In 2015, there was huge excitement in the
space community. That is because up until then, the best image we had of the Pluto system
was this. Hubble also squinted its lens at Pluto, but it is so small and distant, the
best it could see was a few blobs of colour variation. But in 2015, this all changed.
That is because, after a nine-year journey, the New Horizons space probe flew by the dwarf
planet, giving us a detail and fidelity of Pluto and its moons like we had never seen
before. So, the question is, what did the New Horizons probe see and discover during
its flyby of the Pluto system? And what has it been doing since the flyby? I’m Alex
McColgan, and you’re watching Astrum. Stick with me in this video and I will show you
all the highlights from the New Horizons mission to the Pluto system and beyond. Let’s first
of all give you a quick bit of context in case you are new to Pluto, or if it’s been
a while since you last heard about it. Pluto is a remarkably pretty, tiny world, much smaller
than our moon. It’s found in the Kuiper Belt, a disperse belt of asteroid or comet
type objects beyond the orbit of Neptune. Pluto was the last of the traditional 9 planets
to be explored. This was due to its distance from us, but also because – can you believe
this – it wasn’t considered a very interesting celestial object. Thankfully, the team behind
the probe pushed hard for this mission to be approved, and in 2006, New Horizons was
launched as part of NASA’s New Frontier program, for medium budget space missions.
The goal of the mission was to get to Pluto as soon as possible, and as such, New Horizons
was the fastest launch ever, it being a light spacecraft on the most powerful rocket available
at the time – a fully boosted Atlas V. It whizzed past the moon in only 9 hours. The
Apollo missions took 10 times as long. On its way to Pluto, it used Jupiter as a gravity
assist which shaved 3 years off the arrival time. It also used Jupiter as a trial run
for its systems, taking some remarkable videos and images of the planet and its moons. After
this successful trial, New Horizons went into hibernation mode to prevent the wear and tear
of its instruments. Leading up to its approach in 2015, the team turned the systems back
online, and every day the spacecraft sent back images of the Pluto system. This was
an incredibly exciting time for enthusiasts following the story. We began to get hints
of what Pluto could possibly look like, and saw how different Pluto was from its biggest
moon, Charon. Every day, the resolution got higher and higher, and more details could
be made out. Yes, there were other scientific goals for the mission, but the most interesting
thing to me was what it looked like. Soon there could be seen what looked to be a heart
shape on the dwarf planet! On the 14th July, the New Horizons probe made its closest approach,
at only 12,500 km from the surface of Pluto. However, mission controllers didn’t get
a look straight away. Firstly, the probe was too busy taking a lot of photos during the
flyby to send any back immediately. Once data transfer commenced, they had to deal with
the slow uplink speed of only 1kbit/sec. Further to that, there was a 4.5-hour latency between
the spacecraft and the Earth. But what it saw and sent back was spectacular: mountain
ranges, ice plains, glaciers and an atmosphere. It also had a good look at some of Pluto’s
moons. Let’s go into detail about what it actually discovered during this flyby. One
of the first things observed about Pluto is its unusual relationship with its moons. For
a start, Pluto’s biggest moon, Charon, orbits very closely to Pluto, and is also very big
in comparison. This means that the barycentre of the two objects, or in other words, their
centre of mass, is outside of the primary object. They actually both orbit around a
point in space. Not only that, but both objects are tidally locked to each other. This means
that if you stand on one, the other won’t move from that point in the sky. This is very
unusual because while some moons are tidally locked to their parent planet, the planet
is not also tidally locked to the moon. Charon is very different visually from Pluto being
much darker. This implies they are not from the same origin. The rest of Pluto’s moons
are very small, only a few kilometres across. Their orbits are exceptionally circular and
are all coplanar with Pluto’s orbit. The geology of Pluto is very interesting. The
biggest visible feature on Pluto is this giant heart shape, which wowed the world when it
first came into view. It has since been named Sputnik Planitia. It is the size of Texas,
and it has a strong colour contrast to the surrounding area. This is because it is a
giant ice plain. In fact, during the flyby, it was confirmed that 98% of Pluto’s surface
is composed of nitrogen ice. On average, the temperature on the surface of Pluto is -229c,
which means water ice would be rigid and brittle. On the other hand, nitrogen ices at this temperature
act like water ice on Earth, meaning it can flow as glaciers. This can especially be seen
around the edge of the heart, glaciers flowing into the gaps around the craters and mountain
ranges. The ice plains themselves have giant polygon shapes across the entire area. There
are also no craters, which means it must be a relatively new feature, or a feature that
is being continually renewed. It is perhaps only 10 million years old. The polygonal cells
show ridges on them which are likely caused by sublimation, the process of an ice turning
directly into a gas. Ices sublimate and freeze here regularly, creating troughs and pits,
meaning these polygons are likely to be convection cells. These cells are moving and can be seen
pouring into the mountain ranges surrounding the region through slow moving glaciers. Sputnik
Planitia could be compared to Greenland and Antarctica, in that it controls the climate
of Pluto heavily. Although it’s not known for certain, Sputnik Planitia could have formed
from an impact, and ices filled the crater in from a potential subsurface liquid ocean.
