Meteors and FAQs!

Meteoroids, Meteors and Meteorites – Frequently Asked Questions

What is a meteor?

A meteor is actually a phenomenon of light. It is the visible path of a meteoroid as it enters the Earth’s atmosphere. A meteor’s short streak of light may last only for a fraction of a second. It occurs when a meteoroid encounters the Earth’s atmosphere. Meteors have different brightness; the brightness of a meteor will depend on the composition of the original meteoroid, its initial mass, its entry speed and its zenith angle.

How big is a meteoroid?

The International Meteor Organisation (IMO) glossary defines a meteoroid as a solid object moving in interplanetary space, of a size considerably smaller than an asteroid and considerably larger than an atom or molecule.

A typical meteor is caused by meteoroids that are anything from a fraction of a millimetre to a few millimetres in size. To become a meteor, a meteoroid must be larger than about 100 microns in size because objects smaller than this will loose their kinetic energy through thermal radiation rather than mass loss and will not produce a distinct trail. After a fraction of a second nothing of the meteoroid will remain.

What altitude do Meteors occur?

Conversion of the meteoroid’s kinetic energy into thermal energy begins as it interacts with the atmosphere, the process of heating being due to a ‘shock layer’ (ram pressure) rather than friction. When the temperature reaches about 2,000 degrees ablation begins and the meteoroid starts to loose mass leaving behind it a trail of ionised atoms and electrons. This process of ablation for a typical naked eye meteor takes place between 115 km and 85 km.

When does a meteoroid become an asteroid?

If it is large enough to reflect light so that it can be seen in a telescope then it is probably considered to be an asteroid (the IMO definition of an asteroid is an object ranging in size from sub-km to about 1000 km).

What is a meteorite?

If a meteoroid is small – or large – enough to survive its journey through the atmosphere and reach the ground intact, it is called a ‘meteorite’. Most of the meteoroids that reach the ground are small and do not get vaporized because they are light enough that they slow down very easily.

Where do meteoroids come from?

Space isn’t empty; it’s full of ‘stuff’. The majority of meteors we observe are produced by comets and asteroids. Millions of years of collisions as well as debris left by passing comets provide a plentiful supply of meteoroids. For example, the Perseids shower which appears in August is a result of the debris trail left by Comet Swift-Tuttle, a periodic comet with an orbital period of 133 years. Meteoroids can also be created by high speed ejecta from asteroid collisions with larger bodies such as Mars or the Moon.

Measurements suggest that about 3,000 metric tonnes of particles in the 20 micron to 500 micron size range is intercepted by the Earth each year. A small fraction of meteors originate from interstellar space. A clue to identifying these meteors of ‘alien’ origin is their velocities which will exceed 72 kilometres per second but due to small particle sizes these are unlikely to be seen with the naked eye.

Meteoroids are ejected by comets every time their orbit takes them close to the Sun. Sublimation of the icy surface can begin as far as 2AU from the Sun when a comet begins to develop its characteristic tail. The comet is in effect laying down its meteoroid trail with meteoroids being carried away in random directions by its gaseous outflows. Over time they will spread away from the comet but will continue to orbit the sun perhaps long after their parent body has disintegrated. Their orbits will be altered by solar radiation and gravitational influences so that eventually their orbits will differ from that of the parent body.

What causes a meteor shower?

On any ordinary night you might expect just a few meteors per hour from every direction, but at certain times of the year meteor activity increases and we see a meteor shower. Meteor showers occur whenever the earth passes through a meteoroid stream.

Meteors that belong to a shower have the same origin, usually a dust trail from comet, they will have parallel paths and roughly the same initial speeds. Because they are travelling on parallel paths they all appear to radiate from the same local area of the sky (the Radiant). Annual meteor showers occur when the Earth is at the same solar longitude (solar longitude is used rather than date because civil time changes with respect to solar longitude by approximately six hours every year) . For example the Perseids occur at a solar longitude of 139.4º

Meteors further from the radiant have longer trails than those closest to it. With meteoroids in a stream having roughly the same velocity this implies that the speed with which a meteor appears to travel across the sky increases with distance from the radiant. The orbital characteristics of individual meteoroids within a meteor stream are not identical which is why the radiant is s small patch of sky rather than a precise point. Streams are subject to perturbations and so the radiant of an older stream often occupies a larger area than that of a younger stream because older streams have been subject to gravitational influences of planets over a greater period of time.

