‘”The Sky is falling...” some Truths about Near Earth Asteroids’ A talk given by Peter Thomas, ASV editor for the CRUX magazine. 
“Once upon a time there was a tiny, tiny chicken named Chicken Little. One day Chicken Little was scratching in the garden when something fell on her head. "Oh," cried Chicken Little, "the sky is falling. I must go tell the king." So Chicken Little ran and ran, and she met Henny Penny...” and so the story goes on. According to Wikipedia the fairytale originated in Indian folklore at around (or before) the third century CE. The phrase, "the sky is falling," has passed into the English language as a common idiom indicating a hysterical belief that disaster is imminent. Its usage is generally derogatory.
There is ample evidence though, that the threat of asteroids, comets and meteoroids colliding with planets and their moons is real. Even a small Telescope reveals the huge craters with which their impact marked the surface of our airless Moon. Earth must have had a similar bombardment in the past, but erosion and tectonic activity has over time merged the craters with the environment and only the most recent impacts (like Wolf Creek in North Central Australia from about 300,000 years ago), are left to tell their powerful story. Science and modern, satellite imaging technology have helped to piece together information on older and (or) more remote impact sites, like Tunguska, the Great Siberian explosion in June 1908, caused by a 50m stony comet or asteroid, right back to the giant 10km bolide event in the Yucatan Peninsula Mexico, assumed to have caused the end of the dinosaur era, 60 million years ago.
So the thread of the sky falling in on us one day is real. The question remains, how real is real? In other words, how likely is it to happen? Peter Thomas presented a huge amount of details he had collected in this new astronomical field of study of Asteroids or Near Earth Objects: from the means of detecting and monitoring them, to the system of numbering, to the terminology of their orbits around the Sun and their likelihood of colliding with (or passing close by) Earth. Did you know that there are 5055 (as of 8 January 2008) known Asteroids classed as Near Earth Objects? And the list is growing: 646 new ones were discovered just last year. They range in size from 10m right up to the 10km behemoth and in orbital periods from less than a year to the decades of comets. They are grouped into four subclasses according to their orbit in relation to the Earth’s orbit, and type-named (following astronomical convention) after the first asteroid discovered in that class: Apollo, Aten (named for 2062 Aten, the first such asteroid discovered), Amor and those on an Inner Earth Orbit known as Apohele. The first two have Earth-crossing orbits, that is, their perihelion is inside and their aphelion outside the Earth orbit; the difference between the two of them being the size of their Semimajor Axis, which for Apollo is equal or greater than one Astronomical Unit and for Aten it is smaller than one AU. The Amor Class orbit is completely outside the Earth orbit and the Apohele completely within. There are at present only six known Apoheles. That name is still under a bit of debate, since there is as yet no asteroid named Apohele. It is said to be the Hawaiian word for orbit, which was chosen partially because of its similarity to the word aphelion.
Orbits of all the known Near Earth Asteroids have now been plotted for years ahead and the only scare that has arisen so far is a near miss in 2028 and a close encounter with asteroid 2006XG1 in 2041. Remember though, this accounts only for the known and mapped asteroids. There are literally mega-bitsof cosmic rubble in the Main Asteroid Belt between Mars and Jupiter, supposedly left over from the formation of the solar system and estimated to total up to almost half the mass of our moon. Sir William Herschel in 1802 coined the term asteroid (star-like) for such bodies, writing "they resemble small stars so much as hardly to be distinguished from them, even by very good telescopes”. The largest asteroid by far is Ceres. Originally discovered on 1 January 1801, by Giuseppe Piazzi, it is 933 km in diameter and contains about 25% of the mass of all other asteroids combined. Classified originally as a planet, then reduced to an asteroid and now elevated again to the level of a dwarf planet, it was named after the Roman goddess Ceres - the goddess of growing plants, the harvest, and of motherly love. Its astronomical symbol is a sickle. Smaller bits of rubble in the Main Asteroid Belt could be thrown out of orbit at any time by collision or harmonic resonance on a trajectory towards Earth.
To help Chicken Little feel safe, Large Synoptic Survey Telescopes are being constructed to provide digital imaging of faint astronomical objects across the entire sky. Night after night the 8.4m ground-based telescope will monitor with a 10 square degree field of view the trajectories of known asteroids and keep
an eye out for new threats. An amazing bit of statistic here is, that the total amount of material that falls on Earth from space as dust or meteorites is variously estimated between 37,000 and 78,000 tons annually; Is anyone worried about that?
The vote of thanks for this in-depth survey of cosmological things that may fall on our head was given by Barry Adcock, with the customary show of appreciation in a glittering red shopping bag. The moral of the “Chicken Little” fairy story depends on which of the many available endings are used. It can mean, don’t be a ‘chicken’ or it can mean: do not believe everything you hear. And watch out, the-sky­is-falling modus operandi can come in many guises (such as global warming, national security or religious fervour for instance), manipulated to whip up mass hysteria and serve a hidden agenda. Recognising a “the sky is falling” routine may prevent jumping to a hasty conclusion.