The European Space Agency's Giotto and the USSR's Vega spacecraft returned close-up images of comet Halley for the first time in its history, pulling back the curtain on this terror-inducing apparition to reveal its true nature: a passive lump of ice and rock in the shape of a peanut.

Science dissipated our fear. The frightening unknown became the benign known: comets are just passing us by. The influence on human events were only the projections of our own actions, as subjective and meaningless as the patterns we impose on the stars to create the constellations.

But science has also brought about a new fear – this time utterly legitimate – that comets and their asteroid cousins have a far greater ability to alter human affairs than the mere toppling of some local kingdom; they can destroy all human civilization and even all life on Earth via a large impact. The recession of one fear ushered in another: the insecurity of our existence. The knowledge that one day we may face our own demise, that, as Sagan says, "is improbable in a hundred years [but] inevitable in a hundred million." This parallels the path of comprehending our own mortality, the inevitable price of self-awareness that has long troubled the human condition.

The Hebrew bible's Garden of Eden and similar myths from other cultures epitomize this transition. Knowledge drives us out of our comfortable existence. Though as we saw with people's reaction to the appearance of comets, ignorance isn't always paradise, but it sure requires a lot less responsibility.

In Episode 4 of Cosmos, Heaven & Hell, Sagan narrates our own civilization's departure from this Eden. Starting with the destructive comet impact of the Tunguska event (all the more immediate nowadays in the aftermath of Chelyabinsk) and continuing into global nuclear war and climate change, we are reminded again of the price we pay for leaving ignorant superstitions behind.

But this is Cosmos, so there's a lot more than just comets to discuss this week. Sagan takes us through a tour of the inner solar system, describing the difference between gas giants and terrestrial planets. We learn about the electromagnetic spectrum and get a satisfying takedown of Immanuel Velikovsky's strange theory that Venus was a "comet" spat out from the planet Jupiter, responsible for a variety of ancient biblical miracles. I'm assuming this was a more prominent part of the zeitgeist when this was first filmed than it is now. If I had to guess, I would say that we will not see this revisited in the new Cosmos.

One useful thing about Velikovsky's theory is that it provides the thematic bridge between comets and Venus (thanks Velikovsky!) and we are able to spend some time appreciating the hell that is our near neighbor, Venus.

There are a surprising number of people who don't realize that human beings have landed robotic spacecraft on the surface of Venus (I fear this may have to do with a certain NASA-centric chauvinism of many Americans). The Soviets landed on Venus six times, returning all the images we have of the surface of planet. The images are eerie and spectacular, and I recommend you take a moment to read about them on Don Mitchell's comprehensive website.

The first discoveries about the nature of Venus (and Mars) helped create the new field of comparative planetology, one of the more compelling consequences of planetary exploration. We have, next to Earth, two examples of planets gone horribly wrong. Mars's atmosphere has disappeared, and with it, its water. Venus's atmosphere has spiralled out of control, trapping in the heat that led to its hellish environment.