15 February 2013: A small asteroid enters the Earth atmosphere over Russia. Mass ~11.000 tonnes, size ~20 m and speed ~18 km/s. At an altitude between 15-25 km it explodes, causing a shock wave on the ground resulting in about 1500 people hurt (mostly by scattered glass) and ~ 7000 buildings damaged. Estimated energy 440 kilotons of TNT, equivalent to ~25 times the energy of the atomic bomb on Hiroshima, most of it absorbed in the atmosphere. This asteroid had not been discovered before its impact.
On the same day about 15 hours later, asteroid 2012 DA14 passes Earth at a distance of only 28.000 km (that is within the orbit of the telecommunication satellites!). Bigger (about 40.000 tonnes and 30 m), it was discovered in February 2012. Although occurring almost simultaneously, the two events are not related.
Next year, 19-10-2014, comet C/2013 A1 will pass extremely close to Mars, with a small possibility of an impact with the planet. Here we are talking about a different order of magnitude! Estimated size of the comet nucleus is ~ 3km! IF it would hit Mars, the energy released would be in the order of millions of Megatons of TNT. For comparison, the asteroid that struck Earth 65 million years ago and ended the dinosaur era on Earth was only about three times as powerful.
Do we have to get worried? In the aftermath of the Chelyabinsk meteor there was a lot of commotion in the media that action should be taken immediately to protect us from future collisions. Suggesting that nobody had expected this. As if the scientific world was not already aware of this problem!
Those outer space objects (asteroids, meteorites, comets) that can come close to (or even hit) Earth are called Near Earth Objects (NEO’s). Because of the impact risk they are monitored already for decades. The problem is that there are many of them and that they come in all sizes. Here is a graph of their distribution. The blue line gives the number of known NEO’s, the red line is a estimate of their total number. Please note the double logarithmic scale of the graph! For example, only about 100 NEO’s with a size of ~10 meter have been observed, whereas the estimated total number is ~ 10 million!
At the moment about 10.000 Near-Earth objects have been discovered. About 900 of them are asteroids with a size of 1 km or larger.
Here is a table with (statistical) information about the impact of a NEO. The second row gives data for objects with a size of 30 meter. The 2012 DA14 falls in this category. The effect of impact: a fireball, a shock wave and minor damage. Fits quite well the Chelyabinsk meteor, although it was smaller. The second column gives the average time in years between impacts of this size: 300 year.
If comet C/2013 A1 would have been on a (near) collision course with Earth, it would fall in the category: Billions of people dead, global climate change. Time interval: millions of years.
Followers of my blog may remember the post about Apophis At the time of discovery there was worry that it might hit Earth. We know now that the probability of impact is negligible. With its estimated size of 300 m it would create havoc, but no global destruction.
Conclusion: Earth runs a risk of an impact with a NEO. It is not a matter of IF but of WHEN. Early observation of “dangerous” NEO’s is important, so protecting measures can be taken. That is why there are global initiatives, like for example NEOShield
For this post graphs, tables etc have been taken from this very informative site.