Meteoroid

Gayle, J.B., Dalton, C.C., and Martin, H.L., A Bibliography Concerning Aspect of the Meteoroid Hazard, George C. Marshall Space Flight Center Report No. MDWG-63-2, Huntsville, AL, 302 pp., April 1963. 

Rae, W.J., A Critical Review of Impact Theories, Presented at the Highwater Conference on Meteoroid Impact, Highwater, Quebec, Canada, 26 pp., July 14-15, 1966. 

Friend, W.H., Murphy, C.L., and Shanfield, I., Review of Meteoroid-Bumper Interaction Studies at McGill University, National Aeronautics and Space Administration Contract Report No. NASA CR-54857, Cleveland, OH, 187 pp., August 1966. 

Meteoroid - the name given to a meteorite or a meteor before it enters the Earth s (or any other planetary) atmosphere. 

Meteors form when meteoroids enter the atmosphere. The mean orbital velocity of the Earth around the Sun is 29.7 kms-1, so a particle stationary in space will enter the Earth s atmosphere at this velocity. Gravity will further attract the particles up to a maximum of 11 kms-1 (the escape velocity) to make an entry velocity of order 40 kms-1 plus whatever additional velocity the particle may possess. This energy is converted into heat by the drag on the atmosphere, producing temperatures of order 2000 K - the glow seen from a shooting star. The final temperature depends on the properties of the particle, the atmospheric composition and the angle of incidence with the atmosphere. 

Meteor A meteoroid entering the Earth s atmosphere at speeds greater than 70 kms-1 that bums up completely before it hits the ground. 

Meteorite A meteoroid entering the atmosphere that survives the journey through the atmosphere to land on the ground and become a find . 

Meteoroid A particle of interplanetary debris that can enter the atmosphere of a planet to become either a meteor or a meteorite. 

Microcrater spall zones - Ten laser pulses were applied to sites where surface material had been spalled away by nearby meteoroid impacts. 

Cosmic-ray exposure ages are determined from spallation-produced radioactive nuclides. Cosmic-ray irradiation normally occurs while a meteoroid is in space, but surface rocks unshielded by an atmosphere may also have cosmogenic nuclides. These measurements provide information on orbital lifetimes of meteorites and constrain orbital calculations. Terrestrial ages can be estimated from the relative abundances of radioactive cosmogenic nuclides with different half-lives as they decay from the equilibrium values established in space. These ages provide information on meteorite survival relative to weathering. 

Comet A small (typically 1-10 km in diameter) interplanetary body that mostly consists of ice, other frozen materials, and dust. Comets originate from the outer solar system (compare with asteroid and meteoroid). 

Meteorite A meteoroid that has survived impacting the Earth s surface. 

Meteoroid A small (typically having a size of a few meters or less) essentially nonicy interplanetary object (compare with asteroid and comet). 

Bloch, M. R., Fechtig, H., Gentner, W., Neukum, G., Schneider, E. (1971) Meteorite impact craters, crater simulations, and the meteoroid flux in the early solar system. Proc. Second Lunar Science Conf., 3, 2639-52. 

Many specific properties of water are cited to make the case that water is an ideal biosolvent uniquely suited to support life Frozen water floats. Water is an excellent solvent for salts. Water is liquid over a broad range of temperature. Indeed, the concept of a habitable zone, a region around a star where life is presumed to be possible, largely posits a region in which a planet s surface or subsurface might support liquid water. One such planet in a habitable zone could export life to other suitable environments via meteoroids. 

Meteoroids are small bits and pieces of rock left over from the original material that formed the solar system. As they follow random paths through the solar system, some wander close to Earth and enter its atmosphere. Hundreds of meteoroids enter the atmosphere every day, but most are very tiny and do not survive the journey. Only the largest fragments reach the Earth s surface. 

Meteors or shooting stars are meteoroids that fall through Earth s atmosphere. Friction between the fast-moving meteoroid and air molecules in the atmosphere produces heat, which burns the meteoroid and causes the air around it to glow. 

Meteorites are meteoroids that survive their fiery flight and hit the ground. They are classified by their mineral composition and texture. Thousands of meteorites have been identified on Earth and there are undoubtedly many more yet to be discovered. 

Some meteoroids land with such force that they leave a depression or crater. These impact features, technically called astroblemes, may be round or elliptical, depending on the angle at which the meteoroid hit the ground. There are astroblemes all over the Earth. Recent impact sites are easy to spot, but older craters have been subjected to millions of years of erosion and burial, and are harder to identify. 

Some meteoroids shatter upon impact, while others remain intact. Those that break when they hit the ground may be found in fragments around the site of impact. Some of these fragments may weigh a couple of tons All of the meteorites from a single fall are given the same name. Thus, the many hundreds of meteorites that have been found at Meteor Crater in Arizona are called by the name Canyon Diablo. 

When a large meteoroid strikes the Earth, the shock and heat caused by the impact can liquefy pieces of the Earth s crust. The resulting blobs of... 

The luminous entry of meteoroids into Earth s atmosphere, known for millennia as the phenomenon of shooting stars, is quite interwoven with myth and folklore. Good historical and physical surveys may be found in the... 

A recent pictorial review may also be consulted for beautiful displays of this phenomenon.The term meteor comes from the Greek fiereopov meaning things in the air. The terms fireball and bolide are also frequently applied. The former refers to brighter events associated with centimeter size meteoroids. The latter term applies to a meteroid that seems to explode as it gets heated by entry into the Earth s atmosphere. 

The physics of meteoric phenomena can be divided into three parts (1) the dynamics of meteoroid motion in the atmosphere (2) the small scale physical processes occurring in the extreme environment of a meteor and (3) the ionospheric chemical and plasma kinetics induced by atoms and molecules deposited by meteors, particularly metals. These will be briefly discussed in the following. 

Investigations of the dynamical aspects of the meteor phenomenon dated back to the 19th century. The characteristic properties of a meteor have been visual magnitude, the radiant, and entry velocity, size and mass of the meteoroid. While the first two can be directly observed, the latter three properties must be derived with a model for drag and mass loss from the observed deceleration as a function of momentary velocity and visual magnitude. 

Typical visual meteors are associated with particles with representative diameters ranging from 1 mm to 20 cm. ° The size distribution of interplanetary dust, however, peaks at substantially smaller diameters. Meteoroids... 

Table 1. List of the important visual meteor showers. The data was obtained from the International Meteor Organization s Web site (www.imo.org). Details in this table correspond to the best information available in April, 1996. v is the meteoroid velocity, r is a measure of intensity the smaller the number the more intense is the radiant. ZHR is zenithal hourly rate. Some showers have ZHRs that vary from year to year. The most recent reliable figure is given here, except for possibly periodic showers that are noted as var. = variable. Dates of maximum activity refer to meteor rates from the radiant, not the true maxima.

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