Mars - 6

There are millions of asteroids, ranging in diameter from dust particles to 950km. Like most other small solar system bodies, asteroids are thought to be the shattered remnants of planetesimals, bodies within the young Sun’s solar nebula that never grew large enough to become planets. Most known asteroids orbit in the main asteroid belt between the orbits of Mars and Jupiter or co-orbital with Jupiter. However, other orbital families exist with significant populations, including the near-Earth asteroids. In the image above right, Ida and Gaspra (superimposed with each other) are two such rocky, metallic objects. Shown here to the same scale, Ida is about 30km long, and Gaspra about 17km.

Asteroids have played a role in the evolution of life on Earth. Sometimes the orbits of asteroids bring them close to Earth and collisions ensue. There is strong evidence that the mass extinction that killed the dinosaurs 66 million years ago was caused by an asteroid at least 12km wide. If that had not happened, would humans be walking the Earth today?