Does 3I/ATLAS Generate its Own Light?
Does 3I/ATLAS Generate Its Own Light?:
The best image we have so far of the new interstellar object, 3I/ATLAS, was obtained by the Hubble Space Telescope on July 21, 2025. The image shows a glow of light, likely from a coma, ahead of the motion of 3I/ATLAS towards the Sun. There is no evidence for a bright cometary tail in the opposite direction. This glow was interpreted as evaporation of dust from the Sun-facing side of 3I/ATLAS.
Figure 3 of the analysis paper (accessible here) [PDF] shows a steep surface brightness profile of the glow with a projected power-law slope of -3, which implies a three-dimensional emissivity profile with a radial power-law slope of -4. Such a slope is steeper than observed in solar system comets. Together with my brilliant colleague, Eric Keto, we realized that the observed slope of -4 is consistent with an alternative model in which the dust outflow around 3I/ATLAS is illuminated by a central source. This model naturally accounts for the steep brightness profile, since the outflow density slope of -2 is accompanied by the radial decline of the illuminating radiation flux with an additional declining slope of -2.
If 3I/ATLAS generates its own light, then it could be much smaller than expected from a model in which it reflects sunlight. The reflection model requires a diameter of up to 20 kilometers, which is untenable given that the limited reservoir of rocky material in interstellar space can only deliver such a giant rock once per 10,000 years or longer (see the calculation in my paper here).
Last night, we held the annual soccer cup match between the faculty and the students at Harvard's Institute for Theory & Computation, for which I serve as director. Although I scored 2 goals for the faculty team, the students won 3 to 2. Disappointed by the outcome, I focused on 3I/ATLAS as soon as I woke up the following morning.
First, I calculated that the luminosity of 3I/ATLAS needs to be of order 10 gigawatt. Second, I realized that the steep brightness profile around 3I/ATLAS implies that the nucleus dominates the observed light. This must hold irrespective of the origin of the light. In other words, the nucleus dominates over the emission from the glow around it.
The illumination by sunlight cannot explain the steep 1/R⁴ profile of scattered light, where R is the radial distance from the nucleus. This is because a steady dust outflow develops a 1/R² profile which scatters sunlight within the same emissivity profile. Sunlight would dominate the illumination in this model because a rocky nucleus would reflect only a small fraction of the solar intensity from a much smaller area than the 10,000-kilometer region resolved in the Hubble Space Telescope image. Another possibility for the steep brightness profile is that the scattering halo is made of icy particles that get evaporated as they move towards the Sun from the warm Sun-facing side of 3I/ATLAS. This would explain why there is no tail of these scattering particles. The required evaporation time must be of order 10 minutes but it is unclear whether this would lead to the observed 1/R⁴ brightness profile.
The simplest interpretation is that the nucleus of 3I/ATLAS produces most of the light. I calculated that the nucleus cannot be a thermal emitter with an effective surface temperature below 1000 degrees Kelvin or else its peak emission wavelength would have been longer than 3 micrometers with an exponential cutoff at shorter wavelengths, incompatible with the data. At higher effective temperatures, the required luminosity of 3I/ATLAS can be obtained from a source diameter smaller than 100 meters. A compact bright emitter would make 3I/ATLAS of comparable size to the previous interstellar objects 1I/`Oumuamua or 2I/Borisov, making more sense than the 20-kilometer size inferred in the model where it reflects sunlight.
I first calculated that a primordial black hole with a Hawking temperature of 1,000 degrees Kelvin would produce only 20 nanowatts of power, clearly insufficient to power 3I/ATLAS. A natural nuclear source could be a rare fragment from the core of a nearby supernova that is rich in radioactive material. This possibility is highly unlikely, given the scarce reservoir of radioactive elements in interstellar space.
Alternatively, 3I/ATLAS could be a spacecraft powered by nuclear energy, and the dust emitted from its frontal surface might be from dirt that accumulated on its surface during its interstellar travel. This cannot be ruled out, but requires better evidence to be viable.
