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Explosions in the Sky: Fireballs that Produce Meteorites

Tunguska fireball in Siberia, 1908
Figure 1. Artist's reconstruction of a typical fireball. This view is based on reports of the Tunguska fireball in Siberia, 1908.

Everyone has heard the term "fireball," but what is a fireball, really? Scientifically, the term refers to brilliant objects, like balls of fire sometimes as bright as the sun, crossing the sky by day or night. Throughout history, many have been described, often involving explosions and noise. For centuries, such objects were thought of as supernatural events. Today we realize they are connected with interplanetary debris crashing into Earth's atmosphere, and therefore they are connected with another interplanetary phenomenon: meteorites. Meteorites are rocks or fragments of metal that fall out of the sky. Until the 1700s, they, too, were regarded as supernatural, until French academicians got good documentation of a shower of meteoritic rocks that pelted the French village of L'Aigle, France, in 1803. This documentation opened the door to our modern understanding of such events.

Soon it was realized that meteorites are fragments of the stony or metallic asteroidal and cometary debris that enter Earth's atmosphere and produce fireballs. They hit Earth at speeds typically around 10 mile/second, and are heated during entry like a space shuttle, making them glow with a brilliant display of fireball light.

Figure 2. Rapid increase in the number of well-tracked fireballs yielding meteorites is shown in this plot of cumulative cases as a function of time. From such data, we can predict a total of 20 cases within perhaps a decade.

The shock of hitting the atmosphere at such speeds causes most mid-sized objects (meter-scale diameter, hundreds kg mass or more) to break up and explode in a swarm of fragments. Even though a link was known between fireballs and their solid fragments, there was a missing link. Until 1959, nobody had ever picked up a meteorite from a fireball that was well enough observed to say anything about its trajectory or explosive behavior. The problem was that a typical fireball on its slanting path through the air may explode occur 40 to 60 km (24-40 miles) above the ground; therefore it may be best seen as a fireball tens of miles "up-path" from where the meteorite fragments fall. Eyewitnesses are thus often tens of kilometers from the "target" area where meteorites fall. Tracking of the fireball and determination of the target area are typically poor, and the meteorites themselves are not recovered.

Figure 3. Photograph of a fireball Jan 21 1999 from Czech station No. 16 of the European Fireball Network camera system.

Now we have filled in the missing link. PSI is represented on a team has been formed at the International Space Science Institute (Bern, Switzerland) to study a rare but growing class of fireballs: those from which solid meteorites have been recovered on the ground. Starting with the Pr�bram fireball, which fell in the Czech Republic in 1959, the fireball path was well documented by photography, and meteorites and were recorded.

More recently, increasing numbers of video cameras in use at any time, has produced many cases of fireballs photographed by onlookers (as well as meteorite photo-networks in some cases). The dramatic increase in numbers of photo records, due mostly to amateur videos, is shown in Figure 2. The photos from observers allow detailed triangulation of the fireball flight path, and prediction of the meteorite fall site. This in turn has produced a rapid increase in the number of fireballs from which samples have been collected. The total number of fireballs that produced meteorites reached 9 by early 2004, is increasing so rapidly that we can anticipate many more examples within the next decade.

Moravka fireball
Moravka fireball
Figure 4. Two frames from amateur videos of the Moravka fireball of May 6, 2000, over Czech Republic. Six fragments were found. This and two other videos of Moravka allowed measurement of the trajectory.

Examples of photographic records are shown in Figures 3 and 4. Scientists, especially starting in central Europe and North America, have established photo networks with automated night-time cameras that record bright meteors and fireballs simultaneously from various locations; such a photo of a 1999 nighttime fireball is seen in Figure 3.

Photo networks, developed especially in the Czech Republic, image the fireball from several directions. Through triangulation, they provide trajectory data for many fireballs. Today, many such fireballs are also recorded from military satellites (but such data are mostly classified). Figure 4 shows two views of a fireball that fell in 2000 in the Czech Republic.

The rapid accumulation of cases where we have meteorites from specific well observed fireballs creates a new branch of meteoritics -- the comparison of the "ground truth" properties of the solid samples, of various meteorite classes, with their behavior during atmospheric entry.

In recognition of this Olga Popova (Institute for Dynamics of Geospheres, Moscow) organized a team in 2004 under the auspices of the International Space Science Institute (ISSI) to study these cases. Figure 5 shows this group during its first meeting in Switzerland in 2004. They collected and compared data on nine meteorites, including explosive behavior, trajectories though the atmosphere, and preatmospheric orbits around the sun.

Fireball/meteorite team
Figure 5. Fireball/meteorite team, meeting at ISSI in October, 2004. Standing, left to right: Edwin Gnos (Switzerland), Olga Popova (Russia), Josep Trigo-Rodriguez (Spain). Sitting, left to right: William Hartmann (USA), Ivan Nemtchinov (Russia), Jiri Borovičkai (Czech Republic), and Pavel Spurn� (Czech Republic).

The most exciting result is that in all available cases, the first explosions observed in the high atmosphere occurred when the pressure and stress on the incoming body were much less than the strength of the rocks collected on the ground. This means that the meter-scale meteorite bodies in space had very low strength, probably because they were highly fractured from impact events prior to their arrival on Earth. Collisional impacts in the asteroid belt and heavy fracturing of asteroidal material has long been accepted theoretically. The international team proposed that the meter scale bodies were heavily fractured when they were ejected from parent asteroids during collisions, and tend to blow apart along the fractures, forming the more coherent rock specimens found on the ground.

This work thus tells us important stories about the nature of meteoroidal bodies in space. It supports the idea that even modest scale asteroids, which may be visited in future space missions, are fractured. This may make it easier to obtain samples or even useful materials, such as metals, from such bodies.

PSI has had a number of fruitful collaborations with teams hosted by ISSI. ISSI is described further at their web site.

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