[lippel82]

The headline seems to be plainly wrong. It's an interstellar comet that supposedly has not had a close encounter with another star. But it certainly is not the "first interstellar comet to have visited our solar system", or did I miss something?

[UnlockedSecrets]

It is the first confirmed interstellar comet and so is the first under the interstellar comet classification, Unfortunate that the headline does not contain that detail however.

[elihu]

That was my reaction; I'm not sure whether they meant to say the "21/Borisov, the interstellar comet for whom our system is the first solar system it has visited", or (as other comments suggest) "the first confirmed interstellar comment to visit our system". Obviously if something is only now noticed with modern technology, it's unlikely that interstellar comets haven't swung by our solar system many times before. We just don't have a record of it.

[davidcuddeback]

It's somewhat debatable. 1I/Omuamua was reclassified as an asteroid because it didn't exhibit a coma. But it's possible that it's a remnant of a comet.

[firebaze]

Despite reading quite a lot about astronomy, comets and asteroids and the history of all of the three, I didn't get the distinction between asteroids and comets yet. I understand the composition makes the difference, but I didn't get why we expect exactly two (!) distinct categories of intersolar/interstellar objects, and giving them exactly two distinct names. Why not more, or less (i.e. no distinction at all, just a summary of the probable composition)?

Why is this so? Is there a fundamental reason to make this special distinction, or is it just "tradition"?

[AnimalMuppet]

Asteroids mostly have circular orbits. Comets (at least the ones we see) tend to have very highly eliptical orbits. That I think is the origin of the distinction.

But asteroids also tend to be at lower distances from the sun than comets. That in turn leads to them tending to be composed more of rocky material, whereas comets have more ice.

I am not an astronomer, so any and all details may be in error. Corrections are welcome.

[garmaine]

Mostly tradition. Asteroids have had their volatiles baked off their surface, or never had them due to forming inside the frost line. But it’s not a very intrinsically meaningful distinction—it’s more about what they look like from the vantage point of Earth as they come by on close approach.