Question 1

What is being measured — the comet or the tail?

Accepted Answer

The nucleus: the solid body of ice and dust, typically a few kilometres across. The coma and tail can be larger than the Sun and longer than the distance from here to it, but they are thinner than a laboratory vacuum and last only while the comet is near the Sun.

Question 2

Why are so few sizes known?

Accepted Answer

Because a nucleus is small, dark and normally buried inside its own coma. The reliable figures come from spacecraft that have visited one, or from observations made far from the Sun where the comet is inactive and the bare nucleus is what is being seen.

Question 3

What does the size column actually measure?

Accepted Answer

Not always the same thing, and the site says which. A round body has a mean radius; an oblate planet may have an equatorial one; an irregular asteroid has an effective radius, meaning the radius of a sphere that would show the same disc. Each object's page names the kind it holds.

Question 4

How firm is the order?

Accepted Answer

Firm at the top and soft further down. The largest bodies have been resolved or visited and their sizes are known to a per cent or two; the smaller ones are often sized from brightness plus an assumed reflectivity, which can be wrong by a factor of two in either direction.

Question 5

Why are some comets missing?

Accepted Answer

Because no size is recorded for them here. Measuring one needs a resolved image, an occultation, a transit or a radar echo, and most catalogued bodies have had none of those.

Question 6

How big can a comet nucleus be?

Accepted Answer

Most are a few kilometres. The largest known, Bernardinelli-Bernstein, is around 120 km across — big enough that it was first mistaken for a dwarf planet, and it will not come closer to the Sun than the orbit of Saturn.

Question 7

Does size decide how bright a comet becomes?

Accepted Answer

Only partly, and less than people expect. Brightness depends on how much volatile ice is left near the surface and how close the comet passes to the Sun, so a small fresh comet can outshine a large exhausted one.

Question 8

Do comets shrink?

Accepted Answer

Yes. Every pass boils off part of the surface, and a short-period comet loses metres of material each orbit. Eventually it either breaks apart or seals over with dust and becomes indistinguishable from a dark asteroid.

The largest comets

COMMON QUESTIONS