How Big is 3I/ATLAS? The Massive Interstellar Visitor Revealed

The Answer: 3I/ATLAS is at Least 5 Kilometers Wide

3I/ATLAS measures at least 5 kilometers in diameter, making it a giant among interstellar visitors. To put this in perspective, this mysterious object from another star system is 10 times larger than 2I/Borisov, the previous record holder for interstellar objects, and absolutely dwarfs the famous 'Oumuamua.

Recent calculations by Harvard astrophysicists, based on precise tracking data from May through September 2025, have established this as the minimum size for 3I/ATLAS. The actual diameter could be even larger, but we now know with confidence that this interstellar wanderer is truly massive by cosmic visitor standards.

With a mass of at least 33 quadrillion kilograms (3.3 × 10^16 grams), 3I/ATLAS isn't just big, it's the most massive interstellar object ever detected passing through our solar system.

How Scientists Measured an Object From Another Star System

Measuring the size of something millions of kilometers away presents unique challenges, especially when that object comes from beyond our solar system. Scientists used a clever approach: they tracked 3I/ATLAS with incredible precision as it moved through space, looking for any deviation from a purely gravitational path.

Over 4,000 separate observations from 227 different observatories worldwide were combined to create the most accurate trajectory map ever compiled for an interstellar object. By analyzing how 3I/ATLAS moved compared to what gravity alone would predict, researchers could determine how much material the object was losing to space and, from that, calculate its minimum size.

The James Webb Space Telescope provided crucial data on August 6, 2025, measuring that 3I/ATLAS was ejecting 150 kilograms of material every second at speeds of 0.44 kilometers per second. This outgassing rate, combined with the object's stable trajectory, allowed scientists to work backward and determine that 3I/ATLAS must be at least 5 kilometers across to maintain its observed path.

Comparing 3I/ATLAS to Other Interstellar Visitors

The size of 3I/ATLAS becomes even more remarkable when compared to our previous interstellar guests:

While 'Oumuamua was roughly the size of a city block and 2I/Borisov comparable to a small mountain, 3I/ATLAS is more like a small city. If you could place 3I/ATLAS next to Manhattan, it would span from the southern tip of the island nearly to Central Park.

This dramatic size difference has profound implications. Larger objects retain heat longer, can sustain more complex internal processes, and are much more likely to survive the harsh journey between star systems. The fact that 3I/ATLAS arrived intact after potentially millions of years in interstellar space speaks to both its size and robust construction.

What Makes 3I/ATLAS So Unusually Large?

The exceptional size of 3I/ATLAS raises intriguing questions about where it came from and how it formed. Most objects ejected from planetary systems tend to be smaller chunks of rock and ice, similar to the comets we see in our own solar system.

For comparison, 3I/ATLAS is similar in size to some of our solar system's most famous comets:

• Halley's Comet: approximately 15 km long and 8 kilometers wide
• Comet Hale-Bopp: estimated 40 to 60 kilometers in diameter
• 67P/Churyumov-Gerasimenko: about 4 kilometers

Finding an interstellar object this large suggests it came from a planetary system with either very different formation processes than our own, or from a much more violent ejection event that could launch such massive objects into interstellar space.

The Challenge of Detecting Large Interstellar Objects

The discovery of such a large interstellar visitor actually creates a puzzle for astronomers. If objects the size of 3I/ATLAS are common among interstellar wanderers, we should be detecting them much more frequently than we do.

The rarity of finding objects this size suggests either that 3I/ATLAS represents an extremely unusual case, or that our understanding of how planetary systems eject material needs revision. The amount of rocky material that would need to be ejected from star systems to account for objects like 3I/ATLAS far exceeds current estimates of what's available.

This size paradox is one of many mysteries surrounding our cosmic visitor, adding to the growing list of unusual properties that make 3I/ATLAS unlike anything we've encountered before.

Getting Even Better Measurements

While we now know 3I/ATLAS is at least 5 kilometers across, even more precise measurements are coming. On October 3, 2025, when 3I/ATLAS passes within 29 million kilometers of Mars, NASA's Mars Reconnaissance Orbiter will capture the highest resolution images ever taken of an interstellar object.

The HiRISE camera will achieve a resolution of about 30 kilometers per pixel, potentially revealing surface features and giving us our first detailed look at the structure of an interstellar visitor. These observations could refine our size estimates and might even show whether 3I/ATLAS has the irregular shape typical of natural comets or something more geometrically precise.

Why Size Matters for Interstellar Objects

The massive size of 3I/ATLAS isn't just a curiosity, it fundamentally changes how we think about interstellar visitors. Larger objects can:

• Retain internal heat longer, potentially supporting more complex chemistry
• Survive the radiation and particle bombardment of interstellar space
• Maintain structural integrity over million-year journeys between stars
• Carry larger amounts of material from their origin systems

This size also means 3I/ATLAS has been visible to our telescopes for much longer than smaller visitors, giving astronomers unprecedented time to study an interstellar object. The extended observation period has revealed chemical signatures and behaviors that would have been impossible to detect in smaller, more fleeting visitors.

The Bottom Line on 3I/ATLAS Size

3I/ATLAS is definitively at least 5 kilometers in diameter, making it the largest interstellar object ever observed passing through our solar system. This massive size, 10 times larger than the previous record holder 2I/Borisov, places 3I/ATLAS in the same category as substantial comets in our own solar system.

The object's enormous mass of at least 33 quadrillion kilograms and its ability to maintain a stable trajectory while ejecting material at high rates confirms that we're dealing with a truly substantial visitor from another star system. As observations continue through its Mars flyby in October 2025, we may discover that 3I/ATLAS is even larger than current minimum estimates suggest.

This remarkable size, combined with 3I/ATLAS's other unusual properties, ensures it will remain a subject of intense scientific interest as the most significant interstellar visitor humanity has yet encountered.

Sources and Research

This article is based on the latest research by Richard Cloete, Abraham Loeb, and Peter Vereš from Harvard University and the Minor Planet Center, published in September 2025. The size calculations draw from over 4,000 astrometric observations collected between May and September 2025, combined with JWST spectroscopic data. For detailed technical analysis, see the original research paper and related studies on 3I/ATLAS observations.