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## Has Armageddon arrived?

Yesterday an extraordinary coincidence happened. As I blogged about here, a 50m long lump of rock (asteroid 2012 DA14) was scheduled to zip past Earth yesterday evening (15th of February) at some 12 kilometres per second at a distance of “only” 28,000 km. Although this sounds like a lot, it is closer than any asteroid that size has come to Earth to our knowledge for many decades.

But yesterday I awoke to the news that another lump of rock had exploded in the air over an area in the Ural mountains in southern Russia near Kazakhstan. Initial reports were that about 500 people were injured, this figure had risen to closer to 1,000 by the end of the afternoon.

Of course some people wondered whether these two events were connected. Well, I can assure you they were not. Earth is constantly being hit by particles from space, as we whizz around the Sun at 30 kilometres per second. Every day some 100 metric tonnes (a tonne is 1,000 kgs) of material enters the Earth’s atmosphere. Most of these are sand-grain sized particles, with the number of larger and larger objects becoming fewer and fewer. It probably follows what we call a power law size distribution, in much the same way as the interstellar dust grains I studied for my PhD do. In fact, at the very small level the particles are interstellar dust grains, it’s only when they become larger that we start referring to them as asteroids or lumps of rock.

What this power-law size distribution means is that larger lumps are rarer and rarer. NASA has calculated the size of the lump which exploded over Russia yesterday to be about 15m, and the damage gives an indication of what something this size entering the Earth’s atmosphere can do. To my memory, this is the first time I know of a meteor explosion causing human casualties. The 50m sized asteroid which thankfully missed Earth last night would have caused far more damage, enough to wipe out a large city. We believe 50m sized asteroids hit the Earth about once every 1,200 years, larger ones even less often. And, asteorids a few kilometres in size, large enough to cause widespread devastation and even lead to global catastrophies seem to happen as rarely as once every few hundred millon years.

The Earth will be hit by an asteorid large enough to cause global devastation, of that I have little doubt. But the chances of it happening when humanity still exists is very remote in my opinion. And, as long as we are vigilant whilst we still exist as a species, there is much that can be done to deflect such large asteorids which are on a collision course for Earth. Unlike the portrayals in movies, probably what we do not want to do is send a missle to destroy a threatening asteroid. Rather, we want to gently nudge its orbit into one which will miss the Earth.

Gently nudging an asteroid so it will miss Earth can be done in a number of different ways, but possibly the most elegant is just to paint one side of it white. This will cause the Sun’s light to reflect off of that side more than the rest of the asteroid, and we can use the fact that sunlight has momentum to push the asteroid using sunlight!

## The MRN size distribution

The most popular interstellar dust size-distribution used by astrophysicists is one due to Mathis, Rumpl and Nordsieck (MRN), which they published in 1977. It suggests that the number of dust grains of various sizes follows the following distribution – $n(a) = n_{0}a^{-3.5}$ where $n_{0}$ is the number at some size $a_{0}$. So, for example, if you want to compare the number of dust grains which have a size of 1mm compared to the number which have a size of 5mm then the ratio is $\frac{1}{5}^{-3.5} = 280$ or that 1mm sized dust grains are 280 times more numerous than 5mm sized dust grains. It is unlikely that the distribution of lumps of rock follows exactly the same size distribution, but if it did the number of 15m lumps compared to 50m lumps would be $\frac{15}{50}^{-3.5} = 68$, so 68 times more 15m lumps than 50m lumps.

### 11 Responses

1. on 16/02/2013 at 08:53 | Reply Ana Maria

The simplest solutions are not only the most elegant but usually the best.

• Indeed!

2. on 16/02/2013 at 09:19 | Reply Redstone

Great article Rhodri once more. I was wondering , whilst listening to an Astrophysicist on the BBC yesterday about the Space Tractor Theory, using the tractor`s gravity to alter the path of the asteroid. Approximately how big would and what materials would be used in the Tractor to attain the mass needed to be to draw an asteroid away from its original path? And how far out would they need to start altering the path to miss an impact situation?

• Yes, if you add some mass to an asteroid it will alter its orbit because the size and nature of the orbit is determined by its mass. The size of the mass you add needs to be a few percent of the mass of the asteroid. By how much you alter its path depends on how early you are able to do it. Altering an asteroid’s path years before it is predicted to hit Earth would require much less of an alteration than if you were trying to do it a few months before a threatened impact.

3. Thanks Rhodri! I understood what you told.
Vera.

• You’re welcome Vera!

4. Will this take longer to paint than the Forth road bridge?

• Let’s hope not! 🙂

5. […] the meteor that exploded earlier this year over Chelyabinsk in Russia on the 15th of February, I blogged about it here. Well, this story has recently appeared on the BBC news […]

6. […] have discussed meteors several times before, for example here and here. But, just so this background information is all in one place, I’ll repeat myself. A […]

7. […] BL86 is estimated to be between 400 metres and 1km in size, considerably larger than the asteroid which exploded above Chelyabinsk in southern Russia in February 2013. That asteroid has been estimated to have been about 17 metres […]