  The internal structure and strength of small asteroids and large meteoroids
is poorly known. Observation of bright fireballs in the Earth's atmosphere can
prospect meteoroid structure by studying meteoroid fragmentation during the
flight. Earlier evaluations showed that meteoroid strength is significantly
lower than that of the recovered meteorites. We present detailed study of
atmospheric fragmentation of seven meteorite falls, all ordinary chondrites,
and 14 other fireballs, where meteorite fall was predicted but the meteorites,
probably also ordinary chondrites, were not recovered. All observations were
made by the autonomous observatories of the European Fireball Network and
include detailed radiometric light curves. A model, called the semi-empirical
fragmentation model, was developed to fit the light curves and decelerations.
Videos showing individual fragments were available in some cases. The results
demonstrated that meteoroids do not fragment randomly but in two distinct
phases. The first phase typically corresponds to low strengths of 0.04 - 0.12
MPa. In 2/3 of cases, the first phase was catastrophic or nearly catastrophic
with at least 40% of mass lost. The second phase corresponds to 0.9 - 5 MPa for
confirmed meteorite falls and to somewhat lower strengths, from about 0.5 MPa
for smaller meteoroids. All these strengths are lower than tensile strengths of
ordinary chondritic meteorites cited in the literature, 20 - 40 MPa. We
interpret the second phase as being due by cracks in meteoroids and the first
phase as separation of weakly cemented fragments, which reaccumulated at
surfaces of asteroids after asteroid collisions.
