We are very lucky down here on Planet Earth. The chances of a comet impact are actually very low. There is only one comet on the list of potential impactors currently being tracked by NASA’s Jet Propulsion Laboratory, Comet Catalina 2005 JQ5. The comet is estimated to be approximately 1km in size and the resultant impact would be equivalent to 6 billion tonnes of TNT (6 gigatons). With an impact of that magnitude there is only one thing you can do – bend over, place your head between your legs, and kiss your ass goodbye! The chances are that a deep impact would take place over ocean somewhere and the resultant tsunami would be catastrophic with seawater travelling hundreds of miles inland destroying everything in its path. The ejecta from the impact travelling up into the atmosphere would block out the light from the Sun plunging the planet into a nuclear winter that could last many years, causing irreparable damage to the food chain. Continued survival would become a war for anybody who had survived the initial impact. The reality is that the odds of Catalina 2005 JQ5 impacting with our planet are quite small. They were originally put at 1 in 300,000 of a strike on 11/06/2085, and later stretched out to 1 in 120 million. The cut-off point for inclusion on the list of potential impactors is 1 in 10 billion.
Planet Earth is actually quite well protected as a photograph of the back of the Moon demonstrates. It is very heavily cratered in comparison to the face that is always turned towards us. We are also quite well protected by the outer planets in the Solar System which was admirably demonstrated on 16/07/1994 when Comet Shoemaker-Levy 9 impacted with Jupiter. The impact released an estimated 6,000,000 megatons, more than the entire nuclear arsenal on this planet. An impact of that nature down here would be like playing interstellar billiards; we wouldn’t survive it and there is nothing we could do about it. Without Jupiter acting as a cosmic waste collector we would have been wiped out long ago. Unfortunately we can’t rely on Jupiter to save our necks all the time. Occasionally something slips on by and eventually makes landfall on one of the inner planets. There have been several close encounters with deep space objects over recent years but the closest we have probably come to a big impact was The Tunguska Event of June 30th, 1908. The explosion is believed to have been caused by a comet fragment or meteorite that exploded before making landfall. The force of the blast was upward of 5 megatons with a blast area in excess of 800 square miles. It is fortunate that the area was very remote and not at all populated. It took another 19 years before an expedition could be mounted to the area to survey the damage!
Deep space objects enter our atmosphere on a daily basis. Most of these objects are quire small but now and again something large finds its way in. Most of these objects burn up or explode (airburst) before hitting the ground. There is compelling evidence to show that these airbursts happen quite frequently. A meteorite of about 10 metres diameter can produce an explosion of around 20 kilotons similar to the blast of Fat Man, the atomic bomb dropped on Nagasaki in 1945. Data from the U.S. Airforce Defence Support Program suggests that explosions of this magnitude occur high in the upper atmosphere more than once a year. Megaton events such as Tunguska are much less frequent (thankfully!) and estimates by astronomer and comet hunter Robert Shoemaker put these events at around every 300 hundred years.
Projects like The Eden Project (Cornwall, England) and The National Seed Bank (Kew Gardens, England) would become of global importance, much more than they are now. The survival of our species is going to be dependant on these projects, much more so than it is now. It doesn’t matter how big a hole you dig to hide in, or how high you can climb, if there is no green life left on this planet we are dead as a species. Our only other chance of long term survival is for interplanetary travel to become available that allows us to migrate to a colony on another planet that has been terra-formed to make Life is sustainable. Mars is probably the ideal candidate but even if we were to start work tomorrow it probably wouldn’t be ready by the time Catalina 2005 JQ5 arrives on our doorstep. Our best chance of survival is to keep hoping and praying that we don’t have to face up to a deep impact before we are ready and able to deal with the consequences.
