“Scientist issues a frightening update on 3I/ATLAS warning it is ‘not natural’!” “Does 3I/ATLAS pose a threat to Earth?” “Astronomer suspects mysterious object is up to no good!” And my favorite: “Astrophysicist says attack from ‘potentially hostile alien threat’ could take place in just days!”

These are just some of the headlines that were top-ranking when I ran a simple internet search for the interstellar comet 3I/ATLAS in preparation for this article. It’s easy to scoff, but conspiracy theories, pseudo-science and misinformation have a habit of spreading like a virulent disease, particularly in this social-media age.

So the aims of this article are two-fold. First is to describe what we’re learning about 3I/ATLAS, and second is to challenge some of those conspiracy stories and help set the record straight.

Throughout history people have seen comets as portents, but there’s nothing mythical about them. They’re just chunks of ice and maybe some rock mixed in. Scientifically comets are interesting because they are like time capsules, harboring materials that dates from the birth of the Solar System 4.5 billion years ago and which has been untouched since. Comets can clue us in to the conditions in which Earth and the building blocks for life formed. 

To find comets that have come from interstellar space can be even more revelatory because they can tell us about planet-forming conditions around other stars, allowing us to directly compare the birth of our Solar System with others.

Conspiracists and fantasists would like to propagate the idea that there’s something highly unusual about interstellar comets, but the idea of these cosmic icebergs wandering onto our Solar System’s shores from the interstellar ocean did not begin with 1I/’Oumuamua, which in 2017 became the first object to be discovered. Let’s just say that 1I/’Oumuamua, the second interstellar comet 2I/Borisov and indeed 3I/ATLAS have all been expected. As long ago as the seventeenth century, Isaac Newton envisioned comets around other stars, and we know that comets can be ejected from their host systems, either through perturbations triggered by passing stars or by being kicked out via gravitational interactions with giant planets. 

In his seminal 1985 book Comet, co-written with Ann Druyan, Carl Sagan discusses how “If every star in the Milky Way ejects a thousand comets into interstellar space every 4.5 billion years, as ours has, then there may be the equivalent of the mass of a hundred millions suns floating undetected in the space between the stars... the number of interstellar comets in the galaxy is almost 10^24, more than the number of stars in the Universe.” 

These numbers are so vast that our Solar System is bound to encounter some. At least 10,000 interstellar objects are estimated to be passing through our Solar System at any one time. They’re just hard to spot, which is why we’ve only found three so far. The Vera C. Rubin Observatory, which is the most powerful survey telescope ever built and which begins science operations at the end of 2025, is expected to discover on average of one interstellar object per year.

It’s just the current observational limitations that make it seem like they are extraordinarily rare events. The argument that they are all spaceships is going to get very tired, very quickly, when we begin finding these things every year.

For now, what we have in 3I/ATLAS is a comet that is both remarkable and reassuringly familiar at the same time. Frustratingly though, it’s been playing hide-and-seek with our telescopes.

Following its discovery on 1 July by the Asteroid Terrestrial-impact Last Alert System (ATLAS), 3I/ATLAS was visible in telescopes as a very faint smudge that gradually grew a tail. Its path through the Solar System has taken it around the back of the Sun from our point of view, where we cannot see it from Earth. 

That’s a shame, because on 30 October it will be at perihelion, which is how astronomers describe its closest point to the Sun as it follows its trajectory. At perihelion it will be 1.4 astronomical units (one astronomical unit, AU, is 149.6 million kilometers, which is the mean distance between Earth and the Sun) from the Sun, which is just inside the orbit of Mars. Even at. this distance it is warm enough to increase activity on the comet by sublimating volatile ices on the surface of its nucleus, causing an increase in outgassing that lifts comet dust into what we call the coma, which is an atmosphere that clings to the head of the comet, obscuring the solid nucleus. Some of this dust also forms one of the comet’s tails, while the other tail is composed of charged particles that have also come from the comet.

This is why perihelion is the most exciting time to be observing a comet, especially 3I/ATLAS. All that gas and dust spewing from it can be observed spectroscopically, revealing some of the material that the comet is made from. Statistically speaking, this is probably the closest that 3I/ATLAS has come to a star in billions of years, possibly since it left its home system. There could be primordial ices on its surface that have been untouched since its formation that will suddenly sublimate and become detectable.

Although we cannot see 3I/ATLAS from Earth during perihelion, we’re not completely blind because we’ve got spacecraft out and about in the Solar System that have a better view of that hemisphere of the Sun. These include various missions at Mars, NASA’s Psyche mission to the asteroid of the same name, as well as the European Space Agency’s Jupiter Icy moons Explorer (JUICE) that’s currently on its way to the giant planet via fly-bys with Earth and Venus. We also shouldn’t forget that copious observations of 3I/ATLAS were made between its discovery and it slipping into solar conjunction at the end of September.

