A Different Health Risk

For the past 6 months or so, I’ve spent quite a lot of time thinking about the close approach of asteroid 2012-DA14 on 15 February 2013, and the fireball that exploded above Chelyabinsk on the same day.

DA14 had been discovered in 2012, and its return in 2013 had been expected, and telescopes were ready to record it. But the Chelyabinsk fireball was completely unexpected. It shattered windows over a wide area. Fortunately nobody was killed.

I thought that the Chelyabinsk fireball was most likely a companion rock of DA14, but NASA immediately declared that they were unconnected, and it was a freak coincidence that one rock had landed on Russia a few hours before another one passed with 30,000 km of the Earth. They said that video evidence showed that the Chelyabinsk rock had been travelling from north to south, while DA14 had come up from below the south pole of the Earth, and so they couldn’t be related.

But the Chelyabinsk fireball wasn’t the only one in the days around that time. There were also fireballs seen in the sky – but not exploding – over California and Florida and Cuba, and also Japan and Australia. There was even one spotted over Britain. But unlike Chelyabinsk, where the fireball was recorded by multiple webcams, most of these reports consisted of a single video, with little or no information about the location or direction in which the fireballs were travelling. NASA had nothing to say about these other sightings.

But it seemed that there had been an upsurge in sightings of fireballs around the world around 15 Feb 2013, and this suggested to me that maybe they all came from a cloud of rocks accompanying DA14. A number of other people thought the same, because it’s well known that asteroids – particularly ones which are just piles of rocks – can easily break apart when they pass near a large planet.

I decided to investigate further. And I was unusually well placed to do so, because about 20 years earlier, with the assistance of a physicist friend of mine, I’d built a simple computer orbital simulation model, using Newton’s laws of motion and gravitation. And I’d gone on to use this model to demonstrate an Orbital Siphon. And about 10 years ago I’d extended it to model not only the entire solar system, using data available from the NASA Horizons website, but also the Earth as it spun on its axis. So I had quite a sophisticated simulation model which would allow me to look at DA14 and any rock cloud there might have been around it.

I started out thinking that a rock cloud around DA14 would probably be spherical, with DA14 at the centre. But when I started to construct little clouds of rocks in orbit around the Sun, I soon found that, even though I gave all the rocks the same velocities and directions, my little clouds would gradually extend to form not a sphere, but a long thin train of rocks along the orbital path. Even if I constructed expanding clouds, they’d still end up as long thin trains of rocks. And that meant that instead of rocks diffusing slowly apart, there would be concentrations of rocks along the orbital path of DA14.

And then I found that, as I grew long trains of rocks fore and aft of DA14, that the Earth would regularly cross the path of this long train of rocks in February every year, which was when the two orbits intersected. And that meant that it was quite likely that there would be fireballs seen every February. And indeed there are February Fireballs that have been noticed for decades.

Gap opening just after passage of Earth

But the other thing that I found was that, when the Earth passed near to the DA14 rock train in February each year, it would punch a hole in it. As the Earth passed through the rock train, it would draw rocks inwards towards it. Some of these rocks would be speeded up in the direction of orbital motion, and some slowed. And the ones that were speeded up were thrown into a higher orbit with a longer period, and the ones that were slowed dropped into a lower orbit with a shorter period. A gap appears in the rock train, with two arms at each end. On the top right you can see a gap beginning to open just after the Earth has passed, with two arms extending from the rock train. Over the subsequent year, the gap gradually gets larger, as the two arms at either end lengthen.

widening gap

The gap keeps widening until a year later, when the Earth returns, the two arms have folded back next to the rock train to form a double rock train.

And I also found that, after the Earth had disturbed the rock train in February 2012, creating a widening gap in it with a folding arm at each end, when the Earth returned in February 2013, it passed right through one of the folds. And that meant that, in February 2013, the Earth was passing through a region of space which had twice the density of rocks in it as most years.

rock train folded double

So it’s not surprising that there were lots of fireballs seen around that time.

At bottom right the fold in the rock train is shown approaching the Earth (circular body 3) from bottom right, with DA14 (body 7) also shown. It gives an indication of how big these clouds can be.

