How often will Asteroids strike?

Not just NASA but also some other commentators said that asteroids of the size we saw in Russia strike Earth only about once a century. But can we really say that?

The question implies the answer: no. For several reasons.

We cannot make comprehensive observations of asteroids smaller than 100m. We do not know where they are and how many there are. We also don’t have any reliable data on the statistics of their formation and perturbations of their trajectories that might send otherwise harmless asteroids towards earth. Of course, we can make assumptions based on kilometer sized asteroids that have been observed. We also know about centimeter sized meteorites being observed on a regular basis. But we don’t have data in-between.

While some large asteroids were detected using the Vela satellites, we may not assume that all asteroids of this size striking the earth have been detected and correctly identified as such in the last 40 years. Vela satellites were not designed with this purpose in mind. Infrasound and pressure waves also do not give sufficient coverage of the earth. The only pressure wave ever detected from an asteroid was caused by the much larger Tunguska event.

Tunguska also shows that even eyewitness accounts are unreliable. Even if they were, you’d need an eyewitness giving an account first. 70% of the earth is covered by oceans. Much of the land is virtually uninhabited or was historically so weakly connected to the rest of the world, that accounts would not be heard – as in much of Africa or practically all of Antarctica. But you need to rely on eyewitnesses, as such exploding asteroids do not leave much of a trace in nature.

You may wonder how, in our day and age, an asteroid impact could possibly go unnoticed anywhere on earth. But then you are vastly overestimating the capabilities we have using currently deployed technology. Last year, a huge volcanic eruption – possibly the largest of the last 100 years – went by undetected for over 10 days … after which a huge pumice raft was spotted on the ocean and the search for its origin began. Eventually, the eruption itself was found to have been imaged by satellites, but only after a painstaking search through humongous amounts of archived data. But this search would not have been conducted without a pumice raft almost the size of Belgium floating on the Pacific Ocean, giving them a hint or two that something must have happened.

The monitoring stations of comprehensive test ban treaty organisation picked up asteroid. But only by two out of 45 stations did so. A smaller asteroid would not have triggered any of them, nor would a less conveniently placed event have done so. Besides, even those stations are new.

All this leads to the suspicion, that asteroids of this size hitting the earth are probably under-detected and we have no way of telling just by how much. Even assuming that asteroids are hitting earth at perfectly random intervals, a few decades worth of observations wouldn’t tell us much.

As in the above example, we have no idea about the frequency and size of even large underwater volcanic eruptions. Pumice rafts will dissolve at some point and the bottom of the oceans are still mostly unknown. But we do know about large volcanic eruptions on land and they are mostly random.

If we look back 100 years, there is only one large eruption on land (defined here as having a VEI of 6 or more) – that of Pinatubo in 1991. We may come to the conclusion that eruptions of this size are rare and larger eruptions even rarer. We might come to conclude, that the danger of large volcanic eruptions is generally overstated.

But let us go back to the year 1913 and ask the same question again. Then we would find that there were the eruptions of Nova Rupta in 1912, Santa Maria in 1903 and Krakatao in 1883 – each at least as big or larger than Pinatubo. And then there was Tambora in 1815 – twice as large as all eruptions mentioned so far combined. (And Tambora came 32 years after the massive Laki eruption in iceland and about 9 years after an unidentified, but probably just as large, eruption in 1806 somewhere on the globe.)

All of the sudden, the end seems to be nigh. You’d have to have been a fool in 1913 to predict just one major volcanic eruption (and a small one at that) in the next 100 years. Yet, that is what happened.

What needs to be done with respect to asteroids of this size is improving observation, developing and testing equipment to destroy them, in order to avoid a tragedy should a slightly larger asteroid be on a collision course. Larger asteroids disintegrate lower in the atmosphere, leading to stronger shockwaves, even if the energy was the same. The shockwave of a 25m equivalent asteroid would have been enough to destroy brick walls and surely caused hundreds or thousands of deaths.

Fortunately, there is a method to destroy such asteroids that is sure to work – the collision with a solid projectile of two or three tons of weight. Typically, asteroids travel at several km/s relative to earth and rockets routinely send spacecraft of such mass into the outer solar system. At the speeds involved in such a collision, the energy released at the impact is that of a small nuclear bomb – about 100tons of TNT equivalent.  Enough to reduce asteroids such as that involved in the Tunguska event (about 60m) into small enough debris to pose no significant threat.

Before deployment, the device must be tested. And after deployment, it must be ready to be launched. Depending on whether it is supposed to intercept asteroids on short notice or not, it may need to be put on military ICBMs, stationed on several sites around the globe, using storeable rocket fuels to keep them ready to launch when needed.

While this would require some amount money, it is nothing unusual at all for many countries to have rockets of sufficient size for this purpose stationed ready for launch at all times, without any actual intention to launch them. It is quite possible, it would encourage cooperation between countries around the world and we would have a decent chance to prevent some of the most destructive events that nature can throw at us from happening.

If we could prevent volcanic eruptions with as little effort as we could prevent impacts of most asteroids, there is no doubt we would do it.


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