What is an Asteroid Occultation?

Asteroids are a few miles to a few hundred miles across. They exist by the millions in the inner solar system, mainly in the asteroid belt. But we've sent space craft to only a few of them to get close up pictures. They are too small and too far away to see anything about the vast majority except how bright they are as they reflect sunlight. So astronomers have resorted to a clever and inexpensive means to help us understand those that remain only points of light. It's important to understand their structure, their size, their composition, not only for a better understanding of the origin and evolution of the solar system, but because the Earth is sometimes hit by asteroids and mitigation techniques require good knowledge of these things.

Amateur astronomers in the International Occultation Timing Association have written software to generate predictions of exactly when and where asteroids pass in front of distant stars. By timing the exact moment the star disappears and reappears and combining the same such observations from others at different places, a profile of the outline of the asteroid can be reconstructed. Sometimes the asteroid is bright enough to be visible, but usually it is so dim that in small telescopes it is not visible. When the occultation occurs, the brightness of the object goes from being the brightness of the star+asteroid, to being the brightness of just the asteroid itself.


YouTube Videos of Asteroid Occultations
On these videos, see that they have GPS time inserted on each frame, down to the 1/1000 of a second. Such accuracy is necessary for the most accurate profile. On some, you'll see the image fade but not disappear - the asteroid is bright enough to see with their equipment. On others, the image will disappear entirely.

Occultation by Phocaea (note the brief secondary occultation, showing irregularity of surface)
Occultation by Bandusea
Occultation by Roma (of Delta Oph, with pronounced fading, and Tangra analysis)
Occultation by Banberga (this shows the profile from 5 tracks, a quite spherical asteroid it appears)
Occultation by Palma
Occultation by Pulcova (this shows both asteroid and star as they approach, merge, and separate, speeded up)
Occultation by Thusnelda (a very rare grazing asteroid occultation, as the star disappears behind two mountains on the asteroid)
Occultation by Iduna (another grazing asteroid occultation)
Occultation by Penthesilia (shows the light curve after the event)
Occultation by Danae (asteroid itself remains visible faintly)
Occultation by Gezelle (appears to be 3 mountains in a close orbits around each other)
Occultation by Regina (shows the light curve at the end)
Occultation by Pomona (double occultation, with geometric possibilities illustrated afterward, with light curve)
Occultation by Eukrate (star and asteroid about same brightness, showing a fade that's slight by eye, but shows on light curve)
Occultation by Elfriede (with light curve)
Occultation by Xanthe (of bright double star Iota Cancri. One of the two stars disappears)