The luminous streak lasting seconds or fractions of a second and seen at night when a solid, natural body plunges into the Earth's (or another planet's) atmosphere. The entering object is called a meteoroid and, if any of it survives atmospheric passage, the remainder is called a meteorite. Cosmic dust particles (with masses of micrograms) entering the atmosphere and leaving very brief, faint trails are called micrometeors, with the surviving pieces known as micrometeorites. If the apparent brightness of a meteor exceeds that of the planet Venus as seen from Earth, it is called a fireball, and when a bright meteor is seen to explode, it is called a bolide. See also Meteorite; Micrometeorite.
Under normal, clear atmospheric conditions and dark skies (no moonlight or artificial lights), an observer will see an average of five meteors per hour. The spatial distribution of meteoroid orbits relative to the Sun and the circumstances of their intersections with the moving Earth are responsible for pronounced variations in meteor rates.
The average meteor seen by the unaided eye starts with a meteoroid velocity of 18 mi/s (30 km/s) and leaves a luminous trail from 67 to 50 mi (110 to 80 km) high. The meteor trails are rapidly expanding columns of atoms, ions, and electrons dislodged from the meteoroid by collisions with air molecules, and can be excited to temperatures of several thousand degrees Celsius. For a time after trail formation, the free electrons are dense enough to reflect radio waves in the very high frequency range, and therefore can be used to transmit radio messages. See also Radio-wave propagation.
Under the right circumstances, particularly with high-power ultrahigh-frequency (UHF) radars, the ionization right around and moving with the meteoroid itself is seen. This is known as the head echo, and a determination of its velocity is the most accurate way to determine radar meteor speeds. See also Radar; Radar astronomy.
The Earth moves around the Sun with an average speed of 18 mi/s (30 km/s). According to thelaws of celestial mechanics, if a meteoroid comes from beyond the solar system, its velocity at the Earth's distance from the Sun must be greater than 26 mi/s (42 km/s). If such a meteoroid hits the Earth head-on, indications of preatmospheric speeds in excess of 45 mi/s (72 km/s) would be observed. The fact that the vast majority of observed meteoroids have orbits with Earth-approaching velocities of less than 45 mi/s indicates that most of these are comet and asteroid fragments, and are therefore long-term members of the solar system. However, in the 1980s and 1990s, a combination of spacecraft and high-power radar observations indicated that hypervelocity micrometeoids do indeed exist with seeming interstellar dust connections. See also Interstellar matter.
A combination of the meteoroid's and Earth's velocities of travel around the Sun make the meteor itself seem to originate from a specific direction in the sky called the radiant. If there are numerous meteoroids in nearly the same orbit (sometimes incorrectly called meteor streams), the Earth sweeps them up at specific times of the year and a so-called meteor shower is observed. Meteor showers are named after the constellation or single star in the sky from which they appear to radiate. While shower meteoroids are really moving nearly parallel through space and result in nearly parallel meteor trails, the effects of perspective make the meteors appear to diverge from the radiant. Meteors that cannot be shown to be associated with a known shower are termed sporadic meteors.
A number of meteor showers have been observed to be in orbits that are similar to those traveled by known comets. Thus an association between shower meteors and comets has gradually become a firmly entrenched concept. There are numerous theoretical scenarios where vaporization of the more volatile cometary ices ejects small solid particles from the surface of the nucleus. A fair proportion of these fragments, particularly the smaller dust-sized ones, escape and take up their own orbits as meteoroids. Cometary nuclei have been known to split into two or more pieces and, when this occurs, it is likely that particles larger than dust size are released as well.
The strategy of photographic or electronic measurements is to place at least two cameras 10–52 mi (15–85 km) apart over a known baseline, but arranged to examine the same volume of space at a height of about 56 mi (90 km). Each camera has a rotating shutter so that the meteor trail consists of a line of bright dashes. Meteor imaging is one of the most difficult areas of astronomical detection, even with ultrafast cameras. Meteor spectroscopy is even more difficult since the light is spread out over areas hundreds of times larger than the meteor trail itself. See also Astronomical photography; Astronomical spectroscopy.
Radio and radar observations depend on the fact that the initial ion-electron densities in a meteor trail are considerably higher than the average for the ionosphere at an altitude of 56 mi (90 km). For a very high frequency (VHF) or somewhat lower-frequency radar system, the maximum reflected signal occurs when the meteor trail is at right angles to the outgoing wave, with head echoes rarely seen. At ultrahigh frequencies (UHF), radar reflections from the head-echo predominate. From these, high-accuracy radial velocities are determined directly, using the Doppler effect. See also Doppler effect; Radio astronomy.
The parent comet of the Leonid stream, 1866 I, made another of its periodic (33-year intervals) approaches to the Sun in 1999. With the appearance of the comet, it was expected that the strong meteor storm that happened last in 1966 would again make a brief but spectacular appearance. However, perturbations by the outer planets (particularly by Neptune) once again played a significant role in this shower's behavior. The perturbations moved a number of thin meteoroid streams produced by the comet many orbit periods in the past into intersection range of the Earth. This produced a unique succession of strong peaks covering a span of seven years (1996–2003). The scientific yield of this extended display was much more than anyone had hoped. While meteor trails had been previously recorded from the space shuttle and other spacecraft, in 1997 the first above-atmosphere, far-ultraviolet spectrum of a bright meteor was recorded during the Leonid shower. Spectra obtained from the ground also yielded new information. The large number of fireballs in the Leonid streams enabled many details of the ablation processes at higher than average meteoroid incoming velocities to be recorded with high-speed cameras.
