Asteroids are objects made up of mostly metals and rocks. Many of these were mistaken for small planets in the past, as some orbit the sun.

Asteroids Introduction

A majority of asteroids are rocky leftovers from the birth of our solar system 4.6 billion years ago. Airless, they exist as ancient space debris, orbiting between the main asteroid belt and Jupiter and Mars.

Basic Asteroid Statistics:

Asteroids range in size from the largest, Vesta, that is around 329 mi/530 km in diameter to those that are 33 ft/10 m or smaller across. If you added up all of the asteroids they would have a mass that was less than our moon.

The biggest percentage of asteroids exist between Jupiter and Mars in the Asteroid Belt. This is a band where hundreds of thousands of asteroids remain.

It’s thought that the powerful gravity from Jupiter helps to keep them in the asteroid belt and in doing so, protects our Earth from the onslaught of constant asteroid collisions.

Although asteroids are believed to be smaller objects, scientists believe that there are nearing 200 asteroids that are known to be larger than 100 km in diameter.

Some of the more recent theories of asteroid development include the Grand Tack and the Nice Model that suggest that our gas giant planets did a lot of moving around prior to settling to their current positions.

This movement might have been responsible for throwing asteroids throughout the growing Solar System, causing them to hit the various terrestrial planets.

It’s thought that this process not only emptied the asteroids but replenished the originating Asteroid Belt with new ones.

The current number of known asteroids is 795,722.

Asteroids are divided up into three broad composition classes: C-, S-, and M-types; and one V-type.

The C-type (chondrite) asteroids are most common and the most ancient objects in our solar system. C-type asteroids make up 75% of the known asteroids.

They are mostly made up of silicate rocks and clay, and have a dark appearance.

The S-types (“stony”) are made up of silicate materials and nickel-iron. They range in color from reddish to greenish and make up about 17% of the known asteroids.

The M-types are metallic (nickel-iron). These asteroids are made up of a lot of different materials, depending upon how far away from the sun that they were formed.

Some of these asteroids experienced high temperatures after they formed and partly melted, with iron sinking to the center and forcing basaltic (volcanic) lava to the surface.

There are other more rare asteroid types that are made up of different compositions. V-type asteroids such as Vesta, have a basaltic, volcanic crust.

Asteroid Classifications:

Scientists have been studying asteroids for many years and have theorized that they exist in three different areas of the solar system. Their origins, combined with their current location divides them into three classifications:

Main Asteroid Belt: A majority of the known asteroids can be fund orbiting between Jupiter and Mars in the asteroid belt.

The asteroid belt is estimated to have between 1.1 and 1.9 million asteroids in it that are bigger than .6 mi/1 km. in diameter. There are also millions of smaller ones.

Trojans: These are the asteroids that share an orbit with a larger planet but aren’t in danger of colliding because they are in only two special orbital locations called L4 and L5 Lagrangian points.

Their location puts them in a unique position of being balanced between the gravitational pull of the sun and the planet that they orbit around. This balance keeps them from flying out into orbit.

The Trojans around Jupiter have the largest population. Both Mars and Neptune have large numbers of trojan asteroids. In 2011, NASA announced the discovery of an Earth trojan.

Near-Earth Asteroids: These are the objects that have orbits that pass close to the Earth. When they cross the Earth’s orbital path they are called “Earth-crossers.” In 2013, 10,003 near-Earth asteroids were identified.

It’s thought that there were 861 that had a diameter of 1 km or more and 1,409 were classified as potentially being hazardous and could pose a threat to our planet.

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 45 million kilometers (28 million miles) and 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.

History of the Name:

Giuseppe Piazzi thought an asteroid was a planet in 1801. He even gave the object the name “Ceres” after the Roman harvest goddess. However, as Piazzi continued to watch, he found out that these were not planets but instead, some other form of celestial body.

Looking through their telescopes he noted that they shared a characteristic that was similar to viewing stars. It is this reason that the name asteroid was given, because it means “star-shaped or star-like.” Astronomer William Herschel came up with the name “asteroid” in 1802.

Important Information:

The asteroids that exist outside of the main asteroid belt are called “Trojans.” The most common orbit is around some of the larger planets due to the powerful gravitational pull.

The area of orbit is called “Lagrange points,” and Jupiter has the largest number of Trojan asteroids of all of the planets. Neptune, Mars, and even our Earth are other planets that can claim Trojan asteroids, although its believed that Jupiter has almost as many as exist within the asteroid belt.

We call asteroids that are closer to our Earth than the distance of the sun to Earth, NEAs (Near-Earth). The European Space Agency (ESA) has estimated that there are nearing 10,000 NEA asteroids. The NEAs are divided into groups including:

Amor asteroids whose orbits are really close to our Earth, but never cross Earth’s path.

Apollo asteroids that spend most of their time outside Earth’s path, but do cross our path.

Alten asteroids are those that are within Earth’s orbit and are near-Earth.

Our planet has an atmosphere, weather conditions, erosion, and tectonic plate movement that has hidden the many collisions that we have had with asteroids.

Scientists have been scouring the planet, using a variety of sophisticated technologies and have uncovered many impact craters that would normally not be visible.

Euler diagram

They estimate that an asteroid the size of a car enters the atmosphere of Earth once each year. A majority of it burns in the atmosphere, creating a fireball effect in the sky.

