The other evening I watched Green Lantern on DVD. I suppose it’s an OK part 1, back story, introduction to the character type of movie, for what will likely become yet another superhero franchise. Ryan Reynolds is his usual funny, charming self as Hal Jordan (the Green Lantern), the special effects are good, and I was entertained for a couple hours. At least off-duty me was. On-duty, MPC astronomer me, however, nearly choked when the Green Lantern flies away from Earth and, no sooner has our dear planet left the screen, he reaches the Asteroid Belt. Say what? He didn’t even fly by Mars!
That’s not where it ends. When he arrives at the belt, it’s packed with asteroids—you’d think there was a 100m+ asteroid convention going on. Not that I’m surprised, it’s just the well-known asteroid thicket trope so common in movies. I betcha Stanley Kubrick wouldn’t have fallen into that trap! But I digress.
What does the Asteroid Belt really look like? Not like this:
The Green Lantern flies through an asteroid field in artwork unrelated to the movie (©Seth Engstrom).
I couldn’t find an image from the film showing the asteroids (presumably because the film makers are ashamed) but the above illustration by the talented Seth Engstrom comes close. So what would an asteroid field really look like? It would be mostly the blackness of empty space. That’s right—if you were standing on an asteroid in the middle of the asteroid belt and you looked out…you wouldn’t be able to see the nearest asteroid to you! Let’s look at some simple math (no! don’t run away; it’s easy math!):
- Width of the asteroid belt: The main part of the asteroid belt stretches out from 2.1 AU to 3.3 AU (so 1.2 AU wide); 1 AU is the average distance between the Sun and Earth, equal to 149.6 million km, which makes the asteroid belt 179.5 million km wide.
- Thickness of the asteroid belt: ~1 million km.
- Total volume of the asteroid belt: From the above two we derive a volume of 4.56 x 1023 km3 (that’s 456 billion trillion cubic km, in real words).
- Total number of asteroids in the belt: Tough to estimate, and at what size do we stop counting? Let’s stop at 100 m, then it’s estimated there are 1 billion asteroids larger than 100 m in diameter within the asteroid belt.
- Now if we divide the volume of the asteroid belt by the number of asteroids in it we’ll get the average space each asteroid has to roam in, and this value is 456 trillion km3 per asteroid. To help you visualise this volume, imagine a cube of space with each side measuring 7,700 km; you’d find on average only one asteroid larger than 100 m within it.
Let me know if I botched the math, but even if I did, the distance between each asteroid is on average huge, certainly much larger than depicted in Green Lantern.
I really hope they pay more attention to the Astronomy in the second instalment. Or they could just hire me as a consultant—I’m available weekends and can fly coach to L.A. (nudge nudge, wink wink).
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June 12, 2012 at 16:36
This post is so entertaining and actually funny (not just nerdy astronomer funny) that a byline is scarcely required. It was obviously not written by curmudgeonly director Spahr.
September 26, 2012 at 14:27
Yeah, the asteroids are sparse, even in the belt. And my calculator says the cube root of 456 trillion is 77,000. What am I doing wrong?