On 15 February an asteroid passed by at a distance of less than 28,000 km. That is, about three quarters of the 36,000 distance to the belt of geosynchronous satellites providing much of our communications and satellite broadcasting, and seven per cent of the 384,400km distance to the Moon. The object was about 45 metres in diameter, weighing about 130,000 tonnes and moving at 7.8 kilometres a second. Even though by cosmic standards this object was tiny, it would have been a disaster if it had hit us. Fortunately, measurements of the asteroid’s path showed this would not happen. However, the Earth has been hit many times during its history. Indeed, if these cosmic collisions didn’t happen, we would not be here to worry about them.
When our Sun formed about 4.5 billion years ago, from the collapse of a huge cloud of gas and dust, the newborn star was surrounded by an orbiting disc of unused construction material. Little bits collided with other bits, and dust became gravel, rubble and eventually large lumps. These continued to collide and stick together until we ended up with the planets and other objects inhabiting our Solar System today. However, there are still lots of unused dust, gravel, rubble and much larger lumps still moving around the solar system. Planet construction is still happening, but more slowly.
Some impacts greatly affected the history of our world. One of them occurred about 250 million years ago, contributing to the extinction of over 90 per cent of living species on Earth. The trilobites were one of the main casualties. Another impact 65 million years ago contributed to the extinction of about 70 per cent of living species, including the dinosaurs and the ammonites. The end of the reptiles’ dominance of the Earth set the stage for the rise of mammals, and of course us. However, we are not that keen on being kicked off stage to make room for some new species, so we are watching for asteroids that could possibly hit us. There are now multiple telescopes scanning the sky, identifying asteroids and determining their paths. So far though we have not come up with good ways to deal with such threats. A large bomb would not make much of an impression on most asteroids, and at best we would swap being hit by a big bit to being hit by many smaller ones, over a larger area.
The most powerful reliable rocket booster we have ever made is the first stage of the Saturn 5 rocket, which took the Apollo astronauts to the Moon. If we could attach one of those boosters to that 45m rock, we would change its speed by about 0.6 km/sec before the rocket’s engines ran out of fuel. That could change the object’s course enough to make it miss us. However, for something a kilometre in diameter or larger, the course change would be tiny. An alternative could be to install a motor producing a very small thrust over a very long time. That’s possible, but we will then have to identify threats many years in advance, which is still very difficult. However, our planet is not hit by something really big very often, so we probably have centuries or longer to find a way to do what the trilobites, ammonites and dinosaurs couldn’t.
Mercury and Mars are low in the west and getting lost in the sunset twilight. Jupiter shines brilliantly, high in the southwestern sky. Saturn rises around midnight. The Moon will be full on the 25th.
Ken Tapping is an astronomer with the National Research Council’s Dominion Radio Astro-physical Observatory, Penticton.