A number of agencies, including, of course, NASA, are focusing solar system exploration efforts on Mars. At first glance, though, Mars doesn't really seem like the best candidate. Venus is much closer -- at a distance that ranges between 38 million kilometres and 261 million kilometres, compared to Mars' 56 million to 401 million kilometres, it's Earth's closest neighbour.
It's also comparable in size to Earth -- a radius of 6,052km to Earth's 6,371 -- and has similar density and chemical composition.
But everything else about it makes it almost utterly unvisitable. While probes have been sent to the planet's surface, they lasted, at most, just two hours before surface conditions on Venus destroyed them. These conditions include an atmospheric pressure up to 92 times greater than Earth's; a mean temperature of 462 degrees Celsius (863 degrees Fahrenheit); extreme volcanic activity; an extremely dense atmosphere consisting mostly of carbon dioxide, with a small amount of nitrogen; and a cloud layer made up of sulphuric acid.
In short, Venus? Not a top holiday destination, really.
NASA thinks it might have a solution that will allow sending humans up to check it out, though: Cloud City.
The High Altitude Venus Operational Concept -- HAVOC -- is a conceptual spacecraft designed by a team at the Systems Analysis and Concepts Directorate at NASA Langley Research Center for the purposes of Venusian exploration. This lighter-than-air rocket would be designed to sit above the acidic clouds for a period of around 30 days, allowing a team of astronauts to collect data about the planet's atmosphere.
While the surface of Venus would destroy a human, hovering above its clouds at an altitude of around 50 kilometres (30 miles) is a set of conditions similar to Earth. Its atmospheric pressure is comparable, and gravity is only slightly lower -- which would allow longer-term stays, effectively eliminating the ailments that occur during long-term stays in zero G. Temperature is about 75 degrees Celsius, which is hotter than is strictly comfortable, but would still be manageable. Finally, the atmosphere at that altitude offers protection from solar radiation comparable to living in Canada.
The mission would, NASA outlined to IEEE Spectrum, begin with a robotic probe deployed to Venus to perform initial checks and investigations. With the return of this data, a crewed mission would spend 30 days floating above the planet; followed by missions that would see teams of two astronauts spending a year each. The end goal would be a permanent human presence in a floating cloud city.
While this city would be fixed, exploration would be made possible with a mobile unit -- a crewed, 130-metre-long Zeppelin filled with helium, accompanied by a smaller, 31-metre robotic Zeppelin. This Zeppelin would take advantage of Venus' closer proximity to the sun: its top would be adorned with over 1,000 square metres of solar panels for power.
And it's all designed to be built using existing or near-to-existing technology -- although of course it's at least a decade or two from actual implementation. But, should it come to fruition, it may provide another way to see humanity inhabit the universe beyond Earth.
The next step would be performing simulations of Venusian conditions on Earth -- and NASA is already across it, with a paper that outlines the current capabilities and facilities for performing just such tests.
"Venus has value as a destination in and of itself for exploration and colonization, but it's also complementary to current Mars plans," said Chris Jones of the Langley Research Center. "If you did Venus first, you could get a leg up on advancing those technologies and those capabilities ahead of doing a human-scale Mars mission. It's a chance to do a practice run, if you will, of going to Mars."