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 Optical Observations of Asteroids 

Discovery

Discovering asteroids is a bit tricky. Unlike the planets, when you look at an asteroid in a telescope you don't see a disk with features.  It just looks like another star.  In fact, the word asteroid means "star like."  But asteroids can be distinguished from stars by the fact that their orbits cause them to move across the sky. If a telescope makes a long exposure while remaining pointed at a fixed set of stars, any asteroid present in the image will leave a streak as it moves.  The photograph at right was taken by Alain Maury and Derral Mulholland and shows the Earth-crossing asteroid 4179 Toutatis

 
If instead of a single long exposure you make a series of multiple exposures you can actually see the asteroid skipping across the sky.  The animated multiple exposure image at left shows the Earth-crossing asteroid 1620 Geographos.  The images were taken by Petr Pravec.
The search for asteroids has been greatly aided by new technologies such as CCD cameras and computer controlled telescopes.  Four groups at the cutting edge of the search for new asteroids are:

Orbit Determination

Discovering an asteroid is only the first step. Next you've got to figure out its orbit so you know where it will be in the future.  Otherwise it'll get lost.  Physics tells us that an orbiting object moves along an ellipse. Orbit determination involves figuring out which ellipse from observing the angle between the asteroid and other objects in the sky.  These two figures illustrate the idea.  The black orbit represents Earth. Suppose that the blue curve is your guess at an asteroid's orbit while the true orbit is the red one.  You would expect to see the asteroid in the direction of the light blue line, but it will show up along the direction of the magenta line.  If your guess isn't very good, like in the figure at top left, these two directions will not agree very well.  Say you improve your estimate to get the situation in the figure at top right. Now the agreement is quite a bit better, but still not perfect.  So, you can improve your estimate some more, and so on.  In this manner orbits can be determined with enough accuracy to allow us to accurately predict where an asteroid will be hundreds of years from now.

Light Curves

For the purposes of discovery and orbit determination you basically pretend that the asteroid is a point of light.  A real asteroid will probably have an irregular shape and be rotating with some period P, the length of the asteroid's day.  When an irregularly shaped object rotates it will reflect different amounts of light as time goes on, so the brightness of the point of light it leaves on your camera will change with time.  If you look at the Geographos animation above you might be able to detect a change in brightness as time goes on.
 
For example consider the model of asteroid Geographos shown here.  As it rotates we can sometimes see a lot of its surface while at other times we only see a little. So the solid curve at right, which is the plot of  total brightness vs. time,  goes up and down. Graphs of this sort are called light curves. The circles plotted are the actual Geographos brightness measurements made by astronomers.

The time it takes for a lightcurve to start repeating is the length of the asteroid's day, called its rotation period. The lightcurve amplitude (how much the curve goes up and down) gives us some information about the elongation of the asteroid - in other words, it tells us something about how stretched out into a cigar shape the asteroid is. The lightcurve of a sphere would not go up and down at all, so any lightcurve variations immediately tell us that the asteroid is non-spherical and/or possibily has surface features.

Spectrally Resolved Imaging


Spatially Resolved Imaging