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 the description and the code ASTRUM at checkout,
and you’ll only have to pay $14.99 for a whole 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 more of the same. A big thanks to my patrons and
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