Insisting on 3I/ATLAS being a natural object, one might consider the hypothetical case of an object heated by friction on an ambient medium. In this case, the momentum flux of the dust flowing out of the object must exceed the momentum flux of the ambient medium in the rest frame of the object, the so-called ambient ram pressure. Otherwise, the dust outflow would be suppressed by the ambient medium. What does this condition boil to?
Given the mass loss rate (6–60 kilograms per second) and ejection speed of dust (20–2 kilometers per second) that were inferred from the Hubble Space Telescope image, I calculated that this model is marginally ruled-out. In addition, the required ambient medium density is larger by many orders of magnitude than the mass density of the zodiacal gas and dust through which 3I/ATLAS is traveling as it traverses the main asteroid belt.
This leaves us with the interpretation of the brightness profile around 3I/ATLAS as originating from a central light source. Its potential technological origin is supported by its fine-tuned trajectory (as visualized here and discussed here).
The new interstellar object 3I/ATLAS is expected to pass within a distance of 28.96 (+/-0.06) million kilometers from Mars on October 3, 2025. This would offer an excellent opportunity to observe 3I/ATLAS with the HiRISE camera near Mars, one of six instruments onboard the Mars Reconnaissance Orbiter. This morning, I encouraged the HiRISE team to use their camera during the first week of October 2025 in order to gather new data on 3I/ATLAS. They responded favorably. It would be challenging to observe 3I/ATLAS from Earth around the same time because of the proximity of 3I/ATLAS in our sky to the direction of the Sun. The more data we collect on 3I/ATLAS, the closer we will get to understanding its nature.
3I/ATLAS
This is a Hubble Space telescope image of the interstellar comet 3I/ATLAS. Hubble photographed the comet on 21 July 21 2025, when the comet was 365 million kilometres from Earth. Hubble shows that the comet has a teardrop-shaped cocoon of dust coming off its solid, icy nucleus. Because Hubble was tracking the comet moving along a hyperbolic trajectory, the stationary background stars are streaked in the exposure.
[Image description: At the center of the image is a comet that appears as a teardrop-shaped bluish cocoon of dust coming off the comet's solid, icy nucleus and seen against a black background. The comet appears to be heading to the bottom left corner of the image. About a dozen short, light blue diagonal streaks are seen scattered across the image, which are from background stars that appeared to move during the exposure because the telescope was tracking the moving comet.]
3rd Interstellar Comet Just Formed a Bizarre Tail and More Updates
Youtube Video: 3rd Interstellar Comet Just Formed a Bizarre Tail and More Updates [15m33].
[This video has NOT been reviewed by staff.--JR]
« Revolutionary AI Tech Breathes Life Into Virtual Companion Animals | Canadian EV Sales Collapse by 35% as Gas Car Purchases Surge »
Related Stories
A new analysis of our solar system's interstellar interloper, 3I/ATLAS, reveals that it's spewing huge amounts of water — and astronomers can't immediately explain why:
The object, which is widely believed to be comet, showed strong ultraviolet emissions that are unmistakable telltales of hydroxyl gas (OH), a byproduct of water, when astronomers imaged it with NASA's Neil Gehrels Swift space telescope before it disappeared behind the Sun. The emissions could only be spotted from space because the ultraviolet light would get absorbed in the atmosphere.
Their findings, detailed in a new study published in The Astrophysical Journal Letters, argue that the presence of all this OH indicates the comet is ejecting water vapor at a torrential rate of about 88 pounds per second — around the same rate as a fire hose running at full blast, according to a press release about the findings.
[...] Astronomers believe that 3I/ATLAS came from the center of the Milky Way, where it was likely booted out of its original star system by a gravitational disturbance like the close flyby of another star, braving interstellar space before eventually cruising through our solar neighborhood. Based on these inferences, astronomers estimated that the comet must be billions of years old, perhaps three billion years older than the Sun itself. It's not only a snapshot of a different part of the galaxy, but a different era of the cosmos altogether.
Previously: Does 3I/ATLAS Generate its Own Light?
