An interstellar comet older than our solar system emits water vapor every day at a rate equivalent to 70 Olympic swimming pools. Scientists say this may be the most valuable lesson in planetary defense they’ve received in years.
Comet 3I/ATLAS was discovered on July 1, 2025 and has since become the subject of some of the most unusual tracking in ESA’s history.
What makes Comet 3I/ATLAS different from all previous comets?
Comet 3I/ATLAS did not originate here. It formed somewhere in the far reaches of the Milky Way and is estimated to be at least 7 billion years old and possibly 10 billion years old, making it the oldest comet ever identified.
It passes through our solar system once and will never return, giving scientists a narrow window to study it.
The space comet 3I/ATLAS reached its closest point to the Sun in late October 2025, resulting in the spacecraft receiving an intense burst of solar energy.
The heat caused ice in the comet’s core to sublimate into water vapor, leading to the exceptional rate of outgassing that scientists observed in early November during ESA’s Juice spacecraft camera observation of the comet.
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 mission’s main goal is to study Europa, Ganymede and Callisto, as scientists believe these moons have buried liquid oceans beneath their ice-covered surfaces.
The Juice science team used the spacecraft’s NavCam system to observe 3I/ATLAS because it gave them better close-up images that telescopes on Earth couldn’t match. Juice was able 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, which they collected in November 2025, to monitor the comet’s position changes and improve their trajectory calculations.
That tracking ability is the finding that defense planners are paying attention to. Many potentially dangerous asteroids are too far away and too dimly lit to be detected from Earth until they are already approaching.
Juice’s observations from 3I/ATLAS show that deep space probes already operating in the outer solar system could serve as advanced warning sensors that could calculate the orbits of threatening objects long before any ground-based telescope could confirm their existence.
