﻿If you haven’t already heard,
our solar system is currently host to a strange and intriguing visitor.
Comet 3I/ATLAS burst onto the astronomical scene in early July 2025,
when it was first identified by the NASA-funded ATLAS survey telescope,
or Asteroid Terrestrial-impact Last Alert System, operating in Rio Hurtado, Chile.
Since then, scientists across the globe have assembled to collect
every scrap of information they can get about this elusive entity,
leading to a flurry of articles, alerts, papers, (and controversies)
about the first interstellar object to enter our cosmic backyard in six years.
But amid this media frenzy, what have we actually learned?
What information about this comet is true, and what lies in the realms of science fiction?
I’m Alex McColgan, and you’re watching Astrum.
Join me in this video as I give you a much-needed update on 3I/ATLAS,
piercing through the noise and getting to the heart of what makes this visitor so special,
just in time for its 30th of October perihelion.
If nothing else, the buzz around this comet proves how dynamic the world of science research can be.
When I made my first video about 3I/ATLAS back in July,
there were only around 4 published research papers naming it.
Now, at the time of writing in September, there are nearly 70.
That represents a huge number of new discoveries, verifications,
and boosts in our collective knowledge about this object,
but before I dive into the newest findings, let’s do a quick recap of where we left off…
In my previous video, I discussed the initial discovery of comet 3I/ATLAS,
named for its status as the third interstellar object to visit our solar system,
and the ATLAS survey telescope that identified it.
I compared this comet to the famed ‘Oumuamua,
which made its way through our solar system in 2017,
and Borisov, which was discovered in 2019.
In contrast to its two interstellar predecessors, 3I/ATLAS is thought to be much faster,
travelling at a whopping speed of 58 km per second,
and older, with estimates that it is somewhere between 3 and 11 billion years old!
That was a speedy summary, so you should definitely check out my first
video - which you can watch at the link here - for more in-depth information.
But since 3I/ATLAS made its debut on the channel, we have learned a lot more about it, thanks to
countless hours of observation and measurements from astronomy facilities across the globe. So,
what’s the latest information to come from this researching spree?
With the worldwide recognition that 3I/ATLAS has attracted since its discovery, it has managed to
turn the heads of two very prestigious telescopes… It’s famously difficult to schedule observation
time with NASA’s Hubble Space Telescope, with scientists having to submit proposals months in
advance to argue their case for every minute of the telescope’s time. But luckily for 3I/ATLAS,
Hubble always reserves some time for so-called “Targets of opportunity” - things that need to
be observed before they move out of our reach - giving this unexpected
interstellar visitor a free pass to skip the queue. And it was definitely worth it.
Hubble captured this image of 3I/ATLAS on the 21st of July, when it was 446 million km from Earth.
You can get a sense of how fast the comet is travelling from the streaky stars in the
background, and see the teardrop shape of the dust coming off its icy nucleus. These observations
also allowed Hubble scientists to make estimates for the size of 3I/ATLAS’s nucleus, measuring it
between 440 m and 5.6 km in diameter. That makes it a lot smaller than we originally thought,
since original estimates of its size assumed that the comet’s reflectance was coming entirely from
the nucleus, not the coma. But now that the dust has cleared, we can see things in finer detail,
and future observations may be able to pin down these measurements even more.
The other major player that has turned its attention to 3I/ATLAS is the James Webb Space
Telescope. On the 6th of August, scientists used JWST’s near-infrared spectrograph in
an attempt to determine the composition of this comet, in a study led by astrochemist
Dr Martin Cordiner. They found a complex coma morphology, suggesting a range of
different outgassing patterns depending on the elements and compounds being released.
It seemed that carbon monoxide and carbon dioxide were sublimating more at this stage than water,
but this is likely to change as the comet gets closer to the Sun. And while usually comet tails
point away from the Sun, the researchers found a surprising amount of dust emanating from the comet
in the direction of the sun, caused by the dust grains fragmenting as they move further from the
nucleus, and in turn, reflecting more light. This process might be exacerbated by gas production,
as excessive carbon dioxide production could actually be “launching” the dust further away from
the nucleus. Pretty dramatic for a feature that appears to us as nothing more than a hazy glow…
Hubble and James Webb are not the only NASA telescopes getting in on the 3I/ATLAS-fun.
The Transiting Exoplanet Survey Satellite, affectionately known as TESS, has spent more
than 7 years conducting an all-sky survey to discover exoplanets orbiting around nearby,
bright stars. It uses four identical wide-field cameras to map the sky in 90 by 24-degree strips,
and has already discovered thousands of planetary candidates. But as it turns out,
TESS isn’t just good at detecting exoplanets, but smaller objects too. Objects like… comets.
