Telescopic Spectroscopy -- Analyzing Asteroids' Compositions from Earth Telescopes and Probe Flybys
"Reflection spectroscopy" analyzes the sunlight reflected off of the surfaces of asteroids, and can be used to indicate the average composition of the asteroid's surface.
When light is reflected off of an asteroid, its spectrum is changed. This is because the sunlight incident on materials (such as mineral grains) is usually transmitted to some depth within the material before being reflected. The material absorbs part of the spectrum and reflects part due to its molecular nature, resulting in light and dark bands within the spectrum reflected off the asteroid. The telescope is equipped with modern optical electronic sensors and focused on the asteroid for a long enough time to collect a sufficient sample of light to analyze.
By determining which wavelengths of the spectrum were absorbed, and how strongly each band was absorbed relative to other bands, we can get an indication of what mixture of materials are on the asteroid's surface. The more wavelengths we analyze, the greater our degree of confidence. Many ranges of wavelength are needed before we can determine much.
Thousands of asteroids have been analyzed this way to some degree. However, most of the asteroids analyzed were very large asteroids from the Main Belt between Mars and Jupiter, with many of the economically attractive near-Earth asteroids not receiving as much attention.
The data shows that there is a good correlation between asteroids and meteorites, though harder asteroids would better survive the plunge thru Earth's atmosphere and thus be better represented in meteorites.
The data mainly gives an average composition of the surface and cannot see detailed variations over the surface, except hemispheric variations on occasion as the asteroid rotates.
Also, this just tells us the surface composition. For example, if it's a captured comet, there's a good chance its surface volatiles will have been heated off leaving a crust seal, though it may be rich in subsurface volatiles.
One near Earth asteroid was found to still be a live comet, though only sensitive electronic equipment could see its wake and tail.
On a given night, usually about 20 wavelengths in the non-visible or visible part of the spectrum are viewed. The number of wavelengths viewed is limited by the detector size, telescope aperture, and exposure time (hours for small and distant asteroids). Some other night, at some other telescope in the world, an additional 20 wavelengths may be viewed. The data is pieced together in the published literature. Much of this information has recently started appearing on the internet.
Starting in 1975, a system was adopted categorizing asteroids into 3 types according to the telescopic spectroscopy:
Over the years, this system has evolved and in 2002 it was expanded into 24 types, though mostly subtypes of the above three, so that the vast majority of asteroids are still broadly grouped into C, S, and X types, though in more detailed discussion they are categorized as subtypes. There are about a half dozen minor types which are outside of the C,S,X types.
This classification system is subject to change and refinement, and there are some other classifications schemes outside of this traditional system, so for further details, one must follow the literature. The Wikipedia page on Asteroid spectral types is a fairly good starting point.
In reading the volumes of scientific literature on asteroids, one sees that there exist a vast variety of asteroids. Countless asteroids are not going to fall neatly into any single category, if any category at all.
Some of the asteroids of less exotic composition are fascinating to think about. For example, the asteroid named "16 Psyche" is a 249 km diameter (150 mile) asteroid of almost pure nickel-iron metal, probably a core stripped of its silicate crust and mantle by enormous impacts which chipped away its crust and mantle. (It's the biggest M-type. Many smaller asteroids are also M-type.)
The surface composition of asteroids varies on average with respect to the distance of their orbit from the Sun. The more distant ones have more water and carbon on their observable surfaces. However, asteroids nearer the sun stay considerably cooler underneath the surface and thus probably have higher concentrations of volatiles under their surface.
Asteroids whose orbits are closest to the Sun tend to be more stony-iron on the surface, whereas more distant asteroid orbits are more populated with carbonaceous chondrites. However, this is only the average, and some of the families of asteroids are strikingly at variance with their surrounding populations.
Various techniques can also reveal information on the asteroid's size, rotation period, surface features, shape and subsurface consistency, e.g., using radar, radio wave observations, interferometry, polarimetry, and stellar occultation.
Radar has never discovered an asteroid and is unlikely to do so, because radar beams are necessarily tiny (~2 arcmin) and other daunting technical challenges. Nonetheless, once an asteroid is discovered, radar is the most powerful technique for groundbased refinement of orbits and physical characterization. It is not an exaggeration to say that what the Arecibo and Goldstone radars can do for asteroids is comparable to what Hubble Space Telescope can do for the rest of the universe.
Radar reconnaissance reduces the risk and cost of robotic and piloted missions to near Earth asteroids. For example, knowing the shape and spin state of an object helps define the nature of orbits close to it, e.g., any equipment orbiting the asteroid. Orbiting an asteroid is awfully complex and radically unlike orbits close to a large spherical body.
The radar images of many asteroids can be found at Steven Ostro's Asteroid Radar Research website.
The main radars used are the Goldstone, California, and Arecibo, Puerto Rico (USA) radars. Additional radars used occasionally to analyze near Earth asteroids are in Japan, Russian and Germany.
It is easiest to view asteroids if they are further away than Earth because we can point the telescope nearly straight up in the night sky. Viewing a near Earth asteroid on the horizon is not easy from the surface of the earth due to the thick atmosphere's horizon glow. Near Earth asteroids spend much more of their time near the horizon (much like Venus and Mercury), viewable near sunrise and sunset. This creates a bias in our discoveries. A small telescope above the atmosphere, i.e., in orbital space, dedicated to analyzing the spectra of asteroids in orbits near Earth's, would be a step in the right direction. This is exactly what one company announced in 2012 it had embarked on, Planetary Resources, Inc., for mining asteroids.