This filled in basin actually causes a positive gravitational anomaly. A gravitational anomaly
is where the gravity at one point is different from elsewhere on the object. The ice plain
is directly facing away from Charon, which would align it up with the objects’ tidal
axis. Due to the short distance between Pluto and Charon, tidal effects are very strong
on both objects. This could be the reason why Pluto is tidally locked to Charon and
the two objects can’t look away from each other.
Surrounding the ice plains are vast mountain ranges made of water ice, which, when viewed
from the side on, look spectacular. Water ice is the only type of ice detected on Pluto
that would be strong enough to support heights of several kilometres at this temperature.
Among the mountains found on Pluto, there might also be some which are cryovolcanoes,
one of the most likely candidates being Wright Mons. It is 4 kms tall, one of the highest
peaks on Pluto, and a big depression is found in the centre. Cryovolcanoes could be a contributing
factor for Pluto’s young surface. Here is an extremely interesting region called Tartarus
Dorsa. It is an extensive, highly distinctive set of 500-meter-high mountains that resembles
snakeskin or tree bark. They are thought to be Penitentes. If that is true, Pluto is the
only place in our solar system other than Earth where they have been observed. Even
on Earth they are very rare, but some can be found in the Atacama desert and other dry,
high altitude regions. The ones on Pluto are much taller and cover a much vaster area than
on Earth. We can only imagine what they look like close up. Another obvious feature of
Pluto is the dark material that seems to be sprinkled on the surface in some areas. The
biggest such area is called Cthulhu Macula. It is weirdly reminiscent of a whale in shape,
as can be seen in this image. The region on Pluto is much more heavily cratered than the
heart, which implies the surface there is much older. Mountain ranges can be seen in
the middle of Cthulhu Macula, topped with what is thought to be methane ices. Methane
apparently condenses as frost at higher altitudes on Pluto. The dark colour is thought to be
a deposit of tholins, a kind of tar made up of hydrocarbons that have interacted with
sunlight. Similar deposits can be seen on one of Saturn’s moons, Iapetus, so the process
has been seen elsewhere in the solar system. Scientists suspected this substance was tholins
as soon as New Horizons started sending images back, but its distribution over Pluto’s
surface was baffling. Why are only some areas covered? Also, how dynamic are the processes
surrounding the distribution of tholins? Has Pluto looked like this for a while, or is
this a changing environment? It turns out that these tholins may well be connected to
the cryovolcanism found on Pluto. Pluto is one of the few objects in our solar system
where cryovolcanoes are actively shaping its surface. Water from either the mantle or from
pockets of water trapped in the crust erupt over the surface of Pluto, creating a varied
landscape. But it turns out that it’s not just water found in these eruptions, but tholins
are clearly mixed in too. Data from Hubble suggested that Pluto was getting redder, and
New Horizons may have passed by during Pluto’s reddest time of its year. And New Horizons
may have found out why. Here, in a region called Viking Terra, we see a cryovolcano
that fountained this water tholin slurry across the immediate surroundings. Just next to this
region, we see a crater and a trough filled by this slurry during another eruption. By
the trough, you can see where this slurry flowed down and pooled. This can also be seen
in another region by Virgil Fossae, another trough where this slurry has travelled down.
