Shades of Gravity

Interview with Catharine Conley

A planet’s gravity is determined by its mass. The Earth’s gravity is greater than that of less massive bodies like our Moon and Mars, but it is tiny compared to the gravity of more massive bodies like Jupiter or the Sun. All life on Earth has evolved to live in Earth’s gravity, and it is only now, in the era of spaceflight, that we can see how life copes with levels of gravity different from Earth’s, for instance, the “weightlessness” of orbital free-fall (called “microgravity” because we still are affected by Earth’s gravity while in orbit around the planet).

Catharine Conley, NASA’s Planetary Protection Officer

However, despite the past 40 years of spaceflight and the experience of landing men on the Moon, we still don’t know exactly how different levels of gravity influence human health. In this interview with Astrobiology Magazine’s Leslie Mullen, Catharine Conley says that by spinning worms around really fast, we can learn more about our ability to live in space and on other worlds.


Astrobiology Magazine (AM): Before you recently became NASA’s Planetary Protection Officer, you did research on the biological effects of gravity?

Catharine Conley (CC): Yes. I’m currently acting as Planetary Protection Officer at NASA HQ on detail from Ames Research Center, but I still do have a biological laboratory at Ames. When I first got to Ames, I started doing experiments with the nematode worms Caenorhabitis elegans. We put the worms on centrifuges to try to understand how they respond to increased gravity, or hypergravity, with the idea that this could be a model for understanding how organisms might respond to decreased gravity.

AM: How could increased gravity be comparable to less gravity?

CC: Gravity can be considered a continuum. We live at one G, and there’s greater than one G and less than one G. It’s possible that, if animals are living at a certain level of gravity, there are similar responses to a change in gravity regardless of what level they start out at and what level they end up at. Humans have problems at microgravity, but we don’t know at what level of gravity they stop having problems. Will humans have problems on the Moon, which has one-sixth the gravity of Earth? What about Mars, which has one-third Earth’s gravity?

That’s one of the questions I was trying to address with worms. We can’t do experiments on humans where we grind them up and test their gene expression, and we can’t dissect them to examine their muscles. We need model organisms to be able to do that. Worms are fairly closely related to humans: they have muscles, they have a gut. They have an exoskeleton instead of an internal skeleton, but that’s actually good for doing gravity studies. We know that bones respond to gravity — humans lose calcium in microgravity — so not having that system in the way means we can specifically investigate the muscles.

Enduring spinning forces that would kill a human being, tiny worms are used in NASA studies seeking to explore how life adapts to gravity beyond Earth.
Image Credit: NASA

I’ve done a couple of space flight experiments, and from the second experiment, where we actually got back results, we were able to do gene expression studies. We collaborated with a Japanese group, and we were able to show that some of the problems that happen in human muscles also happen in worms.

AM: Was that comparable to the results from your centrifuge studies?

CC: We never completed the centrifuge studies to the extent that I’d like to because we had to prepare for the space flight study. Our first space flight opportunity had a very rapid onset — I only had a couple of weeks between being told I had the experiment and going on the shuttle.

But in the centrifuge experiment, we were able to show through gene expression that the centrifuge was altering their metabolism rather than just causing them generic stress.

AM: So it was affecting how they ate or processed energy?

CC: They were reproducing more slowly, so the cultures grew more slowly. After they came off the centrifuge, they moved very slowly. In additional control studies, we performed mechanical stimulation by growing worms in a shaking water bath. The worms in the centrifuge behaved in opposite ways from them. When you stopped the shaking water bath, those worms were hyperactive, but when you stopped the centrifuge, those worms moved more slowly than they should.

AM: How do you tell if a worm is just dizzy?

CC: (laughs) That’s a very good question. Worms don’t actually balance. They are always crawling on the ground; they don’t try to stand up straight. But they do have responses to mechanical stimulation. If you touch their noses, they’ll back up.

Astronaut Mario Runco, Jr. takes a break from activities on Space Shuttle Endeavour’s mid-deck. Humans who remain in microgravity conditions for extended periods may experience physical ailments.Click image for larger view.
Image Credit: NASA

One of our hypotheses was based on results from rats and humans in space flight. We know that both humans and rats get space sick. One of the reasons is that in the inner ear, which humans and other vertebrates use for balance, there are some nerve endings that respond to the amount of gravity. There are little rocks in your head, called otoliths, that sit on top of a membrane in the inner ear that is sensitive to the weight of those rocks. When you move your head, the rocks roll around. You can feel that, and that’s why you keep your balance. When you go out in space, the rocks are lighter and your inner ear doesn’t perceive them rolling around as much, so you may get space sick. But the nerve endings also become more sensitive in space, so they can perceive the rocks rolling around even though the signal is lowered.

Our speculation was that perhaps the worms had the same sort of system. But because we were increasing the gravity in the centrifuge worms, causing them to be heavier than normal, they ramped down on their sensitivity. That might be why, when they came back to one G, they didn’t move.

