It was a quiet summer morning over the Tunguska river in Siberia on June 30, 1908. Right until an enormous meteor exploded with the energy of a large thermonuclear weapon, incinerating and leveling thousands of square kilometers of forest. An unlucky jackpot, certainly, but the end of June may have better odds than usual.

It was a long time ago over one of the most sparsely populated places on the planet. But imagine such an event over a populated area, a large city, today. The damage and human toll would be much, much higher than the Hiroshima and Nagasaki nuclear blasts. Fortunately this kind of event is rare, but in late June the odds for an impacts are higher – enough for astronomers to be on the lookout for incoming space rocks this year. So what is going on?

“I was looking down, and I thought someone suddenly pointed their car headlights towards me, then I realized it was a fireball in the sky!” Brilliant Taurid meteor in October 2015. Photo: Marko Korošec.

Enter the Taurid meteor shower. It is a fairly typical, well-behaved meteor shower. We encounter the Taurids every year and they produce a moderate meteor shower in November. We also encounter them in June, when they come at us from the direction of the Sun – the daytime Beta Taurid meteor shower. And then there is the not so typical part – the Taurid resonant swarm. It is cloud of larger than average Taurid space rocks (meteoroids), shepherded in their orbits by Jupiter. For every 7 orbits of the Sun that the rocks make, Jupiter makes two – it is called a 7:2 orbital resonance. This orbital dance keeps the large rocks in a relatively compact cloud – that we encounter every once in a while. Like in late June 2019.

Why is the Taurid swarm so important? Because there are some seriously big space rocks in there. In 1975 we encountered the swarm in June. During the exact period of the encounter, seismographs on the Moon left by the Apollo mission astronauts, detected a flurry of seismic activity, most likely caused by large Taurid meteoroids impacting the Lunar surface. The last big encounter with the Taurid swarm was in 2015. In the last days of October and first two weeks of November, bright fireballs from the Taurid stream were noted across the world, many lighting up the sky brighter than the Moon!



Huge Taurid fireball in the skies over Thailand on November 2, 2015 – during the peak of the 2015 Taurid swarm encounter. Caused by a Taurid meteoroid several meters in diameter. 00:10 into the video.

There were some big rocks in the 2015 Taurid swarm. We were under the sky every night for two weeks in the first two weeks of November 2015. There was a steady trickle of brilliant Taurid meteors, caused by pebble to basketball sized rocks. Indeed, the European fireball network captured nearly 200 Taurid fireballs, and the largest was produced by a Taurid meteoroid about 1 m in diameter – about the size of a large washing machine. An even more massive Taurid, about 3-4 m in diameter, disintegrated over the Pacific ocean! But big rocks in the Taurid swarm do not end there. Several larger rocks, proper asteroids, have also been found to be embedded in the swarm: objects 2015 TX24 and 2005 UR are both several hundreds of meters across. These asteroids are super difficult to detect. They are about as dark as charcoal and can only be spotted when they are very close to the Earth.



Rocks this big are a serious threat. The devastating Tunguska blast was caused by an asteroid approximately 50 m in diameter. Much smaller than 2015 TX24 and 2005 UR. A Tunguska-sized blast over an urban area would be unimaginably bad, but an asteroid several hundreds of meters across would be much worse. The blast energy would be in the range of hundreds to thousands of megatons TNT. Unlike the Tunguska blast, they would nearly certainly penetrate through the atmosphere and hit the ground. An asteroid 300 m in diameter would explode with 20x more energy than the biggest nuclear weapon ever detonated. It would produce a crater about 5 km in diameter and cause regional devastation. It would likely kick up enough aerosols to have a short-term effect on climate.

Barringer meteor crater in Arizona: ~1200 m in diameter. Formed about 50 000 years ago by a 50 m asteroid. There are likely many asteroids within the Taurid stream that are significantly larger. Photo: NASA Earth Observatory.

Fortunately large impacts are extremely rare, with centuries and millenia between major events. Knowing as much about the Taurid resonant swarm and looking for the largest objects within it is our best bet for keeping safe. This is why this year astronomers will put a lot of effort into looking for these objects. If a big rock with our name on it was found well in advance of a possible impact, there would be ways to redirect it. From blowing it up, Armaggedon-style, to quirkier and probably better ways of redirecting it – like attaching a small rocket to it, or even painting half of white.

In the mean time, it is best to keep in mind that for every killer asteroid within the Taurid swarm there are millions of pebbles that produce a beautiful, harmless celestial fireworks show. Who knows, maybe you will chance into a bright, colorful Beta Taurid meteor in the dawn skies in the next two weeks. Don’t forget your wish!

Cover image: José Antonio Peñas/Sinc.