Jupiter's Role In Life On Earth: Protector or Ultimate Destroyer?

Jupiter's been in the news a lot recently, as anywhere there's on ocean-sized explosion tends to be. The recent impact (and detonation) of an asteroid against the gas giant's hide has triggered the usual flurry of discussion with the planet cast as everything from cosmic protector to vengeful heavenly killer (both believable aspects of Jupiter, the god the Romans ripped off from the Greeks, but less so for the actual solar system object). Both miss the real answer: it's just there, and sometimes things just happen.

The cosmic protector idea is popular and plays to our egos by emphasising the importance of our solar system setup. The idea is that Jupiter's gravity acts as a giant interplanetary vacuum cleaner shield, sucking in rogue comets and asteroids before they penetrate to the inner planets. After all, it got hit twice in the last fifteen years! It's a cool idea, and can only be countered by, er, applying even the most basic facts. Jupiter's orbital radius is three quarters of a trillion meters - that means the orbit covers almost five trillion meters, while the planet is only seventy million miles across. Even accounting for its gravitational attraction (which is actually a false argument, as we'll see in the next section) it can never "shield" anything even remotely near a single percent of that path. It's like claiming a moth buzzing around your head will shield you from thrown darts.

The vengeful god idea is based on a flaw in the above - just because an asteroid is caught in Jupiter's gravity, it won't necessarily crash into the surface. If it's further out it'll just get slung around into a new path - Jupiter and Saturn create a kind of "planetary pinball" machine, randomly rearranging the vectors of anything coming in and out anywhere near them. Some say that Jupiter slings extinction-level rocks at us the odd time, only missing by the barest of margins.

The first flaw with this is the same as for the protector - Jupiter is absolutely gigantic by planetary scales, but - like any planet - still tiny by inter-planetary scales. If it aims anything it was by accident and - here's the important bit - it's just as likely to redirect something away from us as towards us. Claiming that Jupiter is out to get us is like blaming the weather.

The simple fact is that humans want things to have functions. We'd prefer benevolent forces, but deep down we'll still take cruel and vengeful gods over random chance. It's the origin of spiritism, the polytheism of our earliest societies, and can only be beaten into submission by years of science and advancement. It often takes generations to even hammer down the multitude of imagined forces into one central deity, never mind getting rid of them altogether. Which is why it's possible to look through an infrared image taken by satellites we launched with our own skill, and still try to assign motives to what we see.