Should we give up on artificial gravity?

Posted on December 7, 2015


I think space is awesome. I think going to Mars is awesome. I think it’s important to do. I’m not going to make an argument why, but you can see one here.

A problem for human spaceflight is countering the negative impacts of moving in an environment where gravity is less than Earth’s. This is the predominant impetus for some space posts I’ll have, as there is a ton here which can be applied to everyday people on Earth, and vice versa.

Sometimes I’ll hit on current treatments; sometimes I’ll hit on ways I think things could potentially be improved. Maybe this can be my small way to help the Mars movement.

Finally, I want to stress, even if you’re not interested in space, I think this stuff can be valuable to learn about. For instance, this post will show why trying to avoid exercise / intense physical activity never works.

In my post on minimizing mass for human spaceflight (focusing on the human element), I briefly discussed artificial gravity. That even if we implement artificial gravity, we want to be exercising anyways. But first,

What’s artificial gravity?

In many space movies, you’ll see a rotating ship.

Rotation in the middle. From The Martian.

Rotation in the middle. (From The Martian.)

By rotating the ship, we cause things within the ship, including the people, to be pushed outward. By having to overcome this force, we garner ourselves something analogous to gravity i.e. a force we’re always having to overcome.

Gravity isn’t enough

The most pressing concerns with a microgravity environment are atrophy oriented, such as loss of bone, muscle (including the heart), blood volume, subsequently causing general deconditioning. There are also vision issues. (We don’t seem to know exactly what’s up with the vision issues yet, but women have done fine with them, and we think it might be cardiovascularly oriented. The better in shape you are -> perhaps the less likely vision problems are.)

At least until around middle age, basic movement like the ~5,000 steps a day most average, in a 1g (Earth gravity) environment can largely, if not completely, mitigate these factors.

But we’ve already seen merely overcoming 1g all day, especially in the context of modern life, isn’t enough for ideal health. Hopefully, you have a body which can overcome more than your own bodyweight in 1g all day in more scenarios than only standing. Often the more resistance, the more scenarios, the better. This is what we call strength training and cardiovascular exercise, and we’ve all been berated with how healthy it is for our bodies.

Some benefits from exercise:

  • Improves your chances of living longer and living healthier
  • Helps protect you from developing heart disease and stroke or its precursors, high blood pressure and undesirable blood lipid patterns
  • Helps protect you from developing certain cancers, including colon and breast cancer, and possibly lung and endometrial (uterine lining) cancer
  • Helps prevent type 2 diabetes (what was once called adult-onset diabetes) and metabolic syndrome (a constellation of risk factors that increases the chances of developing heart disease and diabetes; read more about simple steps to prevent diabetes)
  • Helps prevent the insidious loss of bone known as osteoporosis
  • Reduces the risk of falling and improves cognitive function among older adults
  • Relieves symptoms of depression and anxiety and improves mood
  • Prevents weight gain, promotes weight loss (when combined with a lower-calorie diet), and helps keep weight off after weight loss
  • Improves heart-lung and muscle fitness
  • Improves sleep
  • Helps with dementia
  • Helps with parkinson’s disease
  • Benefits fibromyalgia patients
  • Improves self-esteem
  • Strengthens immune system
  • Reduces inflammation
  • Impacts basic cognitive function, such as learning and memory.
  • Decrease macular degeneration (helps with eyesight)

It’s important to reiterate, the older you are, the more important exercise becomes. The average astronaut age is 48. Past middle age. We want them exercising.

-> Astronauts are fit people to begin with. They’re used to exercising; they like exercising. Psychologically, they’ll also want to be doing it.

Even in the realm of sci-fi, where artificial gravity is made to be a panacea, you see some exercising. In the latest, The Martian, there is a brief scene with an astronaut on a treadmill, and there’s the famous one from 2001.

2001 space odyssey running GIF

Mars Direct is a noteworthy plan to get us to Mars. Even in that plan, which uses artificial gravity, you see this:

In the center of the above ship is a safe house, for dealing with radiation during space travel. This a big concern as a spaceship doesn’t give the protection from radiation we get from Earth. Instead, the particles (like from the sun) more easily shoot through the body, breaking things like DNA, giving increased likelihoods of cancer. You know what we’re finding more and more evidence for, helping repair DNA, helping turn down cancer promoting genes, helping turn up tumor suppressor genes? Exercise!

