asteroid spectral types
One of the most important ways to classify asteroids is by features in their reflectance spectra along with their color and albedo. These characteristics are related to the surface composition. A number of different classification schemes have been developed but the one seen most commonly is that proposed by David Tholen in 1984, and based on data fro the Eight-Color Asteroid Survey (ECAS) and albedo measurement. Fourteen different classes are recognized in the Tholen scheme.
A rare type of asteroid, reddish in color, with
a moderately high albedo of 0.13 to 0.35.
The infrared spectra of A-class asteroids suggest that these objects are
rich in the mineral olivine. Examples include the asteroids 246 Asporina (diameter 70 km), 446 Aeternotas
(diameter 52 km), and 5261 Eureka, the first Mars
Trojan to be discovered.
A subcategory of C-type asteroids
whose members have albedos in the range 0.04
to 0.08. Although fairly low, this is at the high end for C-type objects.
Examples include 2 Pallas (the second largest
asteroid), 379 Huenna (diameter 62 km), and 431 Nephele (diameter 78 km).
A very dark and non-reflective type of asteroid,
gray in color, with a composition believed to be similar to that of carbonaceous
chondrites (the "C" stands for carbonaceous). C-type asteroids are
the commonest type known and dominate the outer part of the main asteroid
belt. They have an albedo of 0.03 to
0.09 and a reflectance spectrum that is flat at wavelengths longer than
0.4 micron but shows a feature shorter than 0.4 micron thought to be due
to water of crystallization. Examples include (10) Hygiea and (253) Mathilde. Sub-types include
B-type, F-type, and G-type asteroids.
A very dark and non-reflective asteroid,
reddish in color, probably due to the surface presence of organic materials.
Rare in the main belt, D-type asteroids
crop up with increasing regularity beyond about 3.3 AU from the Sun. They
have albedos of 0.02 to 0.05 and appear to
be made of some of the most primitive material in the solar system.
Examples are found among many of Jupiter's Trojans,
including the largest, Hektor. Also the Martian
moons Phobos and Deimos may well be captured D-type asteroids and the Tagish Lake meteorite has
been confirmed as a probable D-type fragment, the most likely parent-body
candidate being 773 Irmintraud.
|A fragment of the Tagish Lake meteorite, which fell
in Canada in 2000, and is believed to have come from a D-class asteroid
A rare category of asteroid, slightly red
in color and spectrally similar to M-type
asteroids and P-type asteroids,
but of higher albedo (in the range 0.25–0.60).
E-type asteroids dominate the Hungaria
group of the main belt, though some, including the two largest, 44 Nysa
(see Nysa-Polana family and 64 Angelina, are found scattered further out. Their flat reflectance spectra
in the 0.3 to 1.0 micron region and high albedos suggest a connection with
the aubrite (otherwise known as enstatite
achondrite, hence the "E") category of meteorite. However, this link is
problematic because aubrites are clearly of igneous origin, whereas some
E-class asteroids (including Nysa) show an absorption feature at 3 microns
that points to water- and/or hydroxyl-bearing minerals and a non-igneous
The Rosetta spacecraft achieved the first
fly-by of an E-type asteroid when it passed within 800 km of the main belt
asteroid 2867 Šteins on Sep. 5, 2008. Šteins has dimensions of 6.67 ×
5.81 × 4.47 km and a mean distance from the Sun of about 2.4 AU.
A dark asteroid belonging to a subcategory
of C-type asteroids distinguished
by having a weak to absent ultraviolet absorption
feature. F-class asteroids account for most of the Polana faction of the
Nysa-Polana family. They have
albedos of 0.03 to 0.07 and featureless reflectance
spectra across the range 0.3 to 1.1 microns.
A dark asteroid belonging to a subcategory
of C-type asteroids, distinguished
by having a very strong ultraviolet absorption feature at wavelengths shorter than 0.4 microns due to water of hydration
in the surface rocks. G-class asteroids, of which the best known and largest
example is Ceres (now officially classified
as a dwarf planet), have albedos of 0.05
|Ceres: a G-class asteroid
Image credit: NASA/Hubble
A relatively bright and reflective asteroid,
made mainly of metallic iron and nickel (the "M" is for "metal"), typically found in the middle of the main
asteroid belt. M-type asteroids are slightly reddish and have featureless
reflectance spectra over the range 0.3 to 1.1 microns. They are distinguished
from the spectrally similar E-type
asteroids and P-type asteroids
by their moderate albedo of 0.10 to 0.18.
The largest known example is the 248-km-wide (16) Psyche.
A dark type of asteroid, commonest in the outer part of the main asteroid belt at a solar distance of about 4 astronomical units, with a spectrum intermediate between that of C-type asteroids and D-type asteroids. P-type objects are distinguished from the spectrally similar E-type and M-type asteroids by their lower albedo (in the range 0.02 to 0.06). The largest example is the 282-km-wide (87) Sylvia.
A rare class of asteroid of which only Apollo and a few other near-Earth
asteroids are known to belong. Q types are fairly bright and have reflectance
spectra with a strong absorption feature at wavelengths less than 0.7 micron
and a mild one near 1 micron – spectra that are similar to those of
ordinary carbonaceous chondrites.
An extremely red asteroid with a fairly
high albedo and strong absorption features
shorter than 0.7 micron and near 1 micron. Spectroscopic studies point to
a surface rich in basaltic minerals, including pyroxene and olivine. The only recognized example
is the main-belt asteroid 349 Dembowska.
However, the possibility has been raised that the near-Earth
asteroid 2001 XR31 may also be of class R.1
- Marchi, S., Lazzarin, M., and Magrin S. "An R-type asteroid within
near-Earth objects?" Astronomy & Astrophysics, 420, L5-L8 (2004).
A moderately bright, slightly reddish type of asteroid
believed to be composed largely of silicate minerals (the "S" stands for
"silicaceous") such as olivine and pyroxene.
S-class asteroids are quite common in the inner part of the main asteroid
belt, their proportion decreasing at greater distances from the Sun.
They have an albedo of 0.10 to 0.28 and a
reflectance spectrum that is flat at wavelengths longer than 0.7 micron.
Examples include 433 Eros, 951 Gaspra,
15 Eunomia (see Eunomia family),
and the 219km-diameter 29 Amphitrite. S-class asteroids may be the parent
objects of stony-iron meteorites.
|Animation of the rotation of 433 asteroid
A rare type of asteroid with a fairly low albedo (0.04 to 011) and a moderate absorption feature at wavelengths shorter than 0.85 micron. Examples include (114) Kassandra and (233) Asterope.
A rare class of asteroid, of which Vesta is the preeminent member, characterized by fairly high albedo
and a reflectance spectrum with strong absorption at wavelengths shorter
than 0.7 micron and a separate dip near 0.95 micron. The latter suggests
the presence of the silicate mineral pyroxene.
• ASTEROIDS AND OTHER
MINOR PLANETS: TYPES AND GROUPS