The Threat of Asteroids

Arthur C. Clarke’s science fiction masterpiece Rendezvous with Rama opens up with the words:

“Sooner or later, it was bound to happen. …In those days, there was nothing that men could do to protect themselves against the last random shots in the cosmic bombardment that had once scarred the face of the Moon. The meteorites of 1908 and 1947 had struck uninhabited wilderness; but by the end of the 21st century, there was no region left on Earth that could be safely used for celestial target practice.”

Luckily for us, Earth has managed to launch a civilization before the cosmos has managed to launch an asteroid to wipe it out. But, how far into the future can this civilization foreseeably persist until an asteroid finally manages to revert its progress back to its earliest seeds of existence and wipe out any trace of intelligence?

Unlike the dinosaurs, we are lucky to have accumulated enough consciousness to be able to apprehend such a looming threat, perhaps to the chagrin of the next species that would have otherwise filled our shoes. Perhaps our technological progress means that Earth might not really be this scoreboard of celestial target practice, as Arthur C. Clarke would otherwise portray. But, how much of a threat do asteroids really constitute?

The vast majority of asteroids lie in the “asteroid belt” between the orbits of Mars and Jupiter, at 2.2 to 3.3 Astronomical Units from the sun with orbital periods of 3-6 years. 200 of such asteroids have diameters of 100 km or more. Main belt asteroids pose fairly no threat. They have circular orbits that are close to the plane of the ecliptic. Near-earth asteroids (NEA) however have orbits that bring them much closer to Earth and in fact many of their orbits do cross Earth’s orbit. They are thought to be derived from the asteroid belt, in which case gravitational interactions with Jupiter and Saturn diverted them into Earth-crossing orbits. Such near-earth asteroids have more elliptical orbits than their main belt counterparts and it is this which makes them quite a threat because they pass closer to the sun than the Earth does.

9000 NEAs have been detected by NASA, a thousand of which are large enough to have undesirable consequences on Earth. In fact, a 1 km asteroid hitting Earth could have an explosive impact equivalent to 4 million atomic bombs and could leave a crater 26 km wide. It is estimated that within the last 500 million years, the Earth has been bombarded with a number of asteroids over 10 km with impact energies of the order of a hundred million megatons!

Recent close approaches to Earth. Credit: NASA Near-Earth Program

In 2013, NASA’s WISE telescope began a mission to catalogue the location of NEOs and track their pathways. In 2013, it detected 2013 YP139, which was found orbiting  45 million km from Earth. In about 100 years, it is predicted to pass within 480,000 km of Earth.

1999 RQ36 was first spotted in 1999. At 500 metres, It has the greatest likelihood of impact among all other asteroids. It turns out there is a 1 in 1400 chance that it would collide with Earth between 2169 and 2199 at one of its 8 close encounters. The next most risky likelhood of impact is that of the 250-metre Apophis, which has a minute risk of a 1-in-250,000 probability of impact during its approach in the year 2036.

More than 100 million pieces of debris enter Earth’s atmosphere daily but thankfully, they are not large enough to have any noticeable impact. However, 2000 NEAs that are larger than 1 km have been estimated to cross with Earth’s orbit. 400 of these will eventually collide with Earth at some point.

Asteroid DA14 Near Miss – 27,000 km from Earth

Because the orbits of such asteroids are eccentric, even non-hazardous NEAs can become hazardous at some point. 433 Eros is a 20 km near-earth asteroid and in the next million years, there is a 50 % chance that it will be diverted into an Earth-crossing orbit and collide with Earth. An impact would be catastrophic.

Even though the chances are quite minute that an asteroid will hit upon us anytime soon, we have to remain vigilant. According to Asteroid Day’s website, a “million asteroids in our solar system that have the potential to strike Earth and destroy a city, yet we have discovered less than 10,000 — just one percent — of them” despite our technological progress. More asteroids need to be catalogued and discovered!

So, please, join me in signing Asteroid Day’s declaration to accelerate the tracking and discovery of Near-Earth Asteroids up to 100,000 per year within the next 10 years. The dinosaurs did not have the luxury of a space program or a petition. We should make use of that!


Featured image credit: NASA. Photo of Eros as taken by the NEAR spacecraft



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