(Score: 4, Insightful) by JoeMerchant on Tuesday August 26, @01:09PM (2 children)
>which is untenable given that the limited reservoir of rocky material in interstellar space can only deliver such a giant rock once per 10,000 years or longer
Why can't we have a once per 10,000 years event now? Yes it's low odds, but still higher odds than the speculative frequency calculation that this probability is based on being correct.
🌻🌻🌻 [google.com]
(Score: 2) by VLM on Tuesday August 26, @07:17PM (1 child)
Not enough people are looking into this aspect of the story.
We have direct observational evidence for a couple decades that at most we get less than one per decade. Certainly, we don't get more than 1 per decade because we haven't seen any this large or weird for some decades of looking. Perhaps one sailed by in 1950 and nobody caught it. We can't say for certain that one didn't fly by in 1800.
Observational data shows its rarer than once a decade, but this 10K-year stuff comes from statistical analysis of the distribution of smaller ones thats pretty fuzzy logic, apparently.
A lot of that statistical stuff assumes randomness where randomness may not exist. For example you could look at just a couple roulette wheel spin results to estimate remarkably accurately how many "spots" there are on a wheel based on imaginary distribution models and perfect randomness, but nothing statistically could prepare you for the inevitable eventual surprise introduction to the "double zero" spot on the roulette wheel. That's because the distribution of spots on the wheel, and apparently the distribution of interstellar asteroid sizes, are not perfectly random. Why, well ask the Vegas casinos or wait for cool new astrophysics journal papers.
Also, the universe is old; perhaps we've been in a space junk drought for the last couple decades and now we're getting space crap flying by at normal rates again.
(Score: 3, Insightful) by JoeMerchant on Tuesday August 26, @08:56PM
>We have direct observational evidence for a couple decades that at most we get less than one per decade. Certainly, we don't get more than 1 per decade because we haven't seen any this large or weird for some decades of looking.
And that line of reasoning neglects / ignores the possibility of an uneven distribution. Indeed, if you are a comet falling in from the Oort cloud through the asteroid belt, you will see nothing, nothing, nothing for hundreds of years, then suddenly have a very high impact frequency with rocks large and small. Do we really know what's "out there" in interstellar space that might intersect our path?
> perhaps we've been in a space junk drought for the last couple decades and now we're getting space crap flying by at normal rates again.
Exactly. Astrophysicists love to point out how insignificant our lifespans are in relation to the age of the Cosmos, and then they turn right around and publish this rubbish based on observations made in a single human lifetime, or less. Yes, yes, they use all sorts of hocus pocus to extrapolate and try to leverage their tiny eye of the needle view of the infinite, but they seem to perpetually lose sight of their own limitations.
🌻🌻🌻 [google.com]
(Score: 4, Funny) by jelizondo on Tuesday August 26, @02:02PM (2 children)
I appreciate professor Loeb's attempts to widen our understanding of the cosmos but I think he goes too far with his arguments. Close to Earth, Aliens!, Far from Earth, Aliens! Small, Aliens! Large, Aliens!
In my view, he has become a joke: everything is aliens.
(Score: 4, Funny) by pkrasimirov on Tuesday August 26, @02:55PM
You better delete that before the aliens come and see it or they might make you glow from inside too!
(Score: 2) by mcgrew on Tuesday August 26, @06:19PM
Yes, even my sig.
What did you expect when you voted for a convicted felon, peace and rainbows?
(Score: 4, Insightful) by Undefined on Tuesday August 26, @02:59PM (2 children)
FWIW, no, the simplest interpretation is that it's a big rock.
I use a dedicated preprocessor to elaborate abbreviations.
Hover to reveal elaborations.
(Score: 3, Interesting) by VLM on Tuesday August 26, @07:04PM (1 child)
OP is trying to make a funny, I think, but is also technically correct in that this is a ridiculously large interstellar object / rock / comet and clearly the astrophysics does not scale up to this huge size as well as some of the old timers demand. Even "obvious" corrections like volume/area ratios aren't exact if you go deep enough into the decimal places.
