3I/ATLAS: The Story So Far
Comet 3I/ATLAS, the third confirmed interstellar object in history, was discovered on July 1, 2025. The word “interstellar” means it comes from outside our solar system, and is not gravitationally bound to the Sun.
From October: 3I/ATLAS flies through solar system3I/ATLAS reached its closest point to the Sun on October 29. It will be at its closest to Earth on December 19—although it will be much too faint to see with the naked eye.
Thomas Puzia at the Institute of Astrophysics, PUC in Santiago, Chile, has been observing the comet with three of his graduate students: Rohan Rahatgaonkar, Juan Pablo Carvajal, and Baltasar Luco. As an old friend of timeanddate, we caught up with Dr. Puzia and his team for the latest news.
What’s Happening as 3I/ATLAS Flies Through the Solar System?
Having likely spent billions of years in interstellar space, 3I/ATLAS started to warm up as it approached our Sun on its journey through the inner solar system. For astronomers analyzing the comet’s chemical makeup, this created a race against time as possible clues about the comet’s interstellar origin began to turn into gas and disappear into space.
“The comet’s inbound trajectory into the solar system gives you the surface chemistry,” says Dr. Puzia. “The outbound trajectory gives you the nucleus chemistry. You have to catch these things early to see the unprocessed surface chemistry.”
What Interesting Chemistry Have We Seen?
Early on, the team was intrigued by some patterns in the ultraviolet part of the spectrum of light from 3I/ATLAS. The intensity of the patterns increased over time, suggesting it wasn’t just noise.
After some detective work in the library, they concluded it could be an indication of nickel. This element was detected in 2I/Borisov, an interstellar comet discovered in 2019, and has also been seen in comets originating from within our own solar system. Nevertheless, it was unusual to observe nickel vapor when 3I/ATLAS was so far from the Sun, and still very cold.
Rohan Rahatgaonkar takes up the story: “Maybe it wasn’t noise but emission lines. We had to read through the literature to find out what these lines could be. We were going through hundreds of papers, all over a weekend, and found they could be nickel—the same as Borisov.”
How Else Has 3I/ATLAS Been Changing?
Like all comets, 3I/ATLAS sped up as it neared the Sun, and is slowing down as it moves away. Astronomers have to account for these changes in velocity, since they affect the wavelengths of light coming from the comet. “We can see night-to-night changes in the velocity,” notes Baltasar Luco.
What’s the Big Question We Want to Answer?
In a way, the biggest question for astronomers is: Where did 3I/ATLAS come from? “The chemistry of 3I/ATLAS can help us put its formation in the Milky Way at a certain location and epoch,” Dr. Puzia explains. “It’s an ice-rich object, likely from the outskirts of a protoplanetary disk around some star.”
Unwinding the Milky WayWhat’s the Key Message from 3I/ATLAS So Far?
As only the third confirmed interstellar object we’ve ever observed, Dr. Puzia likens 3I/ATLAS to a primordial message in a bottle: “It‘s telling us there’s much more chemical variance in the Milky Way galaxy than in the solar system,” he remarks.
The hope is that this object from outside our own solar system can tell us more about how planetary systems form in the galactic context. “We know about the chemistry of stars, but what about their planetary systems? That’s a different question.”
How Big Is 3I/ATLAS?
Perhaps surprisingly, estimates for the size of 3I/ATLAS vary from around 320 meters (350 yards) up to around 5.6 km (3.5 miles). “It’s a huge range,” admits Dr. Puzia.
The team in Santiago is hoping to play a role in narrowing these estimates using infrared light, which, unlike visible wavelengths of light, can pass through the dust surrounding the comet.
Show the location of 3I/ATLAS on our Night Sky Map
What Happens Next?
A global network of astronomers is continuing to collect data on 3I/ATLAS from many different telescopes. The comet has even been observed by the European Space Agency’s ExoMars and Mars Express spacecraft in orbit around the Red Planet, as well as the Juice spacecraft currently on its way to Jupiter.
