Re: How do you get the diameter of an asteroid or irregular shape?

G'day!

My apologies for not responding sooner. Your question turned out to be one of those that opened up a big can of worms, and the only way to recan that mess is to use a larger can. In other words, finding out the answer to your question turned out to be as educational for me as I hope this answer will be for you.

Asteroid diameters are poorly known, if at all, and very few asteroids have directly- measured diameters. This is primarily because most asteroids are "unresolved," which means they just appear as points of light from the earth's surface. There are very few asteroids which we've been able to take pictures of as three- dimensional objects. If we have such a picture, we can give the "diameters" in different directions or we can just give an average diameter - usually defined as the diameter for which the volume of the sphere would equal the real volume of the asteroid.

In all other cases, we must be more clever. Fortunately, the Infrared Astronomical Satellite (IRAS) observed roughly two thousand asteroids well enough to lead to diameter estimates based on observed magnitudes and colors. The basic idea is that a bigger asteroid will reflect more light and appear brighter.

If IRAS-derived diameters are available, then those can be used for your asteroid diameter.

If IRAS diameters are not available, then the asteroid diameter is estimated from a least-squares fit of all asteroids with IRAS diameters to:

ln D = A + B H

where A and B are parameters to be fit, D is the IRAS diameter, and H is the asteroid absolute magnitude. This is at best a rough estimate (off by up to a factor of 2), but at least in the ballpark.

I had originally thought that the diameter was more-or-less based on rotation about the asteroid's center of gravity. This would include longer-than-wide and kidney-bean shapes, too. But that all falls through when you can hardly see the object in question in the first place, much less how it rotates. Shows how much I knew...

I wish I could be of more help on this subject. Good luck to you!

Steve Cartoon
Internet Applications Engineer
Edmark.com

Things That Can Explain More Than I Can:
IRAS http://www.ipac.caltech.edu/ipac/iras/iras.html

Derived Diameters and Albedos http://pdssbn.astro.umd.edu/SBNast/holdings/IRAS-A-FPA-3-RDR-IMPS-V4.0.html

Direct Measurements of Asteroid Sizes and Duplicity Search by the HST FGS Interferometer http://www.aas.org/publications/baas/v31n4/dps99/226.htm