Artificial gravity has not been proven to be *practically* feasible

Just because something is possible, doesn’t mean it can be done.

Given infinite money, an entire population in love with space travel, and artificial gravity seems like a sure thing. Given human constraints, the reality of a track record of it costing around fifteen thousand dollars to get a kilogram into orbit, it seems anything but.

The module way

This is how we typically see a ship in sci-fi:

Photo by ©Paramount/Courtesy Everett Collection/REX (4116107a) Interstellar Interstellar - 2014

Photo by ©Paramount/Courtesy Everett Collection/REX (4116107a) Interstellar – 2014

A bunch of modules connected together to acquire the needed radius of the ship. What’s hard to appreciate in a movie is how big this radius needs to be.

To simulate Earth’s gravity would require us to build a ship with a radius of 224 meters. For context, a Falcon 9 (currently contracted by NASA) launch vehicle is 68 meters long. The Saturn V, most powerful rocket ever, was 110 meters tall. The International Space Station is 73 and 108 meters long and wide (and damn near one million pounds).

Picture a standard 400 meter track. Stretch out more than half the track, or think the length of more than two football fields, and that’s just your radius of the ship. Double it, more than an entire track stretched out / more than four football fields, and we have the space between opposing modules.

Diameter = 2 * radius = 448 meters

Circumference = pi * diameter = 3.14 * 448 meters = 1407 meters

We are talking an utterly enormous ship, longer than 10x the size of anything we’ve ever built, which means god knows how much mass that thing will be, which means god knows how much money would need to be involved.

-> A bigger ship is also more susceptible to being hit by space junk. A ship which has artificial gravity means a ship more likely to hit something on its journey.

It’s possible we can get by with simulating something less than Earth’s gravity, which would allow us a smaller radius. However, maintaining a physiological adaptation typically requires engaging in that physiological stimulus. If you want to be able to run a four minute mile, or bench press 400 pounds, you need be consistently training at something around a four minute mile stimulus, or a 400 pound bench press stimulus. If you consistently train at 20, 50, 70% less than the stimulus, you eventually adapt to that lesser percentage.

Said another way, if you bench press 400 pounds one day, then for a few years never bench press more than 152 pounds -38% of 400- you aren’t still going to be able to bench press 400 pounds. You will lose that ability.

If we do try to get by with a smaller radius e.g. we try to only simulate two thirds of Earth’s gravity (still a huge ship), we will need something to make up for that loss of stimulus. That something will be exercise.

The tethered way

There is a way to do artificial gravity with much less mass than the module way. It looks something like,

artificial gravity example 0 artificial gravity example 1 artificial gravity example 3

This is an actual attempt we tried, albeit briefly:artificial gravity Gemini_11_Agena

(This doesn’t have to be a tether. It can be a truss. They’re similar enough for this discussion.)

While less mass than a bunch of modules, we’re talking a long and strong tether here. According to the Mars Direct plan, it would be 600 kilograms. According to a NASA paper examining artificial gravity from a tether, we’re talking a system needing 21,000 kilograms, “plus 1,400 kg of propellant.” (A truss system would be even more.)

Part of the discrepancy here seems to be Mars Direct is talking a simpler tether system which, once you get to Mars, you’re done with.

-> Once the tether is let go of, we’re increasing space debris, albeit around Mars this time, which might not be a concern now, but could be in the future.

On the way back, you’d repeat the process. (Space debris around Earth! Currently a concern.) Mars Direct would also use a different rotation rate and length than the NASA plan. Bigger rotation and or faster rate means more “gravity,” but bigger rotation means more material, and there is only so fast you can go without causing motion sickness. Faster rotation increases wear on the structures too. (Again, more money to insure the structure is strong enough.) For the NASA plan, I couldn’t find what the gravity force is intended to be. For Mars Direct, it plans to simulate Martian gravity, at 0.38g.

