It’s always an astonishing feat when astronomers find new celestial bodies that tell different
stories about the universe, but how often do they find ones that hold a lot of information
about our star system that it effectively works as a time capsule?
Today, we are going to explore farthest planetesimal that NASA’s New Horizons’ flyby ever captured:
a floating snowman
Admit it.
I got you curious when I said NASA’s newest discovery is a flying snowman, right?
Because that’s exactly how I felt when I read the headline in BBc back in 2019.
But before we go on and explore the main topic, how about a bit of backgrounder first?
Like all first meetings, it is best that first we get to a bit of history, before talking
about the core stuff.
It’s some kind of a setting of expectations about what we’re about to find out.
Back in 2015, astronomers went completely crazy when NASA’s very own New Horizons
spacecraft fruitfully accomplished flying by Pluto and providing mankind with its first
ever, never before captured, raw, high-definition photograph of this dwarf planet.
This endeavor was so huge of a success that it got astronomers all pumped up and thought,
“Hey, maybe we can do more!
How about we search for new candidates to explore?
The spacecraft still has a lot of fuel, and we can only go further anyway.”
And so the search began for the next object to photograph.
Choosing the next candidate was not an easy task since, when the scientists pointed their
telescopes to where New Horizons could possibly go, there were a lot of other stars and other
objects that were really bright and acted as some kind of “noise” to the celestial
map.
However, the selection was still successful, despite the challenges.
Time went on and an object was chosen.
Catalogued as 2014 MU69, discovered back in 2014, this object found floating about in
the Kuiper belt, 43 AU from the Sun..
So, New Horizons plot its course and took on the task of flying by this new target!
This expedition cost a lot of effort that the spacecraft actually had to do change its
course four times just so they could fly by the object of interest.
That’s a lot of fuel being burned for the sake of knowledge!
Fast forward to the first day of 2019, lo and behold, New Horizons received transmission
containing blocks of information about the photograph it took, and as it was revealed,
the scientists were appalled by what they saw: “Is that Olaf floating in space?”
To be honest, I don’t know if that last bit is true, but that is definitely what anybody’s
first impression is going to be when they see this object: it looks like a giant, sideways
snowman!
What’s more amazing is that this is no simple snowman, apparently, upon studying this object
further, scientists thought that this object could be Solar System’s very own time capsule.
By definition, a time capsule is a storage unit.
You place something inside it, either a memory, a photograph, and information, or a message
to your future self or to whomever might pick this up someday.
Our interesting snowman serves this function to our very own Solar System, and the astronomers
are ecstatic to have found it!
Arrokoth: Solar System’s Time Capsule
The object earlier categorized as MU69 was later named Ultima Thule, taken from a story
in Greek mythology describing a place that is somewhat the “Northernmost Place”,
or essentially the farthest anyone can go to.
However, there was a lot of criticism with the naming since Ultima Thule was also used
previously in Nazi propaganda and fanaticism.
So, to avoid further conflict, Mr. Snowman was renamed to Arrokoth: a word from Powhatan/Algonquian
which translates to “sky” in English.
Upon further investigation, it seems that Arrokoth may hold a lot of secrets about the
formation of other objects and planetesimals in the Solar System.
This is why scientists consider this to be Solar System’s very own time capsule.
A peek into the properties of this celestial thing is a window to the early days of our
star system, and provides a different perspective to insights we never had before.
Arrokoth, is a trans-Neptunian object.
It is a contact binary, meaning this object used to be two individual ones that went so
close to each other that eventually they got stuck.
It has two lobes nicknamed Ultima and Thule.
The larger one, Ultima, is about 21.6 kilometers across its longest axis, while the smaller
one, Thule, measures 15.4 kilometers end to end on its longest axis.
The Ultima appears to be flatter than Thule.
So, I think we can reevaluate our impression of Arrokoth looking like a snowman.
Rather, it most closely resembles a head biting a gigantic cookie.
Besides that, it has no rings, satellites, or other special features.
Finding no satellites was particularly weird since we would expect it to have even a small
amount of debris revolving around it, but this isn’t the case.
Astronomers suspect that there might have been some satellites at some point, but they
just flew away to another direction.
More on this later.
It rotates once every 16 hours, and its axis has a tilt of 98 degrees.
This means that the side exposed to the sun, the day time side, is pretty much as it is
all the time.
It’s like the blades of an electric fan facing the Sun.
The average temperature of the object was really cold, with the maximum being 60 Kelvin
at the poles on the day time side.
The night side can get even colder at 30 K maximum.
Now that’s a cold, cold place.
However, Arrokoth’s “coolness” doesn’t stop there.
This small rock is also what’s called a dynamically cold, classical Kuiper belt object.
See, a lot of objects in the Solar System have orbits that were, at some point, messed
up with other forces around it: sometimes, objects get knocked out of place, sometimes,
a gravitational influence from a more massive object causes its orbit to be more eccentric.
Arrokoth has never experienced this kind of deviation.
It orbits in a circular manner around the sun at practically the same plane as other
planets, and it is extremely likely that where it is now is exactly where it was billions
of years ago.
It never experienced any event strong enough to perturbate it out of its orbit or to be
tossed out to a more eccentric object.
This is what we meant when we said that Arrokoth is a cold, classical Kuiper Belt object...or
CCKBO, if you’re down with the abbreevs.
If the gravity of that hasn’t sunk into you yet, just imagine this: from the moment
that it was created, it has never left its orbit.
It is literally the most pristine object we know in the Solar System so far!
Isn’t it amazing that from all the discoveries mankind has achieved over the years, this
is the first time that we have seen something as awesome as this?
