Asteroid is the name for small bodies in the solar system. Generally asteroids are at least metres across or larger, and which do not show outburst activity (unlike comets). Anything smaller is a meteoroid. (A meteoroid entering Earth's atmosphere is a meteor and when it's found on the ground afterwards it's a meteorite)

The line between asteroids and comets is not clear (see lecture 4). Some asteroids turn out to be comets and comets may turn into asteroids as they run out of volatiles.

The first asteroid was found in 1802 and named Ceres. A handful of others were found soon after, but after that almost 40 years passed before more were found. Now we know over 100 000 asteroids and more are found at an increasing rate with improving survey technology. The second largest asteroid is Vesta.

One way to classify asteroids is based on where they are found. Asteroids can be found all over the solar system, but the vast majority of them are concentrated in a few smaller areas. The main belt is a region of space between Mars and Jupiter, in which a large fraction of asteroids orbit. This is where the first asteroids were discovered and where most of the known ones are. It was believed at one point that the main belt might be the remains of a planet that has been somehow destroyed into small pieces, but the total mass of the asteroid belt is nowhere near even a small planet's mass.

Some asteroids orbit almost in the same orbit as Jupiter, only 60° ahead or behind it in the orbit. These are called the Jupiter trojans and are another large asteroid population. Because of certain laws of dynamics, these kinds of orbits are kept very stable by Jupiter's gravitational influence.

Some asteroids have orbits which bring them very close to the Earth. Some can even intersect the Earth's orbit and thus possibly impact the Earth. These are known as Near-Earth Asteroids/Objects (NEA or NEO). They are an important field of study, as asteroids impacts pose a significant threat to human life, something which has been widely understood since the 1970s.

Another way to classify asteroids is their composition. Because have not been able to analyze many asteroids in situ, this classification is based on their spectroscopic properties. The main division is into S-type, C-type and M-type asteroids. The S-types are silicate, consisting mostly of relatively silicate-rich minerals. The C-types are carbonaceous, consisting of carbon-rich minerals, and the M-types are metallic, consisting largely of iron and other metals.

Dwarf planets

A recent new class of solar system bodies is the dwarf planet. The classification was created after many objects were discovered in the same region as Pluto, which are similar (or even larger) in size to Pluto.

A dwarf planet is defined as a body which is large enough to be spherical (the gravity of a large enough body will pull it into spherical shape) but not massive enough to be the gravitationally dominant body in its orbital region.

The only dwarf planet in the inner solar system is Ceres, but outside of Neptune's orbit there are already several (nine, to be precise) besides Pluto. It is estimated that there may be hundreds or thousands of them, but they are still very difficult to find with current telescopes.


At this point in the lecture I tried to clarify some points abotu spaceflight.

Going into space is not particularly difficult. Space is relatively close, only 100 kilometres straight up. It is relatively easy to build a rocket that can fly into space, and these are used a lot to study the atmosphere. The main problem is staying in space.

Even though astronauts appear weightless while in space, the Earth's gravity is almost at strong at the space station's orbit as it is on ground. An object which is simply launched upwards into space to a typical satellite's altitude will simply drop back down.

The solution for staying in space is moving very fast horizontally, instead of vertically. If you move fast enough horizontally, you move around the Earth faster (or just as fast as) the Earth's gravity pulls you down. When a rocket is launched, it launches almost vertically, but this is just to get above the atmosphere, which slows it down. After a rocket has passed through most of the atmosphere, it has turned so that it is accelerating almost entirely sideways.

A note on time scales

In my lectures I often talk about time scales in the millions or billions of years as the timescale of the solar system is very slow geological time. It is important to remember the difference between millions and billions.

A billion is a thousand million, so a billion years is a thousand times longer than a million years. If a million years is a centimetre, a billion years is ten metres.

Most of the solar system has not changed very much in the last millions of years, so "in the last few millions of years" actually means "in the very recent past" or "still while the solar system was like it is now".

A scale of hundreds of millions of years into the past is already somewhat significant, especially in Earth geology. On many solar system bodies not much has happened.

The solar system is around 4.5 billion years old, so "billions of years ago" means roughly "in the early days of the solar system", though the difference between 1 billion and 4 billion years into the past is huge, too.

If the timeline of the solar system were ten metres long, two billion years would be around the middle.