Thanks to CuriosityStream for sponsoring today’s
video. 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, a disperse belt of
asteroid like objects beyond the orbit of Neptune. 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? I’m Alex McColgan, and you’re watching Astrum, and together we will explore
almost everything there is to know about Arrokoth. 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 object 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 fracture 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/cm³, 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, 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. Alan Stern, the head of the New Horizons
team, gave a fascinating lecture about Pluto and beyond on CuriosityStream if you want to
get an insight on the mission from the inside. His passion for the mission is contagious, and
it really is thanks to him that we had a Pluto mission in the first place. CuriosityStream also
has thousands of really high-quality streamable documentaries about a variety of topics,
including my favourite topics on space and science. They also have apps so you can access
them on your phone, computer, TV and more. So, if you want access to this lecture, or just
have a passion for learning no matter the topic, have a look at CuriosityStream. Use my link in
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year’s subscription. I definitely recommend checking it out. 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
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