Asteroid Occultation 101

Why do some astronomers spend cold dark nights trying to spot faint stars being occulted by asteroids? What is an occultation anyway?

An occultation of a star by an asteroid is basically a teensy eclipse. From our line of sight the asteroid passes in front of a star much like the Moon passes in front of the Sun during an eclipse. From Earth we either see the star dim as the asteroid goes in front (we are just seeing the fainter asteroid) or disappears entirely (the asteroid is too faint to see).

If we know the position of the star accurately, we know precisely the times the star disappears and reappears and we know the position of the observer we can get useful information about the asteroid. Such as its size and orbit. If many observers witness the same event we can even get the shape of the asteroid. This is because each observer will see a slightly different occultation from their different positions.

(135) Hertha
Profile determined by occultation observations compared with the radar modelled shape of (135) Hertha

This is one area of astronomy where amateurs are doing useful work for the professionals. Important discoveries have been made, e.g. the rings of Uranus were discovered when it occulted a star, the atmosphere of Pluto the same way and even satellites of asteroids have been discovered when the star disappears twice during an occultation! One of the most exciting recent discoveries was two rings around the Centaur (10199) Chariklo in 2013. New discoveries await anyone willing to put the effort in to observe these esoteric events.

Predictions of events are published on the internet and you can filter these to determine which you can observe. Often it is cloudy of course but hopefully sometimes you get to observe. Getting a +ve occultation result is a rare event. You may have to observe many -ve events for every +ve. But even the -ve results are important as they tell us that the asteroid wasn’t in that place at that time.

For more information on observing asteroid occultations see IOTA’s and this Introduction to Observing Occultations.

For predictions of upcoming events see Highlights of Asteroid Occultations in Europe.

OccultWatcher is a Windows software program which sorts through the various prediction feeds and graphically shows you which ones are favourable for your location, it is an invaluable tool for all occultation observers.

I maintain the UKOCL feed of occultations predicted to cross the British Isles.

For all European results see