My lay understanding of a comet consists of a relatively small object that orbits the sun and as such it is part of our solar system. Recently, scientists detected a new type of celestial object from outside our solar system that is incredibly large and moving very, very fast. It does not orbit the sun and exhibits a number of abnormal characteristics. Why then do they refer to it as a “comet?”
Source: https://science.nasa.gov/solar-system/comets/3i-atlas/comet-3i-atlas-multimedia/
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You said:
I’m unclear why 3I/Atlas is referred to as a comet when it isn’t solar bound?
ChatGPT said:
Great question — and this gets into how astronomers define “comet” versus how they classify interstellar objects.
Why 3I/ATLAS is called a comet
- “Comet” describes its appearance and behavior, not its orbital binding.
- 3I/ATLAS has a coma (gas + dust surrounding a nucleus) and a tail that develops as volatile ices sublimate when warmed by the Sun.
- That makes it physically and observationally a comet — regardless of whether it’s gravitationally bound to the Sun.
- The “I” in 3I/ATLAS is the new designation for “interstellar.”
- Astronomical naming rules (set by the IAU) label interstellar objects as nI, where n is the discovery number.
- So this is the third confirmed interstellar object after 1I/ʻOumuamua and 2I/Borisov.
- Its full designation is C/2025 N1 (ATLAS), where:
- C/ = a comet not expected to return (non-periodic)
- 2025 N1 = discovered in the first half of July 2025
- ATLAS = the survey that discovered it
- The “comet” label is therefore based on activity (coma/tail), while “interstellar object” is its dynamical class.
The distinction
- Solar-bound comets: These have elliptical orbits (short-period or long-period) around the Sun.
- Interstellar comets: Hyperbolic orbit, not bound; they visit once and leave.
- 2I/Borisov (2019) was the first recognized interstellar comet — it behaved just like a natural comet but was clearly not bound to the Sun.
- 3I/ATLAS follows in that category: cometary in behavior, interstellar in origin.
✅ So: “Comet” = what it looks like and does.
“Interstellar” = where it comes from and how it moves.
Would you like me to sketch a simple chart that contrasts solar comets vs. interstellar comets vs. asteroids so it’s easier to see the taxonomy at a glance?
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