Three Leonid meteors captured in a 5 minute exposure. The brightest is a fireball clearly showing a trail of ionised air molecules as well as the smoke trail.
Credit: Jerry Lodriguss

About one meteor in every thousand is seen to be brighter than the planet Venus. These are known as ‘fireballs’ and are generally associated with larger (centimetre sized) meteoroids which may survive their passage through the atmosphere to land on the Earth as meteorites. Alternatively, they may explode before reaching the ground, an event generally accompanied by a sonic boom. In this case they are known as bolides.
Tens of thousands of fireballs fall to Earth every year (only about 5,000 of these become bolides), but very few of these are actually observed. This is due to many reasons:

• they fall over the sparcely populated areas that make up the majority of the Earth (including the oceans).
• they fall during the day and are masked by the brightness of the Sun.
• they fall during the night when few people are awake to observe them.

Fireballs can develop two types of trails. The most commonly observed is a short-lived (generally only a few seconds), glowing trail of ionized air molecules left behind after the passage of the meteor. Occasionally a smoke trail is also observed. This consists of particles stripped from the meteor during its passage through the atmosphere (ablation) and generally occurs at relatively low altitudes.