I said there was something coming. In the 15 Oct (vol. 251) issue of Planetary and Space Science:
Alvaro Alvarez-Candal A. discussion on estimating small bodies taxonomies using phase curve… Article 105970 .105970
At detection, a new asteroid is… detected! It’s just a dot in the sky. We want to track the dot (“orbit determination), to see what sort of object it is (possibly the Earth-threatening type). We want to see how it brightens and dims (a “light curve”), which indicates rotation, shape and homogeneity, possibly any asteroid moons, etc. We want colorimetry, to see what composition clues we might deduce. If possible, we can get finer spectral resolution, and actually get certain compositions that might be present. And, as the asteroid moves, there’s a subtle light effect. As a body travels across our sky, it goes from ahead or behind us in its track about the Sun. The opposition point is the spot in the sky where a body is exactly opposite the Sun- that is, if you define the Sun as being the ‘noon point’ of the sky, then opposition is the ‘midnight point.’ Bodies then continue traveling, crossing through opposition, into the ahead or behind sky from our point of view.
As small bodies approach, cross, and depart opposition, there’s a light effect from these phases. But different asteroids/comets have shown us different phase effects. We now know that these differences relate somehow to the body’s surface and composition. Most bodies are brightest at the opposition point. Some bodies brighten noticeably; some barely brighten at all. We have guessed that the difference is related to spectral type: asteroids in the S-complex are bright, and show a major phase effect. Bodies in the C-complex, dimmer to begin with, stay pretty dim, even at opposition. But the ‘rule’ isn’t a hard rule, there appears to be more to it than that. Alvaro Alvarez-Candal writes of the more to it.