Tuesday 19 February 2013

Understanding the terms

Asteroids? Or Small Solar System Bodies?
(Image: Wikipedia)
Over the past few days, there has been a bit of confusion over the difference between the Chelyabinsk event (a meteor) and the 2012 DA14 flyby (a small asteroid)

How we classify the objects in our Solar System is defined by the International Astronomical Union (IAU). In 2006, they voted on a modification to the system previously used. This resulted in the nomenclature that scientists now use today.

Starting with the largest, the objects are:

Planets must orbit the sun, must have achieved "hydrostatic equilibrium" (that is, they are more or less round) and have "cleared the neighbourhood of its orbit". In other words it must be the dominant gravitational force in that orbit area. There are currently 8 recognised planets around our Sun (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune).

Dwarf Planets are the next category. These orbit the sun and have achieved hydrostatic equilibrium, but they have not cleared the neighbourhood of the orbit. At the moment the IAU recognises five dawrf planets (Ceres, Pluto, Haumea, Makemake, Eris), although more are likely to be added to the list as we discover them or as other unclassified objects are evaluated.

There there are the Small Solar System Bodies. Essentially, this is everything that orbits the sun, that is not in the first two categories. So this includes comets and asteroids.

Comets are typically made of ice and dust. Their distinguishing feature is that they  have a coma or tail. This is caused by solar radiation vapourising volatile material.

And then the others are asteroids; chunks of inert rock.

There are a few terms around which have been used in the past, but are no longer recommended by the IAU. Examples include "minor planet", "meteoroid" or "planetoid".

2012 DA14 and the Chelyabinsk meteor were both asteroids and, thus, small solar system bodies. The difference is that 2012 DA14 missed the Earth during the recent flyby, whereas Chelyabinsk did not. This brings us to meteors and meteorites.

When an object enters the Earth's atmosphere, the friction of the entry can sometimes be sufficient to heat it to cause it to give off light. This visible "streak across the sky" is a meteor. Particularly bright ones are often referred to as fireballs.

This heating usually causes the object to burn up completely. Thus, it never reaches the ground.

However, if the object survives the burning in the atmosphere, and survives the impact as well, then it is called a meteorite.

The Chelyabisk event is definitely a meteor. At present there are searches being carried out to try to find debris on the ground, with some candidates having been collected. This is an on-going effort and no doubt more news will come from this during the next few weeks.

However, the bulk of the original object was destroyed on entry and, as yet, no substantial fragments have been found on the ground. Thus it continues to be referred to as the "2013 Russian Meteor" for the time being. However, that may change!


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