Asteroids are metallic, rocky bodies without atmospheres that orbit the Sun but are too small to be classified as planets. The term "asteroid” has historically been applied primarily to minor planets of the inner Solar System.

Traditionally, small bodies orbiting the Sun were classified as asteroids, comets or meteoroids, with anything smaller than ten metres across being called a meteoroid.

The term "asteroid” is ill-defined. It never had a formal definition, with the broader term minor planet being preferred by the International Astronomical Union until 2006, when the term "small Solar System body” (SSSB) was introduced to cover both minor planets and comets.

Sometimes known as "minor planets,” tens of thousands of asteroids are found in an area so-called main asteroid belt: a vast, doughnut-shaped ring located between the orbits of Mars and Jupiter from approximately (186 million to 370 million miles/300 million to 600 million kilometers) from the Earth. Gaspra and Ida are main belt asteroids.

The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. The asteroid belt region is also termed as the main asteroid belt or main belt because there are other asteroids in the solar system such as near earth asteroids and Trojan asteroids.

More than half the mass of the main belt is contained in the four largest objects/asteroids: Ceres, 4 Vesta, 2 Pallas, and 10 Hygiea. These have mean diameters of more than 400 km, while Ceres, the main belt’s only dwarf planet, is about 950 km in diameter.

The remaining bodies range down to the size of a dust particles. The asteroid material is so thinly distributed that many unmanned spacecrafts have traversed this region without any incident.

Known asteroids range in size from the largest Ceres, the first discovered asteroid in 1801 at about 1,000 kilometers) in diameter down to the size of pebbles. Sixteen asteroids have diameters of 240 kilometers or slightly greater.

Only one asteroid, 4 Vesta, which has a reflective surface, is normally visible to the naked eye and this is only in very dark skies when it is favorably positioned. Rarely, small asteroids passing close to Earth may be naked-eye visible for a short time. The orbits of asteroids are often influenced by the gravity of other bodies in the solar system.

The majority of known asteroids orbit within the main asteroid belt between the orbits of Mars and Jupiter. This belt is now estimated to contain between 1.1 and 1.9 million asteroids larger than 1 km in diameter and millions of smaller ones.

The mass of all the objects of the Main asteroid belt, lying between the orbits of Mars and Jupiter, is estimated to be about about 4 percent of the mass of the Earth’s Moon.
Various classes of asteroid have been discovered outside the main asteroid belt. Near-Earth asteroids have orbits near that of Earth.

Some scientists have so far started visiting some of these asteroids e.g. a space capsule nicknamed Hayabusa was launched in May 2003 by Japanese to an asteroid nicknamed Itokawa and reached asteroid in September 2005 for three months of research and daring close approaches to the potato-shaped asteroid.

The visit ended with a series of attempts to collect samples from the asteroid, but the spacecraft inadvertently landed on the surface for 30 minutes.

On Sunday 13th June 2010, the small Japanese capsule returned to Earth from the surface of the asteroid travelling at 27,000 mph, and landed in the remote Australian outback in a military installation where more than 50 representatives of the Japan Aerospace Exploration Agency joined several dozen Australian workers to recover the Hayabusa capsule.

The capsule was supposed to jettison a portion of its heat shield and backshell once it reached a point 6 miles above Earth, permitting its parachute to unfurl and slow the craft to a more gentle velocity and very slow touch down speed.

The photo below taken as i twas approaching the earth.
The details of the information gathered by the same craft may take some months for the scientists to publish the findings.

Sunday’s  return of the Japan’s craft  to Earth wrapped up a mission lasting seven years and traversing more than a billion miles across the solar system.

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