Blogs / Bad Astronomy

In my book Death from the Skies!, I talk about the many ways our galactic neighborhood can wipe out life on Earth: nearby supernovae, magnetar hissy fits, episodic oscillations of the Sun carrying us into a stream of dangerous intergalactic cosmic rays, and more.

Given that litany of disastrous scenarios, you wish to breathe a momentary sigh of relief that perhaps one of these things can be scratched off the list: the Earth plowing into the galaxy’s spiral arms.

The Milky Way is a disk galaxy, a flattened circle of stars, gas, and dust. The spiral arms are like traffic jams of material; gravitational speed bumps where gas can pile up and form stars. There aren’t that many more stars in the arms than between them, it’s just where the biggest and brightest tend to be, so they light the place up.

But there are more and denser gas clouds in the arms on average, and every few hundred million years the Sun drifts into and out of these regions. It’s been supposed that wandering into a nebula like this can potentially affect the Earth’s environment. If the dust is dense enough it can block sunlight, cooling the Earth, causing an ice age.

But a new study shows this may not be the case. Universe Today has the details, but basically there appears to be a 140 million year period to changes in Earth’s climate, which was suspiciously close to the period between encounters with our galaxy’s arms. The new study — using actual data of how dense clouds of gas and dust orbit the Milky Way to calculate when our Sun moves into the spiral arms — finds this period does not line up with any galactic goings-on.

Does that mean we’re safe? Well, no. First, all those other impending disasters are still out there, though of course the good news is that they are incredibly rare and basically won’t happen for millions of years (in other words, don’t fret). Second, the period of climate change is still apparently real, it’s just that it’s not tied to the Sun’s orbit around the center of the Milky Way. If it is real, then something must be causing it. We’ve taken one cause off the list, but what else are we missing?

To be honest, I won’t be rushing to edit that chapter in my book just yet. This study looks good, but I’ll wait and see what other scientists say. With another few dozen million years to go, I have plenty of time.

If you can’t take a bloody nose, go home and crawl under your bed.
It’s not safe out here. It’s wondrous,
with treasures to satiate desires both subtle and gross…
but it’s not for the timid.
-Q

In my book, Death from the Skies!, I don’t spend much time discussing magnetars. Although terrifying — able to generate truly mind-numbing outbursts which I’ll describe in a moment — they are simply too rare and too far away to be much of a threat.

Yeah, well, I might’ve been wrong. A little wrong, I mean; there’s no reason to panic. Life on Earth won’t be snuffed out by some rogue magnetar blasting away our atmosphere or anything like that. But one of my main premises for feeling completely safe has been eroded a bit, and to be fair I should talk about it.

Magnetars are neutron stars, superdense balls of tightly packed neutrons left over from the collapsed core of a massive star that’s gone supernova. Neutron stars have about the mass of the Sun but are only a few kilometers across, making them fantastically dense and giving them surface gravities that can be billions of times the force you feel standing on the Earth. They also can posses magnetic fields literally trillions of times stronger than the Earth’s. And in some cases, young neutron stars can be even more powerful: their field strength might be a quadrillion (1,000,000,000,000,000) times the Earth’s! These beasts, called magnetars, probably lose that field strength rapidly, decaying in only a few thousand years. That makes them rare on a galactic scale.

Still, several are known to exist. And they can have a nasty, nasty temper.

See, the magnetic field is coupled to the crust of the neutron star. The crust is extremely rigid and under vast pressure from the gravity of the star. If the crust cracks — a starquake, if you will — the energy released makes the strongest earthquake ever recorded on our planet look like a friendly pat on the back. I once calculated the strength of such a starquake, and it would register as magnitude 32 on the Richter scale. This ultraviolent blast shakes the magnetic field of the star, which in turn reacts by slamming around subatomic particles… the bottom line is that such an event can trigger a phenomenal release of X-ray energy from the star. And by "phenomenal" I mean "pants-wetting terrifying".

In December 2004, the magnetar SGR 1806-20 underwent such a starquake. In one-tenth of a second the subsequent blast released something like 2 times 10⁴⁶ ergs of energy — equal to about 50 trillion times the Sun’s output during that same period.

Holy crap.

This star sits about 50,000 light years from the Earth: literally halfway across the Milky Way galaxy from us. Yet, even from that forbidding distance, this titanic event was able to physically affect the Earth. It compressed our magnetic field and partially ionized our atmosphere, causing it to puff up measurably.

Mind you, it was 500 quadrillion kilometers (300 quadrillion miles) from us at the time.

So you can see why these things are a bit unnerving. But really, this one is so far away! Sure, it can hurt us, but at that distance really all it can do is what it did; we don’t expect it can have a bigger event, so we’re safe enough. Moreover, these objects are so bright in X-rays that we think we’ve found all the really big bruisers in the Galaxy. If one were closer to us, there’s no way to hide it. We’d see it.

