Hawking opens the film with a prediction and a warning: "I am convinced that humans need to leave Earth and make a new home on another planet," he says. "We must do it now before humanity is overtaken by some disaster that we can neither anticipate nor control."
The film then explores what humanity must do to achieve the task Hawking sets before us: from finding habitable planets to developing propulsion systems powerful enough to take humans to these locations in decades rather than millennia. But reaching a new planet is just a sliver of the challenges humans face in expanding into space, the film says. Such a pioneering crew would face the years of space travel that would tax their mental and physical well-being.
After this exhausting journey, they would begin their true task: establishing society on a new world, which is no small feat. "It will take more than explorers to colonize a planet," Hawking says. To create a self-sustaining colony, astronauts would need to build habitats, farm food, and mine and process the planet's resources. Perhaps robots would prep the location for the arrival of the first settlers. Establishing a colony in space would require incredible ingenuity, but the documentary demonstrates that ingenuity is a resource humans have in abundance.
Throughout the film Hawking's iconic, digital voice inspires hope and belief in all who listen as he urges the citizens of the world to reignite the innovation and ambition of the space race.
"Our species' natural curiosity is what will drive us to distant planets," he says. "In the next 100 years, we will embark on our greatest ever adventure. Our destiny is in the stars."
Editor's Note: Smithsonian Channel has announced that they will air "Leaving Earth" Sunday, March 25 at 8 p.m. EDT/PDT rather than April 1 at 9 p.m. EDT/PDT.