The Asteroid Belt

We are leaving Mars and what is known as the inner Solar System. Looking out into the darkness, you can spot a bright star some distance away, almost by Blindernveien. That is Jupiter. To get there, we must first pass through the asteroid belt. It is a region containing billions of small and large rocks. These are called asteroids, and the largest of them, Ceres, is almost 1,000 km in diameter. In August 2006, the International Astronomical Union (IAU) decided that Ceres would become one of the Solar System’s then five dwarf planets. Pluto became another one after losing its status as a planet that same year.

None of the asteroids, including Ceres, are included in the model. They are too small and too numerous. If they were included, most of the asteroids would orbit in a belt 20 metres wide, between 30 and 50 metres from the Sun. Some of them, however, follow very different paths and can even cross Earth’s orbit. They are called near-Earth asteroids (NEAs) and pose a collision risk. Astronomers estimate that there are a couple of thousand such objects with a diameter greater than one kilometre. Fortunately, space is vast, so the risk of a collision remains low in the short term. On geological timescales, however, collisions are inevitable—as the dinosaurs discovered. Asteroids are remnants from the formation of the Solar System and are made of rock and metals.