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Comet Watch: 3I/ATLAS Flies Through the Solar System

“This is the first opportunity to study an interstellar object in such exquisite detail,” says Dr. Thomas H. Puzia, an old friend of timeanddate.

Early image of comet 3I/ATLAS with the first detection of its tail. Taken by Thomas Puzia’s team at Pontificia Universidad Católica de Chile on August 23, 2025, using ESO's Very Large Telescope (VLT).
An image of 3I/ATLAS with an early detection of its tail. Thomas Puzia and his team at Pontificia Universidad Católica de Chile took this image of the comet using the Very Large Telescope on August 23.
©ESO/PUC/Thomas H. Puzia

An Interstellar Visitor

It isn’t something we can see with the naked eye or binoculars—it won’t be lighting up the night sky—but the comet 3I/ATLAS is causing astronomers to catch their breath.

Show the location of 3I/ATLAS on our Night Sky Map

Why? The clue is in the first part of the comet’s name: 3I tells us it is the third confirmed interstellar object in history. An interstellar object is something that comes from outside our solar system, and is not gravitationally bound to the Sun.

In other words, 3I/ATLAS is passing fleetingly through our solar system, on a journey through space that has likely already lasted billions of years. In mid-2023, the comet crossed the orbital path of Neptune, the farthest planet from the Sun.

On July 1, 2025—not long after it reached the orbit of Jupiter—3I/ATLAS was spotted for the first time by a robotic telescope in Chile. (The telescope is part of the ATLAS system, which explains the second part of the comet’s name.)

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Eyes On the Sky

One of the first teams to begin studying 3I/ATLAS was a group led by Dr. Thomas H. Puzia at the Institute of Astrophysics, Pontificia Universidad Católica de Chile (PUC).

3I/ATLAS was discovered on Dr. Puzia’s birthday. This meant he spent most of the day writing a proposal to get observation time on the Very Large Telescope (VLT)—a highly advanced facility run by the European Southern Observatory in Chile’s Atacama Desert.

His group’s observations using the VLT began on the night of July 3–4, and they soon noticed a haze around the object. “We can see how certain parts of the object are puffing out,” Dr. Puzia tells us. “There are one or more mechanisms that liberate dust from the surface.”

The group studied the wavelengths of light being reflected by 3I/ATLAS, hoping to learn more about its chemistry. This is a technique known as spectroscopy. “The chemistry is an important marker for where this planetary building block came from,” notes Dr. Puzia.

“Everything Is New”

Astronomers have had two previous opportunities to study interstellar objects: 1I/ʻOumuamua (which was discovered in 2017) and 2I/Borisov (in 2019). But 3I/ATLAS was discovered when it was farther away on an inbound trajectory, enabling astronomers to witness more of its evolution as it approaches the Sun and warms up.

“This is the first opportunity to study things in such exquisite detail,” says Dr. Puzia. “Everything is new, everything is exotic.”

Dr. Puzia has been observing 3I/ATLAS alongside three of his PhD students: Rohan Rahatgaonkar, Juan Pablo Carvajal, and Baltasar Luco. “We are working overtime for this once-in-a-lifetime opportunity. We balance dissertations and responsibilities, but right now our compass points to one thing: 3I/ATLAS. When the Universe whispers, you answer.”

The group has made use of two other facilities in Chile: The SOAR Telescope (on a mountaintop close to the ATLAS telescope that discovered the comet), and ALMA (a powerful radio telescope near the Bolivian border).

Image of team at Institute of Astrophysics, Pontificia Universidad Católica de Chile: Thomas Puzia (back right), Baltasar Luco (back left), Juan Pablo Carvajal (front right), Rohan Rahatgaonkar (front left).
The PUC team that has been studying 3I/ATLAS, clockwise, from top left: Baltasar Luco, Thomas Puzia, Juan Pablo Carvajal, and Rohan Rahatgaonkar. One of their first papers about the comet was published at the end of August; another is in review and a preprint is available.
©PUC

Closest to Sun on Oct 29

3I/ATLAS passed close to Mars on October 3, giving spacecraft in orbit around the Red Planet the opportunity to take pictures.

The comet will reach its closest point to the Sun—about midway between the orbital paths of Mars and Earth—on October 29. Its closest approach to Earth will be on December 19, although it will be some distance from our planet: A little under twice the distance from Earth to the Sun.

Astronomers will keep their telescopes on 3I/ATLAS for as long as possible, trying to learn as much as possible from this unexpected visitor. “It’s spent billions of years in interstellar space,” says Dr. Puzia. “Who knows what it’s floated through? Now it’s presenting it to us to understand.”

Night Sky Map screenshot showing the position of comet 31/ATLAS, as seen from New York, USA. The comet’s brightness is greatly exaggerated—it will only be visible with a telescope.
3I/ATLAS will only be visible to telescopes—it is far too faint to see with the naked eye. Our Night Sky Map indicates the comet’s position: This screenshot for New York, USA, shows it will be in the same part of the sky as the Moon around November 16.
©timeanddate.com

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 11.5 (estimate)

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 comets
What is magnitude?

The brightness of an astronomical object as seen from Earth is called its apparent magnitude. Two important things to know:

  1. 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
  2. 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 aphelion
What 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