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?