[AstralTraveller (David)]

Originally Posted by Blue Skies

Size of asteroids is fuzzy, as they can only go on how much light it reflects - ideally a large asteroid would reflect lots of light, but what if its got a dark surface? If the asteroid has a moon then you're laughing because the interaction of the two bodies gives the mass away.

There is a way to get a direct measurement of an asteroids size - and amateurs are leading the way. It's easy to know where an asteroid is (how far away) and how fast it is moving. So if one times how long it takes to pass in front of a star one can work out the length of the chord across the asteroid. If more than one chord is observed by observers at appropriate spacing then a good estimate of the size and shape of the asteroid is obtained.

In practice the asteroid is normally much fainter than the star, which is bright enough to be accessible in amateurs' telescopes, and so the observer sees the star blink off for a few seconds. Observers are typically using about 20cm scopes equiped with a video camera and a bit of other gear to give a time signal.

There is a small but active group of observers in Aust and NZ who are getting positive results fairly regularly. The following links may be of interest.

http://occsec.wellington.net.nz/
http://www.hristopavlov.net/OccultWa...ltWatcher.html
http://www.lunar-occultations.com/iota/occult4.htm