This summer, astronomers have an opportunity to support or reject an idea that’s a little unsettling: The risk of impacts from 10-100 meter sized objects from space may be higher than we previously thought. The risk currently is based on random encounters with these objects, but it’s possible some might be affiliated with an annual meteor shower — meaning repeated encounters that increase the risk.
[Before I get going here, I know some people understandably get freaked out thinking we might get hit by a comet or asteroid. I address this issue frankly here. Even if this hypothesis is correct, let me repeat that the chance of an impact may be higher than we thought, but still pretty low. As usual, I’d say it’s a matter of concern — i.e. we should be thinking about it seriously — but not necessarily worry — i.e. don’t panic. As you’ll see, we just don’t know enough yet, but we will very soon.]
The sources of meteor showers are comets: Big lumps of ice and rock that move around the Sun on elliptical orbits. When warmed by sunlight, the ice in a comet turns into a gas, expanding around it and forming the fuzzy head and long tail. But mixed in with that ice are countless grains of dust and pebbles, trailing behind the comet in its orbit. If the Earth happens to pass through that debris cloud, some will burn up in our atmosphere, creating meteors.
Most of the meteors we see are from rocky bits the size of a grain of sand or smaller; although teeny, they’re moving relative to us at dozens of kilometers per second, so when they pass through our atmosphere that velocity is turned into energy — light and heat — and they get very bright.
But… we also know that these rocky bits don’t have to be that small. The vast majority are, but some can be bigger. The question is, how big can these rocks get and how many are there?
This is actually a realistic concern. You may have heard of the Tunguska Impact: On June 30, 1908, what may very well have been a 45-meter wide fragment of a comet slammed into the Earth’s atmosphere over Siberia, exploding due to atmospheric pressure while still a dozen kilometers above the ground. The resulting blast flattened trees for hundreds of square kilometers around it, the equivalent of a 5-megaton detonation.
It’s possible that the Tunguska impactor was actually part of the Beta Taurid meteor shower, the debris left behind by comet Encke as it orbits the Sun. The Earth’s orbit intersects the comet’s orbit twice; once in October, creating the Taurid meteor shower, and again during June and July, creating the Beta Taurid shower.
Tunguska happened in late June. Hmmmm. Also, analyzing the blast pattern, astronomers could trace the trajectory of the impactor back up into space, and it came from the same direction in the sky as the Beta Taurids.
There’s more to this as well. Normally, the debris from the comet would drift away over time, dispersing. But there’s a hypothesis that the debris left by comet Encke as it orbits the Sun may be concentrated by the gravitational influence of Jupiter, which keeps it somewhat more compact. This idea is called the Taurid Swarm.
There’s some evidence for it. In 1975, when the Earth passed close to the center of the proposed swarm, seismographs on the Moon recorded an increase in quakes from impacts. In the 2015 encounter, which again was close to the center of the swarm, a large increase in very bright meteors (called fireballs, usually from objects a few centimeters to a meter or so in size burning up high above Earth’s surface) was reported. This was predicted by the Taurid Swarm hypothesis, so those observations support the idea*.
While this is all unproven, it’s somewhat worrisome. If the Tunguska impactor was part of the Beta Taurids, and they reoccur every year, is it possible another Tunguska-sized chunk of comet is out there with our name on it?
I’ll remind you that space is big and the Earth small, so the odds of such an impact are really low — we expect to see them on the order of once every thousand years or so. It would be nice, however, to survey the Beta Taurids and see what’s actually there.
And now, for the first time, we may be able to do that.
A team of astronomers has calculated that Earth will pass relatively close to the center of the swarm of particles from comet Encke during June and July of this year. They also determined that given the geometry, the phase of the Moon, and other factors, there are two observational windows where conditions are good to look for bigger chunks — 100 meters in size or so — among the dust. Those dates are July 5 – 11, and July 21 – August 10. The earlier dates are best for southern observers, but astronomers anywhere on the planet can look during the second window.
In both cases, the objects are faint, and will likely need professional-class observatories to do the search; the brightest of these objects (if they exist at all) will only be magnitude 22 (the faintest star you can see with your naked eye is a couple of million times brighter) — faint, but well within reach of bigger ‘telescopes. Also, the Beta Taurids come at us from the direction of the Sun, making them extremely hard to observe until they get close and pass us on their way out. Because of this timing is important, but it also means regular folks won’t be able to go out and see a meteor shower from the smaller bits; anything like that would occur during broad daylight, which has some obvious drawbacks.
I urge professional astronomers (and highly qualified amateurs) — if they haven’t already — to either try to get these observations themselves or spread the idea to others who might be able to. The idea that Tunguska might be just one of many such objects in the Taurid swarm is circumstantial, but critical to test. A survey of the sky this summer could put this idea to rest once and for all, or it could show us that we have a bigger concern than random impacts.
Any chance to understand the population of potential impactors is one we should take. It’s not like the movies where you just lob a nuke at the incoming asteroid/comet and then get to celebrate in slow motion while Aerosmith plays. The real science shows us that the kind of impactor matters, the orbit it’s on matters, the geometry matters. If we want to take this threat seriously — and make no mistake, I and many other astronomers most certainly do — then this is a fantastic opportunity we don’t want to miss.
[My thanks to Mark Boslough, who did the calculations to determine the origin of the Tunguska impactor, for bringing this to my attention, and to David Clark, who has been running point on all this (including making the orbital simulation videos above).]
*I’ll note that some folks have extrapolated this idea to try to explain lots of other events in history, like saying the disappearance of the people of the Clovis culture in North America some 13,000 years ago was due to an impact from a cometary fragment. I find these ideas pretty thin; the evidence for them is quite low compared to some of the claims made. If you hear catastrophist claims like those I suggest you take them with a 10 – 100 meter wide grain of salt.