On November 30, 3I/ATLAS was studied for a second time by the Hubble Space Telescope, producing the main image shown at the top of this page.
Juan Pablo Carvajal points out that different telescopes observe the comet at different wavelength ranges, and can therefore see different things. “Astronomers have taken data everywhere and are analyzing,” he tells us. “Corroborating data takes time. More is coming!”
What Would Be the Most Exciting Discovery?
Just as a hypothetical question, we asked the team in Santiago what they would most like to see in the 3I/ATLAS data. “I want to see the currency of life, which is phosphorus,” replied Dr. Puzia.
Phosphorus is a key element in DNA, energy housekeeping in cells, and other building blocks for life as we know it. “Imagine confirming the ingredients for life in a billion-year-old interstellar visitor. It would dramatically expand the chances that complex life could have emerged much earlier in the history of the Milky Way and, by extension, the universe.”
3I/ATLAS Facts & Figures
Note: 3I/ATLAS has another official name, C/2025 N1 (ATLAS), which follows the usual rules for comet names (see the FAQs below).
| Discovery | 1 Jul 2025 |
|---|---|
| Discovered by | ATLAS |
| Perihelion | 29 Oct 2025 |
| Closest approach to Earth | 19 Dec 2025 |
| Brightest magnitude | 9.5 (approx) |
Frequently Asked Questions
What is a comet?
Comets are dusty, icy, rocky objects—often around one to ten kilometers in size—that orbit the Sun.
Most comet orbits are highly elliptical. This means that, at one extreme of their orbit, they pass close to the Sun; at the other extreme, they lie far from the Sun, in the outer part of the solar system.
A few comets have hyperbolic orbits, where the comet swings by the Sun, but doesn’t go into orbit around it.
Read more about cometsWhat is magnitude?
The brightness of an astronomical object as seen from Earth is called its apparent magnitude. Two important things to know:
- Magnitude uses a reverse scale: The lower the number, the brighter the object—in the case of very bright objects, the number can go below zero
- Magnitude uses a logarithmic scale: If the magnitude of an object decreases by 1, it becomes about 2.512 times brighter
So although it may seem as though the Sun (magnitude -27) is only twice as bright as the Full Moon (magnitude -13), it is in fact 400,000 times brighter. (The difference between -27 and -13 is 14, and 2.512 multiplied by itself 14 times is 2.512^14, which is roughly 400,000.)
Below are some approximate examples of the average magnitude of objects as seen from Earth.
- The Sun: -27
- Full Moon: -13
- Venus: -4.1
- Jupiter: -2.2
- Sirius (the brightest star): -1.5
- Polaris (the Pole Star): +2.0
- Limit of naked eye (with good conditions): +6.5
- Neptune: +7.8
- Limit of handheld binoculars: +9.5
What is perihelion?
Perihelion is the point where a comet—or other astronomical body—is at its closest to the Sun.
Perihelion and aphelionWhat are the rules for comet names?
Those complicated-looking names are based on a set of rules from the International Astronomical Union. Let’s take C/2024 G3 (ATLAS) as an example:
- C/ refers to a non-periodic comet. This means the comet hasn’t been observed before, and its orbital period (the amount of time it takes to complete one orbit around the Sun) is greater than 200 years. Alternatively, it may be a comet with a hyperbolic orbit (see “What is a comet?” above). There are other prefixes, too: For example, “P/” is used for comets with an orbital period of less than 200 years; “I/” is used for interstellar comets
- 2024 is the year the comet was discovered
- G indicates the half-month it was discovered—in this case, the first half of April. The first half of January is “A,” the second half of January is “B,” the first half of February is “C,” and so on. Here the letter “I” is skipped to avoid confusion with the number “1”
- 3 means it was the third comet discovered in that half-month
- ATLAS is the name of the instrument that made the discovery—in this case, ATLAS is a system of robotic telescopes. Comets can also be named after people: For example, C/2024 E1 (Wierzchos) was discovered by the Polish astronomer Kacper Wierzchoś