Interstellar comet 3I/ATLAS made its closest approach to Earth on December 19, 2025, passing within 167 million miles (269 million kilometers) of our planet. It is only the third interstellar object ever detected, after 1I/'Oumuamua in 2017 and 2I/Borisov in 2019 (PBS NewsHour, 2025).
The comet was discovered on July 1, 2025, by the NASA-funded ATLAS survey telescope in Chile. It arrived from the direction of the constellation Sagittarius and is traveling on a hyperbolic orbit that will carry it out of the solar system and back into interstellar space, never to return (NASA, 2025).
How old is 3I/ATLAS?
A study published in Nature in June 2026 found that 3I/ATLAS could be up to 12 billion years old, nearly three times the age of our solar system, which formed about 4.5 billion years ago. The isotopic evidence suggests it formed in an environment that was minus 243 degrees Celsius, making it among the coldest objects ever seen in our solar system (CBS News, 2026).
What did astronomers learn?
The Hubble Space Telescope detected the comet's nucleus, estimating an effective radius of 1.3 kilometers, much larger than the nuclei of 'Oumuamua (0.08 km) and Borisov (0.4 km). The James Webb Space Telescope and the Atacama Compact Array observed the comet at infrared and microwave wavelengths, revealing a low abundance of carbon-13 that points to formation roughly 12 billion years ago (IOPscience, 2026).
The comet is now leaving the solar system and will never return, so future observations will become increasingly difficult.
— Astronomers, via AFP
Is there any alien technology?
The SETI Institute conducted extensive radio scans of 3I/ATLAS with its telescope in Northern California and found no signs of otherworldly technology. The object looks to be completely natural. SETI's Sofia Sheikh led the analysis, which gave the all-clear on the "alien tech" question (AP News, 2026).
- Discovered July 1, 2025 by the ATLAS survey in Chile
- Closest approach to Earth: December 19, 2025 (167M miles)
- Closest approach to Sun: October 30, 2025 (1.4 au)
- Nucleus radius: ~1.3 km, per Hubble observations
- Age: up to 12 billion years, per Nature study
What comes next?
Astronomers expect to spot many more interstellar objects in the coming years, particularly via the new Vera C. Rubin Observatory in Chile, which could find dozens in the decade to come. The European Space Agency plans to launch the Comet Interceptor mission in 2028 or 2029, which will park in space and wait for an undiscovered comet to target (phys.org, 2026).
Sources and further reading
- PBS NewsHour — Interstellar comet 3I/Atlas will make closest approach to Earth
- NASA — NASA Discovers Interstellar Comet Moving Through Solar System
- CBS News — Interstellar comet could be oldest object ever seen in solar system
- Starship's Orbital Refueling Test: The Results Are In
- Fusion's Break-Even Moment: Real or Lab Fiction
- The Lunar Economy: Mining the Moon Gets Serious
Bottom line
Astronomers expect to spot many more interstellar objects in the coming years, particularly via the new Vera C. Rubin Observatory in Chile, which could find dozens in the decade to come. The European Space Agency plans to launch the Comet Interceptor mission in 2028 or 2029, which will park in space and wait for an undiscovered comet to target (phys.org, 2026).
What we still don't know
This is a fast-moving story. We update the post as new facts land — and we'll flag it when we do.
Enjoyed this? Pay it forward
Five people forward this newsletter before they finish their coffee. Make it six.
