Fun Science Fact #24: Speed Kills.
When I was nine, my best friend tried to convince me that if you were hit on the head by a penny that had been dropped from the top of the Empire State Building, it would split you in half. When I expressed some skepticism, he insisted that he’d actually seen this happen during a family trip to the city.
“Splat,” he said. “Right in half. Guts everywhere. It was awesome.”
That’s when his sister stepped in and explained that while they had seen someone get hit on the head, it had been in Pittsburgh, not New York, and it had been a loogie spit from an overpass, not a penny dropped from the Empire State Building. She never said whether the victim split in half or not, but she did mention that she learned a bunch of new swear words, which she was more than happy to teach me.
While it’s sadly not true that a falling penny can split you in half, fast-moving objects actually can carry a surprising amount of energy, and if you stop them abruptly enough, they can release that energy in some very unpleasant ways.
The kinetic energy of a moving object is given by its mass times the square of its velocity. This leads to some crazy results as velocity gets really large. An object dropped from orbit, for instance, will come in at about seven kilometers per second. A ten kilogram object (a little heavier than a standard bowling ball) moving at this speed has about 490 megajoules to give up to whatever it happens to land on. In terms of energy equivalent, that’s a bit more than what you’d get from a hundred kilos of TNT. In other words, if you have a weapon moving at that speed, you really don’t need to bother putting a warhead on it. Just make it out of solid iron, and the kinetic energy will give you all the destructive force you need.
Unsurprisingly, this fact has not escaped the attention of military thinkers. In the late eighties, under the auspices of President Reagan’s Star Wars initiative, the US military initiated a program called Brilliant Pebbles. The basic idea behind this was that we would place several hundred of what were essentially bowling balls with little rocket motors attached to them in low earth orbit. Then, when Kim Jong Il decided to nuke Seattle, we could just de-orbit one of these guys onto the missile during the boost phase, when it’s fat and hot and slow.
This would have been a lot easier than the plan they eventually decided to go with, which was using a ground-based missile to hit the incoming warhead during re-entry, when it’s small, cold, and really, really fast. We’ve been working on that system for the past twenty-five years, and it still only works if you can get the incoming warhead to broadcast its trajectory to you. I’m sure we’ve approached Kim Jong Un about this, but I’m guessing he’s not too interested in helping us out.
So, why did Brilliant Pebbles wind up getting cancelled? Well, as it turns out, other folks around the world weren’t too happy about the idea of the US government having hundreds of invisible death nuggets hanging over their heads 24-7. They’re not pleased about our arsenal of missiles either, of course, but at least if we launch those they’ve got ninety minutes or so of lead time to duck and cover. With something like Brilliant Pebbles, death comes literally out of the blue, with zero warning, zero lead time, and zero opportunity to respond. If this system had ever become operational, we could have decapitated any government in the world at will, with almost no fear of retaliation.
If you’re the sort of person who’s currently raging against the Iran nuclear deal, this probably sounds like a pretty good deal to you. Fortunately, our military planners at the time realized that the most rational response by Russia or China to our operationalizing Brilliant Pebbles would be to throw their entire nuclear arsenals at us on the day before the first launch. The idea was tabled, and–as far as I’m aware, anyway–it hasn’t been revived.
Imagine a slightly different scenario, though. Imagine Russia doesn’t know about us, and we don’t know about Russia. Imagine that, without fear of opposition, we both put Brilliant Pebbles style systems in orbit (try not to think about the fact that a country that can put hundreds of little satellites in orbit probably already knows what’s on the other side of the ocean–it’s just an example.) Now imagine that one of us finds out about the other. What do we do?
Unfortunately, this example has cosmic implications. With current or foreseeable technology, it’s likely possible to accelerate an object to around thirty percent of the speed of light. Theoretically, it may be possible to move a lot faster than that. At those kind of velocities, an object doesn’t need to be all that massive to literally destroy a planet. Imagine we discover that there’s another civilization hanging out around Alpha Centauri. We know that they could potentially throw a rock at us at any time. It would come in cold, far too fast to detect or react, and if it hit us, it would end us.
Rationally, we have to hit them first. Why? Because they can figure this out as easily as we can. Therefore, rationally, they have to hit us first too.
Ever wonder why SETI hasn’t found anything?
