3I/ATLAS – the third interstellar visitor and NASA’s big photo hunt

When 1I/ʻOumuamua passed through the Solar System in 2017, followed by 2I/Borisov in 2019, astronomers realized that our neighborhood is occasionally visited by “strangers” from other planetary systems. We now have a third such guest – the interstellar comet 3I/ATLAS, discovered on 1 July 2025 by the ATLAS telescope in Chile.

Within days of its discovery it became clear that the object does not follow a closed ellipse, but a hyperbolic trajectory – a classic signature of a body coming from outside, from interstellar space. Its chemical “fingerprint” also suggests that 3I/ATLAS probably formed in a system older than our own, carrying ice and dust that date back to “before the Sun”.

To make sure they wouldn’t miss this opportunity, NASA launched a true multi-mission “photo hunt”: the comet is being tracked simultaneously by the Hubble and James Webb space telescopes, spacecraft in Mars orbit, and even rovers on the surface of the Red Planet.

Interstellar comet 3I/ATLAS in an image from the Hubble Space Telescope

Image: NASA / ESA / David Jewitt (UCLA); image processing: Joseph DePasquale (STScI).

What exactly is 3I/ATLAS and where does it come from?

The designation 3I means “third interstellar object” (after ʻOumuamua and Borisov), while “ATLAS” comes from the Asteroid Terrestrial-impact Last Alert System, an automated telescope network that constantly searches for potentially hazardous objects. The first detections came from the Rio Hurtado observatory in Chile; follow-up observations quickly confirmed that the comet originates in another stellar system and will pass through ours only once.

The key facts in short:

  • 3I/ATLAS is a comet – a mixture of ice, dust and rock that forms a coma and tail when heated.
  • Estimates of the nucleus size are uncertain, but fall somewhere between a few hundred meters and a few kilometers.
  • It passed closest to the Sun in October 2025, and will make its closest approach to Earth in the second half of December, at a distance of about 270 million kilometers – so there is no risk to our planet.

The chemical composition looks “familiar, yet different”: the coma contains carbon dioxide, water, carbon monoxide, cyanide and an unexpectedly large amount of nickel – a mix similar to some Solar System comets, but with nuances that point to a different chemical history of the stellar system it comes from.

How do we know it is interstellar – and why is it so special?

The main evidence that 3I/ATLAS is not “our” comet is its orbit. Instead of an ellipse around the Sun, it follows a strongly open hyperbola: the comet enters at high speed, bends around the Sun’s gravity and then leaves forever. Astronomers often compare such objects to a “cosmic slingshot” – once they pass perihelion, they never come back.

Moreover, its incoming speed and direction do not match any known family of comets from the Oort cloud. It comes from a completely different part of the Milky Way, and some measurements suggest that its parent system is several billion years older than ours, making 3I/ATLAS potentially the oldest comet ever observed.

For astronomy, that’s a gold mine:

  • we get a sample of ice and dust from a completely different planetary system,
  • we can compare the chemistry of “foreign” and “local” comets,
  • we better understand how planetary systems form and how often they eject objects into interstellar space.

NASA’s multi-mission campaign: from Mars to Hubble and Webb

Instead of relying on a single telescope, NASA has engaged an entire fleet of spacecraft and observatories to follow 3I/ATLAS:

  • Mars Reconnaissance Orbiter (MRO) imaged the comet with its HiRISE camera when it passed about 30 million kilometers from Mars. In the image, the comet appears as a small fuzzy dot with a hint of a tail – but clear enough to track the coma structure.
  • MAVEN, PUNCH, STEREO and other heliophysics missions observed the comet at different wavelengths, measuring how the solar wind “whips” its tail.
  • Hubble captured a detailed view of the coma – a bluish “cocoon” of dust and gas around the nucleus, with bright streaks of background stars.
  • James Webb observed the comet in the infrared, helping to disentangle different types of ices and gases in the coma.

Schematic diagram of the trajectory of the interstellar comet 3I/ATLAS through the Solar System

Diagram of the trajectory of comet 3I/ATLAS through the Solar System. Illustration: NASA / JPL-Caltech.

This strategy – “many eyes, one opportunity” – allows scientists to connect:

  • the chemistry of the coma and tail,
  • the interaction with the solar wind,
  • and the precise orbital dynamics,

into a coherent story of how a comet from a completely different stellar system behaves.

Not a UFO: why scientists keep saying “this is a comet”

As with ʻOumuamua, the internet quickly exploded with theories that 3I/ATLAS might be an artificial object or even a spacecraft. This time NASA addressed the rumors right away during its press briefings: instruments see no signs of any “technosignatures” – no radio signals, no regular pulses, no strange maneuvers or anything that would suggest an artificial origin.

Instead, everything observed is consistent with:

  • normal sublimation of ices as the comet is heated,
  • the formation of a coma and tail,
  • and an orbit that exactly matches what we’d expect from gravity acting on a natural object.

In other words, it behaves exactly as a comet should – the only unusual thing is that this one comes from a distant, foreign stellar neighborhood.

Can 3I/ATLAS be seen from Earth?

Unlike some spectacular comets of the past, 3I/ATLAS will not become a naked-eye object. Even at peak brightness it is expected to remain faint, but:

  • it will be within reach of amateur astronomers with small telescopes,
  • the best observing window will be late 2025 and early 2026, as it moves away from the Sun but remains sufficiently active.

For professionals and space missions, however, this is a true cosmic hunting season – the last chance to extract as much data as possible before the comet disappears into interstellar darkness for good.

Conclusion

3I/ATLAS is far more than just another comet:

  • it is the third confirmed interstellar visitor,
  • probably older than our Solar System,
  • and the first that NASA has been able to monitor systematically with an entire fleet of spacecraft and telescopes.

For science, it is a direct sample of another planetary system, without sending a probe to another star. For us “down on Earth”, it is a reminder that the Solar System is not isolated, but constantly exchanging comets, asteroids and cosmic dust with the rest of the galaxy.

Disclaimer: This article is for informational purposes only. It does not represent an official statement from any space agency and does not constitute any guarantee regarding long-term cosmological models and interpretations.