One of the asteroid craters that scientists have been studying for many years is Meteor Crater, just outside Flagstaff, Arizona.

The many asteroids that orbit around Jupiter are occasionally pulled away when Jupiter gets close to Mars. The variations in gravity can cause the asteroids to get knocked out of the main asteroid belt and spin into space.

Scientists believe that this is the cause of stray asteroids and pieces of asteroids slamming in the planets, including our Earth.

Space Visits:

  • Galileo was the first mission from NASA involving a spacecraft that would fly by an asteroid. In 1991 it flew by asteroid Gaspara and in 1993 it flew by asteroid Ida.
  • The Near-Earth Asteroid Rendezvous (NEAR-Shoemaker) mission by NASA studied the Mathilde and Eros asteroids. The Rosetta mission studied asteroids Steins in 2008 and Lutetia in 2010. Deep Space 1 and Stardust had close encounters with asteroids.
  • The Hayabusa was a Japanese spacecraft that landed on the near-Earth asteroid Itokawa in 2005 and attempted to collect samples. In 2010, Hayabusa was a success at getting back to earth with a small sample of asteroid dust that is being studied by scientists.
  • The 2007 launch of NASA’s Dawn spacecraft enabled it to orbit and explore the Vesta asteroid for over a year. Dawn left in 2012 to then head to the dwarf planet Cere. Both Vesta and Ceres are two of the largest remaining protoplanet bodies that almost became planets. Scientists feel that studying them with the same instruments and technology as those used in asteroid study will allow them to compare the differences and contrasts to help to explain each unique evolutionary path. By studying both astroids and protoplanets, scientists think that it will be a window in helping us better understand our solar system.

Important Events:

There are two robotic spacecraft that are exploring asteroids up close. NASA’s OSIRIS-Rex is orbiting the Bennu asteroid and the Japanese spacecraft Hayabusa 2 will be collecting samples from the Ryugu asteroid.

NASA’s NEOWISE spacecraft is orbiting Earth to monitor and study the most accurate observations of near-Earth objects of all kinds.

Additional missions that are scheduled will include NASA’s Psyche and Lucy missions to explore asteroids.

The Hubble Space Telescope used ground-based radar for observations of asteroids.

Facts about Asteroids for Kids:

  • Ceres was the first asteroid discovered and is classified as a protoplanet. It’s the largest known asteroid at 580 mi/933 km across.
  • The smallest known asteroid is 1991 BA and is on 20 ft/6 m across.
  • Scientists believe that asteroids were the building blocks of our planets that were never used.
  • Asteroids crossing the orbit of the Earth are called “Apollo objects.”
  • The asteroid that hit the earth 65 million years ago caused devastation so great that it led to the dinosaur extinction and affected all life on Earth.
  • An asteroid of around .1 mi/.15 km in width is believed to have exploded over Tunguska, Siberia. No crater was ever found, so scientists believed it exploded overhead, causing damage that expanded out hundreds of miles/kilometers.
  • Most asteroids are too small to have enough gravitational pull to make them spherical so they are irregular in shape.
  • Some asteroids are comets that have blown out after all of the ice is gone.
  • A meteoroid is a piece of an asteroid and one meteoroid the size of a car falls into the atmosphere of the Earth once each year. When this happens a fireball effect occurs that can be seen. The meteoroid usually burns up well before it reaches the surface of our planet.

Pop Culture:

Asteroids have been popular objects in science fiction for many years. From books to movies to television, the list of asteroid-related stories is far too long to detail.

People are often familiar with the tale of asteroids crashing into Earth as they were covered in a few movies. Deep Impact was a film based on Arthur C. Clark’s novel, The Hammer of God, in which an asteroid becomes a comet and heads towards Earth.

The astronauts piloted their ship into the path to keep the impact from happening. Armageddon was another movie about an asteroid impacting Earth as the heroes drilled into the core of the asteroid and detonated nuclear bombs.

It should be noted that scientists do NOT recommend bombs as an answer, as it could create thousands of smaller pieces that would impact Earth.

Some of the asteroids that are included in sci-fi are fictional, however, there are some real asteroids that have been used as a base for storylines and even as backdrops.

Here is just a partial listing:


Television series, Space Angel had an episode where they were required to destroy Eros.

The miniseries Asteroid featured Eros as one of the asteroids heading to Earth.

The television series The Expanse was an adaptation of Leviathan Wakes featuring Eros.


Arthur C. Clarke wrote Summertime on Icarus (aka Icarus Ascending) about an astronaut stranded on Icarus.

Larry Niven and Jerry Pournelle wrote the novel Lucifer’s Hammer and included the passing of the asteroid Icarus.


Mobile Suit Gundam, a Real Robot anime had Juno renamed to Luna 2 as it was placed in orbit around the Earth opposite our moon.


Isaac Asimov wrote Marooned Off Vesta as a short story about survivors of a wrecked spaceship stranded in the asteroid Vesta’s orbit. This was Asimov’s first published work.

Lucky Starr and the Rings of Saturn a novel by Isaac Asimov. Vesta is the site of an interstellar peace conference.

Larry Niven’s Known Space series has Vesta aa the site of one of the larger bases in the belt. It is a media center for the belt, and home of the Vesta Beam.