Meteor

Meteor, in astronomy, small solid body known as a meteoroid that enters a planet's atmosphere from outer space and is raised to incandescence by the friction resulting from its rapid motion. Brilliant meteors, known as fireballs, occur singly and generally consist of a luminous head, followed by a cometlike train of light that may persist for several minutes; some, called bolides, have been seen to explode with a sound like thunder. Fainter meteors, called shooting or falling stars, usually occur singly and sporadically. At intervals, however, hundreds of such meteors occur simultaneously and appear to emanate from a fixed point. These swarms are called meteor showers and are named after the constellation in which they seem to have their point of origin. Some appear annually on the same days of each year and are called periodic showers; others occur infrequently at varying intervals. The periods of meteor showers generally coincide with those of certain comets. Most meteors are dissipated in flight and fall to the earth as dust; a meteor that reaches the surface of the earth or another planet is called a meteorite.

Meteoroid

Meteoroid, solid body orbiting the sun, becoming a meteor, or shooting star, if it enters the earth's atmosphere. The vast majority of meteroids are the size of grains of dust, but they range upward in size without any definite limit. The largest can have masses of thousands of tons. See Asteroids; Comets.

Asteroid

Asteroid, one of the many small or minor rocky planetoids that are members of the solar system and that move in elliptical orbits primarily between the orbits of Mars and Jupiter.

SIZES AND ORBITS

The largest representatives are 1 Ceres, with a diameter of about 1,003 km (about 623 mi), and 2 Pallas and 4 Vesta, with diameters of about 550 km (about 340 mi). The naming of asteroids is governed by the International Astronomical Union (IAU). After an astronomer observes a possible unknown asteroid, other astronomers confirm the discovery by observing the body over a period of several orbits and comparing the asteroid’s position and orbit to those of known asteroids. If the asteroid is indeed a newly discovered object, the IAU gives it a number according to its order of discovery, and the astronomer who discovered it chooses a name. Asteroids are usually referred to by both number and name.

About 200 asteroids have diameters of more than 97 km (60 mi), and thousands of smaller ones exist. The total mass of all asteroids in the solar system is much less than the mass of the Moon. The larger bodies are roughly spherical, but elongated and irregular shapes are common for those with diameters of less than 160 km (100 mi). Most asteroids, regardless of size, rotate on their axes every 5 to 20 hours. Certain asteroids may be binary, or have satellites of their own.

Few scientists now believe that asteroids are the remnants of a former planet. It is more likely that asteroids occupy a place in the solar system where a sizable planet could have formed but was prevented from doing so by the disruptive gravitational influences of the nearby giant planet Jupiter. Originally perhaps only a few dozen asteroids existed, which were subsequently fragmented by mutual collisions to produce the population now present. Scientists believe that asteroids move out of the asteroid belt because heat from the Sun warms them unevenly. This causes the asteroids to drift slowly away from their original orbits.

Pallas

Pallas, the second largest asteroid, the second to be discovered. It was first observed by the German astronomer Heinrich Olbers in 1802. It is about 480 km (about 300 mi) in diameter, and it revolves about the sun in 1684 days.

Vesta

Vesta (astronomy), asteroid orbiting the Sun between the orbits of Mars and Jupiter at a mean distance of 2.36 astronomical units (about 353 million km/about 219.4 million mi) and having a diameter of approximately 385 km (about 240 mi). Vesta is the third largest asteroid and was the fourth asteroid to be discovered. German astronomer Heinrich Olbers found it in 1807. In 1999 the National Aeronautics and Space Administration (NASA) spacecraft Deep Space 1 made observations that suggested that asteroid 9969 Braille, which passes close to Earth, and Vesta are composed of similar materials. Scientists used the information to theorize about the relationship between Vesta and the many meteorites that have struck Earth over time.