Asteroids' shapes and rotation: what is a lightcurve?
by Livia Giacomini -Copyright Tumbling Stone 2001
Asteroids have irregular shapes and usually rotate with some period P (called 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 observable will change with time, depending on the observable area. Time series measurements of the asteroid's brightness variations produce what are usually called light-curves.
||In other words, as
the asteroid rotates it is sometimes possible to see a
lot of its surface while other times, only a little
portion of its surface can be seen. 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 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 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 possibly has
Adapted from Spaceguard Science pages (http://spaceguard.ias.rm.cnr.it)...