  Analysis of laboratory experiments simulating space weathering optical
effects on atmosphereless planetary bodies reveals that the time needed to
alter the spectrum of an ordinary chondrite meteorite to resemble the overall
spectral shape and slope of an S-type asteroid is about ~ 0.1 Myr. The time
required to reduce the visible albedo of samples to ~ 0.05 is ~ 1 Myr. Since
both these timescales are much less than the average collisional lifetime of
asteroids larger than several kilometers in size, numerous low-albedo asteroids
having reddish spectra with subdued absorption bands should be observed instead
of an S-type dominated population. It is not the case because asteroid surfaces
cannot be considered as undisturbed, unlike laboratory samples. We have
estimated the number of collisions occurring in the time of 105 yr between
asteroids and projectiles of various sizes and show that impact-activated
motions of regolith particles counteract the progress of optical maturation of
asteroid surfaces. Continual rejuvenation of asteroid surfaces by impacts does
not allow bodies with the ordinary chondrite composition to be masked among S
asteroids. Spectroscopic analysis, using relatively invariant spectral
parameters, such as band centers and band area ratios, can determine whether
the surface of an S asteroid has chondritic composition or not. Differences in
the environment of the main asteroid belt versus that at 1 AU, and the physical
difference between the Moon and main belt asteroids (i.e., size) can account
for the lack of lunar-type weathering on main belt asteroids.
