Learn – What is an Asteroid?

Learn – What is an Asteroid?

Learn – What is an Asteroid? 460 405 Asteroid Day Asteroid Day

Learning Objective: 

To understand what an asteroid is, and how they fit into the rest of the Solar System. 


What is an asteroid? Asteroids are rocky and metallic celestial objects that orbit the Sun, and are smaller than planets. They exist in large numbers and in a variety of orbital families. Asteroids are also known as minor planets and sometimes, planetoids.


Asteroids are remnants left over from the formation of the Solar System. They are made mostly of rock and metal, and many of them are truly ancient, having survived relatively untouched since the birth of the planets 4.6 billion years ago. 

The largest asteroid is Ceres. It measures 946 kilometres in diameter and was discovered by Giuseppe Piazzi on 1 January 1801.  There are now more than 800,000 asteroids known, all of them are smaller than the planets. For example, Mercury, the smallest planet, is still more than five times larger in diameter than Ceres.

Ceres is the largest asteroid. This image of it was taken by NASA’s Dawn mission in 2015. Credit: NASA / JPL-Caltech / UCLA / MPS / DLR / IDA / Justin Cowart

The vast majority of asteroids are irregular in shape because they are not large enough to generate enough gravity to pull themselves into spherical shapes. Ceres is an exception to this. 

Most of them orbit the Sun in a ‘belt’ of orbits that stretch between Mars’s orbit and that of Jupiter. Other asteroids can be found in more eccentric orbits that bring them close to Earth, or even cross our planet’s orbit. 

Asteroids are also known as minor planets. This is because the term asteroid, which means star-like, is thought to be misleading by some astronomers. Piazzi himself preferred the term planetoid, which is still used today but usually only for the larger asteroids.

A modern sub-grouping known as dwarf planets includes one asteroid.

A dwarf planet is any small body that is large enough to have pulled itself into a nearly round shape. This happens when an object has a diameter of roughly 1000 km, and so Ceres just falls into this category. It is the only asteroid to also be a dwarf planet. The other known dwarf planets are Pluto, and the distant Solar System mini-worlds Eris, Makemake, and Haumea.

Gaspra was the first asteroid to be seen close up. This image was taken by NASA’s Galileo spacecraft in 1991. Credit: USGS/NASA/JPL.

The composition of asteroids is extremely varied but falls into three main types:

  • Chondritic (C-type) asteroids are the most common. These consist of clays and rocky material, and are thought to be largely unchanged since the origin of the solar system. Read more here.
  • Stony (S-type) asteroids are made up of silicate materials and nickel-iron. They are probably fragments of once larger asteroids. Read more here.
  • The metallic (M-type) asteroids are made of nickel-iron. They could be from the core of a large planetesimal that was shattered in a collision. Read more here.

Learn more about this subject by visiting these websites: 
Space.com: Asteroids: Fun Facts and Information About Asteroids
NASA Space Place: What is an asteroid?
Sky & Telescope: Why do we call them asteroids?