Fireballs and bolides are astronomical terms for exceptionally bright meteors that are spectacular enough to to be seen over a very wide area. The following table provides a chronological data summary of fireball and bolide events provided by U.S. Government sensors. Ground-based observers sometimes also witness these events at night, or much more rarely in daylight, as impressive atmospheric light displays. This website is not meant to be a complete list of all fireball events. Only the brightest fireballs are noted.
A meteoroid is generally defined as an asteroid or comet fragment that orbits the Sun and has an approximate size between ten microns and a meter or so. Meteors, or "shooting stars," are the visible paths of meteoroids that have entered the Earth's atmosphere at high velocities. A fireball is an unusually bright meteor that reaches a visual magnitude of -3 or brighter when seen at the observer's zenith. Objects causing fireball events can exceed one meter in size. Fireballs that explode in the atmosphere are technically referred to as bolides although the terms fireballs and bolides are often used interchangeably.
During the atmospheric entry phase, an impacting object is both slowed and heated by atmospheric friction. In front of it, a bow shock develops where atmospheric gases are compressed and heated. Some of this energy is radiated to the object causing it to ablate, and in most cases, to break apart. Fragmentation increases the amount of atmosphere intercepted and so enhances ablation and atmospheric braking. The object catastrophically disrupts when the force from the unequal pressures on the front and back sides exceeds its tensile strength.
Objects causing fireballs are usually not large enough to survive passage through the Earth's atmosphere intact, although fragments, or meteorites, are sometimes recovered on the ground. The approximate total radiated energy in the atmosphere is provided in unit of Joules, a unit of energy given in kilograms times velocity squared, or kg x (m/s)2. An event with an energy equivalent of one thousand tons of TNT explosives is termed a kiloton (kt) event, where 1 kt = 4.185 x 1012 Joules. In the accompanying table, the total radiated energy is given but this is always less that the total impact energy. Peter Brown and colleagues have provided an empirical expression to approximately provide the total impact energy in kt (E), given the optical radiant energy in kt (Eo) (see: Brown et al., The flux of small near-Earth objects colliding with the Earth. Nature, vol. 420, 21 Nov. 2002, pp. 294-296).
E = 8.2508 x Eo0.885
The accompanying table provides information on the date and time of each fireball event, its geographic location, its altitude and velocity at peak brightness, its approximate total optical radiated energy and its calculated total impact energy. The pre-impact velocity components are expressed in a geocentric Earth-fixed reference frame defined as follows: the z-axis is directed along the Earth's rotation axis towards the celestial north pole, the x-axis lies in the Earth's equatorial plane, directed towards the prime meridian, and the y-axis completes the right-handed coordinate system.
http://neo.jpl.nasa.gov/fireball/
