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: 1,113,527.

Most of this ancient space rubble can be found orbiting our Sun between Mars and Jupiter within main asteroid belt. Asteroids range in size from Vesta. The largest at about 329 miles in diameter – to bodies that are less than 33 feet across. The total mass of all the asteroids combined is less than that of Earth’s Moon.

Most asteroids are irregularly shaped, though a few are nearly spherical, and they are often pitted or cratered. As they revolve around the Sun in elliptical orbits, asteroids also rotate, sometimes quite erratically, tumbling as they go. More than 150 asteroids are known to have a small companion moon . There are also binary asteroids, in which two rocky bodies of roughly equal size orbit each other. As well as triple asteroid systems.

Get to know more about Asteroids, its composition, classification and many more facts by Pritish Halder.

Asteroid – image


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.

The orbits of asteroids can be changed by Jupiter’s massive gravity. And by occasional close encounters with Mars or other objects. These encounters can knock asteroids out of the main belt. And hurl them into space in all directions across the orbits of the other planets. Stray asteroids and asteroid fragments have slammed into Earth and the other planets in the past. By playing a major role in altering the geological history of planets and in the evolution of life on Earth.

Scientists continuously monitor Earth-crossing asteroids, whose paths intersect Earth’s orbit. And near-Earth asteroids that approach Earth’s orbital distance to within about 28 million miles. These may pose an impact danger. Radar is a valuable tool in detecting and monitoring potential impact hazards. By reflecting transmitted signals off objects, images and other information can be derived from the echoes. Scientists can learn a great deal about an asteroid’s orbit, rotation, size, shape, and metal concentration.

Asteroid Classifications

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 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. These 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.

How Asteroids Get Their Names

The International Astronomical Union’s Committee on Small Body Nomenclature is not very strict when it comes to naming asteroids. As a result, out there orbiting the Sun we have a giant space rock named for Mr. Spock . There’s also a space rock named for the late rock musician Frank Zappa. There are more somber tributes, too. Such as the seven asteroids named for the crew of the Space Shuttle Columbia killed in 2003.

Asteroids are also named for places and a variety of other things.

Asteroids are also given a number, for example (99942) Apophis. The Harvard Smithsonian Center for Astrophysics keeps a fairly current list of asteroid names.


Only a few robotic spacecraft have encountered asteroids up close.

NASA’s Dawn spacecraft was launched in 2007 to explore asteroid Vesta. The second most massive body in the main asteroid belt. Dawn arrived at Vesta in 2011. Then orbited and explored Vesta for over a year before leaving in September 2012 to explore dwarf planet Ceres.

Japan’s Hayabusa2 was launched in December 2014 on a six-year voyage to study asteroid Ryugu, and to collect samples to bring back to Earth for analysis. Hayabusa2 arrived at the asteroid in June 2018. The spacecraft deployed rovers and landers onto Ryugu’s surface, and collected a sample. Hayabusa2 delivered the asteroid sample to Earth on Dec. 6, 2020.

Launched on Sept. 8, 2016, NASA’s OSIRIS-REx arrived at asteroid Bennu in 2018, and collected a sample of dust and rocks. On April 9, 2021, the spacecraft took one last look at Bennu before beginning its journey back to Earth. It’s on track to deliver the asteroid sample to Earth on Sept. 24, 2023.

NASA’s NEOWISE spacecraft is orbiting Earth to improve on the most accurate survey of near-Earth objects ever undertaken.

The Hubble Space Telescope and ground-based radar observatories also contribute regularly to our understanding of asteroids. Several more missions, including NASA’s Psyche and Lucy, missions are in development to keep exploring these small worlds. Scientists also use ground-based radar to explore nearby asteroids whenever possible.