NASA researchers have come up with a plan to send piloted, helium-filled airships cruising through the Venusian atmosphere. The idea, called the High Altitude Venus Operational Concept (HAVOC), could eventually lead to the permanent settlement of Earth's hellishly hot sister planet, its developers say. You can see how HAVOC might work in a mesmerizing NASA video on the Venus airships.

Establishing cloud cities on Venus "would definitely be a very big technological challenge, but it is something that we envision could be possible down the road," Chris Jones, of NASA's Langley Research Center in Virginia, told Space.com.

Putting boots on Mars is the top long-term priority of the international human-spaceflight community. Indeed, NASA is working to get astronauts to the vicinity of the Red Planet by the mid-2030s.