However, the most interesting thing about tholins is not found on Pluto itself, but
rather on its twin dwarf planet, Charon. In this enhanced colour image of Charon, what
do you immediately notice? The red cap over its north pole. Incredibly, because Pluto’s
gravity is so weak, when it erupts this slurry mixture some of it escapes Pluto altogether
and makes the 19,000km journey to Charon. The tholins are localised here because Pluto
and Charon are tidally locked to each other, they only ever show each other one face. Poetically
speaking, Pluto is always hiding its heart from Charon in this eternal waltz. This means
that more tholins fall on a specific spot on Charon, rather than all over. And speaking
of Charon, some interesting discoveries have been made about it too. It is a water ice
world, unlike Pluto whose surface is predominately nitrogen ice. As such, it doesn’t really
have an atmosphere like Pluto does, as the water ice is locked to the surface. On Pluto,
the nitrogen ice sublimates depending on Pluto’s seasons, meaning Pluto’s atmospheric density
can vary by many orders of magnitude over the course of its year. With this sublimating
and refreezing of the atmosphere, Pluto’s appearance may change dramatically over the
course of its 248-year-long seasonal cycle. For me, the most impressive discovery that
New Horizons was able to confirm was that Pluto has an atmosphere. And not only that,
but the images are incredible. Due to Pluto’s small size and weak gravity, the atmosphere
appears to extend high above the surface of Pluto. Earth’s atmosphere, while being much
more massive and dense compared to Pluto, hugs the planets comparatively tightly as
the gravity is a lot stronger. The atmospheric pressure on Pluto, on the other hand, is exceptionally
low, roughly 10 microbars, or 100,000 – 1,000,000 times weaker than the surface pressure on
Earth. It is theorised that the pressure could increase to as much as 18 to 280 millibars,
three times the surface pressure on Mars and a quarter of the surface pressure on Earth.
This may happen throughout Pluto’s year, at some points in its orbit it is closer to
the Sun than Neptune. This would make the temperature rise causing the surface ices
to sublimate into gases, the process of which there is evidence of in the ice plains. But
the last time Pluto was thought to have an atmospheric density similar to Mars was 900,000
years ago. At this pressure and temperature, the conditions could even be right for liquid
nitrogen to form on Pluto’s surface. Some evidence of this might be found here, in what
appears to be a frozen over lake. At any rate, within just one year, Pluto’s atmospheric
density can vary by a factor of four due to seasonal variations. That is a massive contrast
compared to other solar system objects with atmospheres, which generally stay pretty consistent.
The atmosphere consists of the same ices found condensed on the surface, namely nitrogen,
methane, and carbon monoxide. The other fascinating discovery New Horizons made about the atmosphere
is that it has up to 20 haze layers. Haze layers themselves were not unexpected, but
the amount of them was. They can clearly be seen in some of these images, acting like
layers of a thin kind of fog. Sunlight can be seen streaming through one such layer in
this photo, the shadows from the mountains clearly seen in contrast to the sunlight shining
through the haze. The layers do not appear to be level across the planet. Here you can
see this haze layer high above the surface, but on this side of the image it touches the
surface. On a side note, to me these are the most breath-taking photos of Pluto, and I
purposefully saved them until last. You can truly appreciate depth and the scale of the
mountain ranges; Pluto almost seems like a toy replica due to the extreme topographical
relief, but these mountains appear so high because Pluto is so small, and its gravity
is not strong enough to pull them down. In June 2020, scientists released a paper stating
that under Pluto’s surface is believed to be an ocean of liquid water, very much like
the icy moons of the gas planets. It was originally thought that Pluto formed cold, being so far
away from the Sun. However, evidence from New Horizons suggests that this is not the
case, but rather it started off hot. This means it’s always had an ocean, and if that
is true, then there is a case that habitability on Pluto may be just as good as habitability
on the closer, icy moons. In fact, if Pluto is the standard for dwarf planets found in
the Kuiper belt generally, there may be many more habitable worlds out there. How do we
know it had a hot start? There is evidence of expansion, not contraction on its surface.