To go 0.38g for the entire journey to Mars (~6 months), then stay that way however long you’re there (~18 months), then go 0.38g on the way home (~6 months), the body is unlikely to enjoy that. We already know it can take multiple years to recover from only 6 months in weightlessness, without sufficient exercise. In this scenario, you’ll probably be lucky if you’re able to crawl once back in Earth’s gravity. Setting up to bench press 400 pounds after 2.5 years of only bench pressing 152 pounds, and you’re in for a tough experience.

For the NASA plan, the tether is also retractable, which increases the mass as you need more hardware to help with that. But 21,000 kilograms? When it costs multiple thousands of dollars to launch one kilogram? We’re trying to make space travel cheaper here!

None of this has been thoroughly tested either. We would have to develop all this, launch no one knows how many test flights, try out different tether lengths, materials. This increases costs. Here was the experience of the Gemini crew who tried just to tauten the tether (never mind operate this thing for a trip to Mars (notice the picture above- the tether isn’t taut!)):

“When Conrad tried to start the rotation, he found he had another problem. He could not get the tether taut. It seemed to rotate counterclockwise. Surprised, he reported to Young, “This tether’s doing something l never thought it would do. It’s like the Agena and l have a skip rope between us and it’s rotating and making a big loop.” He continued, “Man! Have we got a weird phenomenon going on here. This will take somebody a little time to figure out.” Strangely, although the spinning line was curved, it also had tension. “I can’t get it straight,” Conrad muttered. For ten minutes, the crew jockeyed, using the spacecraft thrusters to straighten the arc. Finally, they got an even tether, but neither of them could ever recall exactly what they had done to stop the odd behavior of the rope.”

-> I train a rocket scientist from MIT, who worked with Lockheed and NASA. He told me he knew quite a few people who did their PhD on the “skip rope” phenomenon. When you’re doing a PhD on something, it ain’t simple!

Plus we have a rotating spacecraft. It’s harder to communicate with this. It’s tougher to maneuver. It doesn’t sound insurmountable, but it’s certainly more complex ($$$).

The tether is also susceptible to breaking. Whether that be a material failure, or space debris as we have more surface area to get hit. The mission can continue if a failure in the tether occurs, or the crew is unable to properly set it up, but then we need a back up plan…which would be exercise!

Redundancy isn’t always necessary

In engineering, you’ll often have redundant systems in case one fails. It would make sense if our exercise program failed, that we had a back up plan, such as artificial gravity. But other than something like a crew member just not doing it, the exercise program won’t fail. If the exercise equipment all broke, which is unlikely if using enough resistance bands, then even a broken band can still be used. (We could potentially use the crew members as one another’s resistance too.) On a trip to Mars, exercise is, or at least should, going to be happening no matter what. Exercise IS the fail safe.

Regardless of which plan, once we get to Mars, we’re going to be throwing a spacesuit on. Meaning once we’re on Mars we don’t only have to deal with our bodyweight and 0.38 gravity. We need to deal with our bodyweight, 0.38 g, and, what has been so far, a heavy ass space suit. Right now they’re 300 pounds on the International Space Station! We’ll need to deal with this along with whatever we encounter during exploration. Climbing hills, moving rocks, manual labor. Much like the minimal amount of walking one does working a computer job does not get one ready for intense hiking, similarly a journey with 0.38g for six months does not get us ready for 0.38 gravity on Mars. Not without exercise to give us a greater than a 0.38g stimulus on the trip.

Say we have a 145 pound (66 kg) female. If gravity is 1g (9.8 m/s^2) -this is likely fantasy due to reasons discussed earlier- on the trip to Mars,

Force = mass * acceleration

Force = 66 * 9.8 = 647 newtons

-> This is the force of gravity.

Ok, now we get onto Mars. We’ll say a 300 pound (136 kg) spacesuit and gravity is 0.38 g.

Force = (66 + 136) * (9.8 * 0.38) = 752 newtons

752 > 647

A 1g trip to Mars may not get us ready for exploration on Mars either.

