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SYFY WIRE Bad Astronomy

Is Earth being bombarded by teeny ultra-high-velocity meteors moving at near lightspeed?

By Phil Plait
A good idea, but I suspect Photoshop. Credit: Not an Exact Science Show, used by permission

A paper just came out with an interesting thesis: What would happen if the Earth were hit by a tiny meteoroid moving at near the speed of light? And by tiny, they mean a millimeter (like a grain of sand) up to 10 centimeters in size. That's because… it's possible hypervelocity meteoroids like this actually exist.

No one's ever detected one, let me assure you. But physically, it's not a completely ridiculous idea. As they point to in the paper, tiny micron-sized dust grains can be accelerated to relativistic speeds (that is, moving close enough to the speed of light that the theory of General Relativity becomes important) in supernova explosions; the fiercely intense light of an exploding star is sufficient to kick them hard enough to get them moving close to science fiction-like speeds.

Extrapolating this up to macroscopic sizes, they speculate that, given how often supernovae go off in the galaxy, tiny meteors moving at very nearly the speed of light could be hitting the Earth on the order of once per month. If true, they could be detectable via various means, including infrasound microphones and an array of light detectors spread across the Earth.

Could this be actually happening? My guess is probably not — which is to say, I'm highly skeptical — but it's not impossible. Put another way, I'd bet a lot against it, but I wouldn't bet that it definitely isn't happening.

Assuming they have a case — and I'll get back to that — what would happen? They find that this material would ram through our air and decelerate violently similar to the way a normal meteor does, but with very different effects. For one, as a normal meteor plows through the air, the air behind it cools rapidly compared to how quickly the meteor moves, so you typically see a glowing point moving through the sky. But a relativistic meteor moves so rapidly that the air doesn't have time to cool down before the meteor slows, generating what looks like a cylinder of plasma (ionized gas) that then cools all at once.

This has several effects, but the biggest is that it creates a shock wave so intense the air glows all along the meteoroid's path. This happens extremely rapidly, making a flash of light that lasts for just one-tenth of a millisecond.

They calculate how bright these relativistic meteors would be, and find that they would be so bright that they could be easily detectable by small detectors just one square centimeter in size from a distance of 1000 km. They then say that an array of about 600 of these spread out across the planet could detect a majority of these meteors. Not only that, but infrasound microphones (used to detect blasts from volcanoes and nuclear weapons) could also detect them.

OK then, so if these can be made in supernovae and if they can be made in sufficient quantities to be significant and if they can be accelerated to tens of thousands of kilometers per second and if they can travel to Earth and if they hit us… well then maybe we can detect them.

Stated that way, my skepticism is a little more clear.

A meteor flashes over the ALMA observatory in Chile. The different colors correspond to different elements vaporizing and glowing at characteristic wavelengths. Credit: ESO / C. Malin

Let me be careful here: I think this is a fun idea, and worth doing the theoretical work for! I'm glad they thought of this, and worked out some of the physics and math to see what the effects are. But I also think that there are some problems here I'd like to see worked out more carefully.

For example, the idea of generating relativistic meteoroids* in supernovae in the first place is problematic. The examples they use to bolster this claim are interesting, but all rely on different physics. For example, they cite that atoms of iron-60 have been found in seabed material dating back a couple of million years. This is true; the radioactive element decays pretty rapidly, and is only known to be created in supernovae explosions. This means in turn the supernova that created them must have been relatively nearby, and is likely to have been from a young loose cluster of stars a few hundred light years away.

But these atoms are tiny, and are accelerated literally by the blast itself, a different proposition than accelerating something macroscopic by light pressure. They also extrapolate from micro- to macroscopic saying that denser regions of supernova debris are seen where larger material can be made, but it's a different thing to get a grain of dust up to high speeds versus something even the size of a grain of sand — which is a thousand times bigger than a dust grain (so it has a billion times the mass and volume). They simply assume it's possible and move on.

Getting here is another issue. There's a rule of thumb in physics that something moving at high speeds through some material (say air, or the dust between stars) will slow significantly when it comes into contact with a total mass of material equal to about its own mass. There is very little material between stars — about one atom per cubic centimeter of space — so something the size we're talking here could travel the length of the galaxy without slowing significantly. They then cite that supernovae in the galactic center would be a good breeding ground for these relativistic bullets.

But the material near the center of the galaxy is much thicker than between stars on average. Densities can be more like 10,000 times interstellar space there, or even millions of times higher in dense molecular clouds of gas and. So these bullets can't travel nearly as far through that stuff. I'm not saying it's not possible, but I am saying that assuming an overall galactic density of one atom per cc seems overly simplistic to me.

Another issue I have is how bright these will be. They do some calculations and show that a 1 mm relativistic meteor would generate about a billion photons of light in a one-tenth millisecond interval that would reach a 1 cm detector 100 km away. That's pretty bright, even though it's a short flash! A bright star, like say Vega, generates very roughly a million photons per second that hit your eye. So, for a very short time, even a small 1 mm bullet would be 1,000 times brighter than the one of the brightest stars in the sky. An object a centimeter wide (the size of a small grape or a six-sided die) would be 10,000 times brighter yet (10 million times brighter than Vega), and one 10 centimeters wide would be a thousand times brighter than that (10 billion times brighter than Vega). That's literally as bright as the Sun!

I think that would be obvious. A camera flash lasts for about a millisecond and is far dimmer than the Sun, but is still bright enough to be blinding. If something like that came in and burned up in our atmosphere I'd expect someone would have noticed by now. To be fair, that's on the high end of their scale for size so it would be correspondingly rare. But even the small ones the size of BBs would greatly outshine Venus for a short length of time.

Also, they don't talk about after-effects, like persistent trains, where rocky material blown off a meteor can glow brightly for many minutes. My guess is that this would be an incredibly obvious phenomenon from a relativistic meteor, but to my knowledge has never been reported.

As an afterthought, I wonder what would happen is such a thing were to hit an airless body like the Moon? Would the crater generated be distinguishable from one made by a more normal impact? That sort of physics is extremely difficult to work out, but it would be an interesting thing to investigate.

Again, I want to say that I like this type of out-of-the-box thinking, and even encourage it! If nothing else, it's fun to play with numbers and see what's possible and what doesn't make sense. This is borderline for me, at least until I see a more detailed study. I wouldn't want to invest in a global relativistic meteor detection network with just order-of-magnitude numbers… unless it can be shown to have other benefits. And even then, a paper like this should get into the hands of meteoriticists — scientists who study meteors — to see what other effects (like persistent trains) there might be. Any such folks who read this blog: Please take a look! I'd love to hear your opinions.

Until then, I'll file this under "Cool idea, sure, but I'll take it with a subrelativistic grain of salt."

*Meteoroids are the solid particles that, when they hit our atmosphere, become meteors and, if they hit the ground, become meteorites. This video describes the difference.