Spectroscopic observations

Since comets and asteroids are relatively unchanged leftovers of the solar system formation process, it is very important to understand their chemical composition. This is normally done with spectroscopic observations, by collecting and analyzing their spectra (click here to understand more about light and spectra).

How does spectrophotometry work?
The light from an asteroid can be measured at many different wave lengths obtaining what is called a spectrum. This spectrum corresponds to the light (initially being emitted by the Sun!) reflected form the asteroid at different wavelengths. If the asteroid's spectrum is compared to incident spectrum of sunlight, it is possible to know exactly how the asteroid reflects and absorbs sunlight at each wavelength.

In these two graphics are represented the normalized spectra of some of the most brilliant asteroids (on the left) and the normalized spectra of some minerals ( a: iron and nickel, b:olivine, c:orthopyroxene, d: feldspar, e: spinel).

These measures make the determination of the average composition of the surface possible. But how does this technique, called spectrophotometry, work in detail? It is not sufficient to simply photograph the spectrum of an object to have information about its chemical composition: quantitative information about the light output (also named flux at a particular wavelength) is needed. These values obtained at different wavelengths must be compared with the light reaching the asteroid from the Sun. In other words, before analyzing the spectra we need to normalize them to the spectrum of the Sun, obtaining a reflection spectrum. This is done dividing the flux at each wavelength by the solar flux at the same wavelength.
Now, minerals reflect minerals in characteristic ways and each of them is therefore identified by a typical spectrum, that can be obtained in laboratory (see the above picture). So confronting the normalized reflection spectrum of the asteroid with these laboratory curves, and examining which wavelength and how strongly each band was absorbed relatively to other bands, it is possible to get an indication of what mixture materials are on the surface of the asteroid.
The main problem of this method is that it is only possible to analyze the surface of asteroids and not the core of these objects. Furthermore, data refers only to an average composition and cannot indicate detailed variations over the surface.

Spectral classification

Asteroids have been divided in various classes that correspond to different chemical composition: this is the well known composition classification of asteroids that corresponds to an equivalent classification made for meteorites.
The different classes of asteroids are characterized by different kinds of spectra that correspond to the different chemical compositions (from the featureless blue color spectra of C type to the reddish spectra of S type).