Image courtesy of NASA

Asteroids' measurable characteristics

As well as all the celestial bodies which don't emit light (such as planets and moons) asteroids reflect the light emitted from the sun, and are therefore star-like, shiny objects (click here to know more about the basic physics of light ). Asteroids can be observed in many different ways: first of all, by optical observations it is possible to study the visible light reflected, deducing many characteristics of the body (such as brightness, dimension and shape). With spectroscopic observations it is also possible to study the light at different wave length, having indications on the chemical composition of the asteroid. (Click here to know how NEOs are studied and how all these observations are made) .
Of course all asteroids' physical characteristics are not independent, and by measuring or estimating some of them it is possible to calculate the others. In other words, all these parameters depend from each other by a formula. For example, measuring the absolute brightness of the object and giving a first estimation of albedo we can deduce the dimension of the NEO (click here to see the details of the formula).


Brightness
The apparent brightness of an asteroid depends on the object's size and on the distance from the observer and is therefore measured as a relative magnitude. Since asteroids, Earth and all the other objects of the solar system are moving, it is also convenient to define an absolute magnitude to indicate a brightness independent of the distance. The absolute magnitude of the asteroid is the magnitude the object would have if it were 1 AU from Earth. (click here to know more about relative and absolute magnitude).
Other factors are important to determine the value of the apparent brightness of an asteroid: how well the asteroid is illuminated by the sun (also called the phase of the object), its shape and its chemical composition .

Phase
The apparent brightness depends on the degree of illumination of the asteroid called the phase of the object, in analogy to the positions of the moon. In other words with the phase of the object, is indicated the percentage of the surface which is illuminated by the Sun. The brightness of any asteroid, in fact starts to brighten up as it approaches a position of opposition to the Sun, which is called full phase. This effect is called opposition effect.

Shape
Asteroids have usually irregular shapes due partly to the process of collisional fragmentation they have passed through and partly to the fact that they are too small to "pull" themselves in spherical shape by gravitational attraction.


Rotation of the asteroid Geographos (NASA)
(click here for its light curve)
While the asteroid follows its trajectory, it usually rotates around an axis and the area of the face which is turned towards the observer changes in time. As a first result, the apparent brightness will change in time, with a periodical curve that follows the rotation (also periodical). This curve is called light curve (click here to know more about light curves and how they are measured).

Albedo
The brightness also depends on the reflectivity (or albedo) of the object. It is very intuitive that a shiny surface, such as a mirror is much brighter than a black, dull surface made for example of coal. The sunlight which is not reflected is absorbed by the asteroid, causing the body to heat up until it reaches thermal equilibrium and emits in the infrared (click here to know more about light).
Albedo is usually estimated using the asteroid's spectral type (and therefore making some hypothesis on its chemical composition). If aside to albedo the absolute magnitude H of the asteroid can be determined, it is possible to evaluate the dimension of the object.