What 3I/ATLAS really is – and what it definitely is not

Dubbed 3I/ATLAS, it hails from another star system, a visitor from deep space making a single, fleeting pass before vanishing forever. Its discovery has captivated astronomers and flooded social media with wild theories, from alien probes to doomsday predictions.

But behind the noise lies a fascinating scientific story that reveals far more about our universe than any rumour could.

This interstellar arrival is unlike anything seen before. Larger, faster and chemically more complex than its predecessors, 3I/ATLAS challenges many of our assumptions about comets.

It is a genuine cosmic mystery, a natural body behaving in unexpected ways and puzzling astronomers around the world.

What makes its visit truly extraordinary, however, is a stroke of luck: NASA's Europa Clipper spacecraft, currently on its way to Jupiter's icy moon, is expected to pass directly through the comet's invisible ion tail, offering a unique opportunity to sample material from another star system.

Unfortunately, that same mix of mystery and excitement has also sparked a flood of online speculation. Social media has been awash with claims that 3I/ATLAS is an alien spacecraft, a hidden threat to Earth or a sign of something catastrophic – none of which is true. Some videos have even used deepfakes to falsely show well-known scientists confirming such ideas.

In reality, astronomers are in full agreement: 3I/ATLAS is a natural comet, albeit one with an unusual chemistry. As physicist Brian Cox put it, it is "a comet, made of carbon dioxide and water ices, not a starship". NASA scientists have backed this up, stating that "all available evidence overwhelmingly points to a natural origin".

Far from posing a danger, 3I/ATLAS represents a rare scientific opportunity. Its unusual features, from its high speed to its strange metallic signature, could reveal how planetary systems form throughout the galaxy. And if Europa Clipper succeeds in crossing its tail, it may deliver the first direct measurements of matter formed around another star.

Discovery and characteristics of 3I/ATLAS

On 1 July 2025, astronomers using a NASA-funded telescope in Chile detected a faint object moving unusually fast across the sky. Its path immediately showed that it was not part of our Solar System.

While most comets travel in closed orbits around the Sun, 3I/ATLAS follows an open, hyperbolic path. It will pass through once and then disappear into deep space, never to return.

Everything about it is extreme. It is travelling at about 210,000 kilometres per hour – faster than any known comet – and may span between a few hundred metres and several kilometres.

As it approaches the Sun, 3I/ATLAS has shown striking activity. The James Webb and Hubble space telescopes observed jets of water vapour and carbon dioxide erupting from its surface, described by one researcher as "a fire hose running at full blast".

The comet's most puzzling trait is its chemistry. Spectra reveal nickel but almost no iron – a mix never before seen in a comet. On Earth, this ratio is typical of industrial compounds, which fuelled online speculation.

Scientists, however, see a natural explanation: 3I/ATLAS likely formed in a very different kind of planetary system, under conditions that created an unusual chemical balance.

A broader perspective

Until recently, astronomers knew of only two other interstellar visitors. The first, ʻOumuamua (2017), was a reddish, elongated object that showed no visible tail but accelerated slightly, as if propelled by invisible gas.

The second, 2I/Borisov (2019), looked more like a conventional comet, though rich in carbon monoxide. 3I/ATLAS has upended any idea of what such objects should be.

From a rocky, inert fragment (ʻOumuamua) to a classic comet (Borisov), to this hyperactive, metal-rich newcomer, the trio shows how different planetary systems can be. Our own Solar System may be only one variation among countless others.

The role of NASA's Europa Clipper

As 3I/ATLAS sweeps through the Solar System, an extraordinary coincidence has put one of NASA's most advanced spacecraft directly in its path. Europa Clipper, launched in 2024, was built to study whether Jupiter's icy moon Europa could support life. Scientists believe the moon hides a global ocean beneath its frozen crust – possibly with twice the water of all Earth's oceans combined.

Europa Clipper / © NASA

To withstand Jupiter's intense radiation, Europa Clipper follows a complex, looping route. By sheer chance, this trajectory will carry it through the ion tail of 3I/ATLAS between 30 October and 6 November 2025 – just as the comet reaches its closest point to the Sun.

Around this stage, known as perihelion, a comet's activity peaks, producing the densest and most extensive tails. The timing gives Europa Clipper a rare chance to detect interstellar material first-hand.

What scientists hope to learn

Comets have two tails: a dust tail, made of solid particles pushed away by sunlight, and an ion tail, formed when gases are ionised by ultraviolet light and swept away by the solar wind. The ion tail always points directly away from the Sun – and it is this invisible stream that Europa Clipper is likely to cross.

By analysing changes in charged particles and magnetic fields as the spacecraft moves through the region, scientists hope to confirm whether it has passed through material from the comet. If successful, this would mark the first direct contact with matter from another star system.

Such data could reveal what elements and compounds are common elsewhere in the galaxy. If water and carbon-based molecules are detected, it would strengthen the idea that the ingredients for life are widespread in space.

Challenges and the road ahead

Despite the potential breakthrough, success is not guaranteed. A government shutdown in the United States, which began on 1 October 2025, has paused many NASA operations. Unless Europa Clipper's instruments are reactivated in time, the spacecraft could miss this historic event.

Even if the systems are ready, the outcome depends on nature itself - the comet's activity and the strength of the solar wind.

The situation highlights the need for flexible, rapid-response missions such as the European Space Agency's upcoming Comet Interceptor, due for launch in 2029. That probe will wait in orbit for the next pristine comet or interstellar visitor, then move quickly to intercept it.

For now, though, all eyes are on 3I/ATLAS and Europa Clipper. This unplanned alignment between a comet from another star and a spacecraft built to study an ocean world around Jupiter offers an extraordinary chance for discovery.

In the days and weeks ahead, we may not witness the arrival of a starship, but something arguably more profound: our first tangible connection to a world born around another sun, and with it, a deeper understanding of our place in the cosmos.