Three Atlas, the most intriguing cosmic
visitor we've ever had, is leaving the
solar system. This comet has been
entertaining us for a little under a
year now. And in that time, we've seen a
flood of new research, hundreds of
papers published, speculations over it
being an alien invader squashed, and
observed the building blocks of a
distant planetary system up close for
the first time. Surely, one interstellar
interlo can't give us more than that.
Well, it turns out it can. As Threei
Atlas rounded the sun, it exploded to
reveal its inner secrets.
3II is now on its way out past Jupiter's
orbit and slipping beyond the limits of
our instruments. And once it's gone,
it's gone for good. So, this is it. It's
time to take stock of the spectacular
science of this final stretch of three
Atlas's journey through the solar
system.
I'm Alex Mccoan and you're watching
Astramm. Join me today as we head back
to ThreeI Atlas for one last time to
dissect the groundbreaking research that
has emerged since it passed Earth last
December. From the discovery of even
more jets to a rare cosmic alignment and
explore the final courseing encounter
that awaited this weary traveler at the
edge of Jupiter's influence.
As I'm sure you already know, we've been
charting three Atlas's course from the
moment it was first spotted. I've
already made three videos on it myself,
and you can watch the latest one here.
There have been hundreds of
observations, papers, and discoveries
made while three Atlas passed through
our neighborhood, but the last few
months have definitely been the most
dramatic. In late December 2025, as the
Atlas drifted towards its perigee, or
the closest point to Earth, it did not
forget to bring some Christmas gifts
with it. At a distance of 1.8
astronomical units, nearly 270 million
km from us, 3i Atlas put on an explosive
show. Observations from NASA's Sphere X
mission captured the comet fullon
erupting. Usually water ice remains
stable in the deep cold of space. But
once this comet got closer to the sun,
its ice started sublimating, turning
from solid straight into gas in massive
quantities. And the trigger for this
explosion had been building for months.
To understand what was going on, we need
to look beneath the surface. Comets are
roughly 1/3 water ice, much of which is
buried deep in their cores, shielded
from the harsh energetic radiation of
the sun. Even after 3i Atlas crossed its
perihelion in October, that heat hadn't
yet penetrated deep enough to warm the
interior. Think of it like a baked
Alaska. The meringue on the outside was
insulating the frozen ice cream middle.
It wasn't until nearly 2 months later
that the sun's immense energy finally
reached the comet's icy interior and it
began to vaporize violently erupting
from three atlas in huge jets. And
within those jets was more than just
water. Methane, methanol, ethane,
formaldahhide. These were the chemical
fingerprints of the ancient star system
where it formed. It appears that Atlas
had packed an entire hydrocarbon lab for
its interstellar journey. The result was
a dramatic surge in activity with water
emissions showing a tremendous 40fold
increase. And this massive halo of gas
did more than just make the comet
brighter. It offered a vast dense target
for fastm moving particles from the sun
to slam into, creating the perfect
conditions for a phenomenon that
scientists had long been looking for,
but that had so far eluded all attempts
at detection. You see, comets that
originate in our solar system usually
have a telltale X-ray signature. As the
sublimated gases in their coma interact
with the solar wind, a process called
charge exchange reaction occurs,
emitting characteristic X-rays that we
can detect on Earth. However, every
single attempt to detect X-rays coming
from Amuam and two I Boris, the two
interstellar objects that preceded this
one ended without any positive results.
Would ThreeI atlas be any different?
Well, as we have seen countless times so
far, this particular object is quite
fond of surprising us, and it did not
disappoint this time either. Images
taken by Jax's Chrysom satellite in late
November 2025 revealed a faint X-ray
emission signature around the comet
nucleus, marking the first time we have
ever detected these wavelengths from an
interstellar visitor. The data suggests
3EI Atlas could be surrounded by a
diffused cloud of gas extending up to
400,000 km from its nucleus. For
context, that is larger than the average
distance between the Earth and the Moon
and wide enough to fit every planet in
our solar system side by side. And this
discovery goes further than just a
simple first detection. It shows that
even interstellar comets interact with
the solar wind in much the same way as
those in our own system. And in doing
so, it gives us a rare, brand new way to
probe any future visitors. It's amazing
a try. Now, back to learning about ThreeI Atlas.
