Photometric observations and lightcurves

Once a NEO has been discovered, its physical and chemical characteristics need to be somehow measured (click here to know more about asteroids characteristics). This is mainly done with photometry, the study of the asteroid brightness visually, photographically or electronically.


Photometry
What is usually measured by astronomical observations, is the apparent brightness of an object in a particular color.
This value of the apparent brightness depends on many factors such as the distance, albedo, dimension (click here to see the exact formula). So, since there are many parameters, it is impossible from a single measure to determine all of them. If only visible observations are available, as in most cases, the diameter can be somehow estimated from an assumed albedo (based on a previous spectral classification of the asteroid).
Photometric observations can be combined with other techniques to improve results. For example, if both visible and thermal properties are determined (measuring both visible and thermal infrared flux densities) albedo and mean diameters can be evaluated more precisely. This is obtained combining photometry with radiometry observations. Another example: when photometry isn't done in one single color, but measures are available in many different colors, using different filters, this technique is called spectrophotometry and is based on spectroscopic techniques.

 

Lightcurves observations

Photometry is often used to study the brightness variations of asteroids over a period of time, obtaining curves where these variations of brightness are represented: the lightcurves.
But what is exactly a light curve? First of all, it is important to know that asteroids have irregular shapes and usually rotate 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 observable will change with time, depending on the observable area. Time series measurements of the asteroid's brightness variations produce light-curves.



The motion of asteroid Geographos combined with its light curves variations.
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 surface features.