The Solar System:


The main asteroid belt

The term asteroid is used to describe any small, typically rocky, object orbiting the Sun that is not a planet, dwarf planet, or comet. The term is primarily used to describe such objects found within the orbit of Jupiter, to distinguish them from the icy Kuiper Belt objects primarily found outside the orbit of Neptune. Many asteroids are thought to be the shattered remnants of planetesimals, objects in the solar nebula that never grew large enough to become planets. It is likely that the gravitational influence of massive Jupiter prevented planetesimals in this region from forming a single object during the early history of Solar System.

Most asteroids in the Solar System occupy a region between Mars and Jupiter known as the main asteroid belt. Hundreds of thousands of asteroids are currently known., but it is estimated that there are millions to billions in the belt. However, total mass of the asteroid belt is only 4% that of the Moon.

The inner Solar System, from the Sun to Jupiter, including the true relative positions of objects (as seen on 14 Aug 2006) in the main asteroid belt (white), the roughly triangular distribution of Hilda asteroids, which are caught in an orbital resonance with Jupiter (orange ), the Jupiter trojan asteroids that occupy the regions of stable gravitational equilibrium 60 degrees ahead (the Greeks) and behind (the Trojans) in the orbit of Jupiter (green), and the near-Earth asteroids (inside the orbit of Mars).

Credit (image and some text): Mdf (Wikimedia Commons) [link]

The two largest asteroids 4 Vesta (left) and 1 Ceres (middle) shown alongside Earth's Moon. Ceres is also classified as a dwarf planet. The diameter of each object is Vesta = 525 km, Ceres = 945 km, and the Moon = 3474 km.

Credit: Moon image by Gregory H. Revera; Ceres image by Justin Cowart; Vesta image by NASA/JPL-Caltech/UCAL/MPS/DLR/IDA [link]

Composite image, to scale, of asteroids that have been imaged at high resolution (except Ceres). The view of Vesta shown here is from a different orientation (rotated by approximately 90 degrees) compared to the previous image. Vesta is not quite massive enough for gravity to squeeze it into a symmetric spherical shape; its profile is oblong when viewed from one orientation, and more circular when viewed from another.

Credit: NASA/JPL-Caltech/JAXA/ESA [link]

Asteroid 243 Ida and its moon Dactyl. Dactyl was the first satellite of an asteroid to be discovered. This false-color image is made from visible and infrared light images obtained by the Galileo spacecraft about 14 minutes before its closest approach to Ida on 28 August 1993, at a distance of about 10,500 kilometers (6,500 miles).

Credit (image and some text): NASA/JPL [link]

Masses of the asteroids

Comparison of the masses of the twelve most massive asteroids known, as fractions of the total mass of the main asteroid belt. Ceres alone constitutes about 30% of the total mass in the asteroid belt, and the four most massive asteroids amount to almost 50% of the total mass.

Credit: Kwamikagami (Wikimedia Commons), CC BY SA 3.0 [link]

Background information: interior structure of planets

Differentiation = The separation of materials inside a large planetary object according to density. Differentiation is caused by gravity: denser (“heavier”) elements sink to the center. while less dense elements “float” in higher layers.

Earth's interior layers, showing differentiation. The core of the Earth is composed of nickel-iron, while the mantle layers are less dense metals and rocks, and the crust is even lower density rocks.

Credit: United States Geological Survey [link]

Structure and composition of asteroids

A round(ish) asteroid is massive enough for gravity to have squeezed it into a spherical shape (like Earth and the other rocky planets). This means that it probably has a differentiated interior:

    • It became hot enough during formation for its interior to melt.

    • This mixes everything together.

    • Then the heavy stuff sinks to the center, and the lighter stuff floats to the surface.

    • Roundish asteroids have an interior structure like Earth and the other rocky planets.

Vesta formed inside the frost line of the solar nebula (where ices and other volatiles were scarce), so it has a metal core with rocky outer layers.

Ceres formed outside the frost line of the solar nebula (where dense metals were scarce, but water and other volatiles were more abundant), so it has a rocky core with ice outer layers.

Cut and polished piece of the Cumberland Falls stony achondrite meteorite that fell in Whitley County, Kentucky, in 1919. Most stony achondrite meteorites that fall on Earth are believed to come from the crust of the asteroid Vesta.

Credit: Claire H. (Wikimedia Commons), CC BY SA 2.0 [link]

The main asteroid belt is (relatively) crowded.

There are more than 200 known asteroids tht are > 100 km across, and an estimated 700,000 – 1.7 million asteroids that are > 1 km across.

But it's not this crowded!

Credit: Wookieepedia [link]

It's not even this crowded!

Credit: Atari/ [link]

Remember: space is big!

Even assuming there are 1.7 million asteroids that are > 1 km across, the average distance between them is > 3 million km - this is more than 8x the distance between Earth and the Moon.

(Assume asteroids are evenly spaced inside an assumed volume for the main asteroid belt that extends over 2.2 - 3.2 AU from the Sun, and 0.5 AU above and below the ecliptic; i.e., the plane of Earth's orbit around the Sun.)

OK, so the main asteroid belt is basically almost empty.

But there are many asteroids with criss-crossing orbits, so collisions do happen…

An asteroid that is >1 km across is the victim of a collision every few billion years (i.e., 1or 2 collisions over the lifetime of the Solar System so far).

This means that there have been millions of big collisions in the history of the Solar System. And we can infer that there have been many more collisions of the many more smaller asteroids.

Artist impression of a collision between two asteroids.

Credit: Don Davis, Southwest Research Institute [link]

When asteroid collisions happen, some fragments are ejected from the asteroid belt and some of these later collide with Earth. We see them as meteors streaking though the sky (being ablated by their passage through the atmosphere).

ablation = stripping of molten material from the surface

If the meteor is big enough, then piece(s) of it can survive to crash into Earth’s surface – it is now a meteorite.

By comparing the chemical composition of meteorites with data gathered for other solar system objects, we can sometimes identify the origin of the meteor. In this way, we have found meteorites on Earth that came from the Moon, Mars, and even Vesta and other asteroids.

Note: In the video, Phil says asteroid 2010 TK7 is 800 million km from Earth, but it is really only about 150 million km away. Don’t worry – Phil knows he goofed.


Main belt total mass

  • Pitjeva, E. V. 2005, Solar System Research, 39, 176 - "High-Precision Ephemerides of Planets - EPM and Determination of Some Astronomical Constants" (2005SoSyR..39..176P)

Numbers of asteroids

Asteroid collision rate

  • Morrison, D., 1997 (August 5), Scientific American - "In science fiction movies, the "asteroid belt" is always pictured as a very crowded place. How dense is it really: impossible to navigate, risky or just interesting?" [link]