AstronomyNZ

Meteors

Meteor showers recur on nearly the same date of every year, although it might take a day or two to reach maximum intensity and a day or two to fall off. A meteor shower occurs when the Earth, carried by its orbit around the Sun is ploughing through a cloud of meteoric debris. All meteors that belong to a shower will appear to come from the same point in the sky – the radiant. The shower is named after the nearest bright star to the radiant. The brightest meteors are called fireballs and a very large fireball can be as bright as the Moon or even the Sun.

Typical meteorite trails are usually somewhere in the vicinity of 100 kilometres up. At this altitude, the atmospheric pressure is 0.00003% of that at the surface of the Earth. Imagine you are holding a piece of a comet stationary above the Earth, and then let it go. It speeds up as it falls. By the time it reaches the Earth’s upper atmosphere, it will be moving at escape velocity – about 11 km/sec. Ordinarily a comet or meteor will have some velocity relative to the Earth before it is attracted by the Earth’s gravity and will therefore hit at a higher speed. A comet on a highly eccentric orbit, moving in a retrograde direction so it collides with the dawn hemisphere, can be travelling as fast as 72 km/sec.

When a meteor enters the Earth’s atmosphere, it is heated to incandescence by friction with the thin air at an altitude of around a hundred kilometres. Big particles the size of your fist, or larger, streak through the Earth’s atmosphere, heat up by friction with the air, and char, melt, or burn off a thin crust. This crust can protect the interior. Very small particles are able to radiate their heat away because they have such a large area for their mass and so do not melt. They simply slow down at a hundred kilometres altitude. Gently, they fall for years. Particles of intermediate size are too small to survive the charring of even a thin crust, and too large to float down gently. They burn up entirely during entry. These are the meteors.

Typical visual meteors are millimetre sized. A fireball as bright as the brightest star typically weighs less than 100g; but a fireball temporarily as bright as the Sun would weigh a hundred thousand tons.

The Antihelion Radiant

Unlike most of the annual showers the antihelion source is produced by debris from unknown objects orbiting in a direct motion like the earth. These objects are most likely asteroids, which produce stony and metallic debris whose density is much greater than material produced by comets. This material collides with the earth on the inbound portion of its orbit, before its closest approach to the sun. Therefore we best see them just after midnight when we are facing the direction from which this activity appears. The antihelion source is active all year from an area of the sky nearly opposite that of the sun. The center of this source will move approximately one degree eastward per day and travels through many different constellations over the course of a year. It may make sense to list these meteors as antihelions or "ANT" but a majority of meteor organizations prefer that you list them from the constellation in which the radiant is currently located or the constellation where the shower reaches maximum activity.

The Northern and Southern Apex Radiants

Like the antihelion area, both apex areas are active all year long and travel approximately one degree eastward per day. Unlike the antihelion debris, these particles orbit the sun in a retrograde motion opposite that of the earth and are most likely produced by unknown comets. They strike the earth after their closest approach to the sun. Since they are moving in opposite directions these particles strike the earth at tremendous velocities often creating bright meteors with persistent trains. These particles strike the earth on the morning side of earth and are best seen just before morning twilight while the sky is still perfectly dark. This is not really a "shower" per se, but an artificial radiant created by the Earth's motion through space. Meteors from both branches are normally included in the sporadic count. I feel it is a worthy project to see if it is possible to distinguish these meteors from the normal sporadic background. On rare occasions there are meteors with a zero inclination that radiate precisely from the apex point on the ecliptic, exactly 90 degrees west of the sun. In simplistic terms, these meteors are seldom seen since the Earth "sweeps clean" much of the material that shares the same orbit as our planet. Much more material is located just north and south of the earth's orbit with slightly higher or lower inclinations. This creates the northern and southern branches of the apex activity.

June