Yeah, about that…

Astronomers have announced they found a new magnetar, named SGR 0501+4516, and it’s only 15,000 light years away. It turns out to be dark most of the time, emitting very little energy, which is how it escaped detection. But it had an outburst last year that lasted four months, allowing scientists time detect it and to get a good long look at it. This event was far less violent than the one from SGR 1806 in 2004, but still nothing to sneeze at.

Is it capable of an SGR 1806-like event? Probably not — that was an extraordinary event — and I certainly hope not! At 1/3 the distance, the effects on Earth would be nine times as strong. That could damage satellites and possibly even cause some effects on Earth itself — probably nothing that would be too big a deal, but still. Yikes.

The thing is, in Death from the Skies!, I said we’re safe from these things because they’re far away, and it’s not possible to hide any closer to us. Yet here is this one, three times closer than SGR 1806. It makes me wonder if there are any closer still. If one were, say, 5000 light years away and had a blast like the one in 2004, the effects would be 100 times larger! There could be serious satellite damage, and possibly even blackouts on Earth due to electric currents induced in our power grid.

Let me be clear: I seriously doubt there’s anything that close to us. This new one at 15,000 light years is something of a fluke, and it’s entirely possible it’s not capable of the same kind of explosive event as its more distant cousin. The odds of one being even closer are pretty small, so I’m not too concerned about it. If I were, believe me, I’d let you know!

The point here is that we have to be careful when we talk in absolutes, and it’s always good to question assumptions. If there’s one thing we know for sure about the Universe, it’s that it’s capable of some pretty good surprises, and not all of them need be the happy fun kind. We’re almost certainly safe from this particular threat… but maybe a little kick in the complacency isn’t always such a bad thing.

Image credit: NASA.

A 14 year old German boy is claiming he was hit by a meteorite. If true, this is quite a story!

Let me be clear to start: it’s entirely possible that this story is in fact real, and the boy was struck by a meteorite. The odds of it happening are very low, but not zero (a woman in Sylacauga, Alabama was hit in 1954), and while it’s good to be skeptical of things like this, there’s no reason to automatically assume it’s baloney.

However, the way the story is reported almost certainly has some of its facts wrong.

First, the headline: "14-year-old hit by 30,000 mph space meteorite". Bzzzzt! Wrong! If it had been moving that fast a direct hit would’ve killed him. That speed is about ten times faster than a rifle bullet, and had it actually hit him at that velocity a rock the size of a pea would’ve torn a hole through him the size of a basketball. Now, it’s possible the meteorite simply barely grazed him, but the article isn’t clear.

Second, there’s no way it was moving that fast to begin with. Meteoroids — the solid bit of rock, iron, ice, or whatever — move very rapidly in space relative to the Earth, but decelerate savagely as they ram through our atmosphere. Still 100 kilometers above the Earth’s surface, a meteoroid that size would slow within a few seconds from hypersonic to subsonic speeds, then basically fall the rest of the way to the ground. It would be moving at maybe 200 kph when it hit the ground, not 50,000 kph as claimed in the article.

Then things get really confusing. The article claims the boy was hit first, then the object hit the ground, carving out a one-foot crater. I’m having a hard time with that: if it hit the ground hard enough to blast a small crater, then it should’ve done a whole lot more damage to the boy than cause a three-inch scar. I suspect that, if we take the crater and all that at face value, it hit the ground first and then he was hit by shrapnel.

The article says this was confirmed to be an actual meteorite:

Ansgar Kortem, director of Germany’s Walter Hohmann Observatory, said: “It’s a real meteorite, therefore it is very valuable to collectors and scientists.”

However, my friend the Dutch science writer Govert Schilling talked to Kortem who is claiming he never saw the meteorite and was misquoted. Interesting. While this doesn’t negate the story, it does cast some doubt on it.

The boy says the piece was "magnetic", but I suspect he means attracted by magnet, since that’s a common way to say it. That means it’s iron, which is common enough in meteorites… but also in misidentified terrestrial objects, as well. Still, it’s entirely realistic to think this may have been a real meteorite.

So let’s assume this is all real. How can we make sense of this? I have an idea that fits all this.

What I suspect may have happened is that a larger meteoroid, maybe a meter or so across, came screaming into our atmosphere and exploded. This does happen: the fierce ram pressure of it moving through the air compresses the object and breaks it up, and then each piece of that breaks up from the pressure, and so on, with each breakup dumping energy into the air. At some point this happens so fast it’s essentially an explosion, and the object gets blasted apart.

In this case then the boy would’ve been hit by a smaller piece of the meteoroid moving much faster than the usual few hundred kph of terminal velocity, because it would’ve been accelerated by the explosion. Again, it’s likely what actually hit him was shrapnel from the ground, and not the meteorite itself (unless it grazed him). The article doesn’t say if they found the meteorite in the crater or not, but assuming they did my idea makes a lot more sense.

