Calculating Asteroid Size

To calculate the size of an asteroid, we must know its absolute magnitude and albedo. Asteroids don’t emit any light of their own, since they are icy or rocky objects. The light we see from asteroids is reflected sunlight. The absolute magnitude (H) can be thought of as the asteroid’s actual brightness, as opposed to how bright it appears to an observer on Earth. The absolute magnitudes of the brightest asteroids are around 3. Most of the asteroids that come close to Earth are very dim, with absolute magnitude values ranging from 18-24.

An asteroid’s albedo (p) is a measure of the reflectivity of its surface. An object that is pure white reflects 100% of the light that hits it and has an albedo of 1. An object that is pure black reflects 0% of the light that hits it and has an albedo of 0. The albedo for an asteroid depends on its surface material and roughness, but it ranges from 0.05 (5%), which is similar to the amount of light that reflects from charcoal or a blacktop road, to 0.25 (25%), which is similar to the amount of sunlight that reflects from grass.

The equation for calculating the size of an asteroid is given by:

D = 1329000p 10 -0.2 × H

Where D is the diameter of the asteroid (in meters), H is the absolute magnitude of the asteroid, and p is the albedo of the asteroid.