There's a lot of interesting physics to learn and maybe something odd happening but currently all of it has a byline "Avi Loeb" which makes it a nearly one-man-band.
Take the "cold fusion" research as an example. It's pretty damn interesting you can pump hydrogen and heat into a bulk chunk of palladium and release it later, weirdly enough that certain conservation equations seem messed up in the decimal places. The headlines about "mr fusion in every basement" crapped up the actually very interesting materials science stuff. I hope that the cool/interesting astrophysics of F-ing ridiculously huge interstellar objects isn't crapped up by headlines about failed space alien invasions. Because, clearly, the astrophysics of "Big Fing Rocks in Space" is not as well understood as people would like and thats interesting to solve.
My gut level guess: This rock is friggin huge and people are not paying attention to long term gravitational effects. 243 Ida is a classic example of an asteroid JUST barely big enough that puny humans can't jump off / reach escape velocity using normal human legs. With a bicycle and a ramp a puny human could permanently launch themselves off 243 Ida but not bare legs. I remember hearing about this and I just double checked this. This 3I/ATLAS is darn near that big. Crap has been accumulating on its surface for a VERY long time and the cometary dynamics of this when it gets heated up by the sun... clearly are not well understood.
The trajectory of this comet is insane.
My Shitty Sci Fi Book Plot: If I wanted to "test" a civilization I'd redirect a perfectly natural, yet weird, comet on an insane trajectory. Below a certain level the civ won't notice or perform religious observances at most. We're at the holy shit WTF bounce radar signals off it all day level, which must be very interesting from a space alien ELINT standpoint to analyze our screwing around trying to measure it. Above our level we'd send out the Starship Enterprise to figure out WTF crazy nonsense is going on, maybe next time around. I think we're being passively probed. No way to prove or disprove that. I'm sure it would make a fascinating book plot. From the space alien PoV: Send a really weird rock on a really weird yet obviously non-hostile trajectory at a solar system and see what happens to indicate if there's intelligent life out there ... I would not be surprised if they planted ELINT satellites to listen to our planetary radar echos, for example.
(Score: 1, Interesting) by Anonymous Coward on Wednesday August 27, @12:27PM
Aye, and if the AAI behind the ELINT decides, based on the nature of the probing, that we match the 'possible long term threat' parameters as defined by the ETIs who sent it on this mission...the anomalous object then alters trajectory, fires up it's point singularity weaponry/kill-o-zap rayguns/whatever and it's sayonara humanity....again, and the anomalous object carries on to the next target system in it's neverending 'peacekeeping mission'.
Of course, there's always the shiny space rocks plot device used in 'A Better Mousetrap' by John Brunner, but we're not technically advanced and 'verminous' enough yet for that scenario to play out.
(Score: 5, Funny) by istartedi on Tuesday August 26, @03:31PM (1 child)
I asked AI about the existence of free O2 in space, and the possibility of combustion on a body like that, where trapped oxygen gets released and reacts with material as it heats up. It seems to think that's plausible but of course it's AI and to be taken with a giant grain of salt.
It also says that the spectrum of a combustion reaction could be distinguished from reflected sunlight so let's do that. Has anybody observed anything unusual in the spectrum?
Maybe something other than combustion could make such an object glow--like a pocket aurora or something. Pretty much guaranteed it's not aliens though. If it is, I'll eat my own tentacle. Wait... better not say that. Delete last sentience. Send.
Appended to the end of comments you post. Max: 120 chars.
(Score: 2) by Mojibake Tengu on Wednesday August 27, @03:49AM
Never place bets on your last tentacle.
Rust programming language offends both my Intelligence and my Spirit.
(Score: 3, Interesting) by Mojibake Tengu on Wednesday August 27, @03:45AM
In Chinese Buddhist mytology, there is a concept of Sunship: a huge artificial starship tugging very small star in its claws as a source of power.
This concept is related to Vedic solar deities race known as Adityas since Ancient times.
Rust programming language offends both my Intelligence and my Spirit.