Visitors from Beyond: 3I\/ATLAS – The Third Interstellar Visitor

Imagine looking up at the night sky and wondering whether something from another star system could ever pass through our Solar System. In the last few years, astronomers have answered that question with a clear “yes.”

Three unusual interstellar objects have been discovered racing through our cosmic neighbourhood: 1I ʻOumuamua, 2I Borisov, and the newest visitor, 3I/ATLAS. These rare travellers are not bound to the Sun’s gravity and will eventually disappear back into deep space.

🗣️ Vocabulary Builder

These keywords will help you talk about interstellar visitors:

• interstellar object – a natural object from outside our Solar System

• coma – the bright cloud of gas and dust that surrounds a comet’s nucleus

• hyperbolic orbit – a path that allows an object to pass by the Sun and then escape back into space

• non-gravitational acceleration – movement caused by forces other than gravity, like gas jets from a comet

• sublimate – to change from solid ice directly into gas without becoming liquid

• spectroscopic – relating to the study of light to find out an object’s chemical makeup

• carbon dioxide – a common gas released when comet ice warms

• perihelion – the point in a comet’s orbit when it is closest to the Sun

• mass loss – the material a comet loses as gas and dust while it warms

• trajectory – the path an object follows through space

🌠 1I ʻOumuamua – The First Mysterious Visitor

Discovered in October 2017 by a telescope in Hawaii, 1I ʻOumuamua was the first interstellar object ever confirmed. Its name comes from Hawaiian, meaning “scout” or “messenger from afar.”

What made ʻOumuamua so puzzling was its unusual behaviour. It showed no visible coma or tail like a typical comet, yet it sped up slightly as if pushed by a hidden force — something scientists call non-gravitational acceleration.

Its shape was also strange. Some thought it looked like a long cigar-shaped rock, others imagined a flat pancake-like fragment. It was probably only a few hundred metres long, but even today its true nature remains uncertain.

🌌 2I Borisov – A Classic Interstellar Comet

Two years later, in August 2019, an amateur astronomer named Gennady Borisov discovered another interstellar object. This one behaved more like a typical comet.

2I/Borisov had a bright coma, a glowing cloud of gas and dust, and it displayed a tail as it neared the Sun. Its icy nucleus was small, probably under half a kilometre wide, but scientists were excited to study a familiar comet that came from another stellar system.

Its chemical composition suggested it may have formed in a very cold region around a distant red dwarf star, showing higher levels of carbon monoxide ice than most Solar System comets.

☄️ 3I/ATLAS – The Newest Cosmic Wanderer

In July 2025, astronomers using the ATLAS survey telescope in Chile discovered a third interstellar visitor. It was named 3I/ATLAS — the “I” for interstellar, and ATLAS for the telescope that first spotted it.

Journey Through the Solar System

3I/ATLAS follows a hyperbolic orbit, meaning it is not trapped by the Sun’s gravity. It will sweep in, curve around the Sun, and head back out into the galaxy.

Its closest approach to the Sun, called perihelion, will happen near the end of October 2025 at about 1.36 AU from the Sun (slightly farther than Earth’s distance from the Sun). It will remain a safe distance from Earth, at about 1.8 AU away, so there is no threat to our planet.

AU stands for Astronomical Unit.

It’s a standard unit of distance in astronomy that equals the average distance between the Earth and the Sun — about 149.6 million kilometres (or roughly 93 million miles).

So, when we say that 3I/ATLAS will come 1.36 AU from the Sun, it means it will be about 1.36 times farther from the Sun than Earth is.

This unit helps astronomers describe distances in the Solar System more simply without using huge numbers of kilometres.]

A Powerful and Active Comet

Unlike ʻOumuamua, 3I/ATLAS is clearly an active comet. As it approaches the Sun, the heat causes its surface ices to sublimate, turning directly into gas and forming a bright coma and tail.

Its nucleus is hidden by this cloud, but early estimates suggest it could be a few hundred metres to possibly more than five kilometres across. Even more surprising, scientists believe it may be anomalously massive — perhaps weighing tens of billions of tons.

Because of its size and weight, 3I/ATLAS shows very little non-gravitational acceleration, even though it is losing gas and dust. This sets it apart from many lighter comets.

Rich in Unusual Ices

Early spectroscopic studies show that 3I/ATLAS is unusually rich in carbon dioxide, with smaller amounts of water vapour, carbon monoxide, and carbonyl sulphide.

It also emits cyanide gas and traces of nickel vapour, which are sometimes seen in regular comets.

These clues suggest 3I/ATLAS may have formed in an extremely cold outer region of its original star system.

🔭 Why 3I/ATLAS Matters

• A Rare Chance for Study – Interstellar visitors are extremely rare. 3I/ATLAS gives astronomers a precious opportunity to examine material that formed around another star.

• Clues to Distant Worlds – Its unusual composition and massive size may tell us more about how comets form in other planetary systems.

• A Test for Theories – Because of its low non-gravitational acceleration, scientists can test how comet activity and mass loss affect an object’s movement.

• Ancient Origins – By tracing its trajectory, researchers suspect 3I/ATLAS may have come from the thick disk of our galaxy, a region full of older stars. Some estimates suggest it could be older than the Solar System itself, possibly over seven billion years old.

❓ Reading Comprehension Quiz

True or False

1. 3I/ATLAS is the first interstellar object ever discovered.

2. ʻOumuamua had a visible tail like a normal comet.

3. 2I Borisov came from outside the Solar System.

4. 3I/ATLAS will pass safely far from Earth.

5. Scientists believe 3I/ATLAS contains large amounts of carbon dioxide.

Multiple Choice

6. What does the “I” in 3I/ATLAS stand for?

a) Ice

b) Interstellar

c) International

7. When was 3I/ATLAS discovered?

a) 2017

b) 2019

c) 2025

8. What makes 3I/ATLAS unusual compared to many comets?

a) It is unusually massive

b) It has no coma

c) It has stopped orbiting the Sun

9. What is the meaning of the term “perihelion”?

a) The farthest point from the Sun

b) The closest point to the Sun

c) The moment a comet breaks apart

10. Which of the three interstellar visitors behaved most like a typical comet?

a) ʻOumuamua

b) Borisov

c) ATLAS

✅ Answer Key

True/False: 1F, 2F, 3T, 4T, 5T
Multiple Choice: 6b, 7c, 8a, 9b, 10b