In my previous video, I mentioned that the first official discovery of 3I/ATLAS was actually
pre-dated by observations going back to June 14th. However, astronomers working with NASA’s
TESS mission realised that they had observational data for this comet dated all the way back to May!
The comet was captured by TESS from 7th of May to the 2nd of June, and was made visible
in composite deepstack images. With this new data, astronomers had the chance to verify
the earlier measurements made from 3I/ATLAS, and make some new discoveries of their own…
In a paper published on the 11th of September, Adina Feinstein, John Noonan and Darryl Seligman
used data from TESS to measure the brightness of 3I/ATLAS. They found that the average visual
magnitude recorded by one of the TESS detectors was in line with previous estimates made by other
astronomers using the same data. But the comet was about to defy expectations in another way…
Between the 7th of May and the 2nd of June, the distance between 3I/ATLAS and the TESS spacecraft
decreased by 0.9 AU. From this change alone, the authors expected the object to get brighter,
since it would be closer to the Sun and could reflect more of its light.
They calculated that the comet’s flux (or the amount of energy received from it) would increase
by a factor of 1.5, corresponding to a magnitude of 20.5. But the object got much brighter than
that – its magnitude increased to 19.28! This shocking surge in brightness was the result of
3I/ATLAS increasing its flux by a factor of 5, not 1.5! Observations with the TRAPPIST North
and South telescopes reported similar brightness values, confirming there was no miscalculation.
With such a huge amount of energy radiating from this object, 3I/ATLAS is far more active than we
originally thought, and may have been active for a long time. Combining their magnitude values
with the Hubble estimates for nucleus size, the researchers found that 3I/ATLAS may have been
active when it was still a whopping 6 astronomical units away from the Sun! That’s near the outer
limits of where comets usually start outgassing, suggesting that 3I/ATLAS might have been releasing
something more exotic and hyper-volatile at this time, like nitrogen or hydrogen.
In my previous video I said that 3I/ATLAS had a flatter light curve than Oumuamua,
which suggests that it reflects light more evenly across its surface. Well, Feinstein
and her colleagues came to a similar conclusion when they tried to extract a light curve for this
comet using the TESS data. In fact, they struggled to find any patterns in the light curve at all,
so they couldn’t conclusively identify the comet’s rotation period. They did identify
some periodic signals - one every 29 hours and another every 16 hours - but the data was noisy,
and TESS may not have been sensitive enough to measure this kind of parameter for a faint
object like 3I/ATLAS. Luckily, other studies using specialized ground-based telescopes agree
with the 16-hour spin period, so we can be more confident of the comet’s shape and how it rotates.
I’ve only covered space telescope discoveries so far,
but discoveries about 3I/ATLAS have been made by telescopes back on Earth, too.
On the 27th of August, researchers working with the Gemini South telescope in Chile
used the Gemini Multi-Object Spectrograph to visualise 3I/ATLAS in multicolour. Amazingly,
the images captured in this session showed not just the coma around the comet’s nucleus,
but its tail! The tail was measured to span about 1/120th of a degree in the sky,
which is significantly longer than it appeared in previous images using the
Gemini North telescope back in July. This is further proof that 3I/ATLAS is becoming more
active as it approaches perihelion, and hints at exciting developments to come. Who knows how
much more the tail will grow before the comet reaches its perihelion on the 30th of October?
The August session also collected the spectrum for 3I/ATLAS. In this recording,
you can watch the moment when Bin Yang, assistant professor at Diego Portales University, identified
an emission feature in the CN band for the comet, proving that it was releasing cyanide molecules!
This happens when water sublimates upon exposure to sunlight, releasing hydrogen cyanide from the
comet, which is then broken apart by sunlight into its constituent elements. We often start
to see this when a comet is between 2.5 and 3 astronomical units from the Sun,
which is exactly where 3I/ATLAS now finds itself.
Other observatories have affirmed this finding, as the once-weak CN signature is steadily
strengthening over time. A paper published at the beginning of September inferred that
the comet was releasing 800 sextillion, or 8 x 10^23, cyanide molecules per second when it was
just over 3 AU from the Sun, which is actually on the lower end of the scale for many comets in our
solar system. By measuring the ratio of di-carbon (C2) to cyanide in the coma, the authors placed
3I/ATLAS as one of the most carbon-chain depleted comets we know of. But what does that mean?