If a giant X-ray aura wasn't enough, 3
Atlas's heartbeat also underwent a
dramatic shift after perihelion. Comets
have an intrinsic spin, a rotation that
can be measured by watching how their
brightness fluctuates over time. Because
three atlas has multiple jets fanning
out into space, its rotation can cause
almost a 30% variation in its brightness
as seen from Earth. This creates a
rhythmic pulsing, a lighthouse effect
that repeats every few hours, almost
like a long drawn out heartbeat. Before
Parihelion, 3 Atlas had a rotational
period of 16.2 hours. But after passing
the sun, something changed. New
measurements from December 2025 revealed
that its rotation had sped up. Its
period was now just 7.1 hours, meaning
3i Atlas was spinning more than twice as
fast. This spin-up was likely a direct
consequence of its orbit around our sun.
As Threei Atlas approached our star, the
intense heat triggered even more
outgassing and activated new jets. These
jets applied enough torque or rotational
force to the nucleus to wind it up, much
like how astronauts use thrusters in
their space suits to guide their
movements.
Soon after this boost, as our traveler
continued its journey, it headed
straight towards another giant of the
solar system and passed us along the
way.
Ever since we first spotted this
interloper, astronomers worldwide have
been trying to figure out exactly how
big its nucleus is, and early estimates
were a bit all over the place. Figures
for its diameter in those first few
months ranged from as small as 320 m to
as large as 5.6 km.
But shortly after we rang in the new
year, an exceptional alignment finally
put this mystery to rest.
At any point in Earth's orbit, you can
draw an imaginary line from the center
of our planet to the center of the sun.
This is the Earth sun axis. On the 22nd
of January 2026, 3i Atlas came within an
incredibly narrow 0.69°
of this line. This alignment was an
unprecedented opportunity.
Now, because the sun was directly behind
us, there were no shadows cast by three
eye atlases dust particles. Usually,
these shadows would reduce its
brightness, but in their absence, the
comet was lit headon. On top of that,
the light waves reflecting off that dust
combined in what is called constructive
interference, leading to a massive 20%
increase in brightness. This spiking
brightness finally allowed scientists to
isolate the nucleus from the glowing
shroud around it. The Hubble Space
Telescope, which had previously observed
the comet back in August 2025, took
another crack at estimating its size,
and this time it was successful. We now
know that three Atlas's nucleus has a
radius of 1.3 km, which is roughly half
of the previous upper estimates. And not
just that, the opposition also gave us
the perfect opportunity to map out 3
atlas's surface features. By using
special filters to remove the symmetric
glow of the coma, scientists isolated
four distinct jets shooting out from the
core. The most prominent was the Sunwood
jet we had seen before, except now it
was also pointing directly at us. This
was joined by three smaller mini jets.
Curiously, these were positioned almost
exactly 120° apart, and that symmetry
raised eyebrows. To some, it looked
almost too precise, fueling even more
speculation that this might be something
more than a natural object. But in
reality, such patterns can emerge from
the rotation and internal structure of
the comet itself. Even so, the idea of
alien origins once again captured
attention. And coincidentally,
instruments designed to search for life
elsewhere were already observing the
comet.
Three Atlas wasn't just teaching us
about different star systems. It was
also letting us test our technological
limits through opportunistic
observations.
These were unplanned, unexpected
opportunities to observe the comet using
spacecrafts and probes already scattered
across our solar system.
Right when we were blocked out from
Earth-based observations, these robotic
helpers came to the rescue. As I
discussed in my last video about the
comet, missions such as NASA's SOHO,
issa's Mars Express, and even the
Perseverance rover caught crucial
glimpses of the comet right when Earth
was blinded. And they weren't the only
ones. Even missions still on their way
to their targets, deep in their cruising
phase, were commanded to wake up and
turn their sights towards this visitor.
One such mission was the Europa Clipper.
On its way to uncover the secrets of
Europa's hidden oceans.
On the 6th of November 2025, the probe
was tasked with 8 hours of continuous
imaging of three Atlas. The spacecraft
on its billion km journey was paused,
reprogrammed, and redirected all to
study a visitor no one had known was
coming. And the Europa Clipper did not
disappoint. It beamed back this UV image
of the comet taken when the spacecraft
was about one astronomical unit from
ThreeI Atlas. While this might not look
like much to an untrained eye, to a
scientist it is a spectrographic
treasure. This image was one of the very
first direct confirmations that comets
from other planetary systems contain the
same volatiles that we expect to see in
ours. It detected faint dust structures
in three Atlas's tail and confirmed the
presence of hydrogen and oxygen, the
telltale signatures of water ice
sublimation.
