One of the most hotly debated topics related to sending humans to Mars is the health effects of prolonged exposure to microgravity and how these might be mitigated.
Prolonged exposure to microgravity (a.k.a. “zero gravity”) has several serious effects on the human body:
- Without the need to support the weight of the body, the musculoskeletal system atrophies and weakens. Bone and muscle mass tend to decrease at a significant rate. Bone mass can decrease at 1-1.5% per month.
- Under normal gravitational forces blood pressure is highest at the feet and legs, and lowest in the head. In microgravity, however, blood pressure is distributed evenly along the body length. The sustained, higher blood pressure in the head can affect brain function and vision.
- Without the need to pump blood around the body against the force of gravity, the heart does not need to work as hard and can therefore also atrophy and weaken.
The longer a person spends in microgravity the more serious these effects become, and the longer it takes to recover after they return to Earth.
H2M (Humans to Mars) and microgravity
A trip to Mars based on a long-stay, or conjunction-class, mission profile using normal chemical propulsion involves approximately a year in space – approximately 6 months outbound and about the same return. A short-stay mission requires around 20 months total in space. This is a significant amount of time to spend in a microgravity environment.
Only two people, both Russians, have spent more than one year in space: Valeri Polyakov’s 438 days in 1994-5, and Sergei Avdeyev’s 380 days in 1998-9. Therefore, we only have a small amount of data about such long-term exposure to microgravity, and much of this data is more than a decade old. Although a variety of techniques for mitigating the adverse effects of microgravity have been developed during the past decade, we have minimal data about spending a year or more in microgravity with access to these.
In addition to a year in space, the crew must also spend 1.5 years on the surface of Mars, which is also a reduced gravity environment (0.38g). Living on Mars surface will reduce the load on the body by 62% and is therefore expected to have effects proportionally similar to microgravity.
The primary concern is that the crew will return to Earth after 2.5 years in reduced gravity environments, be unable to fully recover, and have to spend the rest of their lives in wheelchairs. Apart from the fact that we wouldn’t wish this on our beloved interplanetary explorers and heroes of humanity, such an outcome would hardly add to the glory of space exploration, and may become a deterrent to future human exploration.
An almost equally important concern is that the crew will spend approximately 6 months in microgravity during the outbound trip, arrive at Mars surface, and be unable to do any useful work – thus invalidating the mission.
The problem of microgravity in an H2M mission is hardly a minor one. In conjunction with the similar concerns about health effects of radiation in interplanetary space, it’s considered by many one of the top reasons why not to send humans to Mars.
H2M and artificial gravity
Because of these concerns, some H2M mission designers include AG (Artificial Gravity) in their architecture. The simplest way that we know of to create a feeling of gravity is through the use of centrifugal force, which we can produce by spinning either part or all of the spacecraft. Unfortunately centrifugal force is not a perfect substitute for actual gravity. From DRA5:
“Adverse physiological changes due to reduced gravity may be prevented by exposure to some level of artificial gravity, but the specific level of gravity and the minimum effective duration of the exposure that is necessary to prevent deconditioning are not yet known. Although artificial gravity should reduce or eliminate the worst deconditioning effects of living in zero gravity, rotating environments frequently cause undesirable side effects, including disorientation, nausea, fatigue, and disturbances in mood and sleep patterns. If artificial gravity is to be employed, significant research must be done to determine appropriate rotation rates and durations for any artificial gravity countermeasures. The decision on whether artificial gravity must be employed to adequately support crews on their transits to and from Mars, as well as the decision on the necessary gravity level and rotation rate, has significant implications for vehicle design and operations.”
So in a sense, trading microgravity for artificial gravity is really just trading one set of health problems for another, albeit less serious. The severity of the negative effects of centrifugal force can be mitigated by increasing the radius or decreasing the rate of rotation. A spinning cylinder such as that shown in the movie Mission to Mars may seem fun, but with such a small radius these effects would be quite noticeable.
The problem is that implementing AG in an early-stage H2M mission adds considerable complexity, mass and cost, and these are exactly the kind of things we want to minimise as much as possible. It’s a classic trade-off, and hence the debate. The question is whether AG will produce a better result than a regimen of PT (Physical Training), food and drugs.
AG in Mars Direct
In Mars Direct:
“Artificial gravity is provided to the crew on the way out to Mars by tethering off the burnt out Ares upper stage and spinning up at 1 rpm.”
In this way a comparatively large radius of rotation of about 340m can be created, which would seem to address the side-effects of centrifugal force. However, consider what this idea means for the architecture:
- The hab is designed for a gravity environment: AG in space and Mars-g on Mars. That means it will have a floor, ceiling and walls, with cupboards, screens, controls, etc. mainly on the walls. The result is a less compact and heavier spacecraft (or one with fewer fixtures/features).