What we’ve discovered so far is that 3I/ATLAS is made from the same stuff, more or less, that our Solar System’s comets are. The James Webb Space Telescope (JWST) identified carbon dioxide, water vapor, water-ice, carbon monoxide and carbonyl sulphide in the comet’s coma. The Very Large Telescope in Chile also detected cyanide and nickel, both found in similar abundances in our Solar System’s comets in. 

The differences between 3I/ATLAS’ composition and those of comets native to our Solar System are both slight and subtle. The JWST detected a greater abundance of carbon dioxide in 3I/ATLAS than is typically found in Solar System comets, and furthermore that much of this carbon dioxide is the ‘heavy’ variety, containing the carbon-13 isotope rather than the regular carbon-12. There’s also a dearth of iron in the comet’s spectrum, although iron might become more apparent if the comet were closer to the Sun and heated even more.

The compositional differences tell us of chemical variations in the nebula from which 3I/ATLAS and its parent star formed compared to the solar nebula that produced our Solar System. The similarity in the composition teaches us that stars, planets and comets all form from pretty much the same stuff as what our Solar System coalesced from. Plus we can compare different interstellar comets, hailing from different parent stars in different parts of the Milky Way, and find that they too have similar origins based on their compositions — for example, 3I/ATLAS and 2I/Borisov both have the a similar abundance of nickel.

What 3I/ATLAS’ chemistry is telling us is utterly profound — that planetary systems elsewhere form just like our own, with similar chemistry and since that chemistry led to life on Earth, it emboldens astrobiologists to think that life could therefore also evolve around other stars.

Unfortunately, the Internet is awash with hyperbolic and frankly made-up nonsense about 3I/ATLAS. The headlines that I gave as examples at the top of this article are just a smattering of the misleading stories, videos and social-media posts on the matter.

So let’s tackle some of those claims and set the record straight.

The first claim is that 3I/ATLAS is traveling towards Earth at 130,000 miles per hour. That last part is actually correct. Indeed, 3I/ATLAS is the fastest moving natural object ever seen in the Solar System, racing through space at 58 kilometers per second (which amounts to about 210,000 kilometers/130,000 miles per hour). Interstellar comets travel fast for several reasons. One is that they had to reach the escape velocity in their home system, and afterwards they encounter other stars, usually at a distance, but still close enough to receive a gravitational slingshot. Given the velocity of 3I/ATLAS, it must have been in interstellar space for at least 7 billion years to rack up enough stellar encounters to have accelerated it.

It is not heading for Earth, or any other planet, nor will it come near. As we’ve seen, 3I/ATLAS is currently on the other side of the Sun. Its high velocity means that rather than orbiting the Sun, its journey through the Solar System is practically a straight line, with just a modest bend where the Sun’s gravity slightly acts on it. There are likely to be some minor deviations as the result of non-gravitational acceleration as outbursts on the surface of the comet produce additional thrust, but nothing to significantly alter its course.

The closest that Earth will get to 3I/ATLAS will be 269 million kilometers (1.8AU) on 19 December 2025. Afterwards, 3I/ATLAS will continue its journey back into interstellar space. Claims that its trajectory has dramatically changed or that it has decelerated are, frankly, wrong.

Another claim by Harvard’s Avi Loeb, who is the chief agitator when it comes to wild proposals about 3I/ATLAS, is that it doesn’t really have a tail, and that its diffuse appearance is a consequence of it looking blurred because it is moving so fast. Yet as any astronomer, professional or amateur, worth their salt knows, telescopes can have two tracking modes. One is sidereal, meaning with the fixed stars on the celestial sphere. This is the tracking that astronomers employ to counter the effects of the rotation of the Earth, otherwise the images of stars would become trails. The other mode is non-sidereal, and this can track objects such as comets, asteroids or planets that move relative to the fixed stars. Images of 3I/ATLAS are tracked non-sidereally, and the giveaway is that the stars appear trailed and 3I/ATLAS appears diffuse because it has a coma and a tail.

Then Loeb changed tack and accepted that 3I/ATLAS does have a tail, but that it is pointing in the wrong direction, towards the Sun. This is actually the truth — it does have what is referred to as an ‘anti-tail’ pointed towards the Sun, as well as its regular tails. Yet despite Loeb’s protestations an anti-tail is not impossible nor is it unprecedented. A recent example would be the anti-tail on the comet C/2014 UN271 (Bernardinelli–Bernstein).

