Asteroids

SIZES AND ORBITS

Asteroid, one of the many small or minor planets that move in elliptical orbits primarily between the orbits of Mars and Jupiter.

The largest representatives are Ceres, with a diameter of about 1,030 km (640 mi), and Pallas and Vesta, with diameters of about 550 km (340 mi). About 200 asteroids have diameters of more than 100 km (60 mi), and thousands of smaller ones exist. The total mass of all asteroids in the main asteroid belt, lying between Mars and Jupiter, 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 are binary (having companions)—for example, (243) Ida.

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 sizeable planet could have formed, but was prevented from doing so by the disruptive gravitational influence of the giant planet Jupiter. Originally perhaps only a few dozen asteroids existed, which were subsequently fragmented by mutual collisions to produce the population now present.

In addition to the asteroids in the main belt, recent research has focused attention on apparently similar objects lying in other regions of the solar system. The so-called Trojan asteroids lie in two clouds, one moving 60° ahead of Jupiter in its orbit, and the other 60° behind. In 1977 the asteroid Chiron, named after a centaur of Greek mythology, was discovered in an orbit between that of Saturn and Uranus, and since then another five objects moving in such orbits have been found. These newly discovered asteroids, some of which may be cometary in origin, are known as Centaurs.

In 1992 a completely different type of asteroid was found, moving in an orbit on the edge of the planetary system, beyond Neptune. This, the first of the so-called Kuiper belt (or Edgeworth-Kuiper belt) objects, represents the tip of a rather substantial iceberg: a population, believed to be more than 30,000 in number, of icy planetesimals with diameters greater than about 100 km (60 miles). They are thought to represent debris left over on the outskirts of the solar system from the time of formation of the planets. By October 1996, 39 such objects had been found, although a few were later “lost”, owing to their extreme faintness and the lack of precise knowledge of their orbits.

At the other extreme are a number of asteroids whose orbits lie largely inside the main belt, crossing the orbit of the planet Mars and occasionally those of the Earth and Venus too. By June 1996 more than 400 of these so-called near-Earth asteroids had been discovered. They fell into several groups, according to their distances from the Sun when they are closest (at perihelion) and furthest away (at aphelion). Each group was named after a representative asteroid. There were 195 known Apollos (with perihelia less than the Earth’s aphelion distance, and orbital periods greater than one year); 185 Amors (with perihelia greater than the Earth’s aphelion distance but with orbits intersecting the orbit of Mars); and 22 Atens (with orbital periods less than one year, but with aphelion distances greater than the Earth’s perihelion distance, allowing a possible collision with the Earth). As a result of long-term planetary perturbations, the Atens and Apollos and about 50 per cent of the Amors are on orbits such that they could collide with the Earth, representing a possibly significant extraterrestrial hazard to life.

One of the largest near-Earth asteroids is Eros, an elongated body measuring 14 by 37 km (9 by 23 mi). Apart from an Aten object designated 1995 CR, the near-Earth asteroid whose orbit comes closest to the Sun is the Apollo asteroid Phaethon, about 5 km (3 mi) wide, whose perihelion distance is about 20.9 million km (13.9 million mi). It is also associated with the yearly return of the Geminid stream of meteors.

Several Earth-approaching asteroids are relatively easy targets for space missions. In 1991, the National Aeronautics and Space Administration’s Galileo space probe, on its way to Jupiter, took the first close-up pictures of an asteroid. The images showed that the small, lopsided body, 951 Gaspra, is pockmarked with craters, and revealed evidence of a blanket of loose, fragmental material, or regolith, covering the asteroid’s surface. In a mission dedicated to asteroid study, the Near Earth Asteroid Rendezvous (NEAR) spacecraft launched by the US National Aeronautics and Space Administration (NASA) in February 1996 went into orbit around Eros in February 2000, the first spacecraft to orbit an asteroid, and is expected to provide a detailed chemical and geological survey of the object in a study lasting a year. Such studies should help to assess the nature of the threat from impact by a near-Earth body, as well as give information on the early chemical composition of the Solar System. Results from the mission already reveal a diverse mineral composition and a complex surface of craters, ridges, and grooves.

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