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Asteroids Worksheet

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Key Facts & Information

  • Asteroids are small, rocky and airless objects that orbit the Sun. Although asteroids orbit the Sun like planets, they are much smaller in diameter than planets.
  • The current known total asteroid count is: 958,915.


  • Asteroids are considered remnants left over from the early formation of our Solar System about 4.6 billion years ago when a big cloud of gas and dust collapsed.
  • When this happened, most of the material fell to the centre of the cloud and formed the Sun.
  • The origin of asteroids started in 1801, when an Italian Catholic priest and astronomer, Giuseppe Piazzi, accidentally discovered the first asteroid. Through his telescope, he saw a small object, approximately 1000 kilometres in diameter, lying  between the orbits of Mars and Jupiter. He named it after the Roman goddess of agriculture, Ceres. Currently, Ceres is designated a dwarf planet.


  • Most asteroids are irregular in shape, but some larger ones are spherical. This is attributed to their difference in size and gravitational force. The smaller mass of asteroids makes them vulnerable to be pulled by the gravitational fields of objects with larger mass. The outer appearance of most of them is usually pitted or with craters.
  • Their size varies from a diameter of 10 metres to as big as 1000 kilometres.
  • The mass of all asteroids combined is less than that of our Moon.
  • The second-largest asteroid seen from Earth is known as Vesta, about 525 kilometres in diameter. Vesta is the brightest asteroid seen from Earth. The third-largest asteroid is Pallas. It was the second asteroid discovered after Ceres.


  • The International Astronomical Union’s Committee on Small Body Nomenclature is a little less strict when it comes to naming asteroids. We have giant space rocks named after Mr. Spock (a cat character from Star Trek), rock musician Frank Zappa, and beloved teachers such as Florida’s Cynthia L. Reyes. Asteroids are also named after places and a variety of other things. They are numbered in order of their discovery.

Types Of Asteroids

  • Asteroids are differentiated from each other on the basis of their composition. They can be classified into C-type, S-type and M-type.
  • C-type asteroids or carbonaceous chondrites, include more than 75% of known asteroids.
  • They are extremely dark in appearance and are made of clay and silicate rocks. They are among the most ancient objects found in the Solar System.
  • The M-type or metallic asteroids belong to a minor fraction of asteroids. They consist purely of nickel-iron and are usually bright relating to how far from the Sun they are formed.
  • S-type asteroids, also known as siliceous or stony asteroids, make up 17% of all asteroids. They are relatively bright and made up of metallic nickel-iron mixed with iron- and magnesium-silicates.
  • Pallasites are stony-iron meteorites believed to form between the silicate mantle, or outer shell, and molten metal core of a differentiated giant S-type asteroid.

Distribution Within The Solar System

  • Various groups of asteroids have been discovered orbiting in the Solar System. Their orbits are perturbed by the gravity of other bodies in the Solar System and other factors. Significant populations include the following:
  • Majority of known asteroids orbit within the Main Asteroid Belt that lies between  the orbits of Mars and Jupiter. The belt is estimated to contain millions of big and small asteroids.
  • Next are Trojan asteroids which share an orbit with larger planets but do not collide with them. The Jupiter trojans form mostly from the population of the trojan asteroids.
  • There are also Near-Earth Asteroids. These objects have orbits that pass close by the orbits of the Earth. If they actually cross the Earth’s orbital path they are called Earth-crossers.

Asteroids, Comets And Meteoroids

  • Asteroids, comets and meteoroids are terms commonly used interchangeably. However, these three words pertain to different objects.
  • The difference between asteroids and meteoroids is mainly the size. Sometimes one asteroid can smash into another, creating meteoroids. These meteoroids are smaller,  having a diameter of one metre or less, whereas asteroids have a diameter of greater than one metre.
  • In the case of comets, one of the differences lies in the composition. Asteroids are mainly composed of rocks and minerals, while comets are primarily composed of dust and ice. This is due to the fact that asteroids orbit the Sun, thus, they are formed closer to it, preventing ice from developing. Also, comets usually have tails made from ice and debris that disappears as they go near the Sun. Asteroids typically do not have this.

Some Asteroids Discovered

  • 101955 Bennu. A small near-Earth asteroid which has a close approach to Earth every six years. Bennu is only slightly wider than New York City’s Empire State Building is tall.
  • 10199 Chariklo. Chariklo was the first asteroid found to have a ring system. It was the fifth ring system found in our Solar System – after Saturn, Jupiter, Uranus and Neptune.
  • 16 Psyche. This asteroid was named after the nymph Psyche. Unlike most asteroids that are rocky or icy, Psyche is an M-type asteroid comprised mostly of metallic iron and nickel, similar to Earth’s core.
  • 4 Vesta. Vesta, the second-largest asteroid in the main belt.
  • 25143 Itokawa – In 2005, 25143 Itokawa became the first asteroid from which samples were captured and brought to Earth for analysis.
  • 253 Mathilde – A C-type asteroid where probably very little changed in the last 4.5 billion years, it is a time capsule from the period when the planets first formed.
  • 433 Eros – Eros is the first-known Near-Earth Asteroids and the first asteroid to be orbited by a spacecraft.
  • 243 Ida – Ida is the second asteroid to be visited by a spacecraft and the first found to have its own moon.
  • 30 June is considered International Asteroid Day. It commemorates the anniversary of the Tunguska asteroid when it landed over Siberia, Russian Federation in 1908.

Explorations By Nasa

  • Gaspara Ida – NASA’s Galileo mission was the first spacecraft to fly past an asteroid. It flew past asteroid Gaspara in 1991 and Ida in 1993.
  • Mathilde Eros – NASA’s Near-Earth Asteroid Rendezvous (NEAR-Shoemaker) mission studied asteroids Mathilde and Eros.
  • Itokawa – In 2005, the Japanese spacecraft Hayabusa landed on the near-Earth asteroid Itokawa and attempted to collect samples. On 3 June, 2010, Hayabusa successfully returned to Earth a small amount of asteroid dust now being studied by scientists.
  • Vesta Ceres – NASA’s Dawn spacecraft, launched in 2007, orbited and explored asteroid Vesta for over a year. Once it left in September 2012, it headed towards dwarf planet Ceres, and returned in 2015.