3I/ATLAS

Marcelo Serafim
Oct 28
6 min read

The object designated 3I/ATLAS was discovered on 1 July 2025 by the Asteroid Terrestrial‑impact Last Alert System (ATLAS) telescope in Río Hurtado, Chile. Because of its trajectory and speed, the object is classified as an interstellar object: it is not bound by the Sun’s gravity in a normal elliptical orbit, but rather follows a hyperbolic path, indicating it arrived from outside our Solar System. This makes it only the third confirmed interstellar object after 1I/ʻOumuamua (2017) and 2I/Borisov (2019).

Its trajectory and origin

3I/ATLAS is travelling at very high speed relative to the Sun and approaches the inner Solar System (though not dangerously close to Earth). According to the European Space Agency, it will reach its closest point to the Sun (perihelion) around 29 – 30 October 2025, at about 1.36–1.40 astronomical units (AU) from the Sun (between the orbits of Earth and Mars). The fact that it is moving on a hyperbolic trajectory strongly indicates an origin outside the Solar System — likely from another star system or the galactic disk.

Physical characteristics and composition

Although exact measurements are challenging (because the object has a coma and dust around it), scientists estimate the nucleus of 3I/ATLAS is perhaps less than 1 km in diameter, based on Hubble imaging. Spectroscopic studies (including from the James Webb Space Telescope) show that its coma is dominated by carbon dioxide (CO₂) and includes water vapor (H₂O), carbon monoxide (CO), and carbonyl sulfide (OCS) in unusually high proportions for a comet. Another unusual result: spectroscopic observations with the Keck II Telescope in Hawaii detected significant nickel (Ni) emissions and cyanide (CN) gas around the object — a combination not typically seen in known Solar System comets.

Why it matters scientifically

Because 3I/ATLAS comes from beyond our Solar System, it provides a rare opportunity to study material formed around another star — essentially of foreign origin. Its composition, behavior, trajectory and age may reveal insights into how planetary systems beyond our own form and evolve. The unusual chemical signatures also suggest that objects from outside our Solar System may differ significantly from typical comets or asteroids here. For example, the unusually high ratio of CO₂ to H₂O might indicate formation in a colder or more radiation-exposed region than Solar System comets.

Some unusual behaviors

Observations show 3I/ATLAS exhibits comet-like behavior: it has a coma (the fuzzy surrounding of gas and dust) and shows jets of material being expelled as it is warmed by the Sun. The tail is seen growing as the object approaches the Sun. However, the combination of its interstellar origin

plus its unusual chemical signatures (for instance, the nickel dominance and iron depletion) make it stand out as different from typical Solar System comets.

Why some say it could be an alien spacecraft

Because of its interstellar origin, its high speed, its unusual composition and behavior, some scientists and media outlets have suggested that 3I/ATLAS might not simply be a natural object, but perhaps an artificial object — possibly a spacecraft or probe of alien origin. For example, astrophysicist Avi Loeb has argued publicly that the combination of nickel emissions and odd tail morphology might hint at controlled maneuvering rather than purely natural out-gassing. The public fascination with “alien probe” scenarios arises because of the unknowns: we have very few interstellar visitors to compare, so anomalous behavior can spark speculative interpretations.

Why mainstream science is cautious

Mainstream scientists stress that while the “alien spacecraft” idea is sensational, there is no definitive evidence for artificial origin. The unusual features could be explained by natural processes we do not yet fully understand — especially for interstellar bodies formed under very different conditions. For instance, the high CO₂/H₂O ratio might simply indicate formation in a cold outer region of another star system or exposure to cosmic radiation. Also, launching a spacecraft to intercept 3I/ATLAS is impractical: the required delta-v (change in velocity) is beyond current capabilities for a post-discovery launch. (Wikipedia) So scientists prefer to gather more data and maintain a prudent stance.

What future observations might reveal

Several dedicated observation campaigns are underway to monitor 3I/ATLAS’s tail, composition, polarization, and trajectory. Results from polarimetric studies (the way it polarizes light) already show features “unprecedented among asteroids and comets” which may hint at a new class of interstellar objects. As it passes perihelion and leaves the inner Solar System, telescopes will track how it evolves, how much material it sheds, and what that tells us about its origin. The hope is that by comparing 3I/ATLAS to more typical Solar System comets, we can refine our models of comet formation and interstellar object populations.