To understand what is happening, let’s first explore what makes regular comet tails. As a comet nears the Sun — and it doesn’t even have to get particularly close, it could be out beyond Mars or even Jupiter — its surface warms up causing volatile ices — nitrogen, carbon dioxide and, when close enough, water vapor — to sublimate. If these ices are just below the surface, they can burst out, raising up cometary dust with them. The gas and dust forms the coma. Large particles of dust are left behind in the comet’s wake, forming the comet’s main, brighter tail. Meanwhile, charged particles in the coma’s gases are stripped away by the solar wind and radiation pressure from the Sun, forming a second, usually fainter tail called the ion tail. Crucially, the ion tail always points, straight as a rod, away from the Sun as it is driven by the solar wind. The dust tail also generally points away, though its path is more curved as the result of being laid down along the comet’s curved trajectory.

Sometimes we see an anti-tail that is a geometric illusion, caused by our point of view making it look like the tail is pointing towards the Sun when actually it is not. Occasionally though, an outburst can occur that belches a cloud of dust in the direction that the comet is moving. If the dust particles are small then the solar wind quickly takes them away, but if the dust particles are larger and more massive, it is not as easy for the solar wind to remove them, and so that dust spreads out in front of the comet, forming a tail that genuinely can point towards the Sun. Research by David Jewitt and Jane Luu — the duo who discovered the first Kuiper Belt object back in 1992 — concludes that many of the dust particles spewed out to form this anti-tail are indeed larger with a radius of about 100 microns, compared to small dust particles that are just a micron or so in size.

Then there’s the issue of the size of 3I/ATLAS’ nucleus. Loeb has argued that because the coma has a low opacity, the brightness of 3I/ATLAS must be coming mostly from the nucleus itself, which would mean it must be large and artificially self-luminous. However, even a thin coma scatters a lot of light so there’s no need to invoke a giant self-luminous nucleus, which Loeb reckons must be 46 kilometers across based on observations by the SPHEREx infrared space telescope that launched in March 2025. If 3I/ATLAS really is this big, then it must be an outlier, otherwise we would have detected interstellar comets of this size before.

Based on Hubble Space Telescope images, Jewett and Luu determine that the maximum size of 3I/ATLAS’ nucleus is 5.6 kilometers. In that case, to be as bright as it appears to SPHEREx means that more than 99 per cent of its light must be scattered from dust in its coma. If there were no coma at all, and all the light was coming from the nucleus, then the nucleus would have to be 46 kilometers across to produce that light, but we know there is a coma, so it’s a moot point.

And on and on. Penn State University’s Jason Wright has done a good job on his website refuting all these claims and more, and it’s worth a read if you want to get into the details.

The most frustrating this is that Loeb says that he doesn’t necessarily believe that 3I/ATLAS is a spaceship, but he does think it is a possibility worthy of investigation. For what it’s worth, scientists have actually done that. All three of the interstellar objects so far have been scrutinized by SETI, with the Allen Telescope Array and the Green Bank Observatory both listening in for any signals just in case. However, as Carl Sagan often stated, extraordinary claims require extraordinary evidence, and 3I/ATLAS the evidence is that it is so clearly a comet that for most astronomers it is case-closed. Loeb, however, believes it is a “fun” line of enquiry to pursue, but this is incredibly reckless when his talk is triggering nonsense stories about the comet being a mothership in disguise — a ‘black swan’ as he calls it — heading to Earth for nefarious purposes. People do believe outlandish scientific claims and become scared by them. We remember the tragedy of the Heaven’s Gate cult who committed suicide because they believed comet Hale–Bopp to be a starship. In 2008 in India, a teenage girl killed herself because of erroneous stories that the Large Hadron Collider was going to create a black hole that would destroy the Earth. This brand of pseudoscience can turn from fun and games into something deadly serious very quickly.

We know that 3I/ATLAS will not be the last interstellar comet discovered. Some will be like 2I/Borisov, mundane in their ordinariness. Others may be unusual in one or more ways, which we shouldn’t be surprised about because these objects are not made from a cookie-cutter. Even in our own Solar System there is a fascinating diversity among the various comets, asteroids and Kuiper Belt objects. Nor do comets always behave as we would expect them to, regardless of whether they are interstellar in nature or natives of our Solar System.

There’s an old saying, which is that comets are like cats — they both have tails and do exactly what they like. It’s worth remembering that the next time an interstellar comet comes along that doesn’t fit neatly into the cookie-cutter paradigm, it doesn’t mean that it must be aliens.