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ABSOLUTE MAGNITUDE (H)

Definition: An asteroid's absolute magnitude is the visual magnitude an observer would record if the asteroid were placed 1 Astronomical Unit (AU) away, and 1 AU from the Sun and at a zero phase angle.

Technical Definition: The magnitude of an asteroid at zero phase angle and at unit heliocentric and geocentric distances.

Conversion of Absolute Magnitude to Diameter: The diameter of an asteroid can be estimated from its absolute magnitude (H). The lower the H value, the larger the size of the object. However, this also requires that the asteroid's albedo be known as well. Since the albedo for most asteroids is not known, an albedo range between 0.25 to 0.05 is usually assumed. This results in a range for the diameter of the asteroid. The table below shows the diameter ranges for an asteroid based on its absolute magnitude, assuming an albedo ranging from 0.25 to 0.05.

Absolute Magnitude
(H)
Diameter
(km = kilometers)
(m = meters)
3.0 670 km - 1490 km
3.5 530 km - 1190 km
4.0 420 km - 940 km
4.5 330 km - 750 km
5.0 270 km - 590 km
5.5 210 km - 470 km
6.0 170 km - 380 km
6.5 130 km - 300 km
7.0 110 km - 240 km
7.5 85 km - 190 km
8.0 65 km - 150 km
8.5 50 km - 120 km
9.0 40 km - 90 km
9.5 35 km - 75 km
10.0 25 km - 60 km
10.5 20 km - 50 km
11.0 15 km - 40 km
11.5 13 km - 30 km
12.0 11 km - 24 km
12.5 8 km - 19 km
13.0 7 km - 15 km
13.5 5 km - 12 km
14.0 4 km - 9 km
14.5 3 km - 7 km
15.0 3 km - 6 km
15.5 2 km - 5 km
16.0 2 km - 4 km
16.5 1 km - 3 km
17.0 1 km - 2 km
17.5 1 km - 2 km
18.0 670 m - 1500 m
18.5 530 m - 1200 m
19.0 420 m - 940 m
19.5 330 m - 750 m
20.0 270 m - 590 m
20.5 210 m - 470 m
21.0 170 m - 380 m
21.5 130 m - 300 m
22.0 110 m - 240 m
22.5 85 m - 190 m
23.0 65 m - 150 m
23.5 50 m - 120 m
24.0 40 m - 95 m
24.5 35 m - 75 m
25.0 25 m - 60 m
25.5 20 m - 50 m
26.0 17 m - 37 m
26.5 13 m - 30 m
27.0 11 m - 24 m
27.5 8 m - 19 m
28.0 7 m - 15 m
28.5 5 m - 12 m
29.0 4 m - 9 m
29.5 3 m - 7 m
30.0 3 m - 6 m
30.5 2 m - 5 m
31.0 2 m - 4 m
31.5 1 m - 3 m
32.0 1 m - 2 m
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