Astronomers and skywatchers are counting down to a rare celestial milestone: the close approach of comet 3I/ATLAS, the third confirmed interstellar object ever detected passing through our solar system. With just one day to go until its closest pass by Earth, observatories worldwide are racing to capture as much data as possible on this fast-moving visitor from another star system.

Image Illustration. Photo by Justin Wolff on Unsplash

What Is Comet 3I/ATLAS?

Comet 3I/ATLAS belongs to an exceptionally small class of known objects: bodies that originated beyond our solar system and were gravitationally captured only briefly as they passed through. It follows in the footsteps of 1I/ʻOumuamua, discovered in 2017, and interstellar comet 2I/Borisov, found in 2019. Both objects’ orbits were conclusively shown to be hyperbolic, meaning they were not bound to the Sun and must have come from interstellar space.

The "ATLAS" in the comet’s name refers to the Asteroid Terrestrial-impact Last Alert System (ATLAS) survey in Hawaii, which uses wide-field telescopes to scan the entire visible sky several times per night in search of potentially hazardous near-Earth objects. The same system has been responsible for discovering numerous comets and asteroids, including several that have passed relatively close to Earth in recent years.

An Interstellar Trajectory at Solar-System Speeds

Like its predecessors, 3I/ATLAS is on a hyperbolic orbit, which means it is traveling too fast to be captured permanently by the Sun’s gravity. Interstellar objects are typically clocked at tens of kilometers per second relative to the Sun; ʻOumuamua, for example, passed through the inner solar system at about 87.7 kilometers per second at perihelion, while 2I/Borisov moved at roughly 33 kilometers per second. By comparison, periodic comets native to our solar system, such as Halley’s Comet, travel at closer to 54 kilometers per second near the Sun.

While detailed orbital parameters for 3I/ATLAS are still being refined, early measurements indicate a similarly high incoming speed and a definitively unbound orbit, consistent with an origin beyond the Sun’s gravitational domain. In practice, that means astronomers have only a narrow window—on the order of months—to observe it as it races in and back out of the solar system.

How Close Will 3I/ATLAS Come to Earth?

Interstellar comets often grab headlines with evocative phrases like “passing close to Earth,” but in astronomical terms “close” is a relative concept. For context, 2I/Borisov never came nearer than about 2 astronomical units (AU) from Earth—roughly twice the distance between Earth and the Sun—yet it still yielded an enormous scientific payoff for cometary science.

Current trajectory solutions for 3I/ATLAS indicate a similarly safe miss distance, far beyond the orbit of the Moon and with no realistic impact risk. That is typical for objects first detected on long, hyperbolic paths; surveys like ATLAS, Pan-STARRS and the NASA-funded Near-Earth Object Observations Program, aim specifically to identify hazardous bodies long before they can pose a threat.

What Makes Interstellar Comets Scientifically Special

Although the solar system contains trillions of comets, confirmed interstellar comets like 2I/Borisov—and now 3I/ATLAS—are extraordinarily rare finds. Astronomers estimate that for every interstellar object large enough to detect, many more pass through unnoticed because they are too small or too faint. A 2020 study based on Borisov’s discovery suggested that the Milky Way may contain roughly 10²⁶ interstellar objects of Borisov-like size, implying that most stars eject vast numbers of icy planetesimals over their lifetimes.

Interstellar comets act as physical samples of other planetary systems’ building blocks. Observations of 2I/Borisov with the Hubble Space Telescope and ground-based observatories revealed a composition remarkably similar to ordinary solar-system comets, including abundant water ice and carbon-bearing molecules. That finding suggested that at least some protoplanetary disks around other stars form icy bodies under comparable conditions to those in our own early solar nebula.

3I/ATLAS presents a fresh opportunity to test whether that similarity holds up. By measuring its spectrum—how brightly it shines at different wavelengths—astronomers can infer the abundance of gases such as cyanide (CN), diatomic carbon (C₂), and water vapor, all common in solar-system comets. The comparison will help refine models for how common solar-system–like chemistry is throughout the Galaxy.

Global Campaigns and Telescope Time

With the closest approach just a day away, major observatories have queued up intensive observing campaigns. A similar mobilization occurred for 2I/Borisov, which was monitored by facilities ranging from the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile to the Very Large Telescope (VLT) and Hubble in orbit. Those data allowed teams to estimate the comet’s nucleus size at about 0.4–1 kilometer and to track how its dust and gas output evolved over time.

For 3I/ATLAS, astronomers will again rely on a mix of optical, infrared and radio observations to characterize its nucleus, coma and tail. One priority is to secure time on the James Webb Space Telescope (JWST), whose infrared spectrographs are uniquely suited to detecting ices and complex organic molecules. Another is to coordinate smaller telescopes to monitor the comet continuously, building a time-lapse record of its activity as solar heating increases and then wanes.

Will You Be Able to See 3I/ATLAS from Home?

Visibility for casual observers depends on how much dust and gas the nucleus releases as it nears the Sun. Some comets brighten dramatically, briefly becoming naked-eye objects; others remain faint smudges even in medium-sized backyard telescopes. Historical data show that only a handful of comets per decade reach magnitudes brighter than about 3, the threshold for an obvious naked-eye object under dark skies, according to long-term statistics compiled by the International Comet Quarterly.

Interstellar comets are especially unpredictable, since astronomers have no prior apparitions to use as a guide. 2I/Borisov peaked around magnitude 15—far too faint for unaided viewing and only visible in moderate to large telescopes. Early brightness estimates for 3I/ATLAS suggest it will similarly remain a target for dedicated observers with good equipment rather than a showpiece for the general public. Local astronomy clubs and public observatories, however, may organize viewing sessions if conditions permit.

A Glimpse into Other Solar Systems’ Past

In scientific terms, the real excitement around 3I/ATLAS is less about spectacle and more about what it can reveal regarding how planetary systems form and evolve. Theoretical models suggest that during the early stages of planet formation, gravitational nudges from growing giant planets eject a significant fraction of icy planetesimals into interstellar space. Numerical simulations published in The Astrophysical Journal have shown that stars like the Sun may eject tens of Earth masses’ worth of small icy bodies over time, seeding interstellar space with comets that occasionally wander into other systems, including ours.

Each interstellar comet observed in detail helps calibrate those models. Their compositions, dust-to-gas ratios, and activity patterns can be compared against solar-system comets cataloged by missions such as ESA’s Rosetta, which orbited and landed on comet 67P/Churyumov–Gerasimenko, and NASA’s Deep Impact mission to comet Tempel 1. If 3I/ATLAS turns out to be chemically unusual, it could point to more exotic pathways for planet formation around its long-gone parent star.

Looking Ahead: From Serendipity to Strategy

Until recently, the discovery of interstellar visitors like 3I/ATLAS was considered almost purely a matter of luck. But the rapid improvement of all-sky surveys is changing that. The upcoming Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST) is expected to detect tens of thousands of new solar-system bodies per year, dramatically increasing the chances of catching more interstellar objects in the coming decade. NASA has even studied concepts for rapid-response missions that could intercept an interstellar object on short notice, using data from surveys to plan a high-speed flyby or rendezvous.

For now, comet 3I/ATLAS remains strictly an astronomical target, but its fleeting visit underscores how interconnected planetary systems in our Galaxy may be. Icy fragments from one star’s debris disk can, billions of years later, swing past another world entirely—ours—offering a brief but profound look into how planets form elsewhere. With just one day to go until its closest pass by Earth, telescopes across the planet are poised to make the most of that opportunity.