Astronomers recently confirmed the discovery of Comet 3I/ATLAS, an interstellar object flying through the Solar System at breakneck speed. Detected on July 1, 2025, by NASA's ATLAS telescope in Chile, it's only the third confirmed visitor from beyond our solar neighborhood after Oumuamua and Borisov. Traveling at nearly 68 kilometers per second, 3I/ATLAS isn't just fast - it's ancient, likely more than 7 billion years old, predating the Sun itself.
Luckily, its current path poses no threat to Earth. It will pass safely between the orbits of Mars and Earth, reaching its closest approach to the Sun on October 29 before shooting back into interstellar space. Still, it's hard not to wonder what would happen if its trajectory changed and it headed our way. Spoiler: it still wouldn't be pretty.
The comet's icy nucleus is calculated to be roughly a kilometer wide, with a total width of over five kilometers including its coma - large enough to cause regional to hemispheric damage if it ever hit. If an unexpected event like asymmetric outgassing or a gravitational nudge redirected it toward Earth, it would arrive with terrifying speed. At 60 to 80 kilometers per second, the resulting impact could release on the order of 5×10^20 to 2.5×10^21 joules of energy - roughly 120,000 to 600,000 megatons of TNT, but much smaller than the impact that killed the dinosaurs.
The effects would still be severe. The fireball would span tens of kilometers across, and a land impact could excavate a final crater roughly 15 to 25 kilometers in diameter and a few kilometers deep, depending on impact angle and target geology. Overpressure from the shockwave could devastate structures within a few hundred kilometers, with intense thermal radiation igniting fires across large areas downrange.
An ocean strike would launch basin-scale tsunamis. Initial wave heights near the source could exceed 50 to 100 meters, with destructive run-up on distant coasts. Inundation is typically tens of kilometers inland for vulnerable shorelines, though local geography can focus waves much farther.
Climate impacts would be serious but more modest than "extinction-level." Dust, sulfate aerosols, and smoke from fires would dim sunlight and cool global temperatures by roughly 1 to 3 °C for a year or a few years, with strong regional variability. Agriculture and supply chains would be under acute stress, but a decade-long 5 to 10 °C plunge is unlikely at this energy scale. All of these calculations are hypothetical, of course, since we still have very little verifiable info about this interstellar visitor.
Thankfully, space agencies are keeping a close watch. NASA's Jet Propulsion Laboratory and the Minor Planet Center confirm that 3I/ATLAS's orbit is stable, with its closest pass at about 1.8 astronomical units - nearly twice the distance between Earth and the Sun. It's currently visible only through high-powered telescopes and will fade after November.
For scientists, 3I/ATLAS offers more fascination than fear. Its unusual composition, including elevated nickel vapor and a thick, radiation-scarred crust, gives insight into how interstellar material evolves over billions of years and helps stress-test planetary-defense models for fast movers. So while the "what if" makes for great cinema, this ancient traveler is safely just passing through.