A bolt out of the blue

Last February I was giving a 101 Astronomy class early in the morning. It was Friday, the 15th. Just by chance, I was talking about the minor bodies of the Solar System when we started speaking about impacts on Earth. I showed some slides of the Tunguska event and how the devastated area was similar in size to the whole Basque Country, where we live. In spite of the astonishing amount of energy delivered in the event, few personal losses were to grieve. At that point I highlighted the unlikeliness of being hit by a meteor. Big fail. After that class I opened my email and had to spend the next hour seeing recordings from a remote place in Russia, Chelyabinsk.

The facts are as following. On 15 February 2013, something entered into Earth’s atmosphere without prior notice creating a superbolide (an extremely fast and luminous meteor) in the early morning of the Russian town of Chelyabinsk (see Figure 1). The shock wave created in the atmosphere while most of the asteroid disintegrated produced injuries to about 1,500 people. Most of them suffered from cuts due to blown glasses but nobody was directly hit by the meteorite, fortunately. At least, I was right about something. The explosion was equivalent to 500 kTons of TNT. A tiny part (roughly 100 grams) of the originally 10-20m sized body was recovered from near Lake Chebarkul.

This was what we call a “one in a century” event. However, current technology allowed many people to record the bolide, most of them while driving to work. Security cameras in the medium-sized town also provide valuable data as we will see shortly. When something like this happens, everybody wants to know two things: where it came from and how likely is to happen again.

The scientific community has been making a huge effort in the last six months to analyze available data and scientific papers started to see the light recently. Let’s focus on those trying to determine the orbit of the impactor. Basically, we assume that the body was originally orbiting the Sun when it was gravitational disturbed by a close pass and then entered into orbiting the Earth and eventually fell into the surface. We first need to know where the shot came from, and then integrate backwards in time to reconstruct the original orbit. The better we determine the ballistic trajectory the more likely we are to determine the origin of the body.

A very notable paper by Proud has been published in Geophysical Research Letters magazine ¹. Instead of using images taken from the ground, radar or sound waves (which are the usual means to determine the trajectory), Proud takes advantage of the images taken by three Meteosat Second Generation satellites (MSG), which image the Earth every 15 minutes (Figure 2). By using observations recorded when the meteor was still in the high atmosphere he was able to determine the orbital elements of the body. This is a good leap forward in the field, since meteorological satellites provide a good background for future events of this type or even smaller.

Other works have used the abundant footage from people and public security systems. Zuluaga and collaborators ² have refined their initial orbital estimations by using about one hundred of such recordings. In the very best of them it is possible to determine the ballistic trajectory by analyzing the shadows cast by the bollide. Very interestingly, their results agree pretty well with those retrieved from above the atmosphere.

Zuluaga et al., as other scientists, have deduced the original orbit of the asteroid. It seems to be part of the well-known Apollo family of Near Earth Asteroids (see Figure 3). Moreover, a work which is about to be published in the Monthly Notices of the Royal Astronomical Society ³ has been able to go one step further. They designate some potential candidates to be the parents of the Chelyabinsk asteroid, most likely it could be a fragment of the asteroid 2011EO40. This kind of asteroid are commonly perturbed by close encounters with the Earth-Moon system as well as with Venus, Mars or Ceres. Carlos and Raúl de la Fuente Marcos (from the Universidad Complutense de Madrid) note in their work that should the Russian meteor have been delayed a few minutes, the impact would have happened in central Europe, a much more densely inhabited region with more potential people affected.

So it seems very likely that bodies pertaining to those families of asteroids will regularly fall into the Earth. Why we did not noticed until it was too late? We should be aware of them, even if they are only able to produce a local catastrophe. Well, the answer is that Chelyabinsk asteroid approached from the line of sight to the Sun. None of our present systems is able to look into this direction. For preventing this to happen again in the future, Dunham and collaborators ⁴ propose an observatory at the L1 Lagrangian point in the Earth-Sun system, same location as the SOHO Sun observing satellite, in order to prevent such “bolts out from the blue”. The spacecraft would be looking straight to the Earth and should give us at least a 2 days margin. Apart from the evident decrease in damage and injuries that this time would provide, Dunham et al. also point that this would also prevent the triggering of a regional or global nuclear war, given the nuclear powers in the region that could be mislead by the explosion. Even it is possible that some scientists or wealthy amateur astronomers would be interested in witnessing such an event, resulting in a science driven tourism.

In spite of being fooled by Nature and have to eat my words, I can only be captivated by the way the natural world behaves. We live under the constant threat of being hit by something out there, and we tend to forget about that. It is a good moment to wake up and use science and technology (our minds, in short) to prevent it or, at least, to minimize damage.