Another thing that I noticed about these fireballs was that they were very often seen around sunrise or sunset. The California fireball was seen just after sunset. Same also with the Cuban one. And this fitted in with the DA14 rock train, which was not only approaching from below the south pole, but also from the sun side of the south pole. Because when there’s a cloud of rocks approaching the Earth, a few of them will skim over the surface of the Earth without impacting it, and these will be seen at sunrise or sunset if they’re approaching from the direction of the sun. These rocks may take several seconds to skim over the surface of the Earth, and so are much more likely to be seen than ones which come straight down and strike the Earth, and be visible for only a second or two, and which might be interpreted as lightning flashes.

So it was to be expected that, where these fireballs were seen skimming slowly over the surface or the Earth, high in the atmosphere, it would be around dawn or dusk. And that seems to be pretty much exactly what happened. I only came across one report of someone in Indonesia seeing a sudden bright flash. All the others were slow-moving fireballs high in the atmosphere, with a few of them exploding.

Given some 20 or so reports of fireballs around 15 Feb 2013, and assuming that only one fifth of these were slow-moving earth-skimmers, and that the other four-fifths impacted the Earth in the southern hemisphere oceans, I estimate that at least 100 rocks landed on the Earth during that period, with only one (Chelyabinsk) doing any damage.

I tried very hard to get one of these rocks to land on Chelyabinsk (at 9:20 am local time 15 Feb 2013, the sun rising on the horizon), but Chelyabinsk was too far north. Also, the Chelyabinsk fireball had come in at an angle of around 100° east of north, and I couldn’t achieve that either. So if the Chelyabinsk asteroid was a DA14 companion, it was a highly unusual one.

I haven’t yet managed to reproduce the Chelyabinsk fireball using a DA14 companion rock, but I may not be far away from doing so. Because I’ve found that a few rocks which come very close to the Earth in February 2012 also get very close to the Earth in February 2013, and come in over Russia on 15 Feb 2013 at an angle not too far off 100° east of north. In fact, DA14 is itself a rock which was disturbed slightly in February 2012 and moved into an orbit that brought it very close to the Earth in February 2013.

I’ve got rather exhausted of the whole thing now, after firing thousands of rocks all over the place. The current scientific consensus is that the Chelyabinsk rock was indeed a quite separate rock, disturbed from the asteroid belt between Mars and Jupiter. But my view is that it’s quite likely to have been a DA14 companion, disturbed in 2012.

And as far as I can see, there’ll never be any way of proving whether the Chelyabinsk fireball came from the asteroid belt, or was a DA14 companion, or was some other completely different wandering rock. Because there are millions and millions of rocks which could have landed on Chelyabinsk, approaching at slightly different velocities and from slightly different directions.

Does it matter? Well, if the Chelyabinsk fireball was a random event, a single rock dislodged from the asteroid belt, then it was just a bit of bad luck, and it doesn’t matter. But if the Chelyabinsk fireball (and all the other fireballs) were actually entirely predictable DA14 companions, then we ought to be a bit concerned about fireballs and impacts whenever any big rock like DA14 passes close to the Earth. DA14 had a diameter of 45 metres and a mass of 130,000 metric tons. But there are quite a few bigger rocks around. The asteroid 99942 Apophis, which has a diameter of 325 metres and an estimated mass of 40 million metric tons, is predicted to come within 30,000 km of the Earth on 13 April 2029.

The danger in this case, it seems to me, is not that Apophis will strike the Earth (as was feared when it was first discovered), but that – if it is accompanied by a rock train – there are quite likely to be some pretty spectacular fireballs and impacts in April 2029. 

For if the relative masses of DA14 and Apophis are taken as indicators of the relative sizes of the rock clouds around them, then if DA14 brought a rain of 100 rocks down onto the Earth in February 2013, the close approach of Apophis in April 2029 can be expected to be accompanied by 30,000 fireballs, and about 300 Chelyabinsk-type events.

I think that’s a little  bit more of a risk to human health than the environmental tobacco smoke that has emptied pubs and cafes. But I do not expect any attention to be paid to it.