And Europa Clever wasn't alone. ESA's
Jupiter Icy Moons Explorer or Juice, the
mission bound for Jupiter's Moons,
beamed back its own images taken on the
very same day on the 6th of November. As
you can see from how bright Atlas looks
here, Juice was a lot closer. From a
distance of 66 million km, it took 120
images of 3 Atlas using five of its
instruments. Now, you might wonder why
we're only just discussing and analyzing
these images that were taken in November
last year. Well, due to its orientation,
Juice is currently using its massive
main antenna as a heat shield, meaning
it had to rely on a much smaller
secondary antenna to send this data
home, leading to a painfully slow
transmission rate. The full extent of
what Juice observed in November 2025
only made it back to Earth by February
2026.
The data, however, was worth the wait.
Look at this video made from 53 images
of three Atlas cruising through space.
This was taken by Juice's Nav Cam or
navigation camera. As the name suggests,
this camera is more of a guide for the
spacecraft, meant to snap images of
Jupiter's moons only to be processed on
board and used to update the trajectory.
It was never really meant for complex
space science, let alone observing
interstellar comets. So, this really is
a case of right place, right time.
In another nav cam image, we clearly see
the glowing coma of gas around 3II
Atlas. The plasma tail, a long
projection of charged fast particles,
can also be seen stretching out from the
comet, and so can its dust tail. And if
these two Jupiter explorers weren't
enough, more missions like NASA's Parker
Solar Probe and the transiting exoplanet
Survey Satellite, or TESS, also joined
the effort.
The Parker probe, speeding away from the
sun during its 25th solar flyby,
captured 10 images of Syria Atlas every
day for 18 consecutive days, giving us
these incredible videos of the comet.
But if you felt a bit dizzy trying to
spot the comet in these shots, don't
worry. TESS has you covered. This wasn't
the first time TESS had seen ThreeI
Atlas. Back in May 2025, it had
unknowingly observed the comet 3 months
before its official discovery. But this
time, it knew exactly what it was
looking for. Using 28 hours of test data
from mid January, MIT's Daniel Mutrishna
compiled a series of images into a video
that showi atlas as a bright dot with a
tail moving steadily against a backdrop
of distant stars. Together, these
observations represent something rare. A
moment where the entire space community
came together. NASA, issa, JAXA, ISRO,
multiple agencies, multiple instruments,
all focused on a single visitor. Each
mission contributing its own piece to
the puzzle. And together, they have
revealed how an interstellar comic like
ThreeI Atlas behaves. But to understand
what it is, we have to ask a different
question. Where did it come from?
This is where things get a bit more
complex. We can't really reconstruct the
comet's travel history more than a few
million years into the past. So, we have
to resort to what we can see, the
physical and chemical features to make
educated guesses about 3 Atlas's home
and age. Back in August 2025, one of the
first estimates of ThreeI Atlas's age
came from something called the age
velocity dispersion relation.
Essentially, the relationship says that
the older objects in the Milky Way are,
the faster they travel through space.
3II Atlas was clocked at more than
200,000 km/h, nearly twice as fast as
the Earth is moving and 2.2 times Amuam
Mua's velocity. This meant that threeey
atlas was ancient.
A study led by Aster Taylor from the
University of Michigan estimated its age
to be between 3 and 11 billion years
old. But a more recent analysis led by
NASA's Martin Cordner and based on the
comet's carbon isotopes pushed that
estimate even further between 10 and 12
billion years old. That is just 1.8
billion years after the Big Bang. It is
entirely possible that the star that
birthed this comet is already dead.
So, we have somewhat of a range of how
old this comet could be. But where in
our galaxy did it come from, and what
kind of star did it orbit?
Back in August 2025, Matthew Hopkins and
his team wanted to answer exactly these
questions. They used Gaia's catalog of
nearly 2 billion stars and reconstructed
trajectory of 3 atlas to project a
radiant into the galaxy. essentially the
path that the comet took to get to our
solar system. Their results suggest that
Threra atlas came from the region of the
galaxy called the thick disc. First
identified in 1982, the thick disc of
the Milky Way is an over density of
stars that is spread far above and below
the galaxy's main plane. A region much
more extended than the thin disc or the
flatter region where most stars,
including our sun, reside.
