Science Sunday: Near Earth Objects

Inspired by this Sci-News article, I was going to write about Near Earth Objects (NEOs) and what would happen if one hit us.

But then Radiolab did it so much better than I could. Take a listen. Or watch it here, with dino-puppets!

So I want to take a different approach to the topic: deflecting a NEO. There’s a lot on the topic on Wikipedia, including links to sources. What I find interesting here are these crunchy bits:

  • “It has been estimated that a velocity change of just 3.5/t × 10−2 m·s−1 (where t is the number of years until potential impact) is needed to successfully deflect a body on a direct collision trajectory.” I don’t want to minimize the challenge here, but that’s not nearly as bad as I’d feared. 0.035 meters per second, divided by the number of years of warning. That’s 1.3 inches per second. Suddenly the task seems feasible. Except…
  • A typical NEO of interest is 140 meters in diameter. Assuming for simplicity that it’s a sphere, that’s 1.43676E+12 cubic centimeters. Assuming a density similar to Earth’s (5.5 grams per cubic centimeter), that’s 7,902,152,721 kilograms. For round numbers, call it 8 million tonnes. We don’t have to move it fast, but there’s a lot of it to move.
  • That means that with one year of warning, we need to impart 4.84E+06 joules of kinetic energy. If it’s one month, we need 6.97E+08 joules. Two years: 1.21E+06. In case it’s not obvious yet, early detection makes a big difference in the cost of deflection!
  • For those (like me) who aren’t accustomed to thinking in joules – especially large numbers of joules – here’s a comparison: “The terajoule (TJ) is equal to one trillion (10^12) joules. About 63 TJ of energy was released by the atomic bomb that exploded over Hiroshima.” So that’s 6.3E+13 joules from a small, primitive atomic blast. That’s 90,000 times what we need for an average NEO with a month’s warning!

We can do this. It’s a matter of engineering, politics, diplomacy, commerce, logistics, and orbital mechanics, but we can do this.

Now for Science Sunday, I want to look at the story potential in the science, so here are some thoughts…

The figures above are for the smallest NEOs we’re currently tracking. They come larger – more rare, but they do. If I change my spreadsheet for a dinosaur killer (10km across), the energy requirement is 1.76E+12 joules (with one year warning). That’s a little bigger, about 3% of a Hiroshima blast. Still pretty feasible. But with one month warning, it gets a lot worse: 2.54E+14 joules… something like 4 Hiroshimas, assuming 100% of the energy went into moving the rock. It won’t. I’m not a nuclear engineer nor a rocket scientist, so I can’t guess what the actual efficiency would be. No better than 50%, I’m sure, since half the blast points away from the rock. A big nuclear blast ought to do it, if used right, so it’s still possible.

But when we talk about “warning”, what we’re really talking about is time between the blast and the possible impact. If we see that dinosaur killer two years out, but it takes us 22 months to decide to act, we are dead as the dinos. Somehow we have to get that nuclear device out to where the rock is. Right now we spend years – decades, even – planning relatively simple space missions. This one won’t be simple, and it has to be done right the first time. So a major source of story conflict can be the diplomatic and political effort to get people to act when they don’t believe they have to – until it’s too late.

Now I’ve been talking about nuclear deflection because it’s the simplest to calculate and explain, but that’s only one of many proposed methods. Kinetic impact, rocket engines, ion drives, gravity tractors, mass drivers… They’re all different ways to add that tiny delta V to that great big rock; and no matter which method you use, the required change in kinetic energy is the same: a lot, but not impossible.

Of course, as Carl Sagan warned, if you can deflect an asteroid away from the Earth, you can also direct it toward the Earth. That seems suicidal, but it might work for a doomsday weapon.

And always remember: this isn’t fiction, it’s probability. A dinosaur killer hit us before. If we take no action, one will hit us again. It’s only a question of when. We can’t answer that question without data, so watch the skies!

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