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2001 space station
The concept of artificial gravity conjures up visions of giant rotating space stations like in 2001, but even simpler designs to produce artificial gravity may be more complex than what NASA is willing to take on. (credit: A.M.P.A.S.)

The weak pull of artificial gravity

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Weightlessness can be both delightful and dangerous. On one hand, it allows astronauts to perform feats not possible on Earth, like moving large equipment with the touch of a finger, and even stunts like flipping upside down at the end of a downlink with students. And, of course, microgravity is of serious interest to many scientists interested in its effects on topics ranging from biology to material sciences to fluid dynamics.

“We can maintain bone and muscle, cardiovascular fitness pretty well,” Barratt said. “We probably don’t need artificial gravity.”

But extended time in weightlessness has been known for decades to have serious health effects on astronauts. Researchers have cataloged a long list of medical issues, from bone and muscle loss to vision changes. With NASA planning long-duration missions beyond Earth orbit, including trips to Mars that will take six to nine months each way, developing ways to counteract those effects, like hours of exercise each day, has been a priority for the agency.

The ultimate countermeasure to microgravity, though, is not to experience it at all. Artificial gravity, created by spinning spacecraft, has long been proposed as a means to provide at least partial gravity, perhaps enough to eliminate those health concerns. While spinning spacecraft bring up visions of the giant space station from 2001: A Space Odyssey, other concepts have been as simple as linking a spacecraft to its upper stage with a tether, and spinning them around their combined center of mass.

Yet, surprisingly, artificial gravity is a low priority at NASA and elsewhere. What concepts that do exist of spacecraft for Mars missions, developed either by NASA or companies, don’t make use of spinning in one form or another to create artificial gravity. Even SpaceX’s giant Interplanetary Transport System spacecraft, capable of carrying 100 people, will not produce artificial gravity on its missions to Mars and back.

Michael Barratt, a NASA astronaut and medical doctor, said part of the reason NASA hasn’t adopted artificial gravity is that other countermeasures appear to be sufficient. “We can maintain bone and muscle, cardiovascular fitness pretty well,” he said during a panel session at the AIAA Space 2016 conference in September in Long Beach, California. “We probably don’t need artificial gravity.”

That view is shared by NASA executives. “The bone loss, the vestibular, the muscle wasting, those kinds of things we can control with the exercise protocols,” said Bill Gerstenmaier, NASA associate administrator for human exploration and operations, during a panel at the International Astronautical Congress (IAC) in Guadalajara, Mexico, in September discussing the recent “one-year” mission on the ISS.

One of the people who flew that mission, Russian cosmonaut Mikhail Kornienko, was also on the panel at the IAC, and said he found the recovery from the long-duration missions to be unexpectedly easier than an earlier, shorter one, claiming he was back to normal within weeks, rather than months. The difference, presumably, was from a far more rigorous exercise program he undertook during his 340 days in space.

“The recovery was much faster and easier after the long-duration mission, perhaps surprisingly,” he said, speaking through an interpreter. “I swam a kilometer the very next day after I arrived back on Earth.”

“We’ve looked at a lot of vehicle designs to provide artificial gravity in various ways. That really just doesn’t work,” Gerstenmaier said.

Gerstenmaier and Kornienko spoke at the IAC a day after Elon Musk rolled out his Mars mission architecture, and, in questions after his talk, he deflected any concerns about the effect of microgravity on his spaceships’ crews. “I think those are essentially solved problems,” he said, arguing that long-duration ISS missions are much longer than his planned Mars transits. “It’s fairly straightforward.”

Another issue is the technical design of spacecraft that would be able to provide artificial gravity, either through rotating the entire spacecraft or through the inclusion of a section that rotates on its own or has a centrifuge of some kind.

“We’ve looked at a lot of vehicle designs to provide artificial gravity in various ways. That really just doesn’t work,” Gerstenmaier said. “It’s a major redesign of the spacecraft. It’s a lot of work, and the challenge of just getting to Mars is tough enough as it is.”

He added that putting in one section of the spacecraft with artificial gravity could actually make matters worse, because astronauts will have to regularly readapt between weightlessness and normal gravity, which could trigger reoccurrences of space adaptation syndrome.

“If you had a situation where there’s a partial gravity section of the spacecraft, and another one that’s not, you’re going to be going through this transition from zero-g to one-g, back and forth,” he said. “That may be more problematic than just staying at zero-g.”

Barratt noted he and his colleagues have technical concerns about spacecraft designs that implement artificial gravity. “Astronauts fear artificial gravity. Why? We don’t like big moving parts. They break.”

The vision problems seen in some astronauts, though, could lead to a reassessment of the importance of artificial gravity. While the cause of the impaired vision isn’t known, it’s clearly something that current countermeasures for dealing with other adverse effects of microgravity aren’t handling.

“It’s a very fascinating time for AG because of that,” Barratt said, referring to artificial gravity. “We have a new paradigm now that forces us to look at AG in a new way.”

“We’ll take a look and see if there’s some magic partial gravity that actually mitigates most of the concerns of the zero-gravity levels,” Gerstenmaier said.

There’s no guarantee, though, that artificial gravity could solve the vision problem, since it may not be created by the absence of gravity. “There are lots of ideas about why that’s occurring. One is elevated carbon dioxide levels,” said Gerstenmaier, who noted the carbon dioxide levels on the ISS are ten times higher than in normal atmospheric conditions on Earth.

Kornienko thinks future astronauts heading to Mars or elsewhere can handle several months of microgravity without the need for artificial gravity. “It would be cost-prohibitive,” he said of trying to implement artificial gravity on a future mission. Exercise and related countermeasures “are doing their job, they work, and are sufficient.”

Gerstenmaier, while skeptical of the need for and ability to accommodate artificial gravity, didn’t rule it out entirely. He noted that there’s very little information on the effectiveness of partial gravity, including the minimum levels needed to offset the deleterious effects of microgravity. Some of that research is being done with a small rodent centrifuge on the station’s Kibo module. “We’ll take a look and see if there’s some magic partial gravity that actually mitigates most of the concerns of the zero-gravity levels,” he said.

Until then, astronauts on the ISS and future exploration missions will have to learn to take the good with the bad when it comes to weightlessness—and hope they love to exercise for a couple hours every day.