Hawking was clearly troubled that we were putting all our eggs in one basket - that basket being Earth. For decades, Hawking had been calling for humans to begin the process of permanently settling other planets. It made news headlines again and again.
Hawking's rationale was that humankind would eventually fall victim to an extinction-level catastrophe - perhaps sooner rather than later. What worried him were so-called low-probability, high impact events - a large asteroid striking our planet is the classic example. But Hawking perceived a host of other potential threats: artificial intelligence, climate change, GM viruses and nuclear war to name a few.
In 2016, he told the BBC: "Although the chance of a disaster to planet Earth in a given year may be quite low, it adds up over time, and becomes a near certainty in the next thousand or 10,000 years.