.
When the Cassini spacecraft performed
its close flybys of Hyperion in 2005,
the images it returned were unlike
anything seen before. The surface was
covered in deep, sharp edge pits, making
it resemble a wasp's nest or a sea
sponge. On a typical solid moon such as
our own, an impacting asteroid strikes
the surface at several kilome/s.
The kinetic energy is released in an
explosion that creates a crater,
blasting material. otherwise known as
ejector outward to form a rim and a
surrounding debris blanket. On Hyperion,
the process is fundamentally different.
Because the moon is so porous, the
impact it doesn't blast as much as it
compresses.
It's like firing a pebble into a block
of styrofoam. The material is pushed
downward and is compacted, creating a
deep pit without the typical rim or
ejector rays. Because Hyperion's gravity
is so weak, any material that is kicked
up during an impact also has a very high
probability of reaching escape velocity.
Instead of falling back to the surface
to fill in old craters or coat the
landscape in a layer of dust, it simply
flies off into orbit around Saturn. This
leaves the surface clean, preserving the
original crisp and deeply etched shapes
of the craters for billions of years.
The result is one of the most crater
saturated surfaces in the Saturnian
system. They are spectacular to look at.
Hyperion really does look like a sponge
to me, but they're certainly not the
only strange thing this moon has to
offer. Hyperion moves like no other body
in the solar system.
Hyperion does not exist in a vacuum. Its
existence is sculpted by its much larger
neighbor, Titan. Now, Titan is a
bermoth, the second largest moon in the
solar system, and its gravitational
shadow looms large over everything in
its vicinity. Hyperion and Titan are
locked in a 43 orbital resonance, which
means that for every four orbits Titan
completes around Saturn, Hyperion makes
exactly three. This resonance is a
delicate gravitational tugofwar. As the
moons pass each other, Titan's immense
gravity gives Hyperion a periodic kick.
This interaction has two major effects.
First up, Titan's gravity prevents
Hyperion's orbit from becoming
completely circular. Instead, it follows
an eccentric eggshaped path that brings
it closer to and further from Saturn at
various points in the orbit. Secondly,
the combined influence of Hyperion's
irregular elongated shape and the
varying gravitational poles from Saturn
and Titan create a state of dynamical
chaos.
In most of the solar system, gravity
acts like a clock, allowing us to
predict the positions and orientations
of planets centuries into the future,
but not on Hyperion.
In 1984, planetary scientist Jack Wisdom
and his colleagues at MIT created new
mathematical models to predict the
motion of Hyperion.
Trying to explain why it tumbles rather
than spinning smoothly. They found that
the moon's rotation is so sensitive to
its environment that its orientation is
unpredictable over time scales as short
as a month, a period known as Laapunov
time. In other words, we literally don't
know how it's going to spin for more
than a few weeks into the future. It is
the only large moon in the solar system
known to lack a stable spin axis like
this and is one of the first confirmed
examples of chaotic motion in the
cosmos. If you were to stand on
Hyperion, the sun wouldn't rise in the
east and set in the west in a
predictable pattern. It might zigzag
across the sky, move backward, stay
stationary for days before suddenly
plunging below the horizon. But beyond
its physical structure and its erratic
movement, Hyperion holds an even deeper
secret.
One of the most important instruments
the Cassini spacecraft was fitted with
was the visual and infrared mapping
spectrometer. Its job was to map the
composition, temperature, and structure
of Saturn and in this case its moons.
When it turned its gaze to Hyperion, it
revealed a complex mix of water ice and
organic compounds. The bright crater
walls are primarily composed of
crystalline water ice, similar to what
we might find on Earth. However, the
dark material at the bottom of the
craters is something else entirely. a
slurry of hydrocarbons and organic dust.
Dale Krookshank, a planetary scientist
at NASA, has pointed out that these
hydrocarbons are the same basic chemical
building blocks found in comets and
meteorites. But where does this dark
material come from? To solve this
mystery, we must look further out to the
moon Phoebe.
Phoebe is a dark retrograde moon that is
believed to be a captured object from
the Kyper belt, the same region that
gave us Pluto. I've made a video about
it you can watch here if you want to
know more. It is also the source of a
massive nearly invisible ring of dust
that orbits Saturn. As this dust spirals
inward towards the planet, it is
intercepted by the other moons.
Iapotus, Saturn's yin-yang moon, catches
most of this dust on its leading
hemisphere. But some of it gets pastus
and lands on Hyperion.
Because Hyperion is tumbling
chaotically, it doesn't have a single
leading side, which means the dark
material is instead spread more evenly
across its surface, though it naturally
settles into the low points of the
craters. One of the other most
intriguing findings was that carbon
dioxide on Hyperion is not pure dry ice.
Instead, it is chemically complexed or
attached to other surface materials.
This makes the CO2 more stable,
preventing it from evaporating into
space even over the 4.5 billionyear
history of the solar system.
The presence of these molecules,
especially when exposed to ultraviolet
light, can lead to the formation of even
more complex molecules of biological
significance. While this doesn't mean
Hyperion harbors life, it proves that
the essential chemistry needed for life
is far more widespread in the universe
than we once imagined.
Now, this is all really interesting and
certainly paints a picture of Hyperion
being a rather odd place. But there is
one discovery that Cassini made that I
personally find quite shocking, and it
came not from its cameras, but its
plasma instruments. Hyperion has no
atmosphere, and so it is bombarded by
solar radiation and exposed to the harsh
environment of Saturn's magneettosphere.
It's constantly pelted by a rain of
charged particles from the planet's
magnetic field. Combined, these cause
massive electrostatic charges to build
up on the surface, particularly near the
moon's terminator, the boundary between
day and night. This is similar to the
static electricity you experience when
rubbing a balloon against your hair, but
on a planetary scale. And this spelt
trouble for Cassini. In 2014, a new
analysis of data from a 2005 flyby
revealed that Hyperion had actually
zapped the Cassini spacecraft with a
beam of electrons. At a distance of
about 2,000 km above the moon's surface,
Cassini passed through the magnetic
field lines connected to Hyperion.
Briefly, the probe and the moon were
magnetically connected, and Cassini
received the equivalent of a 200V
electric shock, roughly the same as
sticking your fingers into a plug
socket, which you should not do. This
was the first time a charged surface has
ever been detected on any moon other
than our own. Thankfully, the shock
didn't damage Cassini, but the discovery
has serious implications for future
exploration.
Static charging can cause dust to
levitate and move across a moon's
surface, potentially coating instruments
and even escaping into space. For future
robotic or human explorers, these
electrostatic discharges could pose a
hazard to sensitive electronics,
requiring us to design spacecraft that
are immune to the electrical charges of
the worlds they visit.
So there we have it. Hyperion shows us
that the solar system is far more than a
collection of orderly spheres. It is a
place of ancient violence. mathematical
chaos and unexpected chemistry. From its
bizarre sponge-like surface that absorbs
the impact of the cosmos to its
unpredictable tumbling rotation that
defies our best equations, Hyperion is a
world that keeps us humble. The
discovery that a small icy moon can
reach out across 2,000 km to zap a
visitor from Earth shows just how much
we still have to learn about the
interaction between celestial bodies and
the environments they inhabit. As we
continue to gaze out at the ring planet
and its diverse family of moons, Ian
remains one of our most fascinating
targets. A world that refuses to sit
still, refuses to be predicted, and
refuses to be anything other than its
own strange, beautiful self. If you
enjoyed this deep dive into Hyperion, be
sure to subscribe to Astramm for more
explorations of our amazing solar system
and beyond.