Bigelow's BEAM (the big bump at center-right) is just one of several experimental space habitats being developed for NASA. Image source: Bigelow Aerospace.

Sometime in the next two decades, NASA plans to land humans on Mars. In the meantime, several companies are making efforts to put fresh footprints on the Moon. But here's the thing: If astronauts want to spend any significant amount of time exploring these heavenly bodies, they're going to need someplace to live.

As spaceflight pioneer Elon Musk has explained, even SpaceX's rather roomy Dragon 2 space capsule only has about as much interior volume as a family SUV, while Boeing's (BA -0.24%) CST-100 Starliner capsule isn't much bigger. The Orion crew module that Lockheed Martin (LMT 1.71%) is building for NASA is somewhat roomier -- but still probably not big enough to move its crew of four astronauts to Mars and back in comfort.

Don't get me wrong. All three of these spaceships are probably comfortable enough for short hops. But for any mission of any real length on a foreign planet(oid) -- and especially for the 18-month round trip to Mars and back -- astronauts are going to need some better extended-stay housing options.

So last week, NASA announced a new project to try to develop such options.

Round up the usual space suspects

On August 9, NASA expanded the parameters of its NextSTEP program (Next Space Technologies for Exploration Partnerships) for development of new space technology by private industry. In NextStep part 2, the agency has invited six U.S. companies to try to come up with improved options for long-term deep-space living.

These six companies include publicly traded space giants Boeing (BA -0.24%), Lockheed Martin (LMT 1.71%), and Orbital ATK (OA). In addition, up-and-comer Bigelow Aerospace, private aerospace specialist Sierra Nevada, and relative newcomer NanoRacks of Webster, Texas also won contracts. (Although actually, NanoRacks will partner with Boeing's and Lockheed Martin's United Launch Alliance joint venture. So in a sense, you could say that Boeing and Lockheed won this NextSTEP competition twice).

Living off campus -- way off campus

What will they be building for NASA? The range of potential space habitat ideas is pretty wide. We've already written at length about Bigelow's inflatable "BEAM" series of inflatable space habitats -- one of which is already undergoing testing as an add-on to the International Space Station (ISS).

In addition, both Boeing and Orbital ATK are developing new purpose-built habitation modules for NASA. Lockheed and Sierra Nevada both envision habitats comprising repurposed cargo capsules (sent into orbit on ISS resupply missions, for example). The NanoRacks team will try to do something similar, refitting a launch rocket's second-stage fuel tank to see if it can be converted into living quarters (a la Skylab).

What it means for investors

NASA hasn't divulged the value of its initial six NextSTEP-2 contract awards, which is problematic for investors trying to gauge the investment opportunity here. On the other hand, neither has NASA stated that it will pick just one of the solutions proposed by its contractors for building future deep-space habitats. To the contrary, it might make more financial sense to build as much outer-space real estate as possible, utilizing all available resources -- spent fuel tanks, empty cargo containers, purpose-built modules, and inflatable structures as well.

Potentially, all six of these companies could come out of the NextSTEP-2 competition looking like winners. For now, though, you can still only invest in three of them: Boeing, Lockheed, and Orbital ATK.