A Background in Asteroids, Comets and NEOs


Asteroids are rocky objects which orbit the Sun in our Solar System, but are too small to be considered planets. They are in fact, commonly known as Minor Planets due to their size. The majority of the asteroids in our Solar System orbit the Sun in what we call the ‘Asteroid Belt’. This is located between Mars and Jupiter as can be seen in the diagram below. 

However, there are some asteroids that have left this region after being influenced by the gravitational forces exerted by the planets in the Solar System, and are on paths which bring them near to the Earth. If they  approach the Earth at a distance of less than 1.3 AU (1AU is the distance between the Earth and the Sun), then they are considered to be a Near Earth Object, or NEO. Although the chances of such asteroids hitting the Ea[[{"attributes": {"alt": "asteroidbelt3.jpg", "style": "float: left; height: 196px; width: 250px; ", "class": "media-image medium asset_lightbox", "rel": "asset_lightbox", "typeof": "foaf:Image"}, "view_mode": "media_preview", "type": "media", "fid": "596"}]]rth are very slim, our planet has been struck in the past. An example of an asteroid impact creating a crater [[{"attributes": {"alt": "Barringer", "style": "float: right; height: 61px; width: 100px; ", "class": "media-image small asset_lightbox", "rel": "asset_lightbox", "typeof": "foaf:Image"}, "view_mode": "media_preview", "type": "media", "fid": "597"}]]on Earth is shown in the image of the Barringer Meteor Crater in Arizona on the right.

Asteroids come in different shapes and sizes, ranging from about 1000km across, to a few cm. They are thought to be the leftovers  of the formation of the Solar system which didn’t quite make it into a planet, but remained in orbit around our Sun. Most of our knowledge of asteroids comes from our study of meteorites found on Earth. An asteroid which is on a collision path with the Earth is called a meteroid. If the meteoroid then enters the Earth’s atmosphere, as it burns up due to the speed of its entry, it will leave a streak of light in the sky, and it becomes known as a meteor (or a shooting star!). What is left of the meteor, when it hits the Earth, is known as a meteorite.

One of the largest asteroids, and also the brightest, is Vesta. Discovered in 1807, this asteroid is 530km in diameter. The smallest asteroid known to date is approx. 3-6m in diameter. It passed very close-by to the Earth in 2003 - in fact, it was the closest approach by an asteroid that didn’t hit the Earth that has ever been documented, at a distance of 88,000km. This is less than a quarter of the distance to the Moon, and therefore, much less than 1.3AU, earning it NEO status. The largest asteroid in the asteroid belt used to be Ceres until it was given ‘dwarf planet’ status in 2006.


Comets are commonly known as ‘dirty snowballs’ due to their composition - they are made up of rock,frozen gases and ice. There are two areas of space where these objects can originate from - the Kuiper Belt and the Oort cloud.
The Kuiper Belt lies beyond the orbit of Neptune up to a distance of about 500AU. It is thought that the short period comets originate from this area - these are comets that orbit the Sun in less than 200 years and return again in predictable intervals. Halleys Comet and Comet Hale-Bopp are two examples of short period comets.

Most newly discovered comets however are long period comets. These originate from a region called the Oort Cloud which extends to about 50,000AU from the Sun. The comets from this cloud have very elongated orbits, and therefore spend a lot of their time far from the Sun.

Unlike asteroids whose orbits are mainly circular, comets orbit the Sun in an ellipse.  Short period comets in the Kuiper Belt orbit the Sun in the plane of the ecliptic whereas long period comets have orbits which are inclined to the plane of the ecliptic. This is shown in the diagram below. The Sun is at the centre of the diagram as this is the object in the Solar System that the comets and asteroids orbit around.

The Asteroid Belt can be seen between Mars and Jupiter, and their typical circular orbits are shown with the arrow. It is clear from this diagram that their orbits are on the same plane as that of the Solar System planets.

The positions of the Kuiper Belt objects can also be seen in the diagram below. Again, the objects in this area, the short period comets, orbit the Sun in the plane of the Solar System.  The long period comets which come from the Oort cloud however have elliptical orbits which are highly inclined to the plane of the Solar System - this is also illustrated in the diagram on the right.

Since the majority of comets are only a few kilometers across, they are very difficult  to see until they come close to the Sun.  As they get closer to the Sun, the Sun’s heat evaporates the icy parts of the comet, freeing up the gases and dust particles in the comet. 

The gases begin to glow as they evaporate, forming a region which can reach up to millions of km from the nucleus of the comet. This cloud is called the ‘coma’. The solar wind then blows these gases and dust particles away from the direction of the Sun, resulting in 2 long tails being produced. The dust tail shines by reflecting the sunlight, and points away from the Sun.  

The gas or ion  tail is made up of gases which have been broken apart into charged molecules and ions by the radiation from the Sun. Since the most common ion, CO+ scatters the blue light better than red light, to observers, this ion tail often appears blue.  This can be seen in the photograph of Comet Hale-Bopp to the right, taken by Malcom Ellis in England. The dust tail extends to the right of the comet’s coma and appears white. The ion or gas tail, extends upwards as it is swept away from the direction of the Sun by the Solar wind’s magnetic fields. This tail clearly looks blue. The tails of the comet can extend the same distance as the distance between the Earth and the Sun (1AU)! 

The diagram below shows the changes in the comet’s appearance as it orbits the Sun.