Asteroids and Comets
The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets. The asteroid belt region is also termed the main belt to distinguish it from other concentrations of minor planets within the Solar System, such as the Kuiper belt and scattered disc.
More than half the mass of the main belt is contained in the four largest objects: Ceres, 4 Vesta, 2 Pallas, and 10 Hygiea. All of these have mean diameters of more than 400Â km, while Ceres, the main belt's only dwarf planet, is about 950Â km in diameter. The remaining bodies range down to the size of a dust particle. The asteroid material is so thinly distributed that multiple unmanned spacecraft have traversed it without incident. Nonetheless, collisions between large asteroids do occur, and these can form an asteroid family whose members have similar orbital characteristics and compositions. Collisions also produce a fine dust that forms a major component of the zodiacal light. Individual asteroids within the main belt are categorized by their spectra, with most falling into three basic groups: carbonaceous (C-type), silicate (S-type), and metal-rich (M-type).
The asteroid belt formed from the primordial solar nebula as a group of planetesimals, the smaller precursors of the planets. Between Mars and Jupiter, however, gravitational perturbations from the giant planet imbued the planetesimals with too much orbital energy for them to accrete into a planet. Collisions became too violent, and instead of sticking together, the planetesimals shattered. As a result, most of the main belt's mass has been lost since the formation of the Solar System. Some fragments can eventually find their way into the inner Solar System, leading to meteorite impacts with the inner planets. Asteroid orbits continue to be appreciably perturbed whenever their period of revolution about the Sun forms an orbital resonance with Jupiter. At these orbital distances, a Kirkwood gap occurs as they are swept into other orbits. Asteroids are material left over from the formation of the solar system. One theory suggests that they are the remains of a planet that was destroyed in a massive collision long ago. More likely, asteroids are material that never coalesced into a planet. In fact, if the estimated total mass of all asteroids was gathered into a single object, the object would be less than 1,500 kilometers (932 miles) across, less than half the diameter of our Moon.
The asteroid belt lies in the region between Mars and Jupiter. The Trojan asteroids lie in Jupiter's orbit, in two distinct regions in front of and behind the planet.
What are Comets
A comet is an icy small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena are due to the effects of solar radiation and the solar wind acting upon the nucleus of the comet. Comet nuclei range from a few hundred metres to tens of kilometres across and are composed of loose collections of ice, dust, and small rocky particles. The coma may be up to 15 times the Earth's diameter, while the tail may stretch one astronomical unit. If sufficiently bright, a comet may be seen from the Earth without the aid of a telescope and may subtend an arc of 30° (60 Moons) across the sky. Comets have been observed and recorded since ancient times by many cultures.
What are Near Earth Objects (NEO's)
Near-Earth Objects (NEOs) are comets and asteroids that have been nudged by the gravitational attraction of nearby planets into orbits that allow them to enter the Earth’s neighborhood. Composed mostly of water ice with embedded dust particles, comets originally formed in the cold outer planetary system while most of the rocky asteroids formed in the warmer inner solar system between the orbits of Mars and Jupiter. The scientific interest in comets and asteroids is due largely to their status as the relatively unchanged remnant debris from the solar system formation process some 4.6 billion years ago. The giant outer planets (Jupiter, Saturn, Uranus, and Neptune) formed from an agglomeration of billions of comets and the left over bits and pieces from this formation process are the comets we see today. Likewise, today’s asteroids are the bits and pieces left over from the initial agglomeration of the inner planets that include Mercury, Venus, Earth, and Mars.
The Kuiper Belt And The Oort Cloud
The Kuiper Belt is a disk-shaped region past the orbit of Neptune extending roughly from 30 to 50 AU from the Sun containing many small icy bodies. It is now considered to be the source of the short-period comets. It is often referred to as our Solar Systems 'Final Frontier'. This disk-shaped region of icy debris is about 4.5 to 7.5 billion km (2.8 billion to 4.6 billion miles) from our sun.
No spacecraft has ever traveled to the Kuiper Belt, but NASA's New Horizons mission, planned to arrive at Pluto in 2015, might be able to penetrate farther into the Kuiper Belt to study one of these mysterious objects.
In 1950, Dutch astronomer Jan Oort proposed that certain comets came from a vast spherical shell of icy bodies near the edge of the Solar System. This giant swarm of objects is now named the Oort Cloud, occupying space at a distance between 5,000 and 100,000 astronomical units. (One astronomical unit, or AU, is the mean distance of Earth from the Sun: about 150 million kilometers or 93 million miles.) The Oort Cloud contains billions of icy bodies in solar orbit. Occasionally, passing stars disturb the orbit of one of these bodies, causing it to come streaking into the inner solar system as a long-period comet. These comets have very large orbits and are observed in the inner solar system only once. In contrast, short-period comets take less than 200 years to orbit the Sun and they travel along the plane in which most of the planets orbit. They come from a region beyond Neptune called the Kuiper Belt, named for astronomer Gerard Kuiper, who proposed its existence in 1951.