These cracks show that the crust is moving apart, not folding over itself. If this is
true and Pluto had a hot start, perhaps with bombardments from other planetesimals heating
it up during the early stages of solar system, it could be that shortly after it was formed
it would have had enough thermal energy that it was once an ocean world. This really puts
a new perspective on how the solar system formed. While the total absence of craters
is limited to Sputnik Planitia, it is amazing how few craters there are on Pluto and Charon
generally. This might not just be because their surfaces are young, but perhaps the
Kuiper Belt is more devoid of smaller objects than we may have first thought. The flyby
was over in a matter of days, and New Horizons started heading deep into the Kuiper belt.
New Horizons had travelled so far from Earth at this point that when it looked at our closest
star system, Alpha Centauri, it was in a clearly different place from New Horizons’ perspective
than from ours. This is due to the parallax effect, something I’ve done a video about
here if you want to see more astronomical examples. It’s just mind-boggling to me
to think about how far New Horizons has travelled relative to us, so much so that Alpha Centauri
has moved from New Horizon’s perspective. Conversely, however, New Horizons has travelled
all that way, and that’s the only difference it’s made to the view of our closest neighbour.
Space is just so big. When New Horizons made its flyby of Pluto back in 2015, it barely
slowed down at all. Its trajectory after the encounter actually took it further into the
Kuiper Belt. Given that this region is so far from Earth, it is largely unchartered
territory, a place where no man has gone before! So, did the New Horizons team know of an object
they could visit next? Yes, they did. And its name is Arrokoth. But incredibly, they
didn’t even know of its existence before New Horizons was launched. So, what is Arrokoth?
What does it look like? And what makes it unlike anything we have ever seen before?
You see, the year previous to the Pluto flyby, time had been given to the New Horizons team
with the Hubble Space Telescope so that they could locate an object for New Horizons to
visit after Pluto. Hubble actually discovered three new objects reasonably close to where
New Horizons would be going, and after studying the data, the 35 km long object now known
as Arrokoth was chosen. As a result, Arrokoth would be the first object visited that was
discovered after the spacecraft visiting it was launched. New Horizons was healthy and
well after the Pluto flyby, with propellent left in its tank and years left in its RTG,
and so commands were quickly sent to New Horizons by the mission team to adjust its course so
that it could rendezvous with the promising new target. Being so small and far away, we
didn’t know much about the object, all Hubble could detect was its colour, and the dips
and peaks in brightness as it rotated. However, scientists also observed Arrokoth’s occultation
of a star. Incredibly, from this occultation, they were able to predict the shape of Arrokoth,
and as you will see later, this prediction was almost exactly right. At the very least,
they knew it would be an elongated object, so potentially a contact binary or simply
a long asteroid-type object. It was up to New Horizons to confirm their predictions.
Three years after leaving Pluto, in August 2018, New Horizons began its approach phase
at a distance of 172 million kilometres. At this distance, Arrokoth was barely visible
to New Horizons against the backdrop of distant stars. But by December 2018, it was bright
in New Horizon’s view. Travelling at 51,000km/h, New Horizons was rapidly gaining on Arrokoth,
and science data at this point was already beginning to be collected. As New Horizons
got closer and closer, Arrokoth’s shape could start to be resolved. It was bizarre
looking, what appeared to be a contact binary, and it was relatively crater free, with a
lumpy surface. It was unlike any of the asteroids or comets we had ever seen up close before.
On the 1st January 2019, New Horizons made its closest approach at a distance of only
3,500km from its surface, and it was on this day that it captured most of its science data.
