MIAC

Fireball - Meteorite link

A Brief Guide to the Link Between Fireballs and Meteorites

The purpose of this document is to provide some basic guidance regarding which fireballs are likely to have produced meteorites. While the meteorite recovery is not the only reason to compile fireball reports (see e.g. J. Roy. Astr. Soc. Canada, 87, 140), it is nonetheless true that much more effort should be expended in obtaining accurate eyewitness reports when a meteorite might have resulted from a fireball. Much of what is written below is based upon the fireball parameters which appear to be linked to meteorite falls in the analysis of MORP data by Halliday et al. (Meteoritics, 24, 65).

1. Fireball brightness is not a valid predictor of probability of meteorite production. The Halliday study found that, perhaps surprisingly, meteorites resulted from fireballs at least as faint as -6 apparent magnitude, and a typical meteorite producing fireball is only -9 magnitude. Many of the brightest and most spectacular fireballs did not produce meteorites.

2. Meteorites do not result from fast meteors. While it is difficult to define the precise limit, most researchers suggest that meteorites never result when the geocentric velocity is above about 28 km/s. While it is difficult to determine, from eyewitness reports alone, the velocity of the meteor with much precision, at times it is possible from the apparent movement and duration to establish that the velocity is too high for meteorite production. In interpreting such observations, remember that typical fireballs start at heights about 80 km.

3. Meteorites do not result from cometary parent meteor showers such as the Orionids, Perseids, Leonids, Taurids, etc. It is felt that these objects do not have the required strength to survive atmospheric flight, and, in any case, the geocentric velocities are too high. Often the date and apparent direction can be use to establish a shower link to a fireball (although it appears that extremely bright fireballs are not produced by meteor showers).

4. Meteorite producing fireballs typically end their luminosity at heights of 25-35 km. If, from triangulation using several spaced observations, it can be established that the meteor ended at 50 or 60 km it is very unlikely that a meteorite resulted. The typical meteorite producing fireball has slowed to 8 km/s or so at the height where luminosity ends.

5. There is a slight dependence of meteorite production and zenith angle, with vertical flights being more likely to produce meteorites.

6. Fragmentation significantly reduces the probability of meteorite production (although most meteorites do also fragment). It appears that meteorites with masses above 10 kg are partially fractured by interplanetary impacts, and readily fragment.

7. Production of nonelectrophonic (typical thunder like) sound is a powerful indicator that meteorites have been produced, since it indicates that the meteor has gotten to the lower atmosphere. The time between fireball and sound (typically tens of seconds to minutes) can be used to establish a distance to the meteor. The occurrence of electrophonic (near instantaneous) hissing or staccato sounds which occur simultaneously with the fireball is not an indicator of low penetration and likely meteorites (it is believed that these sounds are electromagnetic waves transduced to sound by nearby objects).


By Robert. L. Hawkes