 a Horizon Guide To armageddon (BBC4, 9pm) anyone dissatisfied with their lot might find it comforting to tune into this, a depressing kind of greatest hits from Horizon. Drawing on doomsday scenarios the science flagship has covered over the years, Dallas Campbell hosts a guide to the end of the world that will make any little human worries seem trivial.  Climate change might be the biggest threat, but there is a chance an asteroid will make its presence felt before the destruction of the Gulf Stream causes disaster. Super-volcanoes, influenza, nuclear war: the possibilities aren't so much endless as horribly final. Yet those who don't want to scare themselves to death can take comfort from the work scientists are undertaking to ensure the planet's survival, with the story of how smallpox was eradicated a humbling insight into humanity's resilience.