An interstellar comet older than our solar system is releasing water vapor at a rate equivalent to 70 Olympic swimming pools every day, and scientists say it may be the most valuable planetary defense lesson they’ve learned in years.
Comet 3I/ATLAS was discovered on 1 July 2025 and has since become the subject of some of the most unusual tracking operations in ESA’s history.
What makes Comet 3I/ATLAS different from any comet before it?
Comet 3I/ATLAS was not built here. It originated somewhere in the far reaches of the galaxy and is estimated to be at least 7 billion years old and possibly as much as 10 billion years old, making it the oldest comet ever identified.
It is passing through our solar system once and will never return, giving scientists a narrow window to study it.
Space comet 3I/ATLAS reached its closest point to the Sun in late October 2025, resulting in the spacecraft receiving an intense solar burst.
The heat caused ice within the comet’s nucleus to turn into water vapor, causing the extraordinary rate of outgassing that scientists observed during observations of the comet by ESA’s Juice spacecraft camera in early November.
The Jupiter Icy Moons Explorer, Juice, is currently traveling through the solar system to reach its destination at Jupiter, which will occur in July 2031.
The main objective of the mission is to study Europa, Ganymede and Callisto as scientists believe that these moons have buried liquid oceans beneath their ice-covered surfaces.
The Juice science team used the spacecraft NavCam system to observe 3I/ATLAS because it provided them with better close-up images that Earth-based telescopes could not match. JUICE had the ability to detect the comet because it appeared during a period when Earth observers could not see it.
ESA’s Planetary Defense team used NavCam images they collected during November 2025 to track changes in the comet’s position and improve their trajectory calculations.
That tracking capability is what defense planners are focused on. Many potentially dangerous asteroids are so distant and have so little light that they cannot be detected from Earth until they are already close.
The 3I/ATLAS observations of Joos show that deep-space probes already operating in the outer Solar System could serve as advanced early warning sensors capable of calculating the orbits of dangerous objects long before any ground-based telescope can confirm their existence.
