The H Parameter and Getting a Diameter

For most small bodies, we never have a ruler. We estimate size from light. The standard proxy is the absolute magnitude H: the brightness the object would have if it were 1 AU from the Sun, 1 AU from Earth, and seen at zero phase angle. Brighter H = more light = bigger (or shinier).

The H-d-p relation

Diameter, H and geometric albedo p obey:

d = 1329 / sqrt(p) * 10^(-H/5)

Plugging in a typical albedo of 0.2, an H of 15 gives a diameter of roughly 550 m. The uncertainty in p - most asteroids range from 0.02 to 0.3 - is the dominant error term, which is why diameters derived from H are usually quoted with a wide range.

When you have a real number

A radar pass or a spacecraft flyby gives a measured diameter, and those values land directly in SpaceAtlas’s diameter column when available. The “largest by diameter” ranking uses exactly that column, which is why it only lists objects with a known size - most objects have no diameter yet.