Some observational techniques...
by Livia Giacomini -Copyright Tumbling Stone 2001
|First of all, scientists have to discover NEOs, which isn't an easy task. NEOs, in fact, look like stars (at least for asteroids). Normally this is done with optical images (as photographs) that allow to recognize NEOs by their motion. In fact, being nearer than stars, NEOs appear to move much faster, on the background sky.|
Orbits have to be tracked, in order to know where the object will be in the future. To do this, normally many observations of the objects need to be combined.
An important problem in the determination of the orbit is that the position of the object can't be perfectly determined: all we know is a region of space where the object is like to be, called region of uncertainty. Each point inside this region is a possible position of the asteroid and is therefore called virtual asteroid. Each virtual asteroid will have a different trajectory that can be calculated as time passes by and confronted with later observations of the position of the asteroid...
|A photometric observation is
the determination of the apparent brightness of an object
(in a particular frequency or color).
The object's properties (such as dimension, albedo, etc) are all dependent and it is impossible from a single photometric measure to determine all of them. Anyhow, 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).
Photometry includes the study of how the light emission varies in time, or in other words, the study of light curves to determine the shape and how the NEO revolves.
These techniques can also be combined with radiometry to obtain more results.
Radiometry is the determination of the infrared emission of an asteroid. In this way it is possible to determine how much the asteroid has been heated up. By confronting the infrared flux with the amount of absorbed light, it is possible to deduce which fraction of the incident is absorbed and which fraction is reflected. In this way, the albedo of the object can be evaluated.
Usually, photometry and radiometry measures are combined.
The heating up process of the asteroid depends mainly on the fraction of light that is absorbed by the asteroid. So, by confronting the infrared flux with the amount of absorbed light, it is possible to deduce which fraction of the incident is absorbed and which fraction is reflected. In this way, the albedo of the object is evaluated. In a second moment, knowing the apparent brightness by photometry, distance and albedo of the object, it is possible to evaluate the object size.
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.
To understand the chemical composition of an
asteroid, usually it is possible to collect and analyze
its spectrum or in other words, 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
These measures make the determination of the average composition of the surface possible. In fact, minerals reflect minerals in characteristic ways and each of them is therefore identified by a typical spectrum, that can be obtained in laboratory. 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.
|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).|
Adapted from Spaceguard Science pages (http://spaceguard.ias.rm.cnr.it)...