- Although designed for a gravity environment, after launch the hab will be in microgravity until the AG is set up. In addition, if the AG system fails and the tether must be dropped for any reason, the hab should continue to Mars in a microgravity mode, despite not being designed for that.
- There is a risk the counterweight could crash into the hab, and it may not be possible to prevent this even by dropping the tether.
- The mass of the cable must also be launched (more mass, more fuel, more cost).
- Communications with Earth are more difficult with a spinning spacecraft, as it’s more difficult to keep antennas pointed in the right direction without sacrificing signal strength. Communications with Mars are already up to one million times more difficult than with the Moon, since Mars is up to 1000 times as far away (signal strength decreases with the square of the distance).
- With a spinning spacecraft it’s more difficult for navigation sensors to track the position of Earth, the Moon and stars.
- Collecting solar energy with solar panels is more difficult and possibly less efficient, as they should ideally be always directly facing the Sun.
- The hab and the counterweight both require an RCS (Reaction Control System), which must work in tandem in order to produce a stable spin around a common centre of gravity, or for any course manoeuvres.
- Additional fuel is required for RCS on both the hab and the upper stage.
- Additional fuel is required to send not just the hab, but also the upper stage, on TMI (Trans Mars Injection).
- Course corrections and other manoeuvres are more difficult, as both the spacecraft and counterweight must be pushed in the same direction at the same time, without overstressing and breaking the tether, or causing it to stretch and rebound, or causing it to become slack. Plus, the dynamics of the spinning masses must also be accounted for.
- As the assembly approaches Mars it must be spun down and the tether dropped before aerocapture. However, this will leave the spacecraft on a trajectory that is difficult to predict in advance with precision, making it more difficult to calculate the required burn to place the spacecraft on an aerocapture trajectory. It may have to be re-calculated in real time, which is risky. Yet for aerocapture to work, the hab must hit the atmosphere at exactly the right angle and altitude. EDL (Entry, Descent and Landing) is already a very technically challenging and dangerous process, especially in a hab – this additional factor makes it even more dangerous.
- Once the assembly must be spun down, the interior of the hab will return to a weightless environment, despite not being designed for that.
- If the tether breaks for some reason (e.g. micrometeoroid) the hab and counterweight will fly off in different directions. The hab must carry additional fuel to make a course correction if this happens. In addition, the ground crew must be prepared to track and communicate with the spacecraft (which may be on an unknown trajectory) and assist with on-the-fly calculations to determine course correction manoeuvres. Otherwise it will mean LOC (Loss Of Crew).
Many of these items will also apply to other AG solutions. Several could be addressed by using a rigid telescopic truss instead of a tether, but that would have a high mass and therefore be an even more expensive solution. There’s got to be a better way! The rest of Mars Direct is comparatively much simpler.
Note that Mars Direct does not use AG for the return trip, but only the outbound portion of the mission. Therefore the crew still must spend about 6 months in microgravity.
AG in DRA5
DRA5 doesn’t commit to using AG for a long-stay architecture. It only states that AG should be used if a short-stay mission class is selected, due to the longer time (~600 days) spent in space.
Microgravity in Blue Dragon
AG is not used in Blue Dragon. The crew will travel through space for about 8-12 months in microgravity. There are several reasons for this decision.
One of the main obstacles to sending humans to Mars (other than concerns about microgravity health effects) is cost. The cost of a Mars mission is driven by two main factors:
- The amount of mass to launch and send to Mars, which determines fuel requirements, rocket sizes, number of launches, and size/mass of major components.
- The complexity of the architecture and hardware, which drives cost of development time, including engineering, manufacturing and human resources.
As the above review of the AG solution presented in Mars Direct shows, using a tether and centrifugal force to produce AG requires greater mass in several ways:
- Mass of the tether.
- Mass of additional fuel for RCS.
- A spacecraft designed for a gravity environment must be bigger/heavier, or have less features, than one designed for microgravity.
- The spacecraft must be more complex (see below), which will also make it heavier.
It increases complexity in numerous ways:
- Navigation systems.
- Solar panels.
- Major manoeuvres: TMI and MOI (Mars Orbit Insertion)
- Tether design and operation.
It’s difficult (at least, for me) to quantify exactly what the additional cost would be to implement an AG solution like the one proposed in Mars Direct. However, what we can assume is that Blue Dragon will likely cost billions of dollars in hardware and development costs. While AG will surely be a feature of future missions once launch costs decrease and launch vehicle capabilities increase further, omitting it from at least the first few will greatly reduce costs without greatly increasing risk.
