Article written by: Rok Nežič
During the summer every year, we observe the International Asteroid Day (“Asteroid Day” for short) on 30th June. The United Nations has proclaimed it will be observed globally on that date “to raise awareness about asteroids and what can be done to protect the Earth, its families, communities, and future generations from a catastrophic event.”
While their topics certainly have some overlap, the date for the Asteroid Day was not chosen in acknowledgment of the film Armageddon (which was released on 1st July 1998), but to commemorate a much more real and to this day somewhat mysterious occurrence: the Tunguska event (which would also make a good movie title!). This summer marks the 110th anniversary of what is believed to be the largest impact event on Earth in recorded history.
Early in the morning – a little after 7am local time – in the plains of central Siberia populated only by a handful of natives and some Russian settlers, “the sky split in two and fire appeared high and wide over the forest. The split in the sky grew larger, and the entire northern side was covered with fire.” This is an account from an eyewitness who was at Vanavara trading post, some 65 km (40 miles) south of the epicentre. He went on to say that his shirt felt as if it were on fire, the ground shook, and shortly afterwards a great blast pushed him off his chair and threw him a few metres away. Soon after, a hot rushing wind came, “which left traces in the ground like pathways, and it damaged some crops. Later we saw that many windows were shattered, and in the barn, a part of the iron lock snapped.”
Anyone familiar with the details of the events in Japan in August 1945 might recognise this sequence as being extremely similar to the events close to ground zero of the nuclear explosions above Hiroshima and Nagasaki, except of course the Tunguska event – named after the nearby river called Stony Tunguska (“Podkamennaya Tunguska”, literally “Tunguska under the stones”) – took place several decades earlier. Indeed, this has led to some science-fiction theories about nuclear-powered alien spacecraft exploding over the Siberian skies. More down-to-Earth commentators note that any large explosion will exhibit similar patterns. A notable feature of nuclear explosions, but missing from the account above – because the eyewitness was too far to see it – is the razing to the ground of any buildings or trees near the epicentre, possibly combined with spreading of fires. The flattening of more than 80 million trees over 2150 square kilometres (830 square miles) in a radial pattern around the epicentre was indeed confirmed by the expeditions, which went out to explore the origin of this extraordinary event.
These local phenomena – while crucial for resolving the mystery – were not the only indications that something extraordinary had happened at the end of June 1908. Seismographs across Europe and Asia have detected a tremor of the ground, the aftereffects of the shockwave (fluctuations in atmospheric pressure) were detected as far away as London, and over the next several days, a strange glow was seen in the night skies in Europe and Asia, apparently brightly enough to read newspapers at night in some places.
In the decades that followed, most scientists have agreed on two most likely explanations for what had occurred at Tunguska. It is quite clear that an object from outer space entered the atmosphere over central Siberia. Because no crater was ever found on the ground, and no meteorite either, it is thought that the object disintegrated (exploded) before reaching the surface. The blast area and other effects point to that conclusion, too. The two options, then, are what the object itself might have been. Some argue that it was a small asteroid, others that it must have been a small comet (or a comet fragment). The glowing skies – suggesting high levels of water vapour in the atmosphere – are more likely to occur due to a comet, but some investigations of the resin of the impacted trees noted presence of material common in asteroids and rare in comets. There are numerous other indications, too, which go in favour of one hypothesis and against the other. 110 years later, the matter still hasn’t been resolved. If it was an asteroid, it was probably about 36 m (120 ft) in size and 100,000 metric tons (220 million pounds) in weight. A comet – being less dense – would have to be slightly larger.
Whatever the object from space may have been, the explosion was extremely strong, although not as strong as first thought. Early estimates from the 1940s and 50s deduced the energy by scaling the effects of nuclear explosions to those observed at the Tunguska event and range from 10 to 30 megatons of TNT, depending on the exact height of the explosion. The upper limit is almost 2,000 times more energy than was released by the atomic bomb dropped on Hiroshima, although the largest nuclear weapon ever detonated, the Tsar Bomba, released energy equivalent to 50 megatons of TNT (which is really rather terrifying if you think about it!), exceeding even the upper limit on Tunguska event’s energy output. Later investigations have lowered that number significantly, noting that the speeding space rock would focus most of its energy in a cone-like shape towards the ground (where we can measure its effects) rather than in all directions, so the equivalent of 3-5 megatons of TNT (still 200-300 times the strength of the Hiroshima bomb) would be enough to cause the observed damage.
Central Siberia is a very remote and sparsely populated place, even for other Russians, so the first expedition to the area to investigate what might have happened was not mounted until 1927; almost 20 years after the event. The leader of this and several subsequent expeditions was a Russian mineralogist by the name of Leonid Kulik, who was surveying the nearby regions in the 1920s and heard local accounts of this strange explosion.
