The asteroid countdown is on. Twenty years from now micro-planet Apophis comes visiting and half the planet will see it pass, perhaps unsure if it will collide with Earth seven years later.

Why is this important, even if its misses? What can science do if this giant 300 metre wide rock passes through an elusive “keyhole” the sky and has its orbit bent enough to send it on a collision course in 2036?

On 13 April 2029, Apophis, named after Apep, a destructive ancient Egyptian demon, will be visible from central Asia, Africa and Europe as a bright star speeding rapidly toward its closest point to earth of around 29,470 kilometres above the mid Atlantic.

(The name isn’t frivolous. Apep is summoned by loud clapping and when the Tunguska meteorite blew apart over central Siberia in 1908 people too far away to see the explosion reported a set of sharp clapping noises in the sky.)

Apophis was discovered five years ago and first catalogued as near Earth orbit crossing object 2004 MN.

That’s when it started getting bad press, because based on the first few observations of its track against background stars; it was assigned a risk as high as one in 37 of hitting earth in 2029. Real cosmic roulette.

Those were the shortest odds of a destructive collision with an Earth crosser ever calculated.

But then there were more observations, the orbit was refined to a non-impacting event and everyone went “whew”. Until it was realised that in coming so close to Earth, the trajectory of Apophis would be bent or distorted and there was a “slight” chance of around one in 48,000 it would collide with us on Easter Sunday, 13 April, 2036.

Since almost everyone expects to win Lotto at odds of around 380 million or more to one, scoring a vastly less improbable hit from a mini-planet suddenly grabbed the attention of astronomers world wide and contingency studies sprang up.

Objects like Apophis strike the planet at velocities of at least 17 kilometres per second in most scenarios. Sometimes they arrive even faster.

The kinetic energy released is the mass of the object times its velocity squared. Current estimates of a direct hit by Apophis involve the release of energy equivalent to more than 65,000 Hiroshima sized atom bombs.

It wouldn’t be the end of the Earth, just a chunk of it. And if it impacted an ocean is would generate tsunami waves of more than 130 metres high, hundreds of kilometres away in some simulations. The destruction would dwarf that caused by the Tunguska comet shard which is variously estimated to have been around 30 metres in diameter when it exploded over what was at the time an uninhabited part of central Siberia.

The trouble with objects like Apophis that modern astronomy can now find more easily than before, is that in some orbits the other planets and larger asteroids exert a very significant cumulative effect that confounds longer term predictions.

The uncertainty factor for Apophis in 2036 ranged in a recent study as between zero kilometres, or 49 million kilometres. Whether it will be a bull’s eye or a very wide ball depends on further observations.

It will come close enough to be studied with optical telescopes in 2011 and measured using radar telescopes in 2013.

The Hollywood scenarios of anti asteroid missile launches and dangerous missions to blow rogue comets or asteroids apart are non-starters. Over a period of even a few years, they can be nudged away from Earth by such simple measures as painting one side with a highly reflective coating to absorb more of the pressure of light and particles streaming from the sun. It’s all a matter of precision and patience and a lot safer than risking fracturing the incoming object into a cluster of giant rocks.

Whatever the fate of Apophis, it may help future generations if they have to deal with the second most dangerous asteroid known to be a probable collider, which is 1950 AD.

This 1100 metre wide object is more inherently dangerous than Apophis and has been intermittently tracked since its first discovery 69 years ago.

But the calculations suggest its date with planet Earth will not fall due until 2880, if at all.

A sample animation of Earth seen from an incoming Apophis can be seen here.