3I/ATLAS facts for kids
This photo by the Canada–France–Hawaii Telescope shows 3I/ATLAS (center) appearing fuzzy and elongated on 2 July 2025, indicating it is a comet.
|
|
| Discovery | |
|---|---|
| Discovery site | ATLAS–CHL (W68) |
| Discovery date | 1 July 2025 |
| Designations | |
| C/2025 N1 | |
| A11pl3Z | |
| Orbital characteristics | |
| Epoch | 1 July 2025 (JD 2460857.5) |
| Observation arc | 19 days |
| Earliest precovery date | 14 June 2025 |
| Orbit type | Interstellar (hyperbolic) |
| Perihelion | 1.377±0.020 AU |
| Semi-major axis | −0.260±0.004 AU |
| Eccentricity | 6.30±0.15 |
| Max. orbital speed | 68.3 km/s with respect to the Sun @ perihelion |
| Inclination | 175.12±0.01° |
| 322.31±0.16° | |
| Argument of periapsis |
127.78±0.20° |
| Next perihelion | 29 October 2025 |
| Earth MOID | 0.38 AU |
| Mars MOID | 0.01 AU |
| Jupiter MOID | 0.24 AU |
| Physical characteristics | |
|
Mean diameter
|
up to 24 km, but likely closer to 5 km |
| Comet total magnitude (M1) |
8.7±0.7 |
3I/ATLAS, also known as C/2025 N1 (ATLAS), is an amazing comet that comes from far beyond our Solar System! It's called an interstellar object because it travels between stars.
Scientists discovered it on July 1, 2025, using the Asteroid Terrestrial-impact Last Alert System (ATLAS) in Chile. When it was found, it was about 675 million kilometers (420 million miles) from the Sun. It was moving incredibly fast, at about 61 kilometers (38 miles) per second!
This comet follows a special path called a hyperbolic trajectory. This means it won't orbit the Sun like our Solar System's planets or regular comets. Instead, it will visit us once and then leave, heading back out into space forever. 3I/ATLAS is only the third interstellar object ever confirmed by astronomers. The other two were 1I/ʻOumuamua and 2I/Borisov.
The main part of a comet is called its nucleus. For 3I/ATLAS, it's hard to know its exact size. This is because it's an active comet, meaning it's surrounded by a big cloud of dust and gas called a coma. This coma makes it look fuzzy and bright. Scientists think the nucleus of 3I/ATLAS is probably smaller than 24 kilometers (15 miles) across. It might even be closer to 5 kilometers (3 miles) wide.
3I/ATLAS will get closest to the Sun on October 29, 2025. At that time, it will be about 206 million kilometers (128 million miles) from the Sun.
Contents
How Was This Comet Found?
The Discovery Moment
3I/ATLAS was first spotted on July 1, 2025. It was found by the NASA-funded Asteroid Terrestrial-impact Last Alert System (ATLAS) telescope in Río Hurtado, Chile.
When it was discovered, the comet was quite dim. This means it was too faint to see without powerful telescopes. It was speeding into the inner Solar System at about 61 kilometers (38 miles) per second. It was about 530 million kilometers (330 million miles) from Earth and 675 million kilometers (420 million miles) from the Sun. At first, scientists thought it might come very close to Earth.
Confirming Its Interstellar Journey
Soon, other telescopes and astronomers, both professional and amateur, started watching 3I/ATLAS. Their observations showed that it would not come close to Earth. Instead, its path suggested it was an interstellar visitor.
Scientists even found older pictures of the comet from before its official discovery. These "pre-discovery" observations confirmed its unique path. For example, the Zwicky Transient Facility had seen it in June 2025. One amateur astronomer, Sam Deen, found even more early ATLAS observations. He thought the comet might have been hard to spot earlier because it was passing in front of a very crowded part of the sky near the center of our galaxy.
Is It an Asteroid or a Comet?
