On the afternoon of Tuesday, November 8, an asteroid about 400 metres in diameter passed close by the Earth. At its closest point it was about 300,000 km away, which is closer to us than the Moon. Since our Earth is less than 13,000 km in diameter, this sounds like a huge distance. However, when we consider the size of the Solar System and the number of objects moving around in it, how fast they are moving, and the uncertainty in determining their orbits we need to pay attention.
Our Solar System was born about 4.5 billion years ago as a rotating cloud of gas and dust. Over time, dust particles collided and stuck together, forming bigger lumps, and these lumps accreted into bigger lumps and so on. Eventually some of the lumps grew to form the planets, including ours. Some smaller lumps became moons, like our Moon. However, some construction material remains unused, and is still wandering around the Solar System as lumps of various sizes, ranging from fine dust to bodies up to hundreds of kilometres in size. These days any of these bodies larger than a few tens of metres is referred to as an asteroid.
Fortunately, most asteroids tidily circle the Sun between the orbits of Mars and Jupiter; however the rest ñ still many thousands ñ follow paths that are highly elliptical, crossing the orbits of one or more planets. This raises the possibility of collisions. These days a lot of attention is being dedicated to identifying asteroids following paths that cross the Earthís orbit. This interest is more than academic, because the consequences of being hit by a rock even tens of metres in diameter moving at several or more kilometres a second would be serious, and the impact of one a kilometre in diameter or larger would be devastating.
This leads to two obvious issues. Firstly, how do we detect possible threats with enough lead time to do something about them, and secondly, having identified possible threats, what do we do about them? We are rapidly getting better at the first problem, but have a long way to go with the second. We can spot asteroids by watching the sky for moving objects. We do this by repeatedly photographing the sky using sensitive cameras, looking for objects that have changed position.
A near miss today does not mean there is no threat from this object. It will cross the Earthí’s orbit over and over again, for millions or billions of years until either it hits us, or it has an encounter with something that deflects it into a completely new, non-threatening orbit. So an additional task for us is to calculate the movements of each object far off into the future. This can be very difficult because the orbits of the asteroids get perturbed by other planets they get close to, and a small perturbation over a long time could put it on a collision path.
The last big impact took place about 50,000 years ago, when an object about 50 metres in diameter hit what is now Arizona at about 10 km/sec. It made a crater over a kilometre in diameter and released the energy of a 10 megaton bomb. Bigger impacts are rare, but their potential consequences for us and our planet are too serious to ignore.
Jupiter rises at 4 p.m., Mars at 11 p.m. and Saturn at 5 a.m.. The Moon reaches last quarter on the 18th.
Ken Tapping is an astronomer with the National Research Council’s Herzberg Institute of Astrophysics, and is based at the Dominion Radio Astrophysical Observatory, Penticton.