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Artists illustration of an asteroid field. Credit: Virtualastro Artists illustration of an asteroid field. Credit: Virtualastro

Space Rocks – Comets, Asteroids, Meteorites and More

Sen—Comets, asteroids and meteors are well used terms for objects travelling through the Solar System. The recent impact of an asteroid in Chelyabinsk and the anticipation of seeing comets PANSTARRS and ISON this year have raised interest in the different types of space rock that we can see flying around the Solar System or crashing down to Earth. 

All of these objects are some form of space rock and most of the time they are harmless. However, as shown by the asteroids that hit Siberia in 1908 and Chelyabinsk in 2013, and the one believed to have caused the extinction of the dinosaurs, they can cause widespread devastation. In the same weekend that the asteroid injured Chelyabinsk, another asteroid was making a close flyby of Earth, 2012 DA14. The close passing of 2012 DA14 - a 50 metre wide asteroid - was unrelated to the Chelyabinsk one but happening at about the same time as the Russian impactor certainly raised awareness of the potential danger of space rocks and generated public interest in the topic!

Generally the Solar System and our corner of it is a pretty quiet place, with major impacts and collisions with space rocks being very rare. However, on a daily basis thousands of small objects hit Earth’s atmosphere without us being aware of them, or they simply give the lucky observers a fantastic light show.

Nearly all of these objects are remnants of the creation of the Solar System and are part of it. Many space rocks were originally from areas such as the Asteroid belt, the Kuiper Belt and the Oort cloud, which extends almost half way to Alpha Centauri, the next closest star system.

The names or classification given to space rocks refer to their characteristics and behaviour, and can be summarised as follows:

Asteroids
Large rocky airless objects that resemble huge boulders, they range in size from a few meters to many tens or even hundreds of kilometres in diameter. The largest asteroid in the Solar System is Ceres with a diameter of 952km – it looks like a small moon and is classed as a dwarf planet! NASA's Dawn space probe is now making its way to study Ceres having recently left another massive asteroid, Vesta. The information gathered on Vesta has led scientists to conclude it was formed in the same way as the inner rocky planets.

Asteroids are usually found in the Asteroid belt between Mars and Jupiter, and the Kuiper belt beyond the most distant planet, Neptune. But there are many that drift through space orbiting the Sun. Some of these rogue asteroids pose the biggest danger to Earth as some pass quite close Asteroids have in the past collided with Earth and will inevitably (with much warning hopefully) collide with the Earth in the future. According to scientists it was a 10 - 15km wide asteroid that caused the extinction of the dinosaurs. Asteroids that pass close to Earth are called Near-Earth Objects (NEOs).

Comets
Large objects made up of rock, dust and ice, tens of kilometres in size that originate from the Oort cloud at the extremes of the solar system, they are remnants of its creation billions of years ago. Comets are often referred to as dirty snowballs that have been knocked out of the Oort cloud by a collision, or the gravity of a passing star sends them hurtling toward the Sun. They comprise of a nucleus – the head or rocky core which resembles an asteroid and a coma – a fuzzy atmosphere created by solar radiation heating the nucleus as it enters the inner solar system, causing water ice to vaporise along with other gasses. The coma can form a tail as it passes through the solar wind and some comets can be seen with the naked eye every few years on average and can linger in the sky for many weeks. Many comets are small and do not survive their encounter with the sun and are often swallowed up, but some continue to orbit for thousands of years. Comets can leave a legacy behind them with their trails of dust and small particles of rock known as meteoroids, which give rise to meteor showers.

Virtualastro meteor

Artist’s Illustration of a comet in the night sky. Credit: Virtualastro

Meteoroids
A small particle of rock or dust left behind by an asteroid or comet and is significantly smaller in size, ranging from a grain of sand to roughly a meter wide.

Meteors
The visible streak of light caused by an object such as a meteoroid, asteroid or other space debris as it enters Earth’s atmosphere. Heat causes the object to glow as it heats up and the streak of light is also known as a shooting star.

Meteorites
A meteor that has impacted the ground, an object can only be classed as a meteorite after impact and can be identified by varying techniques including; testing the object to see if it is magnetic at the impact site or looking for elements such as iridium in a lab. Some meteorites can fetch big money if sold to collectors.

Artist illustration of a meteor shower

Artist illustration of a meteor. Credit: Virtualastro

Meteor Showers
Numerous meteors observed radiating from a fixed point in the sky, caused by meteoroids from the debris trails of comets. Meteor showers are annual events where the Earth encounters a particular comets debris trail in its orbit. Well known meteor showers include the Perseids in August and Geminids in December – both of these showers can produce up to 100 meteors per hour at their peak. Click here to read my article for Sen on the Geminids. One of the most famous meteor shower events was the Leonids in November 1966 where the shower became a storm with thousands of meteors falling like rain.

That’s pretty much it; space rocks are classified and named as the above, and for much of the time they are something exciting and sometimes fun to look at - not to be afraid of. At present there are no large asteroids or comets known to come dangerously close or impact the Earth in the future; however there are some that may look spectacular in the night and even daytime skies.

Hopefully this article has helped you to be able to identify what the different types of space rocks are and how they behave. Good Luck

Highlights

Asteroid: space rocks that range in size from a few metres to many tens or even hundreds of kilometres in diameter. Usually found in the main asteroid belt between Mars and Jupiter, but some have evolved different orbits and may pass close to Earth

Comet: Large objects made up of rock, dust and ice, tens of kilometres in size that originate from the Oort cloud

Meteor: meteoroid falling through a planet's atmosphere, also known as a shooting star

Meteoroid: smallest space rock or dust up to 1 metre wide

Meteorite: meteor that lands on the surface and is retrieved and classifed

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