Question 1

How large is the largest?

Accepted Answer

Ceres, at about 940 km across, holds roughly a third of the mass of the entire asteroid belt and is round enough to be classed as a dwarf planet. Below the first few, sizes fall away very quickly.

Question 2

Why are most of them not round?

Accepted Answer

Because they are too small for their own gravity to overcome the strength of rock. Below a few hundred kilometres a body keeps whatever shape collisions left it, which is why most asteroids are lumps rather than spheres.

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 asteroids 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 is an asteroid's size measured?

Accepted Answer

The best figures come from spacecraft, radar, or watching one pass in front of a star. Otherwise size is inferred from brightness plus an assumed reflectivity — and since a dark asteroid and a small bright one look identical, that estimate can be wrong by a factor of two.

Question 7

Do the big ones ever come near the Earth?

Accepted Answer

No. The largest asteroids are all in the main belt between Mars and Jupiter and stay there. Near-Earth objects are a separate and much smaller population, tracked precisely because their orbits cross ours.

Question 8

Could the belt have been a planet?

Accepted Answer

There was never enough material. Everything in the belt together comes to about 3 per cent of the mass of the Moon, and Jupiter's gravity stirs the region hard enough that collisions there break bodies up rather than build them.

The largest asteroids

COMMON QUESTIONS