Carbon-chain depletion occurs when a comet’s coma or tail is lacking in longer chain carbon
molecules like C2 (di-carbon), or C3, (tri-carbon). This feature is used to
classify comets found across the solar system and is thought to reflect the conditions in
which they were first formed. So, using an upper limit for the comet’s C2 production,
scientists concluded that 3I/ATLAS produces less than 0.09 C2 molecules
for every CN molecule, and so it is carbon-chain depleted.
Interestingly, Borisov also shared this feature, so this may point to a pattern
in the composition of these comets. However, it’s important to note that the interstellar
sample size is still very small, so we should try to avoid over-generalising. Either way,
we should keep an eye on 3I/ATLAS as it approaches our Sun, as di-carbon production could rise,
the comet could start outgassing more H2O, or something even more dramatic could happen.
As astronomer and comet expert Karen Meech has said, these comets are “really very fragile,
dirty snowballs”, and can often break apart. If that happens, we could see a
renewed bout of emissions as the break exposes untouched water ice to the Sun’s radiation.
In recent days, researchers have analysed not only how 3I/ATLAS emits light, but how
it polarises light. On the 5th of September, a paper was published by Zuri Gray and colleagues
presenting the first-ever polarimetric observations of 3I/ATLAS, and by extension,
the second-ever observations of this kind for an interstellar object, the first being Borisov.
Polarimetry is used specifically to probe the physical properties of space objects,
like the size of their dust grains, the structure of their surface regolith,
and the shape of their coma, in a way that cannot easily be done with
other techniques. Gray and her colleagues used data from telescopes across Chile,
the Canary Islands and Bulgaria, and managed to discover something about 3I/ATLAS that
set it apart from any other interstellar object, asteroid or comet we’ve seen before…
They found that 3I/ATLAS has an exceptionally deep and narrow negative polarisation branch.
That may not sound like much, but it means that 3I/ATLAS is scattering light in such a way that
has never been seen before for any other comet or asteroid in our solar system. This behaviour
sets it apart from even Borisov, which showed positive polarisation of light, and could place
it in a distinct class of small planetary bodies that we know next to nothing about.
To give some more context, particles in and around space objects, like dust or gas, can cause
incoming light to scatter in specific directions. This is known as light polarisation, and it can
be measured by telescopes here on Earth. Experts look at the scattering plane, which is the line
formed when looking from the observer to the comet to the Sun, and record whether the direction of
polarisation is perpendicular to the scattering plane (positive), or parallel to the scattering
plane (negative). So, the deep and narrow negative polarisation of light from 3I/ATLAS means that
the light is being strongly polarised in a very localised, parallel direction to the scattering
plane. This pattern of polarisation is somewhat comparable to some Trans-Neptunian Objects and
Centaurs, but further research is needed to pin down exactly why this might be the case.
If you thought the news-worthy observations of 3I/ATLAS were reserved for academics,
you’d be wrong – science really is for everyone, and this next discovery proves why. From the 7th
to the 8th of September, parts of the globe were treated to a stunning lunar eclipse,
or “blood moon”, as it passed through the Earth’s shadow. Naturally, this was
a great opportunity for amateur astronomers to dust off their telescopes and have a look,
but some space-enthusiasts weren’t just looking at the moon.
Michael Jäger and Gerald Rhemann were taking photos of the night sky over Namibia when
they noticed our friend 3I/ATLAS, coloured with a surprising yet beautiful green hue.
Now, this finding hasn’t been verified by scientists at the time of writing,
but comets in our solar system can turn green when they begin to release di-atomic carbon (C2),
which gets energised by sunlight and fluoresces near 518 nanometres.
This behaviour may be normal for other comets in our solar system, but as you’ll remember, 3I/ATLAS
was recently found to be highly carbon-depleted, so it shouldn’t really have much C2 to fluoresce.
Maybe it’s a sign that C2 production has ramped up in recent days, but again, we’ll need further
research to confirm or deny this theory. Whatever the case, it’s a beautiful image,
and this latest mystery has captured the attention of amateur astronomers and scientists alike.
At this point in the video, there is still one major area of development in the news
surrounding 3I/ATLAS that we haven’t touched on yet, but it’s one I’m sure you’ve heard about.