Kulik was also the one to collect the eyewitness testimonies from the locals, an endeavour which did not go as smoothly as he might have hoped. Don Yeomans, head of the Near-Earth Object Office at NASA’s Jet Propulsion Laboratory, explained in an interview for the 100th anniversary of the Tunguska event: “At first, the locals were reluctant to tell Kulik about the event. They believed the blast was a visitation by the god Ogdy, who had cursed the area by smashing trees and killing animals.” Apart from two unconfirmed reports there are thought to have been no human casualties caused by the Tunguska event, but hundreds of reindeer on which the local tribes depended had died in the blast.
It is chilling to think what damage such an impact might cause if it fell on a metropolitan area. Greater London (1,572 square km or 607 square miles) covers only three-quarters of the area where the trees were completely knocked down and has a population of nearly 9 million! It is lucky, then, that about 71% of the Earth is covered by water, and only about a tenth of the remaining 29% is thought to be urban land. This means we would have to be very unlucky indeed to see such an impact hit a densely populated region (this is one thing Deep Impact – the film, not the spacecraft – got right, and Armageddon did not).
This is what Asteroid Day is all about: raising awareness about potential dangers from space. It should first be noted that such events are very rare and extremely unlikely to affect us directly: an object as large as the one responsible for Tunguska event is expected to hit the Earth only once every 300 years or so, and it is definitely most likely – as it has done in 1908 – to hit an uninhabited region. But this does not mean we shouldn’t try to learn as much as possible about potentially dangerous objects if we have the knowledge and the resources to do so. And we do: scientists have been monitoring our Solar System for decades and have so far discovered around 800,000 small Solar System bodies (this includes all asteroids, comets, and trans-Neptunian objects like Pluto), about 18,500 of which have been classified as Near-Earth Objects (NEOs).
Near-Earth Objects are any objects whose orbit brings them very close to the Earth. More than 99% of them are asteroids, and the rest are comets. Since 1998 the NEOs have been systematically searched for, and the collective effort has been dubbed the Spaceguard. We have catalogued over 90% of NEOs of at least 1 km (about 0.62 mi) in size – and which could therefore cause a global catastrophe if they impacted the Earth – by 2011 already. We are now working on cataloguing all objects above 140 m (460 ft) in size, which could still cause large-scale damage.
We do occasionally spot objects coming from unexpected directions just before they pass very close to the Earth, for example a small asteroid discovered in April 2014 which passed us at 1,250,000 km (777,000 mi; about three times the distance to the Moon) in June the same year, or another discovered in 2015 just 21 days before its closest passage at 490,000 km (304,000 mi; about 25% further away than the Moon). The Chelyabinsk meteor, which exploded over the Russian skies close to the city of Chelyabinsk on 15th February 2013 and is thought to have been about 20 m in size, was not detected before its entry into the atmosphere, either. As it happens, the Chelyabinsk meteor is one of the best well-recorded, modern analogues to the Tunguska event, although it was much less destructive (0.4-0.5 megaton of TNT equivalent, as opposed to 3-5 megatons of Tunguska event), despite thousands of shattered windows, damaged buildings and many injuries – those are a result of the Chelyabinsk meteor falling over a more populated area than the Tunguska event. A different micro-asteroid – about 30 m across, proving we do have capabilities to detect even such small objects – which passed very, very close to the Earth (27,700 km or 17,200 mi) the very same day as the Chelyabinsk meteor, however, was detected a full year in advance. In any case, we are very glad to inform you that none of the currently catalogued NEOs present a threat to the Earth.
There are also a few missions being prepared that will show the viability of real-life asteroid deflection – somewhat in the style of Armageddon and Deep Impact (again: the film, not the spacecraft), except of course unmanned, because it would be quite silly to send humans out to such missions in real life. The benefit of knowing all there is to know about the NEOs becomes clear now: if we do find one which might impact the Earth in the future, we could do something about it! As the AIDA (Asteroid Impact and Deflection Assessment) mission aims to demonstrate on a harmless asteroid, just as a proof of concept, we could send a probe to the offending object and slowly but surely deflect it in its orbit. Depending on the object’s size and the amount of time we have, this can be extremely effective even using the resources we have today: a small nudge early enough would do the job, no blowing up of space rocks needed. Naturally more time would be needed to deflect a large asteroid or comet than a small one, but the larger the object, the more likely we are to detect it early, so it all works out in the end. No immediate plan for this is in place at the moment, however, so it is important that we keep supporting these efforts until viable emergency plans become a reality.
The message to take home with you is this: Tunguska event occurred 110 years ago and was very remote, very strange, and very interesting. And still not entirely explained. And the Asteroid Day – celebrated annually on the anniversary of the Tunguska event – is here to remind us that space is strange and wonderful and potentially dangerous to us on Earth as well – not very much, but just enough that we should pay close attention to our immediate surroundings. Keen and vigilant scientists and engineers, however, are not only on the lookout for such dangers, but are already devising means of preventing any potential future “Deep Impacts” or “Armageddons” so that we can all sleep more peacefully at night and enjoy the occasional meteor shower rather than worry about asteroid impacts.