Terms and Concepts for Gaffey et al. (2002)

The proper way of referring to an asteroid is with a number-name format, as in "4-Vesta" or "4 Vesta" (where the number refers to the order in which the asteroid was discovered).  Each asteroid has a different number, but (unfortunately) some names are used more than once.  Usually there is no confusion and a simple name suffices to identify a particular body.  The largest asteroids tend to have the lowest numbers.  For example, 1-Ceres is the largest asteroid (diameter ~ 1100 km) and was the first to be discovered (in 1801).

Lightcurve - For an asteroid, this is the variation in light intensity that results from looking at different sides of a rotating object, caused either by a non-spherical shape or by variations in the brightness of surface materials.

A.U.  -  Abbreviation for Astronomical Unit.  This is the common unit used for distances in the solar system.  1 A.U. ~ average distance between the Earth and the sun ~ 150 million km ~ 93 million miles.  Technically, 1 A.U. = the Earth's semi-major axis, which is equal to half the diameter of the long axis of Earth's elliptical orbit.  This is approximately equal to the average distance between the sun and Earth.

Semi-major axis  - The semi-major axis of any orbit (abbreviated by the letter "a") is half the long axis of an elliptical orbit (all orbits are at least slightly elliptical).  The object being orbited (such as the sun) is located at one of two focal points.  The semi-major axis is usually given in terms of A.U. (as in Table 3) for objects in orbit around the sun.

Zenith - The point in the sky directly above an observer.  Astronomers like to observe objects at the zenith, because it is here that the effects of the Earth's atmosphere are minimized (the airmass is lowest).

NIR - near-infrared.  This is the portion of the infrared corresponding to wavelengths of ~0.6 to ~4 microns that is especially useful for asteroid spectral studies.  For comparison, visible light ranges from blue (wavelength ~0.4 microns) to red (wavelength ~ 0.6 microns).

Band I - the name given to an oft-observed spectral absorption feature at ~0.9-1.0 micron (900-1000 nm) wavelength.

Band II - the name given to an oft-observed spectral absorption feature at ~2.0 micron (2000 nm) wavelength.

Albedo - The brightness of a planetary surface as quantified by a number.  An object with an albedo of 1 (or 100%) reflects all of the light that falls on it and is totally bright; an object that has an albedo of 0 (or 0%) absorbs all of the light that falls on it and is totally black.  In visible light, asteroids have albedos that vary from ~0.04-0.45 (e.g., 4% for Ceres to 38% for Vesta).  In reality, objects have different albedos at different wavelengths; this results in color.

Reflectance spectrum - Planets and asteroids absorb some of the sunlight that shines on them, and reflect the remainder.  A reflectance spectrum is a diagram showing what proportion of sunlight is reflected by an object at a given wavelength.

Figures from: http://marswatch.tn.cornell.edu/nis_facts.html - The top image shows the reflectance spectra for minerals
identified in S-type asteroid spectra.  The lower image show the spectra for two hemispheres of Eros (an S-type asteroid).

S-type asteroid - An asteroid characterized by a certain range in NIR-spectrum and albedo.  S-type asteroids have both band I and II absorption features and somewhat high albedos.  The band 1 and band II absorptions are caused by the presence iron-bearing silicate minerals.  So an "S-type" asteroid is considered to be dominated by a "silicate" (or stony, or stony-iron) mineral assemblage.  Other asteroids are grouped into different spectral types such as M ("metallic"), C ("carbonaceous"), V ("Vesta"), E ("enstatite"), A, P, D, R, Q, etc., depending on the spectral features they show (or don't show) and their albedos.  The different spectral types are supposed to correspond to different lithologies exposed at the surface of an asteroid.

Space weathering - This refers to various processes that modify a surface exposed to a space environment.  It includes radiation damage caused by photons or cosmic rays (high-speed protons), meteoroid bombardment (brecciation with or without shock-melting), and implantation of solar wind (the stream of ionized gases emitted by the sun), etc.  Space weathering can affect the spectra of an object because it damages, alters, or destroys minerals.  It can also be effective in eroding materials on an airless body.  For example, mountains on the Moon have a rounded appearance because of space weathering.  Finally, space weathering may be involved in contributing to the formation of asteroidal sedimentary deposits ("ponds") on small asteroids such as 433-Eros.

"Ponds" (smooth deposits filling low areas) on Eros.  Image mosaic obtained by the NEAR-Shoemaker spacecraft.

Soil - This refers to the impact-reworked, loose material found at the surfaces of airless bodies.  The layer of brecciated material near the top of the airless body is also called a regolith.  A different definition of soil (a mixture of rock fragments and organic material) applies for the Earth.

Gas-rich and shock-blackened meteorite - This is a meteorite (usually chondrite) that appears to have resided in a regolith.  A gas-rich meteorite contains excess solar wind-implanted gas; a shock-blackened meteorite appears dark as a result of shock effects (melting and the formation of tiny metal and troilite inclusions).  Most gas-rich meteorites contain shock-blackened portions.

NEA - near-Earth asteroid.  These are asteroids that have orbits that cross the orbits (semi-major axes) of Mars, Earth, or Venus.  Objects in these orbits are considered to be dynamically unstable, because they can crash into a terrestrial planet, or because their orbit can change as a result of gravitation interaction with (perturbation by) a terrestrial planet.

Orbital resonance - This is an orbital condition that allows repeated gravitational nudges (perturbations) between different objects.  There are different types of resonances.  Resonances between asteroids and Jupiter in the asteroid belt tend to nudge objects out of certain orbits, forming Kirkwood Gaps.