Next month, astronomers will scan the Beta Taurid meteor shower in search of asteroids that might someday threaten a potentially catastrophic collision with Earth.
There are likely two key sources of space rocks that are capable of posing such a dire threat to our planet. First are the NEOs (near Earth objects), such as asteroids and comets that occasionally come close to our planet and whose orbits can even cross paths with ours. Astronomers have already discovered and determined the orbits of tens of thousands of these objects, but there's never a guarantee we can find them before they're already uncomfortably close to Earth. It would be nice to have advance knowledge of such objects well before they get precariously close to us, and astronomers are working on that task.
But there may be a second source of potential collisions, and one that's more easily predicted and monitored. A new study, soon to be published in the Monthly Notices of the Royal Astronomical Society of Canada, suggests that the Beta Taurid meteor stream may camouflage dangerous space rocks.
The Beta Taurid is a stream of meteoroids that regularly produces a modest display of meteors each year in late June, and the idea is that embedded within it may be some much larger chunks that might possibly pose a danger to Earth. Canadian astronomers David Clark, Paul Wiegert and Peter Brown at Western University speculate that there could be a connection between the Beta Taurid meteor stream and the massive airburst that occurred over Tunguska, Russia, in June 1908.
Most meteor showers are generated by comets, cosmic litterbugs that are primarily composed of frozen gases; as they approach the sun, they warm up and are made to glow by the sun's light. As the gases warm and expand, the solar wind blows the gases, as well as innumerable grains of dust and fine particles, off the comet's nucleus to form the appendage popularly referred to as the tail.
These tiny bits are left behind by the comet, traveling along the comet's orbit and leaving in the comet's wake a "river of rubble." If Earth happens to pass through that rubble river, some of those tiny comet bits will burn up high in our atmosphere as meteors.
Even during our most prolific annual meteor displays, such as the August Perseids and the December Geminids, there is absolutely no chance of any meteor shower rubble making it all the way down to the ground. As the comet's leavings dash through our atmosphere at speeds ranging between 20 and 40 miles per second (32 to 64 kilometers per second), the extreme velocity is converted, within the span of a heartbeat, into light and heat energy, creating a bright incandescent streak of light in the sky. Even a meteoroid the size of a pebble or a child's marble will still be consumed dozens of miles above the Earth's surface. No worries here.
Big chunks ... big worries!
But the Beta Taurids could be different, Clark, Wiegert and Brown hypothesize. They argue that within the Beta Taurid meteor stream there could be a dense cluster of much larger meteoroids, perhaps anywhere from 100 to 300 feet (30 to 91 meters) in diameter.
Scientists believe the Tunguska explosion, which occurred on June 30, 1908, was caused by a chunk of cosmic material roughly 150 feet (46 m) across, which slammed into our atmosphere over northern Siberia and exploded about 10 miles (16 km) above the ground with the force of a 5-megaton bomb. The resultant air blast created a sonic boom to end all sonic booms, blowing down 80 million trees over an area of 830 square miles (2,150 square kilometers).
Thankfully, due to the remoteness of the location, no human fatalities were officially reported. Of course, it would have been a far different story had this episode taken place over a major metropolitan area.
Comet Encke is the culprit
Today, astronomers are beginning to accept the idea that the Tunguska event is directly related to the Beta Taurid meteor stream. This isn't a new concept: In 1978, Slovak astronomer Ľubor Kresák suggested that the cosmic projectile responsible for Tunguska was a fragment of the progenitor of the Taurids, Encke's comet. Encke's has the shortest known orbital period for a comet, taking only 3.3 years to make one complete trip around the sun.
Then, in 1992, Victor Clube, an English astrophysicist and an expert on comets and cosmology, suggested that the Beta Taurid meteor stream contains perhaps a half dozen full-size asteroids whose orbits place them squarely in the stream. Clube and his colleagues argued that the Taurids' range of orbits indicates they were all shed by a huge comet, originally 100 miles (160 km) or more across, which entered the inner solar system some 20,000 years ago. By 10,000 years ago, the comet was parched and brittle.
Encke's comet might actually be the biggest leftover chunk of that original parent comet. The Earth's orbit intersects this comet's orbit twice; once in late June, creating the Beta Taurids, and again during early November, creating the South Taurid meteor shower. Earth can periodically encounter swarms of larger particles in certain years, and 2019 is predicted to be one of those years.
