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Asteroid Size Estimator

Very few asteroids have “known” sizes (or shapes). Most asteroids have irregular shapes (that is, very few are close to being spherical). However, the size (diameter of an equivalent sphere) of an asteroid can be estimated from its absolute magnitude H and an assumed geometric albedo .

Estimate size from H and albedo

must be between 0 and 40
must be between 0.01 and 0.9

Estimated Diameter
[[ result ]]

Table of Approximate Asteroid Diameters for a range of H and Albedo values

The following table shows approximate asteroid size (diameter in km) as a function of absolute magnitude H and visual geometric albedo . Please see the text below the table for a discussion on the limitations of these data.

albedo 0.30 0.25 0.20 0.15 0.10 0.05
H            
30.0 0.0025 0.0027 0.0030 0.0035 0.0043 0.0060
29.5 0.0031 0.0034 0.0038 0.0044 0.0054 0.0076
29.0 0.0039 0.0043 0.0048 0.0055 0.0068 0.0096
28.5 0.0049 0.0054 0.0060 0.0069 0.0085 0.012
28.0 0.0062 0.0068 0.0076 0.0087 0.011 0.015
27.5 0.0078 0.0085 0.0095 0.011 0.013 0.019
27.0 0.0098 0.011 0.012 0.014 0.017 0.024
26.5 0.012 0.014 0.015 0.017 0.021 0.030
26.0 0.016 0.017 0.019 0.022 0.027 0.038
25.5 0.020 0.021 0.024 0.028 0.034 0.048
25.0 0.025 0.027 0.030 0.035 0.043 0.060
24.5 0.031 0.034 0.038 0.044 0.054 0.076
24.0 0.039 0.043 0.048 0.055 0.068 0.096
23.5 0.049 0.054 0.060 0.069 0.085 0.12
23.0 0.062 0.068 0.076 0.087 0.11 0.15
22.5 0.078 0.085 0.095 0.11 0.13 0.19
22.0 0.098 0.11 0.12 0.14 0.17 0.24
21.5 0.12 0.14 0.15 0.17 0.21 0.30
21.0 0.16 0.17 0.19 0.22 0.27 0.38
20.5 0.20 0.21 0.24 0.28 0.34 0.48
20.0 0.25 0.27 0.30 0.35 0.43 0.60
19.5 0.31 0.34 0.38 0.44 0.54 0.76
19.0 0.39 0.43 0.48 0.55 0.68 0.96
18.5 0.49 0.54 0.60 0.69 0.85 1.2
18.0 0.62 0.68 0.76 0.87 1.1 1.5
17.5 0.78 0.85 0.95 1.1 1.3 1.9
17.0 0.98 1.1 1.2 1.4 1.7 2.4
16.5 1.2 1.4 1.5 1.7 2.1 3.0
16.0 1.6 1.7 1.9 2.2 2.7 3.8
15.5 2.0 2.1 2.4 2.8 3.4 4.8
15.0 2.5 2.7 3.0 3.5 4.3 6.0
14.5 3.1 3.4 3.8 4.4 5.4 7.6
14.0 3.9 4.3 4.8 5.5 6.8 9.6
13.5 4.9 5.4 6.0 6.9 8.5 12
13.0 6.2 6.8 7.6 8.7 11 15
12.5 7.8 8.5 9.5 11 13 19
12.0 9.8 11 12 14 17 24
11.5 12 14 15 17 21 30
11.0 16 17 19 22 27 38
10.5 20 21 24 28 34 48
10.0 25 27 30 35 43 60

The expression for diameter d in km as a function of absolute magnitude H and geometric albedo a is given by the following equation.

d = 100.5[ 6.259 - log10a - 0.4H ]

The above expression assumes a spherical object with a uniform surface (no albedo variation). When using this expression to estimate the size of an object, it is important to consider the uncertainty in H (typically 0.5 mag.) as well as the uncertainty in albedo (typically assumed based on some spectral class corresponding to an assumed composition of the object - e.g., S-class asteroid with an assumed albedo of 0.15).

As is evident in the table above, an error in the assumed albedo can result in a significantly erroneous diameter. For example, let’s say you assumed an albedo of 0.15 for H=22 but the actual albedo was much closer to 0.05, your estimated diameter would be too small by a factor of almost 2 (~1.7).

References