So ABC's V is here (episode 2 airs tonight), reminding us for the zillionth time that gigantic, mile(s)-wide flying saucers are THE conveyance of choice for any kind of alien visitors. (See: District 9, Independence Day, the original V, Alien Nation, Close Encounters...)
The trouble is, aside from providing an excuse for awesome "reveal" shots where the giant ship blots out the sun, it's actually a completely nonsensical way for extraterrestrials to make an entrance. Here are four reasons why huge-ass space discs would be a dumb idea in the real world:
1. We'd totally see them coming
In V, 29 massive ships suddenly appear in our airspace with nothing but a few minutes of ominous rumbling noises to tip everybody off. What, was everyone at NASA asleep for the last six months? Their Near Earth Object Program has a whole mess of telescopes dedicated to tracking space objects that might be headed for earth, like the asteroid Apophis pictured above.
According to NEO chief Lindley Johnson, any bright object more than 1 kilometer across (and from the looks of things, the new V motherships appear to be between 1 and 3 km wide) will show up on their 'scopes before it even gets past Mars. (Granted, they're only looking for objects that follow natural orbits around the sun, but anything that big heading straight for us would surely stand out.) Assuming that real-world visitors wouldn't have any laws-of-physics-defying tricks up their sleeve (like faster-than-light travel or cloaking devices), we'd have at least a few months to prepare before they got close. Not so great for a sneak attack, or even a surprise visit.
2. They'd wreak havoc in our atmosphere just by showing up
A couple of dozen mile-wide spacecraft entering the Earth's atmosphere all at once would be enough to cause "mesoscale" weather effects, says Jeff Weber, a meteorologist with the University Corporation for Atmospheric Research. These storm systems can be a hundred kilometers across, encompass anything from thunderstorms to tropical cyclones, and have "an equivalent energy of multiple nuclear bombs," Weber says. That's no big deal if the aliens are intending to freak us out, but if they're friendly — or at least wish to appear so at first, like the Visitors — they'd be smarter to keep their rigs out of the troposphere.
(Interestingly, this is one thing that Independence Day actually got right. And here's a fun fact: certain kinds of atmospheric pressure perturbations can cause insane-looking clouds, pictured above, that meteorologists actually call "motherships.")
3. They'd cost too much to keep aloft
Ah, anti-gravity: the sci-fi scriptwriter's best friend. As characters in V handily point out, the Visitors use some sort of magical control over gravitation to keep their monstrous ships aloft. Fine. But in the real world, that kind of feat requires thrust, and thrust requires energy — lots of it. I did some back-of-the-envelope estimates using Wolfram Alpha and some standard high-school physics equations, and discovered that it would take about 368 quadrillion joules of energy to propel a 3km-wide, 500m-tall steel ellipsoid 1000 meters into the air. (And that doesn't include keeping it up there.)
How much is 368 quadrillion joules? About one and a half Tsar Bombas, the most powerful nuclear weapon ever detonated; or twice the total amount of solar energy hitting the earth every second. Even if you had that kind of power at your disposal, why waste it hovering menacingly above our skyscrapers when you could just apply it to, oh I dunno, frying all of us with death beams?
4. If we bring one down, it's game over for the planet (or, at least that continent)
This is more our problem than theirs, but still: You know that scene at the end of Independence Day when Will Smith is gloating over a giant destroyed ship that crashed into the desert? Yeah, well in real life he and everything else would be vaporized. An uncontrolled impact from an object that big (15 miles across, according to some ID4 geeks) would be like "hundreds of nukes," says Alexander Pavlov of NASA's Goddard Space Flight Center. And that's just if it hits from a few miles up, where jets fly. If it came crashing down all the way from orbit, even a smaller ship (like the ones in V) would trigger an extinction-level event. Congratulations: you defeated Evil E.T. so cockroaches can inherit the earth.