This flyby made Arrokoth the most distant object ever visited by a spacecraft, being
6.5 billion km from the Sun at the time, or roughly 45 times further away than the Earth
is from the Sun. Being this far away, the data transfer speed was abysmally slow between
Earth and New Horizons at only 1 kbit per second (although I will mention that it’s
incredible to me that the technology was there for them to communicate with New Horizons
at all). This slow data transfer speed has meant that it’s taken around 2 years to
send all of the data it collected around Arrokoth back to Earth. The highest priority data was
sent back first, namely the images, although I do remember at the time that the highest
resolution images took a while to arrive back. Only low-resolution images were available
when all the media outlets were publishing stories of the flyby, meaning I would guess
that most of the general public never saw Arrokoth in all of its glory. So, here it
is, the highest resolution images we have of this fascinating object, in true colour.
What you’ll immediately notice about Arrokoth is that it is reddish in colour, unlike most
asteroids nearer to home, which are greyer and darker. It’s red because of a similarity
it shares with Pluto, it has an abundance of tholins on its surface. Tholins are organic
compounds that have been broken down by solar and cosmic rays. Organic compounds on the
surface probably included methane and ammonia at one point, however Arrokoth does not have
any of these substances left, probably due to its low mass. What Arrokoth’s spectra
does reveal is that it has methanol, hydrogen cyanide and water ice on the surface. The
abundance of methanol on Arrokoth’s surface is the main factor behind its red colour,
as irradiated methanol is likely the cause of the tholins. However, there is a bit of
a mystery in Arrokoth’s spectra, as interestingly, there is also an absorption band at 1.8 μm
in Arrokoth’s spectra, and scientists do not know what this compound is. It is yet
to be identified, it’s nothing we’ve seen before. It’s a shame we weren’t able to
get a sample of its surface to be able to say for sure. The next thing you’ll notice
about Arrokoth compared to asteroids closer to home is the absence of small impact craters.
It is believed that this is due to the nature of the Kuiper Belt itself. It could have 20-200
times the mass of our asteroid belt, but a lot of this mass is also contained within
large Pluto like bodies which dot the belt. While we can’t say for sure what the population
of the Kuiper Belt is, it is definitely more spread out than our asteroid belt simply because
it’s 20 times as wide and has a much bigger circumference. Being this far from the Sun
means orbital speeds are much slower, so even if an impact does occur, it will be at a low
velocity. Meteorites you see creating shooting stars in the Earth’s atmosphere may hit
us at around 75km/s, whereas impacts in the Kuiper Belt may only be at speeds of 300m/s.
This depression here, which looks like a crater, may not actually have been formed from a collision,
but it could be a sinkhole caused by the escape of volatile substances just under the surface.
The lack of collisions means that what we see of Arrokoth now is like a time capsule
from the early solar system, an object that has been preserved for billions of years.
