Asteroids, sometimes called minor planets, are rocky remnants left over from the early formation of our solar system about 4.6 billion years ago. The current known asteroid count is: 1,100,048. Most of this ancient space rubble can be found orbiting our Sun between Mars and Jupiter within the main asteroid belt. Asteroids range in size from Vesta – the largest at about 329 miles (530 kilometers) in diameter – to bodies that are less than 33 feet (10 meters) across. The total mass of all the asteroids combined is less than that of Earth’s Moon.
The three broad composition classes of asteroids are C-, S-, and M-types.
- The C-type (chondrite) asteroids are most common. They probably consist of clay and silicate rocks, and are dark in appearance. They are among the most ancient objects in the solar system.
- The S-types (“stony”) are made up of silicate materials and nickel-iron.
- The M-types are metallic (nickel-iron). The asteroids’ compositional differences are related to how far from the Sun they formed. Some experienced high temperatures after they formed and partly melted, with iron sinking to the center and forcing basaltic (volcanic) lava to the surface.
Main Asteroid Belt: The majority of known asteroids orbit within the asteroid belt between Mars and Jupiter, generally with not very elongated orbits. The belt is estimated to contain between 1.1 and 1.9 million asteroids larger than 1 kilometer (0.6 miles) in diameter, and millions of smaller ones. Early in the history of the solar system, the gravity of newly formed Jupiter brought an end to the formation of planetary bodies in this region and caused the small bodies to collide with one another, fragmenting them into the asteroids we observe today.
Trojans: These asteroids share an orbit with a larger planet, but do not collide with it because they gather around two special places in the orbit (called the L4 and L5 Lagrangian points). There, the gravitational pull from the Sun and the planet are balanced by a trojan’s tendency to otherwise fly out of orbit. The Jupiter trojans form the most significant population of trojan asteroids. It is thought that they are as numerous as the asteroids in the asteroid belt. There are Mars and Neptune trojans, and NASA announced the discovery of an Earth trojan in 2011.
Near-Earth Asteroids: These objects have orbits that pass close by that of Earth. Asteroids that actually cross Earth’s orbital path are known as Earth-crossers.
Potentially hazardous asteroids- NEAs that are of greatest threat to Earth, which have chances of colliding with Earth are listed as potentially hazardous asteroids or PHAs.
Missions to asteroids
- OSIRIS-REx – Sample Return Mission to Asteroid Bennu (2016)
- Hayabusa2 – JAXA Sample Return Mission to Asteroid Ryugu (2014)
- PROCYON – JAXA Small Satellite Asteroid Flyby Mission (2014)
- Dawn – NASA Orbiter of Asteroids Ceres and Vesta (2007)
- Rosetta – ESA Comet Mission, flew by asteroids Steins and Lutetia (2004)
- Hayabusa (Muses-C) – ISAS (Japan) Sample Return Mission to Asteroid 25143 Itokawa (2003)
- Genesis – NASA Discovery Solar Wind Sample Return Mission (2001)
- Stardust – NASA Comet Coma Sample Return Mission, flew by asteroid AnneFrank (1999)
- Deep Space 1 – NASA Flyby Mission to asteroid Braille (1998)
- Cassini – NASA/ESA Mission to Saturn through the Asteroid Belt (1997)
- NEAR – NASA Near-Earth Asteroid Rendezvous with 433 Eros
- Galileo – NASA Mission to Jupiter via asteroids Gaspra and Ida