The images from the New Horizons spacecraft transmission also revealed Arrokoth’s color
to be mostly red.
This is due to the major composition, which is a complex organic compound called tholins.
What’s so interesting about the tholins is that they are present in a lot of other
celestial objects in the Solar System.
What this tells us is that the same compounds that formed our Solar System family also formed
the planetesimals.
When organic compounds are exposed to sunlight, they tend to break apart due to its energy.
Later on, these broken down parts gather together and create more complex compounds, specifically,
tholins.
Keep in mind that this is a very sensitive process, since the environment can get really
hot and the tholins wouldn’t form.
In the case of Arrokoth, it has tholins most probably because of its position in the outer
region of the Solar System.
The absorption spectra of Arrokoth tells us that this is the most dominant compound there,
followed by methanol, H20, and some unknown compound.
Which is kinda weird considering that KBO’s (Kuiper belt objects) usually contain some
decent percentage of ammonia, acetylene, and some volatile elements.
It’s so weird that it’s closest matching object -- in terms of chemical composition
-- was one of Saturn’s moons.
The surface of Arrokoth is also particularly way smoother than the surface of other heavenly
bodies.
The only evidence of impact on its surface was Maryland, a crater that’s about 4 kilometers
from end to end, but that’s about it.
This tells us that Arrokoth didn’t really experience a lot of stress while it’s orbiting
around the Sun.
Although there were small troughs and crests that are visible, this might only be an indication
of an internal structural collapse due to, maybe, vapors or gas coming out from inside
Arrokoth.
Let’s talk about density next.
To be able to gauge an object’s density, spacecrafts measure the gravitational effects
of the object of interest, say an asteroid, and then calculate the mass backwards, and
consequently have the mass.
However, Arrokoth, being already weird as it already is, didn’t exert that much gravitational
force to New Horizons, so the calculations couldn’t be precise or maybe even couldn’t
be done at all, with such a high room for error.
Now, if somehow they have found an object that used to orbit Arrokoth, it would lead
the scientists closer to the goal, but that’s just wishful thinking.
The object’s density would tell you it’s a bad candidate if you’re aiming to build
a gravitational machine.
Now, we know a lot of awesome stuff about our newest discovery, Arrokoth.
But how exactly does this object serve as a time capsule for the Solar System?
To answer this question, we have to recall what the current understanding is on how planetesimals
form in our Solar System.
About 4.5 billion years ago, our Sun was still a protostar surrounded by a swirling cloud
of dust particles and clouds.
As these particles move fast, they bump into each other, and due to gravity, cluster together
to form bigger pebbles.
This process continued to scale up until planetesimals such as asteroids, comets, and even planets
like our very own Earth were formed.
As you can imagine, this process requires a lot of energy and masses moving in high-velocities.
Now, this is where Arrokoth fits into the picture.
If we imagine high speed collisions, it is most likely that two objects crashing onto
one another would break, but from the looks of Arrokoth, this is obviously not the case.
Instead, we have a snowman looking formation.
According to the team studying this object, it is possible that the two lobes of Arrokoth
were the ones that formed first.
However, unlike the high intensity, high stress process of planetary formation, Arrokoth formed
in an opposite process.
Let’s go back to a billion of years ago once more, as we did earlier.
At the outer region of the protoplanetary disc, the dust particles were moving at really
slow speeds.
Of course, it is definitely possible that some of these particles collide and stick
with one another, forming up pebbles.
As they continue to move, a few of these pebbles might have clustered together to form the
two lobes of Arrokoth, Ultima and Thule first.
It is possible that the two lobes might have shared an orbit together, forming a cute little
binary system.
As time went by, the two lobes lost angular momentum, and slowly came into contact with
one another.
It might be because of the density of gas in that region, or either of the lobes could
have possibly had satellites, which, because the density of either lobes were small, eventually
escaped its orbit.
Whatever actually happened gave birth to this weird Kuiper belt object.
How do we know this to be true for Arrokoth?
We can go back to its characteristics to answer this question.
If we can remember, the surface of the object is relatively smooth.
We have already mentioned earlier how a violent environment couldn’t have resulted in a
smooth surface.
There is also the density of Arrokoth being really low.
This is another indicator of low velocity collisions resulting in object formation.
There is also the alignment of the longest axes of Ultima and Thule.
In high speed collisions, we wouldn’t expect this to happen.
Unlike when objects are formed at really low speeds, this is exactly what we expect to
happen.
This is exactly what the time capsule held by Arrokoth provided to our current understanding:
Back then, we thought that a lot of planetesimals in our Solar System required a stressful environments,
but, as was demonstrated by the characteristics of Arrokoth, we now know that objects can
also form under opposite conditions.
This solves a lot of questions especially in the case of Kuiper belt objects.
Scientists, for a long time, have been finding objects with some characteristics of Arrokoth,
but this study really puts the nail into the coffin.
We can say that we now have a better understanding of our Solar System, particularly its amazing
history.
So what happens next for New Horizons after accomplishing this fly by?
To the current day, New Horizons still has enough fuel to do another one this year, but
they are still looking for suitable objects in the Kuiper belt that they can photograph.
There’s also the task of taking an image of the Earth from this far in the Solar System.
Regardless of what new knowledge comes next, you can count on us to cover all of these
new cool stuff that will satisfy the thirst of our astronomy freaks out there!
That ends our episode for the amazing Arrokoth, holding secrets about the history of our Solar
System!
What Kuiper belt object do you have in mind that New Horizons could possibly fly by and
take a photograph of?
Do you think there are more secrets about the system that were in that we still can
find out?
Let us know in the comment section below!
We’re always looking forward to reading comments from you guys.
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Till next time, stay insanely curious!