Although, a slow collision is one of the ways this object may have come into being. When
asteroids in the asteroid belt impact each other at high speeds, they either cause craters
or cause the body to completely fragment. But a slow collision, like those in the Kuiper
Belt, may cause both objects to simply merge. It may also be that the two lobes of Arrokoth
formed side by side in a swirling cloud of ice fragments that coalesced into two orbiting
bodies. Eventually these bodies got closer and closer until they joined together. In
any case, the merging would have happened very slowly because there really aren’t
many fractures and stress lines to speak of, so the max speed of the collision would be
no more than 2 m/s, plus the two objects would have also had to have been tidally locked
to each other before merging too. The fact that both lobes of Arrokoth look very similar
gives weight to the theory that they formed in the same region. Before Arrokoth got its
formal designation, you may have known it by a different name, as it was originally
nicknamed Ultima Thule. Now, the individual lobes are known as Ultima and Thule. You’ll
also notice some very bright regions on the surface. The ones in the crater are probably
from avalanches as material fell inward after the sinkhole appeared. The other major bright
patch is found around the connecting point between the two lobes. It’s not known with
certainty why this region is brighter, but theories suggest that this region sees the
least amount of sunlight, so perhaps volatile substances can build up here, like ammonia
ice. It could also be that because this region would be the centre of gravity of the object,
loose material rolls down the lobes to collect in the centre. With a density of only 0.5g/cmÂł,
Arrokoth is not going to be densely packed, but it is probably porous. Volatile materials
would have escaped the interior of the object over time due to an internal heat source,
but then these materials would freeze on the surface, leaving behind only rocky remains
inside. This heat source can still be detected to some degree, as models suggested that Arrokoth
should only be 12-14 K, however, New Horizons found that it was in fact 29 K. That is still
extremely cold, just not quite as cold as we were expecting. There’s one last mysterious
characteristic of Arrokoth that isn’t immediately apparent from these images, that only got
discovered after trawling through the New Horizons data, and that is that Arrokoth is
in fact much flatter than we would have expected. We didn’t notice it at first because Arrokoth
rotates like this, meaning we didn’t see too much of it lit up from a side angle. We
don’t really know why it’s flat. Maybe it was due to centrifugal forces when the
individual lobes formed, implying it was spinning a lot faster than it is today. Or maybe it’s
due to the way Arrokoth orbits and rotates, meaning one side of the object is constantly
exposed to the Sun for decades at a time. This would cause volatile substances to escape
only on one side, until later in the year when the other side is exposed to the Sun.
Research is still underway to model the cause. As New Horizons left Arrokoth, it looked back
and caught one last glimpse of its silhouette against the backdrop of stars. Who knows if
Arrokoth will ever be visited again, so it may be that this is the last up-close view
of it that we will ever have. What’s next for New Horizons? Well, it still has life
in its battery, and 11kg of fuel still onboard, so the hunt is now underway to search for
any additional targets. Beyond that, it will follow in the path of the Voyagers, passing
through the heliosphere of the solar system in the 2030s. Even if no other Kuiper belt
object can be discovered close enough to its current trajectory that it can do a third
flyby, the New Horizons team is already submitting proposals for an extended mission that will
have a completely different focus. They want to convert New Horizons into a highly-productive
observatory conducting planetary science, astrophysics and heliospheric observations
that no other spacecraft can — simply because New Horizons is the only spacecraft in the
Kuiper Belt and the Sun’s outer heliosphere, and far enough away to perform some unique
kinds of astrophysics. Those studies would range from unique new astronomical observations
of Uranus, Neptune and dwarf planets, to searches for free-floating black holes and the local
interstellar medium, along with new observations of the faint optical and ultraviolet light
of extragalactic space. Beyond that, New Horizons has already given us a wealth of data on Kuiper
belt objects that we would not have known about otherwise. Who knew that this is what
Pluto would look like? That Charon has a red cap? That Arrokoth would be flat? And considering
these are the only Kuiper belt objects we’ve ever seen up close, there’s bound to be
a lot more out there that’s still waiting to surprise us. Thanks for watching! If you
liked this New Horizons video, you should check out some of the other spacecraft videos
I’ve made here for more of the same. Thanks to my patrons and members for supporting the
channel too. If you want to help me make more videos, and have your name added to this list,
check the links in the description below. All the best, and see you next time.
Amazing that the probe took nine years to get to Pluto and it few by in a matter of hours
Excellent video. Still can't get over how breathtaking that side view shot of the ice mountains is.
Very solid video. Astrum is 100% a channel fellow space nerds should subscribe to as he does a great mix of surface level videos for topics you’re not familiar with and heavy deep dives such as this one.
I made a 3d viewable image of Akkoroth if anyone wants to check it out!
Just cross your eyes till the images overlap, and it will pop into full 3d :D https://imgur.com/a/zHTTygi
That was a fascinating watch. Didn't even know about Arrokoth until today.
Sorry if this is a dumb question, why would this be the last time we'll see images of Pluto and Arrokoth
Thank you for posting this. Arrokoth is new to me. :)
And final images of those penitentes in the Atacama. They will be gone in a decade.
Anybody have a link to the thumbnail image? Looks amazing