The name “Avi Loeb” pops up a lot in the media coverage of this comet – and he has a lot to
say about where, or who, it came from. Avi Loeb is a renowned astrophysicist
and Harvard professor who has stirred up his fair share of controversy throughout
his career. His most recent obsession with 3I/ATLAS has received mixed responses from
his fellow scientists and the wider public. Back in July, Loeb was already publishing his
comments on the discovery of 3I/ATLAS, and at first, it all seemed pretty astrophysical. One
of the main points from this paper and from Loeb is that 3I/ATLAS is too large to be an asteroid.
Early estimates of the comet’s radius put it at around 10 km.
Loeb pointed out that this would give the comet a far greater mass than we would have thought
possible. Given our current understanding of how much material there is in interstellar space,
an object this large entering our solar system should be an extremely rare occurrence.
In light of this argument,
Loeb suggested that 3I/ATLAS may be smaller than we originally thought,
but he also suggested a controversial alternative: that alien technology could be the explanation.
Just days later, Loeb submitted another paper,
along with two collaborators representing non-profit organisations for interstellar
exploration. And the title of this paper? “Is the Interstellar Object 3I/ATLAS Alien Technology?”
While Loeb’s first paper only briefly floated the idea, this follow-up placed the alien speculation
front and center. In it, the authors conducted an analysis of the astrodynamics of the comet,
exploring whether it could be technological and possibly hostile.
They reference the “Dark Forest” resolution to the Fermi Paradox. If you’re not familiar,
we’ve covered this in previous videos, but the Fermi Paradox poses the question of why we
haven’t found aliens yet, considering the size of the universe and the number of stars that
share similarities with our Sun. The “Dark Forest” idea suggests that perhaps aliens are out there,
but are choosing to remain hidden, like prey would hide from a hunter.
The authors stress that they are exploring this idea for educational purposes, and because it
“is fun to pursue, irrespective of its likely validity”. So, let’s see what they had to say…
Firstly, in this paper Loeb still maintains that 3I/ATLAS is too large to be an asteroid.
As Loeb and his colleagues should know, estimates for any newly discovered object
are liable to change, especially as more independent measurements are taken and
results are peer-reviewed. After all, the 10 km value was estimated before we could
properly measure 3I/ATLAS’s coma, and as we now know, that coma is responsible for most
of the reflectance we see. A press release from the Hubble team on the 7th of August
said that 3I/ATLAS measured 5.6 km across at most, but Loeb continues to use the outdated
measurements in his alien hypothesis. In a Medium post on the 13th of August, Loeb
directly mentions the Hubble images, but still bases his argument around the older estimates.
My point is not that scientists should be banned from pursuing alternative lines of research,
but they should at least make sure that the research uses credible data. Especially for new
discoveries like 3I/ATLAS, where the state of our knowledge is still a constant work-in-progress.
As for Loeb’s other points, well, they’ve been rebutted by other scientists, particularly in two
papers by AKM Eahsanul Haque, a geoscientist working at Petronas University in Malaysia.
He rebuffs Loeb’s arguments for non-gravitational acceleration to achieve a non-coincidental flyby
of Venus, Mars, and Jupiter, by referring to the most recent trajectory data for 3I/ATLAS,
which can be explained far better with natural dynamics than engineered manoeuvres.
For many of Loeb’s critics, this is a lesson against jumping to conclusions.
As “fun” as it might be to cry “aliens”, we should at least wait for the dust to settle
on each new discovery before we start looking for whether it could be a sign of intelligent
extraterrestrial life. That way, we’ll be far more efficient at coordinating a response if,
and when, the evidence truly points to an alien encounter.
We’ve come to the end of our reunion with 3I/ATLAS, but as I’ve shown in this video,
this research moves almost as fast as the comet itself, so we’re by no means finished making new
discoveries. On the 30th of October, the comet will reach perihelion around 1.4 AU from the Sun,
though sadly we won’t be able to see it. At the time this video is released,
it will be too close to the Sun to observe, but don’t panic! It will re-emerge by early December,
ready for a renewed bout of in-depth analysis. Missions that may be able to observe the comet
include NASA’s SOHO, SPHEREx and MAVEN, not to mention continued observations from Hubble,
James Webb and the Gemini Observatory, so there will be plenty more to come.
As 3I/ATLAS takes a bow and disappears behind the solar curtain, it leaves plenty of questions
for us to ponder. How will its activity change as it reaches perihelion? Will it
break apart? How long has it been travelling to get here? And can we trace its elusive origin?
It’s a race against time to answer these questions, but astronomers are showing
no signs of slowing down, and I can’t wait to see what they discover next.
Solving the mystery of comet 3I/ATLAS is only possible by finding patterns in the data.
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