How are we going to safely transport astronauts to and from Mars in the next few decades? That’s a question NASA has been pondering for quite a while – and now it’s seeking help to find an answer.

The agency already has several of the components for a mission to Mars in the works. These include the giant rocket that will be necessary to launch humans and equipment there – the Space Launch System (SLS) – and the Orion capsule they will launch and return to Earth in.

But, with a crew of about six, the car-sized Orion capsule won’t be sufficient to house the astronauts for the seven months or so of each leg of the transit to Mars. Instead, they’ll need some sort of additional habitat to live in during their prolonged time in space, where they’ll also be able to exercise to keep their bone and muscle strength up before walking on the surface of Mars.

What this habitat might be is up for debate at the moment, so NASA is asking for proposals as part of its Next Space Technologies for Exploration Partnerships-2 (NextSTEP-2). Public and private organizations are invited to submit proposals to NASA by June 15, with notices of intent needed by May 13.

These habitats might first be tested on missions to the Moon. NASA also wants ideas for habitats to use on the surface of Mars, which astronauts can live in while they operate on the ground.

“NASA is increasingly embracing public-private partnerships to expand capabilities, and opportunities in space,” said Jason Crusan, director of NASA’s Advanced Exploration Systems (AES) Division, which sponsors NextSTEP, in a statement. “Our NextSTEP partners commit their own corporate resources toward the development – making them a true partner in the spaceflight economy.”

There are already some tentative ideas for the habitats humans will use on their way to Mars. One of the most promising is the thought of using inflatable habitats, which will launch in a compact form and then expand for the journey, providing astronauts with a large habitat in which they can live. Such technology is currently being tested on the International Space Station (ISS) by Bigelow Aerospace.

^{An artist's impression of Bigelow Aerospace's inflatable module. Bigelow Aerospace}

Another potential idea is to use a rotating ring structure, which would provide artificial gravity for the astronauts during the transit to Mars. This would prevent bone and muscle loss, and would mean they could start operations on the Martian surface without any of the debilitating effects of prolonged time spent in space.

After returning from the ISS, for example, astronauts are barely able to walk on Earth. This won’t be ideal on Mars, which still has two-thirds Earth’s gravity. However, NASA is still a bit unsure about using a rotating structure on a spacecraft.

“There are a couple of problems with that,” Julie Robinson, NASA’s chief scientist for the ISS, told IFLScience recently. “The first is that it makes the habitat much larger and heavier than it would otherwise be. The more things weigh, the harder it is to get to Mars. We’re very weight-limited with the Mars mission.

“The other thing is you have to design your engineering systems to deal with all that spinning the entire time, or what if the mechanism breaks, and they have to work in a microgravity environment.”

So, with NASA still unsure about the best way to house astronauts on the way to Mars, perhaps someone can come up with a proposal that provides an attractive option. With those first missions not planned until the 2030s, there’s still plenty of time to put something together – but it would be good to have a solid plan sooner rather than later.