1. Asteroids On-line Lesson

2. What are they? Asteroids are solid pieces of rock that have been left around the Solar System from the time when the planets formed. They range in size from one metre ( or perhaps smaller ) up to 1000 km. Asteroids are quite irregular in shape and therefore the light they reflect from the Sun fluctuates as they move and rotate. On-line Lesson: Asteroids

3. What do they look like ? We have never seen any asteroids close up but in a telescope they look like a tiny point of light. Unlike comets, they have no cloud of diffuse light around them. Because they are very cold they don’t emit much radiation themselves but they can be seen by very sensitive Infra-Red telescopes such as ISO. On-line Lesson: Asteroids

4. Asteroids are very irregular in shape as these pictures taken from a spacecraft nearby show. They are quite similar to small planets or moons like Phobos On-line Lesson: Asteroids

5. How Many Are There? Recent surveys using Infra-Red telescopes indicates that in the main Asteroid belt around the Sun there are at least 1.1 million asteroids larger than 1 km. On-line Lesson: Asteroids

6. Different types By studying both the brightness and the reflectivity of certain colours it becomes clear that asteroids fall into several different groups. M are believed to be metallic C are thought to be carbonaecous S contain more silicates On-line Lesson: Asteroids

7. Where are they? Asteroids circle around the Sun in orbits that trace their origins in the Solar System. The orbits of asteroids are more or less circular and they have orbital periods of three to six years. Not all orbits around the Sun are populated with asteroids because the motion of the main planets interferes with some orbits and makes them unstable for asteroids. The majority of asteroids are in the Asteroid Belt with distances from the Sun between 2.2 and 3.2 astronomical units. On-line Lesson: Asteroids

8. Detection Looking for asteroids in a telescope requires a search for objects which move from one night to the next and are point like in appearance. To be sure that it is an asteroid rather than a planet or comet, enough observations must be taken to be able to measure the mathematical parameters of the orbit. On-line Lesson: Asteroids

9. Are Asteroids Safe? Asteroids pass through the Solar System in their orbits, which can change with time as they are affected by passing planets. If an asteroid was deflected from its orbit onto a path that intersected that of the Earth, the results would be very serious. An object 100km across hitting the Earth at 20 km/sec would could immense damage. On-line Lesson: Asteroids

10. Have they hit us before ? Many scientists believe that asteroids have hit the Earth many times in its history. On-line Lesson: Asteroids Artists impression of a giant asteroid impact

11. Because the effect of such a collision would be so dramatic, the signs of this can be seen in the geological record sometimes as huge craters. An event of this kind may have caused the atmospheric and climatic disturbance which led to the destruction of the dinosaurs. On-line Lesson: Asteroids