Although you could say that AG reduces the risk that the crew might reach Mars or Earth in less than tip-top physical condition, the Mars Direct solution adds other risks, such as:
- Hab crashing into counterweight.
- Tether breakage.
- More complex course manoeuvres could send the spacecraft in the wrong direction.
- Harder to track and communicate with spacecraft.
- More risky/complex EDL.
More risk means more failure points, potential emergency situations, abort modes, development costs, and training.
The risks associated with microgravity health effects, however, are comparatively well-known and easy to address.
- The AG solution described in Mars Direct is only valid for the outbound trip anyway, so it’s not a complete solution. Maybe they’ll get to Mars well-adapted to Mars gravity, but on return to Earth they will be in almost the same condition as if no AG was provided at all.
- The Mars Direct AG solution only produces Mars-level gravity. This is still a reduced-gravity environment and therefore the crew will still experience some degree (perhaps ~62%) of the usual effects of microgravity. You could say that overall, this idea only goes about 19% (38% * 50%) towards addressing the microgravity problem.
- By this stage we have many human-years of experience with microgravity thanks to the International Space Station, Mir, and other space stations and missions before that. Similarly, we have experience with many astronauts and cosmonauts fully recovering from their time in microgravity. Commander Chris Hadfield returned a few days ago from the ISS after 5 months in microgravity, and is experiencing dizziness and weakness. However, he is in good humour, giving interviews, and reports feeling better “by the hour”.
- By constructing Abeona (our MTV) on orbit rather than sending a hab on TMI from a single launch, Abeona‘s cruise stage can be provided with additional fuel to reduce the total time spent in space to less than one year and perhaps even less than 10 months.
- NASA and JAXA successfully demonstrated in 2011 that osteoporosis drugs can be used to mitigate bone density loss. Without the drug Fosomax, the average bone density loss was 7% in the femur and 5% in the hip; with the drug, the loss in the femur was only 1%, and hip bone density actually increased by 3%.
- If the crew travel to and from Mars in the same vehicle and in the same gravity environment, the return trip will be much more comfortable, because they’ll already be used to Abeona and will have made it homely. Personal solutions and routines for dealing with microgravity will already have been developed during the outbound trip.
- On arrival at Mars, it’s estimated that it will only take 1-2 weeks at the most for the crew to adapt from microgravity to Mars gravity. Since they have 18 months on the surface, this is only 2.5% of the total time on Mars and quite acceptable. The most important thing is to design the architecture so that the crew will not be required to do any strenuous work during the first few weeks. This is exactly what we’ve done with Blue Dragon, in which the hab is landed 26 months earlier, powered up, checked out and inflated.
- Considering the current state of medical research, especially in nanotechnology, prosthetics and replacement organs, the exponential pace of technological development, and the hero status of the first Mars astronauts, it’s safe to say that in 20-30 years, in the very unlikely event that that do arrive back at Earth in a really messy, unrecoverable state of health, there will be a multitude of techniques to restore them to full health, strength and mobility.
The conclusion is hopefully clear: attempting to implement an AG solution for an early stage H2M mission simply isn’t worth it.
Benefits of travelling to Mars in microgravity:
- More compact and/or full-featured MTV (Mars Transfer Vehicle).
- Microgravity is fun.
There are, of course, other ways to create AG, for example, using a spinning cylinder, or by separating the MTV and a counterweight with a fixed steel truss; however, these each come with their own similar set of drawbacks. The most important are cost and complexity. For the first few human missions, we’ll be just fine without AG.
While not a complete solution to the effects of microgravity, the most important strategy is daily exercise.
As a qualified personal trainer and lifelong fitness and bodybuilding enthusiast, I’m fortunate to have some specialised knowledge about resistance training, also known as strength training. The simple fact is that the human body responds to loading. If you unload it – for example, by spending time in microgravity – the body will adapt by losing muscle and bone mass, as this isn’t needed to carry the weight of the body. If you load the body – for example by lifting weights several times per week – the body will adapt by increasing muscle and bone mass.
Although there’s a vast body of knowledge around fitness and bodybuilding, once you trim away all the cruft there are really only a few basic principles:
- The body responds to loading. This is a function of two things:
- The amount of weight it’s loaded with. The more weight the body is loaded with, the more it will be forced to grow stronger.
- The amount of time it’s loaded (also called “TUT” or “time under tension”). The more time the body spends loaded, the more it will be forced to adapt.