AM: How long did it take them to get back to normal?

CC: Not very long. A couple of hours. That suggests it’s a protein expression phenomenon rather than a cell reproduction phenomenon or something that takes longer than that.

AM: Do you think we’re closer to understanding if humans will be able to live in different states of gravity?

CC: We haven’t had a chance to test it really, because we only have the microgravity of low Earth orbit, one gravity on Earth, and hypergravity from a centrifuge. Humans don’t survive very well in hypergravity because all the blood falls to your feet and your brain doesn’t get any oxygen. So you can’t make someone live in a centrifuge for years and years. Worms can live for months at 100 G, though, so they are much more resistant to gravity effects.

The Human Powered Centrifuge (HPC) was developed as a research tool to provide exercise and gravitational forces (up to 5 G) simultaneously. The HPC is used to obtain physiological, psychological, and human performance data for the life science community.
Credit: Ames CGBR

Between one G and microG, we don’t have a good set of tests. The astronauts on the space station, if they go on human-powered centrifuges or if they do exercises, can regain some of the losses due to microG. But we don’t have any information about, for example, long-term physical effects from Moon levels of gravity.

AM: What are other effects of reduced gravity on humans?

CC: One of the most obvious things is that because there’s less weight, your bones and muscles atrophy because they’re not resisting that weight. It’s the same thing that happens with prolonged bed rest. In addition, you have muscles in the veins of your legs, and when you stand up they have to pump the blood to get it back up to your heart. In microgravity the veins keep pumping, but there’s not enough gravity to pull the blood down. So astronauts end up looking puffy-faced in space — they have more fluid above their heart because that pumping action is still going on. That also may be a contributing factor to space sickness.

Although it’s not well studied, there are some reports that intestinal function is different in microgravity. That’s another thing we can look at in worms, although we haven’t done it yet. It also has been reported that there are changes in the immune system, but it’s not clear whether that’s stress related, or related to a general metabolic shift, or whether they’re also related to the reduction of mechanical stimulation. There have been some studies of muscle cultures that show some form of atrophy.

AM: Other than the centrifuge or exercising in space, are there any other ways to counter the negative effects of microG?

CC: The type of exercise might make a big difference. There are some hypotheses that it’s the maximum impact rather than the total duration of the exercise that strengthens the bones and muscles. So perhaps astronauts should jump up and down instead of using the Stairmaster, for example. Other than exercises, there are some drugs we can use. We also have pressure suits to try to pull the blood back down to the feet, and that may benefit space sickness.

Humans on Mars may experience effects of living with less gravity than typically experienced on the Earth.

AM: Knowing what goes wrong with the human body in microG, how optimistic are you about human colonies on the Moon or Mars?

CC: Oh, I’m sure that partial G will be better than microG. We just don’t know what level is required for optimal health. It may be that if you’re on the Moon you’re somewhat better and if you’re on Mars you’re even better, but you really need to be on the Earth to be perfectly healthy. And that’s one of the other things we’re trying to explore with the worms – the adaptation over multiple generations to microgravity.

AM: In Kim Stanley Robinson’s books about the human colonization of Mars, he talks about how humans adapted to the lower G there wouldn’t be able to come back to Earth.

CC: That could happen if you have enough generations of reproduction and if you’re not adaptable enough. Humans can survive at 3 Gs. There’s about that much difference between Mars gravity and Earth gravity — it’s only a relatively small fraction of one G. That suggests that even if you’re living on Mars for multiple generations, you could adapt to come back to Earth. It might be difficult, but it’s not going to take generations to adapt back, there probably will be some physiological adaptation but it’s not going to completely prevent you from returning. Although with the developmental aspects of human growth, such as bone development, it may require a lot of exercise and a lot of work to be able to come back.

I think Robert Heinlein also used that idea in his novels; it’s a recurring idea in science fiction and an interesting concept. Of course, if there were multiple generations of humans in microgravity, we know that’s different.

Cut-away view of a hypothetical Toroidal space colony. Click image for larger view.
Image Credit: NASA Ames Research Center

AM: There have been some ideas about having hotels in orbit around Earth. You could potentially have generations of families up there, unless you required that they come down in shifts.

CC: It might be like cruise ships where they rotate out, or airplanes where you only have so many days of work. Certainly the astronauts or cosmonauts who have stayed up for a year or more have had some health problems.

AM: In the movie “Contact”, the rich guy, S.R. Hadden, goes on MIR because he says the low oxygen, microG environment slows the spread of his cancer. I wondered if that could be true, and if it would be beneficial to have hospitals in space.

CC: I would be surprised that cancer would grow more slowly in space simply because the radiation environment is much more harsh and would cause more mutations. It’s not unlikely that for cardiac problems it might be easier to live in space, because the heart doesn’t have to work against gravity to pump the blood around. And one of the problems with older people on Earth is they lie in bed a lot, so they get bed sores and other problems. I can imagine that people with arthritis or other problems with mobility might really enjoy being in space.