What is a sporadic meteor?

Sporadic meteors are random meteors that are not associated with a recognised shower. We know that organised showers can become dispersed over time due to gravitational perturbation and solar wind so it is quite possible that a sporadic meteor may once have been part of a meteoroid stream; the process of dispersion could take hundreds or perhaps thousands of years. If the maximum hourly rate of a stream falls to close to (or below) the expected sporadic rate a shower becomes difficult to identify.

There are differences between sporadic rates by time of year. Sporadic meteors are less frequent in the early part of the year.

How fast does a meteor travel?

The speed with which a meteoroid first encounters the atmosphere will vary from 11.2 kilometres per second to 72.2 kilometres per second . The meteoroid will then begin to decelerate.

The minimum speed of a meteor is due solely to the earth’s gravity. A meteoroid with zero speed relative to the Earth, will be drawn in by gravity until it is travelling at a little over 11 kilometres per second when it reaches meteor height (approximately 100km).

The theoretical maximum speed is due to the meteoroid’s motion around the Sun, and can range up to 42 kilometres per second at the Earth’s orbital distance (a meteoroid moving faster than this is not in orbit around the Sun, and must be passing through the solar system or is being ejected as a result of an interaction with a solar system object). In a head-on collision with the Earth, such as occurs with the Leonids which are in a retrograde orbit, we need to consider the Earth’s motion. The Earth is orbiting the Sun at about 30 kilometres per second, the speed relative to the earth is about 72 kilometres per second (42 kps + 30 kps)

What is a boilide?

A bolide is a very large meteor which is sometimes accompanied by loud sonic booms. Bolide is not a term that is favoured by the International Astronomical Union.

What is a fireball?

A fireball is a very large and bright meteor often leaving a smoke trail. The IMO defines a fireball as a meteor which is brighter than any planet or star (i.e. brighter than magnitude -4).

Why are meteor rates difficult to predict?

Predicting meteor showers can be fraught with difficulty. For example, the evolution of a meteoroid stream is hard to predict (for example the effects of perturbation since the last appearance), there will be variation in when the peak activity occurs (it may occur in daytime at your location) as well as variation in meteoroid density.

The Zenith Hour Rate or ZHR is number of shower meteors per hour one observer would see if his limiting magnitude is 6.5mag and the radiant is in his zenith. It is an estimated ideal of how many meteors per hour an observer can expect to see assuming that the radiant for the shower is directly overhead, the observing conditions are perfect, and the horizon is clear, in all directions. The best showers such as the Quadrantids and Geminids have a ZHR of over 100. Rarely, however, do we get ideal conditions. Other factors affecting what you can see include.

The limiting magnitude at your observing site (the faintest meteor that you can see given atmospheric conditions and the local environment)
The size of your field of view
The observing direction
The time (rates improve after midnight).

The limiting magnitude of your observing site can be affected by many factors – including local light pollution, seeing, and of course the moon.

When do annual showers occur?

There are many identified showers and the list is being added to as new showers are identified. The major annual showers are:

Name	Maximum	Radiant	ZHR
Quadrantids	Jan 04	15:20 / +049º	120
Lyrids	Apr 22	18:04 / +034º	18
Eta Aquariids	May 5	22:32 / -016º	20
Delta Aquariids	Jul 25	23:36 / -001º	60
Perseids	Aug 12	03:04 / +058º	100
Orionids	Oct 21	06:20 / +016º	23
Leonids	Nov 17	10:12 / +022º	Variable
Geminids	Dec 13	07:28 / +033º	120
Ursids	Dec 22	14:28 / +076º	10

By Peter Campbell Burns

Nov 2012