On June 30, 1908, an object the size of an apartment building left the sky and exploded in the atmosphere above Siberia. The Tunguska event, named after a river, knocked down trees for 800 square miles. It occurred in one of Asia's least populous places and no one was killed or injured. But the Tunguska air blast remains the most powerful impact event in recorded human history, and remains puzzling because scientists do not know the origin of the object or whether it is an asteroid or a comet.
One hypothesis: it was a Beta Taurid.
The Taurids are meteor showers that occur twice a year, in late June and late October or early November. June meteors are the Betas. They attack during the day, when sunlight washes "shooting stars" that are visible during the nighttime meteor shower at the end of the year.
A new calculation by Mark Boslough, a physicist at the Los Alamos National Laboratory, shows that the pattern of tree fall in Siberia is consistent with an asteroid coming from the same area in the sky as the Taurid meteor swarm. Boslough and physicist Peter Brown of the Western University of London, Ontario, gave a presentation at the fall meeting of the American Geophysical Union in Washington this month in which they called for a special observation campaign in June to search for objects of the Tunguska class or larger Taurids
In a few years, Earth passes close to the denser clump of material in the Taurid stream – and 2019 will be that year. Scientists say it potentially holds the richest batch of material received since 1975 when seismometers left on the moon by Apollo astronauts recorded an increase in impacts during the Taurid swarm.
There are no objects in our catalog that have a significant probability of impact in the next 100 years
"If the Tunguska object was a member of a Beta Taurid stream. . . then the last week of June 2019 will be the next high-probability collision or near-accident event at Tunguska, "said the AGU presentation.
"While we are not predicting another explosion of Tunguska, an improved population of small NEOs [near-Earth objects] in Beta Taurids would increase the likelihood of another such event next year in or near Tunguska, "they concluded.
To be clear, no one is saying that June should be declared National Wear at Helmet Month. Even though there is an "enhanced" number of Tunguska class objects in the Taurid stream, the probability of one reaching Earth remains very low. Space rocks rarely come as close as our moon.
The experts have a simple explanation for this: space is great. It is much easier to lose the earth than to attain it. Of course, this can happen, and it happened in 2013, when an object smaller than the Tunguska impactor struck the atmosphere in Russia near the city of Chelyabinsk, creating a fireball and a shockwave that broke windows and injured more than 1,000 people .
Throughout recorded human history, the number of people killed by asteroid impacts is zero.
"This is not something that should keep you awake at night," Brown said.
Boslough and Brown do not know whether there is, in fact, an "improved" population of relatively large asteroids lurking in Beta Taurids. It's a conjecture.
Boslough puts the risk of the impact of the asteroid in perspective: "It is one of those very low probability risks, but potentially high consequence, which is difficult to quantify and speak. The probability of many people dying of an asteroid impact is super, super low, but not zero. "He adds:" There are so many other risks that are at greater risk. "
Astronomer Amy Mainzer, who is looking for asteroids at NASA's Jet Propulsion Laboratory and is the lead investigator for the NEOcam proposal, an infrared space telescope that would explore Earth's orbit for potentially dangerous asteroids, said scientists have identified more than 90 percent of objects large enough to cause disaster on a global scale.
But down the size scale, the census is much more spottier. Only about 30% of medium-sized objects – 140 meters (460 feet) in diameter or larger – have been seen. And she said that only 1% of the objects were found the size of the impact of the Tunguska, which was about 40 meters in diameter. She said she welcomed the idea of a special effort to look for objects during the Taurid swarm in June.
Another comforting note: the large asteroids hitherto identified do not pose a significant threat to Earth as far as anyone can discern.
This is not something that should keep you awake at night
"There are no objects in our catalog that are likely to have any significant impact in the next 100 years," said Paul Chodas, manager of the Center for the Study of Earth Objects at the Jet Propulsion Laboratory. He noted that the asteroid Bennu – currently scrutinized by NASA's Osiris-REx space probe – has a very small chance of reaching Earth in a few hundred years. "The one we're keeping an eye on," he said, but added, "There are no highly disturbing asteroids."
The geometry of the Taurid current is somewhat difficult to visualize. Imagine it as a ring around the sun, a kind of miniature asteroid belt with a highly elliptical shape, such that the orbit carries material as close to the Sun as the first planet, Mercury, but also beyond of Earth's orbit.
This ring of material is approximately but not exactly in the same plane as the Earth's orbit. This means that the Earth crosses the Taurid River twice a year. The crossing of June crosses the Taurid material that moves away from the sun, and the crossing of October crosses the material traveling towards the sun. That is why you can see the October Taurids as they hit Earth's atmosphere. June Taurids are washed by sunlight, but can be seen by radar.
Boslough and Brown are suggesting that the secret to finding large objects among Beta Taurids is to look the other way – to the night sky where the material is moving away from Earth. It would not create shooting stars, of course – this is a phenomenon of meteors hitting the atmosphere – but any large object could be seen with telescopes. As these large space rocks move away from the Earth, they focus on "vanishing point" geometry, a kind of "ideal point" in the night sky, Boslough said.
If they are there, that's it.