Meteoroids, Meteors and Meteorites

Meteor. Photo courtesy of Scienceblog.com
Meteor. Photo courtesy of Scienceblog.com

Little chunks of rock and debris in space are called meteoroids. They become meteors — or shooting stars — when they fall through a planet’s atmosphere; leaving a bright trail as they are heated to incandescence by the friction of the atmosphere. Pieces that survive the journey and hit the ground are called meteorites.

Gibeon Meteorite in its original form. Credit: http://www.towercrystals.com
Gibeon Meteorite in its original form. Credit: http://www.towercrystals.com

Shooting stars or meteors are bits of material falling through Earth’s atmosphere; they are heated to incandescence by the friction of the air. The bright trails as they are coming through the Earth’s atmosphere are termed meteors, and these chunks as they are hurtling through space are called meteoroids. Large pieces that do not vaporize completely and reach the surface of the Earth are termed meteorites.

Scientists estimate that 1,000 tons to more than 10,000 tons of meteoritic material falls on the Earth each day. However, most of this material is very tiny — in the form of micrometeoroids or dust-like grains a few micrometers in size. (These particles are so tiny that the air resistance is enough to slow them sufficiently that they do not burn up, but rather fall gently to Earth.)

Where do they come from? They probably come from within our own solar system, rather than interstellar space. Their composition provides clues to their origins. They may share a common origin with the asteroids. Some meteoritic material is similar to the Earth and Moon and some is quite different. Some evidence indicates an origin from comets.

Several “shooting stars” or meteors per hour can usually be seen on any given night. Sometimes the number of meteors seen increases dramatically: these are termed “meteor showers”. In fact, some meteor showers occur annually or at rather regular intervals. For example, there are generally more visible “shooting stars” during the autumn and winter of the year. The number always increases after midnight and is usually greatest just before dawn. Perhaps the most famous are the Perseids, which peak around August 12 every year.

Meteor showers are usually named after a star or constellation which is close to the radiant (the position from which the meteors appear to come). Many of the meteor showers are associated with comets. The Leonids are associated with comet Tempel-Tuttle; Aquarids and Orionids with Halley, and the Taurids with Encke.

Meteorites may look very much like Earth rocks, or they may have a burned appearance. They may be dense metallic chunks or more rocky. Some may have depressioned (thumbprint-like), roughened or smooth exteriors. They vary in size from micrometer size grains to large individual boulders. The largest individual meteorite found is the Hoba meteorite in southwest Africa, which has a mass of about 54,000 kg and mostly consists of iron.

Considering the vast infall of meteorites, one cannot help but wonder if anyone has been hurt or killed by meteorites. There are only a few documented cases on record. A shower of stones fell upon Nakhla, near Alexandria, Egypt on 28 June 1911, one of which allegedly killed a dog. On 30 November 1954, Mrs. Hewlett Hodges of Sylacauga, Alabama was severely bruised by an 8 pound stony meteorite that crashed through her roof. This is the first known human injury.

Most meteoritic samples are either iron (actually nickel-iron alloy); stony, which are predominately rocky-silicates; or stony-iron.

While most meteors burn up before reaching the Earth’s surface, many meteoroids break apart in the upper atmosphere, and become “fluffy meteors.” This “fluffy” nature indicates a loose structure or vapor grown crystal aggregates. This gives rise to theories that some meteoroid material was aggregated — that some were subjected to heating-vaporization-condensation. This contrasts with the idea that meteoroids originated from an exploded planet, planetoid or asteroid.

Sixteen meteorites have been found in Antarctica that are believed to have originated on the planet Mars. Gases trapped in these meteorites match the composition of the Martian atmosphere as measured by the Viking spacecraft, which landed on Mars in the mid-1970s. Controversy continues about whether structures found in one of these meteorites, known as ALH 84001, might be fossil bacteria or geologic structures.

Much remains to be learned about meteorites and their origins.

Major Meteor Streams Peak Night
(may vary by +/- 1 day)
Time to Watch*
(24 hour clock)
Maximum Rate**
(per hour)
Parent Body (Asteroid or Comet)
Quadrantids January 3-4 23:00 to dawn 60-200 (196256) 2003 EH1
Lyrids April 21-22 21:30 to dawn 10-15 typical Comet C/1861 G1
Eta Aquarids May 5-6 01:30 to dawn 40-85 Comet 1P/Halley
Delta Aquarids July 27-28 21:30 to dawn 15-20 Unknown sungrazing comet
Perseids August 11-12 dusk to dawn 60-100 Comet 109P/Swift-Tuttle
Orionids October 20-21 22:00 to dawn 25 Comet 1P/Halley
Leonids November 17-18 23:30 to dawn 10-15 Comet 55P/Tempel-Tuttle
Geminids December 13-14 19:00 to dawn 60-120 (3200) Phaethon
* For observers in the northern hemisphere.
** Under perfect conditions