[pmulcahy11b]

There is an episode of the new Science Channel show Curiosity which in my mind gives a plausible reason for aliens to invade earth and wipe out humans: for whatever reason, they want our planet, with the biosphere intact, so they can live on it.

They wouldn't want the Earth to strip it of resources -- for a technological civilization that can travel the stars, resources can be found a-plenty in any asteroid belt or on moons, and water is easy to find and even make. Intact biospheres of the type that Earth has aren't as common by a long shot.

So, why have there been scattered reports of alien abductions and UFOs? They need test subjects and air samples. They've probably been taking animals, particularly birds, as well.

First, they need to take away from us the one thing we need and the rest of the biosphere doesn't: technology. The show had the example of a rain of nanobots -- trillions of them -- which produced EMP effects. The nanobots are only of the molecular size. Sort of shut off the Earth like a light switch.

Then, the reason for abductions and air samples and test subjects: the aliens would engineer diseases targeted only at humans, using animals as carriers (more reliable than simply spraying the atmosphere with disease). If we figure out a cure for one disease, they have many more standing by.

The show gave some ideas on how we could stop the aliens, centered around asymmetric warfare and making their victory too expensive (sort of like what the Viet Cong and North Vietnamese did to us), but it gave the strong impression that we really didn't stand a chance against an alien invasion.