A fireball is a very bright meteor, similar in brightness to the planet Venus as seen in the morning or evening sky. We also have bolides, which are special types of fireball which explode in a bright terminal flash at its end, often with visible fragmentation. What both of these phenomena indicate is the passage of either a fast or large (or both) meteoroid through the atmosphere. Several thousand meteors occur in the Earth’s atmosphere each day. The vast majority of these, occur over the oceans and uninhabited regions, and many are masked by daylight. Those that occur at night also stand little chance of being detected due to the relatively low numbers of persons out to notice them.
Typically velocities and masses needed to produce fireballs are 100g at 20 km/s – that’s 72000 km/hr! Fortunately, however, once a meteor enters the atmosphere it loses a lot of its velocity to atmospheric drag. Similar to shooting a bullet into water, the mass will lose all of their cosmic velocity while still several kilometers up. This is because 90% of the earth’s atmospheric mass is within 12 km of the surface.
As meteor travels through the atmosphere, many witnesses report a broad range of colours from red to bright blue and sometimes violet. The composition of a meteoroid (body coming into the atmosphere) plays an important part in the observed colours of a fireball, with certain elements displaying signature colours when vaporized. For example, sodium produces a bright yellow colour, nickel shows as green, and magnesium as blue-white. The speed of the meteor also affects the colour as high speeds mean more kinetic energy will intensify certain colours.