Why the “many people” scenario persists

Part of the reason so many people say “alien spacecraft” is plausible is psychological and cultural: when a truly unknown object arrives from outside our Solar System, it triggers wonder and speculation. The combination of unfamiliar behaviour (compared to known comets), sensational headlines, and public appetite for “aliens” generate and amplify such ideas. Moreover, because interstellar objects are so rare (only three confirmed), our data set is too small to draw firm conclusions — meaning gaps in knowledge can be filled by imaginative hypotheses. That doesn’t make the alien hypothesis likely, but it demonstrates how cultural and human factors play into scientific interpretation and public perception.

What we should keep in mind

In summary: 3I/ATLAS is fascinating, unique, and scientifically important. It offers a rare window into material from outside our Solar System. Yet, the claim that it is an alien spacecraft remains speculative and unsupported by definitive proof. It is far more likely (though not guaranteed) that it is a natural interstellar comet or object. As data accumulate, scientists will refine their understanding, update models, and perhaps revise our view of what interstellar visitors look like. For now, it is best to maintain both excitement and scientific caution.

Questions

What key pieces of evidence lead scientists to conclude that 3I/ATLAS comes from outside our Solar System?
Why is the high CO₂/H₂O ratio in 3I/ATLAS considered unusual compared to typical Solar System comets?
What are the major arguments made in favour of the idea that 3I/ATLAS might be an alien spacecraft, and what are the counter-arguments?
How do polarimetric observations and spectroscopy help scientists study objects like 3I/ATLAS?
What are the challenges or limitations in sending a spacecraft to intercept or study 3I/ATLAS more closely?

Vocabulary – ten difficult words and their meanings

Interstellar – occurring or existing between stars (i.e., outside a given star system).
Hyperbolic trajectory – a path followed by a body moving at such speed that it is not gravitationally bound to the central object (in this case, the Sun).
Coma – the fuzzy envelope of gas and dust surrounding the nucleus of a comet.
Perihelion – the point in the orbit of a body where it is closest to the Sun.
Spectroscopy – the study of the interaction between matter and electromagnetic radiation as a method to identify composition, motion, etc.
Polarimetric – relating to polarization; measuring the orientation of waves of light or other radiation to infer properties of an object.
Delta-v (Δv) – a measure of the required change in velocity for a spacecraft manoeuvre (often used in orbital mechanics).
Outgassing – the release of gas previously trapped, from a solid body (like comet ice) when heated.
Manoeuvring – making controlled or strategic movements (especially in space or aircraft).
Anomaly – something that deviates from what is standard, normal, or expected.

Phrasal verb related to the text

“pass through” – meaning: to move across or traverse something (often used for moving through a region or medium).Examples:

3I/ATLAS will pass through the inner Solar System before heading back out into interstellar space.
The spacecraft is expected to pass through the comet’s tail to collect data.
When a comet passes through perihelion, it heats up and releases gas and dust.

American idiom related to the text

“Out of this world” – meaning: extraordinarily good, impressive, or beyond ordinary experience. Example:

The composition of 3I/ATLAS is out of this world compared to what we normally observe in comets.
That idea of an alien spacecraft certainly sounds out of this world to many scientists.

English grammar tip related to the text

Tip: Use the present perfect tense when describing recent discoveries that have relevance now. Explanation: The present perfect (have/has + past participle) is used when the action occurred at an unspecified time in the past and has an effect on the present. Examples:

Scientists have discovered 3I/ATLAS in July 2025.
Researchers have observed unusual nickel emissions around 3I/ATLAS.In contrast, when you mention a specific past time, use the simple past:
It was discovered on 1 July 2025.
It reached perihelion in October 2025.

Listening

Homework proposal

Choose one of the following tasks:

Write a 500-word essay summarizing the significance of interstellar objects like 3I/ATLAS, discussing what they can tell us about the galaxy beyond our Solar System.
Prepare a mini-research poster (digital or on paper) presenting 3I/ATLAS: include images, composition data, trajectory, and the “alien spacecraft” hypothesis—with your own evaluation of how plausible you find it.
Interview a friend or classmate: ask them what they think 3I/ATLAS might be (natural object vs. alien spacecraft) and why. Then write a one-page reflection on how public perception differs from scientific caution, using what you’ve learned from this article.