As I said, I don’t know what a spacesuit on Mars will weigh. (I do know developing a new spacesuit costs money though.) NASA has a prototype that, when fully loaded, is 162 pounds (74 kg).

Force = (66 + 74) * (9.8 * 0.38) = 521 newtons

This is more in the neighborhood. What gravitational force would we need on the trip then?

521 = 66 * 9.8 * ?

? = 0.805g

About 80% of Earth’s gravity. For 0.38 g and a rotational rate of 1 rpm, we’re talking a 1500 meter, about a mile, 600kg tether. With 80%, we are talking roughly double that length and mass. Again, that just gets us ready to have the suit on and not be immediately exhausted. If we’re talking be able to do useful work in the suit, gravity would need to be higher / we need to fill that gap with exercise.

“What if we’re staying on Mars? Does it matter then? We could adapt to that loading and leave it at that.”

While we don’t know for sure, it seems unlikely once we’re in 0.38 g, our bodies will adapt to that loading 100% positively. I’ve been surprised how contentious a topic this is. Some think 0.38 g will be enough to potentially offset any negative adaptation. The body is pretty simple here. Load it X amount regularly, and beyond extreme circumstances, it will adapt to X amount of loading. We don’t know for sure, but it’s highly unlikely loading the body more than 60% less isn’t going to have repercussions. You get what you train.

Said another way, start loading your body 60% less on Earth. For instance, use crutches and load your legs 60% less. They’re going to get smaller once you do that, and quickly.

Short of us building a giant centrifuge on Mars, we won’t be able to have artificial gravity there. Maybe after being there for a while we could, but initially we won’t have it. Exercise will again have to fill that gap.

After being on Mars for a while, maybe health wise it’s ok to have less bone density and less muscle, but with less blood volume you have a lesser immune system, and lesser ability to heal. Physical activity can improve our ability to prevent and or delay and or handle a cancer diagnosis. This is where we can go back to our long list of exercise benefits earlier. You want to be doing it regardless of virtually any scenario. Especially in those exposing their body to the unknown.

Given sufficient stimulus from our exercise programs, and how exercise will be done on the journey to Mars, and should be done while on Mars, artificial gravity may very well be an unnecessary redundant measure.

From my little aerospace knowledge, the biggest hurdle right now to spaceflight is cost. (Of course, there are technical reasons why that is so.) I see a good amount of cost being spent on things geared around the human element of all this. We’ve sent some ships to Mars at this point, but sending people is where things seem to really jump up in cost and complexity, which is why we’ve yet to do it. One has to wonder if we need to approach this more as a business move, not as many space projects seem to do -“for the adventure,” “in the name of science and discovery” (there is only so much money the public is willing to give when this is your justification!)- where cost is an afterthought. Sometimes I think NASA getting its big start with the Apollo program has been a hindrance. “If only it were like that again.” But complaining about the Apollo period, and how great the funding was, isn’t going to make it come back. (When you’re currently getting 17+ billion a year, you’re not going to get much sympathy either.) Nor is cost is always a hindrance. Sometimes forced frugality means forced creativity.

Much like any business’ R & D department, coming up with new stuff means $$$. If we can do this with what we already have, what we already know how to do, what we already know can be done, we should. From a business perspective, it’s silly to try and do the same thing for more cost.

Some engineers, and much of the American public, love our gadgets, our fancy approaches, making science fiction movies come true, but we know, and have known for a long time, how to keep the body in shape. There have been no found shortcuts, and there don’t appear any on the horizon. People on Earth are falling apart and a lack of gravity isn’t the problem. Furthermore, short of taking the elusive magic pill, there isn’t going to be a way to do this for less cost and mass than exercising (with bands). If that pill were developed in America, where we allow a guy to buy a drug for AIDS and overnight increase the price from $13.50 to $750 #FreeMarketForTheWinAmIRight?, exercise would probably still be cheaper.

It will likely be that even in space, with humans potentially illustrating themselves as the most advanced species there is, we can’t avoid the necessity of regular, moderate and significantly intense, loaded more than the force of even Earth’s gravity, movement.

Enter your email address to follow this blog and receive notifications of new posts by email.