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Cosmic collision

by Clive Jackson [email protected]

Clive Jackson is the Director of the Camera Obscura (next to the Castle in Tavira), specialising in education and public outreach.

In our day-to-day lives, collisions would generally be regarded as a bad thing, something in fact to be avoided. But in the microscopically small world and in the astronomically large world, collisions are of benefit, indeed they are essential to the health of the entire universe in which we live.

Stars produce their energy from a nuclear fusion that requires an energetic collision between two atomic particles with enough force to overcome their natural repulsive tendencies in order to enable them to fuse together to produce a third particle that has slightly less mass than the sum of the originals.

This mass-to-energy conversion liberates the power necessary to run the entire universe.

Going up in scale slightly to the size of our solar system, we will find that it exists principally because of multiple collisions between the dust and gas particles in a pre-solar nebular that triggered a gravitational collapse, which resulted in the Sun and its orbiting collection of planets.

It did not stop there as originally there were more than a hundred proto-planets that existed in our solar system, and collision between them reduced the number down to the eight we have today.

The ideal conditions for life on our planet Earth is almost certainly due to a large impact between the young Earth a hundred million years after its formation, and a proto-planet the size of Mars.

This resulted in the formation of our Moon that slowed our planet’s rotation from its initial six hour period down to 24 hours and stabilised our rotational axis to ensure habitable climatic conditions for billions of years.

It is widely believed that an asteroid impact 65 million years ago ended the age of the dinosaurs on our planet and this allowed mammals to prosper and consequently permitted the evolution of humans.

Also, our planet is blessed with an abundance of water that would not be present initially when the Earth formed. It is quite possible that asteroid and cometry collisions from objects that formed in the low temperature zones far from the Sun, that contain large quantities of water in the form of ice, deposited this water on the surface of our planet.

There is also an intriguing possibility that primitive life first started on Mars as it possessed the right conditions for life long before our planet was suitable, and that meteor impacts broke off parts of the surface of Mars that eventually crashed into us.

These first organic molecules would be hardy enough to survive the journey and this would help to explain the fact that primitive life appeared on the Earth immediately that conditions were suitable.

Moving much further afield and out into our galaxy it may surprise you to know that galactic collisions are common and indeed vital for star formation.

Galaxies start off their lives as clouds of hydrogen gas surrounding young bright short lived stars, but the rate of star formation depends critically on how much the gas clouds get stirred up, and the best way to do this  is to collide two galaxies together.

This is almost always entirely beneficial to both involved as the existing stars take very little part in the collision because the overwhelming volume of a galaxy is in the gas clouds and dark matter between the stars.

Our own Milky-Way galaxy has undergone dozens of collisions during its 13 billion year life-time and it is at the moment colliding with two small galaxies called the Magellanic Clouds. There is a much more exciting encounter in store for the future, as the giant Andromeda galaxy is heading towards us at about 120 km/second and will arrive here in approximately three billion years from now.

This encounter will rejuvenate both our galaxy and the Andromeda galaxy with a wave of new star formation that will create what is called a starburst galaxy with tens of billions of new baby stars for the parent galaxies involved.

The chance of our planet Earth undergoing a major cosmic collision sometime in the 21st century is very high but, from destruction comes rejuvenation and we will survive!  

Cosmic collisions may be rare, but their influence can be mighty. Take, for instance, when a Mars-sized object smacked into a young Earth – an event scientists think led to the formation of the moon. Credit: NASA

To contact Clive Jackson, please call  281 322 527, fax 281 321 754, email [email protected] or visit www.cdepa.pt.