Topics covered on this page:

What is an asteroid?

What are the characteristics of asteroids?

Where are asteroids found?

Have there been space missions to asteroids?

What is an asteroid?

Eros is one of the largest near-Earth asteroids.

Eros is one of the largest near-Earth asteroids.

Asteroids are leftover debris from the Solar System’s formation, so they did not go into forming larger bodies like the Sun, planets or moons. They are important to study because their chemical and physical properties were not affected by the extensive heating processes that planets have experienced.  So, they provide a window into the original composition and physical properties of the dust and gas that made up the Solar System. They are also important because they pose a real threat to human life since they occasionally collide with the Earth.  Asteroids are the source of most meteorites.

What are the characteristics of asteroids?

Asteroids to scale

Here are some of the largest asteroids in the Solar System. Vesta, is also classified as a dwarf planet

Asteroids consist of mainly rock and metal.  Like Earth’s Moon, they do not have atmospheres. Most of them (in the billions) are very small, less than 1 kilometer.  There are about 1 million asteroids that are larger than 1 kilometer.  From the image above, you can see that asteroids tend to be angular, elongated and heavily cratered.  They also tend to have a “rubble pile” structure. This means that the materials that make up many asteroids are loosely held together, like — e.g. Itokawa.  Another major characteristic of asteroids is that their orbits have a greater incline and are more elliptical in shape compared to the orbits of planets.  Interestingly, if the mass of all the asteroids were combined, it would make up only a small fraction (around 4 %) of the mass of the Moon!! The image below shows the sizes of asteroids Mathilde, Lutetia, Vesta and Ceres compared to the Moon. Vesta and Ceres are also classified as dwarf planets, which are  planetary bodies that orbit the Sun and are large enough to have spherical or semi-spherical shapes, but that have not cleared their orbital region of other objects.  Pluto is also classified as a dwarf planet. Click here to watch a video about why Pluto is considered a dwarf planet and not a normal planet like the Earth. *Update (07/2015): The New Horizons NASA mission visited Pluto, providing stunning new beautiful images of this world! Click here for video showing more details.* 

Comparing the sizes of asteroids, dwarf planets and the Moon

Comparing the sizes of asteroids, dwarf planets and the Moon

 

Where are asteroids found?

Locations of the three main populations of asteroids in the Solar System.

Locations of the three main populations of asteroids in the Solar System.

Most asteroids live in the Asteroid Belt, which is located 2-3 AU (1 AU=approximately the distance between the Sun and the Earth) from the Sun between Mars and Jupiter (white dots in the image above).  Other large populations of asteroids exist outside of the Asteroid Belt, they include the Near-Earth asteroids, which are found within 1.3 AU and the Trojan asteroids, which share Jupiter’s orbit (green dots in image above). The Near-Earth asteroids can cross Earth’s orbit and occasionally collide with the Earth.  They pose serious threat to life on Earth.  A Near-Earth asteroid was likely responsible for the Chicxulub impact that caused dinosaurs to become extinct (see figures below).  On February 15th 2013, a piece of an asteroid hit an area near Chelyabinsk, Russia.  Watch the video of the fireball the meteor produced before the impact.

bigboom_tchicxulub

Have there been space missions to asteroids?

The Galileo spacecraft (launched in 1989) was the earliest space mission to study asteroids. It was operated by NASA (National Aeronautics and Space Administration). Galileo took images of asteroids Ida and Gaspra. This mission discovered that Ida actually has a small “moon” or satellite that orbits it. They named it Dactyl.

Ida and Dactyl

Ida and Dactyl

Gaspra

Gaspra

NEAR (Near Earth Asteroid Rendezvous) Shoemaker was the first NASA mission to land on an asteroid.  It launched in 1996. It imaged asteroids Mathilde (shown below) and Eros (first image on this page).

Mathilde

Mathilde

Hayabusa mission launched in 2003 was operated by JAXA (Japan Aerospace Exploration Agency). Hayabusa visited asteroid Itokawa. This was the first space mission to transport material from an asteroid to Earth (sample-return mission). They collected about 1500 particles from the surface of the asteroid and brought them to Earth for further analysis. The images of Itokawa give a good example of the “rubble pile” structure of many asteroids (see image below).  JAXA is currently organizing their next mission to an asteroid Hyabusa Mk2.

Itokawa

Itokawa

Dawn is currently an ongoing mission operated by NASA to the two largest asteroids in the Solar System, Vesta and Ceres.  This spacecraft was launched in 2007. It has already provided beautiful images of Vesta (2011). It is currently on its way to Ceres and should reach there  in 2015 (see images below). *Update (01/30/06): Dawn is now orbiting Ceres! Click here to see a fascinating video of its surface features.*

Dawn

Osiris-Rex is another sample-return mission to asteroid Bennu. It is operated by NASA. The anticipated launch date is in 2016.

bennu

Simulated image of asteroid Bennu

 

How do unmanned spacecraft successfully navigate around so many asteroids?

A common misconception is that asteroids are so close together that navigation is difficult and dangerous (watch this first video)!  But actually, asteroids are hundreds to thousands of kilometers apart. Here is another video, it shows the actual approach of the Dawn spacecraft to asteroid Vesta. There is no treacherous dodging of asteroids involved here!

Most meteorites come from asteroids.  They provide a relatively easy way to study asteroids without having to physically go to an asteroid.  Combining research on meteorites and research from space missions to asteroids will continue to provide the resources we need to uncover truths about the history of the Sun and the history of the disk of dust and gas that formed around the Sun, where the planets formed.

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