Abstract.
Light curves and spectra of two of the brightest photographed
bolides were studied. The Sumava bolide reached -21.5 absolute
(i.e. 100 km distance) magnitude. This fragile cometary body of the initial
mass of about 5000 kg exhibited
many flares and disappeared at the altitude of 59 km above the surface.
The Benesov bolide (-19.5 mag) was a stony body
of initial mass of about 13 000 kg which radiated down to 17 km of
altitude
and exhibited bright flare at 24 km accompanied with severe fragmentation.
It is shown that the fragmentation was the main mode of ablation. By modeling
the lightcurve of Sumava it is found that about 85% of the initial
mass was lost in five subsequent breakups. Even between the breakups,
the ablation coefficient is found to be much higher than the theoretical
value for melting and vaporization.
The spectra show that the radiation of both bolides was
produced by a gas heated to 4000-5000 K. At this temperature only the
ablated material radiates; the radiation of the atmosphere was negligible.
Atomic and molecular emissions are dominant for most of the trajectory,
but near the maximum light thermal continuous radiation is also present.
The results were generalized for the entry of the comet Shoemaker-Levy 9
into the Jupiter atmosphere. Due to the high ablation, the cometary nuclei
are not expected to
penetrate below the jovian clouds. As the radiating volume of the
associated bolides is optically thick, the bolides remain relatively faint
and only weak dependence of the
bolide brightness on the impactor mass is produced. The results are in
accordance with the bolide and fireball observations from the Galileo
spacecraft.
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