Astronomical Feature of the Month
Falling Meteors

In honor of the Astrium workshop, this month's Feature is about falling meteors. Meteorites, fireballs, and bolides are fragments of extraterrestrial material that enter the Earth's atmosphere. A meteoroid is a pebble or stone in space. A meteor is the bright flash of light that a meteoroid produces as it streaks across the sky, and also refers to the stone while in the atmosphere. A meteorite is the rock that impacts the ground.
    There are three major types of meteorites: stony, iron, and stony-iron. The classification is based on composition, and by far the most common is the stony meteorite though most people think of iron masses when the word "meteorite" is mentioned. Irons are alloys of iron and nickel. Stony-irons are meteorites that are mixtures of metallic iron/nickel and silicate rocky material. Stony meteorites very much resemble terrestrial igneous rocks (and thus, are hard to identify). Stonys are made primarily of silicate minerals, many of which are common in the Earth's crust.
    A fireball is defined as any meteor whose brightness exceeds that of magnitude -3 (about the brightness of Venus at maximum brilliance). Fireballs and bolides are more massive than the objects that produce ordinary meteors. Meteors are the size of a grain of sand and weigh less than a gram. Fireballs range in size from as big as a pea and weigh several ounces to a foot across or more and weigh many pounds. Fireball light is generated by the heating of the body as it enters the atmosphere, ionizing the atmosphere up to 100 feet around the meteoroid. It is this atmospheric effect that you see from the ground. The colors are usually white near the start of the trail, and fade to red as the fireball has been slowed to low enough speeds where the efficiency of ionizing the atmosphere is small. The colors can tell something about the composition of the fireball: nickel produces a green color, sodium a yellow color and magnesium a blue-white color.
    If a fireball exhibits an exploding characteristic it is termed a bolide. Bolides make dazzling displays dozens of miles above the planet. Fortunately, most explode into thousands of pieces or burn up entirely before they reach the surface. Bolides that strike ground are extremely destructive. A very large one created Meteor Crater in Arizona.

Bolides Over 51 Pegasi
by John Whatmough

Here’s a nice Bryce image of a cluster of bolides falling on a hypothetical planet around 51 Pegasi.

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International Association of Astronomical Artists