When 3I/ATLAS was first seen, it wasn't clear if it was an asteroid (a rocky body) or a comet (an icy body with a tail). But on July 2, 2025, telescopes in Chile, Arizona, and Hawaii saw a faint coma (the fuzzy cloud) and a short tail. This showed that it was indeed a comet, actively releasing gas and dust.
On the same day, the International Astronomical Union officially announced 3I/ATLAS. They gave it the special "3I" designation, meaning it's the third confirmed interstellar object. They also named it C/2025 N1 (ATLAS).
What We Learned Next
More observations quickly followed. Scientists like David Jewitt and Jane Luu confirmed that 3I/ATLAS was "clearly active" with a fuzzy tail. Another team saw a tail at least 25,000 kilometers (15,500 miles) long!
Measurements showed that the comet's coma had a reddish color, likely from dust. This was similar to the second interstellar comet, 2I/Borisov. It was hard to figure out how fast the comet's main body (nucleus) was spinning. This is because the bright dust cloud around it made it difficult to see the nucleus clearly.
Comet 3I/ATLAS's Amazing Journey
3I/ATLAS is on a very unusual path around the Sun. It has an extremely high orbital eccentricity of about 6.3. This number tells us how "stretched out" an orbit is. For comparison, planets have eccentricities close to 0 (nearly circular). A number greater than 1 means it's a hyperbolic trajectory, so the object will escape our Solar System.
This comet's eccentricity is the highest ever seen for an interstellar visitor! It's much higher than 1I/ʻOumuamua (1.2) and 2I/Borisov (3.4). 3I/ATLAS will make its closest approach to the Sun, called perihelion, on October 29, 2025. It will be about 206 million kilometers (128 million miles) from the Sun.
At its fastest, near the Sun, the comet will zoom at about 68 kilometers (42 miles) per second! When it's far away from the Sun, it still travels at an incredible speed of about 58 kilometers (36 miles) per second. The comet's path is also tilted quite a bit, about 175 degrees, compared to the flat plane where most planets orbit. Scientists think it came from the thin, flat part of our galaxy called the galactic disk.
Passing By Our Planets
As 3I/ATLAS gets closer to the Sun, it will pass Mars on October 3, 2025. It will be about 31 million kilometers (19 million miles) away from Mars. After it passes the Sun, it will then fly by Earth on December 19, 2025. It will be about 274 million kilometers (170 million miles) from Earth. Later, around March 16, 2026, it will pass about 46 million kilometers (29 million miles) from Jupiter.
When it's near Mars, the comet might be bright enough for spacecraft orbiting Mars to see it! However, from Earth, we won't be able to see 3I/ATLAS when it's closest to the Sun. This is because Earth and the comet will be on opposite sides of the Sun. But don't worry, it should become visible again from Earth in early December 2025.
How Big and Bright Is Comet 3I/ATLAS?
Scientists use something called absolute magnitude (H) to estimate how bright an object truly is, no matter its distance. For 3I/ATLAS, its absolute magnitude is about 11.9. If 3I/ATLAS were just a dark rock (an asteroid), this brightness would suggest its nucleus could be up to 24 kilometers (15 miles) wide.
But since it's an active comet with a bright coma (dust cloud), its actual nucleus is likely much smaller. The light we see comes from both the nucleus and the reflective dust around it. Even though it's active, 3I/ATLAS seems less active than the previous interstellar comet, 2I/Borisov. This makes scientists think that 3I/ATLAS's nucleus might be about ten times larger than 2I/Borisov's. For example, 2I/Borisov's nucleus was estimated to be about 0.4 to 0.5 kilometers (0.25 to 0.3 miles) across. So, 3I/ATLAS's nucleus could be around 4 to 5 kilometers (2.5 to 3 miles) wide.
See also
- 1I/ʻOumuamua–the first interstellar interloper discovered
- 2I/Borisov—the second interstellar interloper and the first interstellar comet discovered