While one would expect that dust and larger objects would gradually disperse with time around the orbit of a comet, Encke's comet proves to be a special case. Seven revolutions of the comet around the sun (7 x 3.3 = 23.1 years) nearly matches two revolutions of the giant planet Jupiter (2 x 11.8 = 23.7 years), a phenomenon known as resonance. As a result, astronomers believe that the dross left behind by comet Encke may be concentrated by the powerful gravitational force of Jupiter, which keeps the Taurid swarm more compact.
There is circumstantial evidence to support this hypothesis. In 1975, when Earth passed close to the center of this resonant swarm, seismographs left on the lunar surface by Apollo astronauts recorded an increase in moonquakes, apparently due to impacts from meteoroids.
And in 2015, we were again close to the center of the Taurid swarm when a large increase of fireball meteors in November was observed. In fact, high-precision cameras recorded over 100 exceptionally bright meteors, all fanning out from the constellation Taurus, and which nicely fit the orbit of the proposed Taurid resonant swarm.
Finally, after careful study of the trajectory that the Tunguska impactor had taken through the atmosphere, scientists determined that it, too, could very well have emanated from the Taurid swarm.
Best opportunity in 44 years
According to the analysis by the Western University astronomers, Earth will approach to within about 5.6 million miles (9 million km) of the center of the proposed Taurid resonant swarm in the coming weeks; the closest such encounter since 1975. The calculations also show that this will be the best viewing time of the Taurid swarm until the early 2030s.
"There has been great interest in the space community since we shared our results at the recent Planetary Defense Conference in Washington, D.C.," Clark said in a university statement. "There is strong meteoric and NEO evidence supporting the Taurid swarm and its potential existential risks but this summer brings a unique opportunity to observe and quantify these objects."
For their study, Clark, Wiegert and Brown constructed a computer simulation of asteroids 328 feet (100 meters) in diameter with orbits similar to the Taurid swarm and calculated their positions for the next 1,000 years. By analyzing each object's position and motion over time, the Canadian astronomers calculated two optimal viewing times and telescope-pointing locations for the Taurid swarm to properly investigate its overall risk potential. Those viewing times run from July 5 to 11, and July 21 to Aug. 10. The first set of viewing times favor those in the Southern Hemisphere, while the second set of times allows those north of the equator to also join the search.
But it won't be easy to spot any of these objects. Any large Taurid chunk will be extremely faint; somewhere on the order of magnitude +22. That's 2.5 million times fainter than the faintest star visible with the unaided eye. So only observatories with large telescopes and capable of making long-exposure images of the sky will have any chance of picking up on one of these potentially hazardous space rocks. Members of the Western University Meteor Physics Group plan to observe the Taurid swarm using the Canada-France-Hawaii Telescope at the University of Hawaii in August.
And if you're hoping to catch a few bright Beta Taurids streaking across the sky, that's not likely to happen because their emanation point (called the radiant) is currently aligned with the sun, so any prospective bright streaks would likely be hidden in the daytime sky.
Finally, I want to stress that the odds of Earth colliding with a 328-foot-size object is exceedingly small. Keep in mind that Earth and such a potential impactor need to arrive at the exact same point in space at precisely the same moment in time. The Tunguska explosion is considered to be a 1-in-1,000 year event, but that assumes a random distribution of events spread over time.
However, when we then consider the Taurid swarm and its hypothesized dense cluster of large objects within the Taurid meteor stream, through which Earth periodically interacts with … well, that could change the odds significantly. It also heightens the possibility of encountering a cluster of large impacts over a short period of time.
That's why astronomers are on call this summer to try to survey the Taurid swarm and see what might be lurking out there.
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Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers' Almanac and other publications, and he is also an on-camera meteorologist for Verizon FiOS1 News in New York's lower Hudson Valley. Follow us on Twitter @Spacedotcom and on Facebook.
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Joe Rao is Space.com's skywatching columnist, as well as a veteran meteorologist and eclipse chaser who also serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers' Almanac and other publications. Joe is an 8-time Emmy-nominated meteorologist who served the Putnam Valley region of New York for over 21 years. You can find him on Twitter and YouTube tracking lunar and solar eclipses, meteor showers and more. To find out Joe's latest project, visit him on Twitter.