The stars in the thick disc are some of
the oldest in the galaxy. Almost all of
them are more than 10 billion years old
with low metalicities similar to those
we see in older population stars. And
because they have low metallicities,
Hopkins expected to see a higher water
content in three Atlas and predicted a
massive outpouring of water as the comet
got closer to the sun, which we now know
was right. 3i Atlas was spewing water
like an open fire hose at a rate of 40
kg per second and hidden within. This
was a crucial clue.
A paper released in March 2026
investigated the detium enrichment of
the water released by 3II Atlas. Detium
is a heavier isotope of hydrogen and
studying the detium to hydrogen ratio of
an object acts as a fingerprint of where
an object was born. 3i Atlas's detium to
hydrogen ratio was more than 40 times
higher than what we find in Earth's
oceans and more than 30 times what we'd
expect to see in solar system comets.
This is highly unusual. Such high levels
of detium are typically seen in only the
coldest regions of the Milky Way,
suggesting that Threeey Atlas was likely
formed in the frozen outer fringes of
whatever star it was orbiting, which
would also make it more prone to being
kicked out of said stars system. But
what did three Atlas actually do for
those billions of years since it left
its home star system? If we could
hypothetically trace its trajectory back
billions of years, what could we expect
to see? Surely in that time, given the
trillions of stars in our galaxy, three
Atlas would have flown by another star.
After all, our sun can't be the very
first star three Atlas has ventured so
close to.
In September 2025, scientists tried to
find a definitive answer to this
question, looking for, in their words,
the witnesses to ThreeI Atlas's voyage.
They integrated its orbit back in time
for 10 million years and using Gaia's
catalog tried to find out if it had
indeed encountered any odd stars on its
journey. And the answer is yes, but not
in the way you might expect. They
identified 62 significant encounters all
where threeey atlas had passed within
two parexs or 6.5 light years of a star.
Remarkably, none of these interactions
were strong enough to meaningfully
change its path. The strongest of these
was a K-type star 70% the mass of our
sun and even that produced a change of
just 50 cm/s in the comet's velocity. So
3i Atlas is largely untouched. Our sun
might be the first star it has
encountered this closely. The first star
to truly alter its orbit. And our sun
wasn't the only local object to give it
a tug. On the 16th of March 2026, Threer
Atlas passed within roughly 0.35
astronomical units or 53.6 million km of
Jupiter. That might sound like a
comfortable margin on human scales, but
in celestial terms, this was a near
miss. The distance placed throughout
Atlas incredibly close to Jupiter's
hillphere, the region of space where the
planet's gravitational pole is stronger
than the sun's. As the comet swept by,
Jupiter's immense gravity gently tugged
at its trajectory. Not enough to capture
it or tear it apart, but just enough to
alter its course. Early orbital
reconstruction suggested a deflection on
the order of a few meters/s.
That might seem insignificant, but out
here in the slow ballet of the Milky
Way, even the slightest nudge can
reshape a journey that spans billions of
years. This was in all likelihood the
single most significant planetary
interaction three Atlas has ever
experienced since it was first ejected
from its home system.
Researchers are still combing through
the data. In the coming months, we can
expect a wave of detailed studies
reconstructing this encounter in full
and telling us exactly what happened to
this interstellar comet at the very edge
of Jupiter's domain. At the time of
writing, this object has had nearly 500
papers and studies dedicated to it.
Thousands of articles, millions of eyes
hooked onto news channels waiting to
find out more about a rare visitor from
deep space. But here's the kicker.
Astronomers now estimate that these
visitors might not be rare at all. In
fact, calculations suggest that there
are likely hundreds of them passing
through our solar system at any given
moment. We simply don't have the eyes to
see them. With our detection technology
finally catching up, the data suggests
we should be spotting a new interstellar
visitor every 1 or 2 years. It might not
be long before I'm back here talking to
you about four eye.
To the unwitting eye, Three Atlas might
just seem like another space rock, and
they wouldn't be wrong. But through our
Atlas is also a relic of a distant
world. A fragment that formed in a giant
swirling protolanetary disc of a star
likely trillions and trillions of kilome
away. It is a fragment that was ousted
from its home way before anything around
us, including our own star, had even
begun to exist. And for a brief moment,
it held the attention of an entire
planet as it rushed past us like a
cosmic bullet. Three eye atlas is
heading back into the dark now, but the
story it told us may only just be the
beginning.
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