3I/ATLAS compared to Mount Etna

Third known interstellar visitor, and the largest and fastest of the three so far, possibly one of the oldest comets ever observed.

Can you see it?

No small telescope will show you this one; it is here to be understood, not spotted.

How big is 3I/ATLAS compared to Mount Etna?

3I/ATLAS is roughly one thirteenth as wide as Mount Etna. That is for a width of about 2.6 km; its real size is only known to lie between 440 m and 5.6 km.

Worth knowing about 3I/ATLAS

Its size: nucleus measured by Hubble on 21 July 2025; the earlier 15–20 km figure was ground-based photometry that could not separate the nucleus from its coma, so it measured the coma.

Why it stands out: Some estimates suggest it may be older than our own solar system (up to 11 billion years). Webb detected methane in its coma after it passed the Sun — the first ever found in an interstellar object.; How we know: Scientists worked backwards from its speed and trajectory to estimate where in the galaxy it likely formed, combined with its unusual chemistry — together suggesting it formed around a very old star, long before our own Sun existed. Rung 3 of the cosmic ladder — how the solar system is arranged

3I/ATLAS — No spacecraft has been there, so nobody knows what its surface looks like. What you see is only what has been measured of it: its size, and its shape and colour where anyone knows them.

The numbers

Mean diameter	440 m to 5.6 km
Discovered	2025

How anyone knows this

Games it appears in

Put them in size order

Show 3I/ATLAS in the model ☀️ Everything the Sun holds 🌙 Plan an evening outside