Probably more than any other natural threat to our existence, a mass extinction event

involving a collision with a large near-Earth object may be the most likely. Asteroids and

comets are the primitive, leftover ingredients from which the solar system formed; they are

strewn haphazardly throughout our neighborhood, thrown about by gravity like trash over a

windy landfill.

There are a great number of these small, rocky bodies circling the Sun in menacing, highly

elliptical orbits that can cross the relatively circular orbits of the planets. It is these

orbits that make them so dangerous, they can all too easily cross our path as they make

their own way around the Sun.

They are very small and dark relative to planets, and they don't reflect much sunlight which

makes finding these elusive rocks difficult. Many small asteroids passing close to the

Earth aren't found until they are almost right over our heads.

Of course, these close encounters occur almost daily and we never notice. More and more however,

thanks to NASA's Near Earth Object Program, more are being seen earlier. In fact, the

week this video was made, on February 9th, 2011, an asteroid the size of a small car

flew a mere 93,000 km over our heads.

NASA is currently keeping track of almost 8,000 known near-earth objects, both asteroids

and comets. The vast majority are small, less that one kilometer in diameter.

One of them, an asteroid about 400 meters in diameter has had our attention for some

time. An asteroid known as Apophis.

Named after the ancient great adversary of the Egyptian sun God Ra, Apophis was the embodiment

of dissolution, darkness and non-being. Apophis was the un-creator.

It isn't surprising that with such a name, Apophis the Asteroid has started to garner

attention.

NASA first announced the discovery of the asteroid in December 2004, and initial calculations

suggested that this body had an excellent chance of colliding with the Earth on April

13, 2029 with a chance of 1 in 37 that it would hit us. This announcement got a lot

of attention.

Followup radar observations at the Arecibo Planetary Radar Telescope, performed almost

immediately after the discovery, ruled out that possibility.

These radar observations, coupled with more detailed analysis of historical images eliminated

the possibility that Apophis would collide with the Earth in 2029, but it will come extremely

close: it is currently projected to come so close that it will fly within the orbit of

geosynchronous satellites, less than 40,000 km over our heads.

While this close encounter with Earth won't be cause for alarm, the story isn't over.

The exact path Apophis follows in April, 2029 is crucially important because it will determine

whether it hits the Earth when it circles around a second time in 2036.

If, on April 13th 2029 Apophis passes through a very narrow window, known as a keyhole,

as it passes by the Earth, its orbit will be affected such that when it returns in 2036,

will directly cross that of the Earth and collide with it on Sunday, April 13th.

So will Apophis pass through this keyhole in 2029? In order to answer this, we need

much more precise measurements. As of early 2011, Apophis is very close to the Sun to

make any accurate observations and won't be in a good position until late 2011.

In order to accurately determine whether the Earth is in for a bad day on April 13th, 2036,

we also need to know more about Apophis' spin direction and other physical parameters. These

will be collected from a variety of projects at NASA as well as the Arecibo radar telescope

in 2013. These radar observations in particular, will allow astronomers to determine the location

of Apophis through 2070.

Throughout its history, life on Earth has been at the mercy of these wandering, early

building blocks of our Solar System. Early in its life, Earth was bombarded much more

than it is now. It is easy to understand how ancient people viewed the arrival of a comet,

these "un-creators", with dread and dispair.

Paradoxically, life on Earth may owe its very existence to the early rain of comets and

asteroids across the surface. They may have been the source of water and other primordial

building blocks, bringing the spark from which early life emerged.

As the solar system ages, these events will become fewer and fewer as asteroids and comets

are either thrown out of the solar system or collide with other planets.

The question is, how many will hit us before they are gone, and will we find them in time?