But the story is maddeningly light in important details! How soon after the flash of light was the boy hit? What made the bang, the actual impact? Or did he hear it seconds before the impact? He says the flash and bang were after he felt the sting in his hand, but I wonder. Eyewitness reports are notoriously unreliable. I wish we had other witnesses to this event! It says this happened on his way to school; I would expect other kids would’ve seen this as well!

I have received a lot of emails and Twitter notes about this, and a lot of folks doubt the story as a whole, and are asking about parts of it. So to be brief my take is this: the story is plausible that this boy was the victim of a meteorite impact. He may have been grazed by one, or hit by shrapnel when it hit the ground. This could have been a piece of a larger object that exploded in the air, heating up and propelling lots of smaller pieces, one of which caused the event. If this happened close enough to the ground the explosion could have knocked him down as he claims. But the lack of details in the story — like other eyewitnesses — makes the actual event difficult to pin down. Hopefully we’ll get more details soon.

Over at Cosmic Variance, John Conway ponders the idea that a meteor might have taken down the Air France flight that crashed over the Atlantic the other day.

John’s a physicist, so he goes through the math. You have to make some assumptions, but most of them look solid to me, or at least not crazy. In the end, the odds of any one flight getting hit by a rock substantial enough to do catastrophic damage to a plane is extremely low. That should be obvious, because if the odds were high then we’d see it happen a lot!

But over time, the odds of one flight getting hit are just high enough to make a single disaster in the past 20 years just within the realm of possibility.

Still, I wonder. When it comes to meteoroids (the solid body that forms the meteor), smaller rocks are more common than larger ones. So for every rock that hits a plane that’s large enough to take it down, we should see lots of damage from smaller ones. Yet I have never heard of such damage being reported (and I think it would be huge news if it did, but to be honest I have not done the research).

But I continue to wonder. There is another complication I think people have forgotten. Airplanes fly at an altitude of roughly 10 km. Small meteoroids move at transsonic speeds when they enter our atmosphere, but slow rapidly, and become subsonic in seconds while they’re still about 100 km up. Once they’ve slowed they are no longer ballistic, and really just fall the rest of the way to the ground. By the time they’re at the same altitude as planes, they may only be moving a few hundred km/hr at most.

That changes the physics a lot. Why? Because of relative areas and speeds.

To calculate the odds of a plane getting hit, you need to know how many meteors burn up over the Earth per day, and what percentage of the Earth’s surface is covered by airplanes. If you have millions of airplanes in the air, the odds of them getting hit are pretty high, but if you only have one, it’s a pretty small target.

But that assumes the plane occupies a couple of hundred square meters of area; that is, that’s the area of an airplane as seen from above.

Imagine you are a rock moving at 25,000 km/hr. To you, the plane is essentially standing still, and the amount of area you see it occupy is really just its area as seen from above. In that case, the assumption is correct.

But now let’s imagine you have zero motion, and so you’re motionless, hovering in the sky. In that case, the plane hits you! You don’t see the plane from above at all, you see it coming head-on, and so it occupies less area, only a few dozen square meters.

That latter case is the one that is more physically realistic, because most meteoroids will be falling relatively slowly when they are in the air lanes. That means that the statistical odds of a plane getting hit by a meteoroid (or really the other way around) are far smaller, maybe by a factor of ten or more.

I suspect that may be why we have never definitively seen an airplane taken down by a meteor. Once you realize the rocks are moving slowly, planes are a lot harder to hit. And that might explain why we don’t see much damage even from smaller rocks. It really is a rare event.

And I should add that it’s incredibly unlikely in any case that this was the fate of Air France 447. We know it was flying into heavy thunderstorms, and in this case when you hear hoofbeats you think horses, not zebras. When you have such an obvious culprit, reaching for an incredibly rare one isn’t terribly parsimonious.

Still, as John points out, it’s really just a matter of time before a plane is hit. But because of relative speeds and cross sections, I suspect that time period is longer than most people think.

Airplane image from Marina Avila’s Flickr stream.

I did an interview chock full o’ DEATH with fine folks from the Cosmic Tea Party podcast, talking over all manners of cosmic catastrophes. I’m unusually lucid in this particular interview, perhaps because I love Australians so much. Besides Minties, Australians really are the best thing about Australia itself.

Well, I guess die Katze is aus dem Sack jetzt. My book Death from the Skies! will be coming out in Germany in January under the title Tod aus dem All (Death from Space). I’m really happy about this (marred only by the lack of an exclamation point in the title), because I think the book will have broad appeal in Europe; there are lots of space enthusiasts there and I get email from them all the time.

It’ll be coming out in at least one other language soon too, but I won’t give that secret away just yet. And since so many people (well, three of you) have asked about a paperback version, I’ll say that one will be coming out, but not for a while.

For now, you can pre-order Tod aus dem All from Amazon.de. Vielen Dank!

I was interviewed by friend, pro radio guy, and fellow skeptic Paul Harris for KIRO radio in Seattle on Thursday. We talked Jenny McCarthy, Oprah, Star Trek, Hubble, and the upcoming TAMs. Paul posted the audio online, or you can download the MP3 directly.