- Nutrition is a critical factor, especially protein. Protein should be the primary macronutrient in the diet, as it provides the building blocks (amino acids) to synthesise new muscle tissue for growth and repair. A balance of other nutrients is also essential: complex carbohydrates for energy and recovery, minerals for muscle function, essential fatty acids (EFA’s), vitamins, and lots of water.
Someone can get started in bodybuilding knowing just two things: lift weights, eat protein.
Strength training involves performing a range of exercises to train all the muscles in the body. But is there an exercise that simulates loading due to gravity? Yes, and this exercise is well-known to bodybuilders as being the number one exercise for loading and therefore building almost the entire body: squats. Squats are called the king of exercises because they load the entire load-bearing kinetic chain of the body – the same muscles used in standing, walking, jumping and running. Squats load the body in almost the same way as normal gravity (vertically downwards), but with a greater load. They will surely be revealed as the number one exercise for reversing the deconditioning effects of microgravity.
The intention for Blue Dragon is for the crew to spend 1-2 hours per day doing strength training. Not only squats, because that would be a bit boring and would lead to over-training, but all the so-called “Big 5” exercises: squats, deadlifts, chest press, shoulder press, and chin-ups.
Spending only 5% of each day with the body partially loaded may not seem like enough to offset the effects of microgravity, unless the body is loaded with a larger force than normal Earth gravity. This is, of course, what we do in the gym in order to stimulate muscle growth. By loading the body with heavy weights during training, it becomes stimulated to increase muscle mass. We can use the same principle in space.
Usually when we talk about resistance or strength training, the goal is muscle growth and fat loss. But one of the major problems with microgravity is loss of bone density. Strength training will help with this, too. Research shows that resistance training produces noticeable increases in bone density, and is therefore sometimes recommended for women suffering from osteoporosis. Actually many other conditions have been shown to benefit from strength training, including cardiovascular disease, diabetes, and high blood pressure.
But how can we lift weights in microgravity? Resistance doesn’t have to come from lifting lumps of iron against gravity. There’s plenty of commercial gym equipment based on hydraulics, which will work perfectly well in a microgravity environment.
There’s something else we can do in microgravity to stay fit: yoga. Yoga is well-known to have a huge range of benefits for the entire body. Can we do yoga in microgravity? Yes, as long as we have space. Also, yoga positions that emphasise balance won’t apply in microgravity. But flexibility, breathing, clearing the lymphatic system (detoxing), releasing stress stored in the spine and muscles, and the many other benefits of yoga can be gained.
Because it’s easier to push blood around the body without gravity, the heart doesn’t have to work as hard. This can cause a decrease in heart mass. But this can be mitigated by regular cardiovascular activity. The heart must be exercised like any other muscle, but it’s exercised differently, using cardiovascular exercise instead of resistance training. The heart rate must be elevated. Although this is achieved for intermittent periods with strength training, cardiovascular exercise serves to sustain an elevated heart rate for longer periods.
EHA (Extra Habitat Activity) on Mars
While on Mars for 1.5 years, the crew will be engaged in considerable additional exercise in the form of regular, perhaps daily, EHA. This will frequently last several hours, perhaps all day in some cases, and during this time they’ll be wearing a MSS (Mars Surface Suit) also known as a marssuit.
The marssuits will probably weigh about 50kg. The spacesuits used by the Apollo astronauts weighed about 100kg, but in the lunar gravity, which is only about 1/6 of Earth’s, they felt like only about 17kg. This is almost what a full traveller’s backpack weighs, and perhaps half what a soldier’s backpack weighs. It’s hard to imagine spending hours walking around and doing hard-core science and exploration on Mars while carrying more than around 20kg, which is why one of the goals for H2M is a marssuit design of 50kg or less. A 50kg marssuit will feel like 19kg on Mars.
The average human weighs 62kg. With a marssuit on, an astronaut on Mars will weigh about 110kg – but they’ll feel like they weigh a total of about 42kg. Thus, they’ll be carrying about 2/3 their normal weight instead of only about 1/3. Going on EHA will be an important part of the necessary physical conditioning to mitigate bone and muscle atrophy. And let’s be honest: if you have a choice between going EHA on Mars and doing squats in the hab, which would you choose?
Blue Dragon daily program
The health and fitness program for the Blue Dragon crew will be something like this:
- PT 6 days, rest 1 day.
- Morning exercise: yoga or cardio (alternating days).
- Afternoon exercise: resistance training or EHA.
- Appropriate osteoporosis drugs and supplements (e.g. calcium) to mitigate bone loss.
- A high protein diet to minimise muscle loss and support muscle growth and repair.
- Plenty of minerals, especially calcium and magnesium.
- Plenty of water, and a healthy balanced diet.
- Sufficient sleep.