First, I think the challenge of getting the mass where you want to go is the least of the challenges facing us. We may well be able to do that by the late 2020s. I think by far the biggest challenge is reliable life support, supply of food, oxygen, scrubbing CO2, and also safety in case of fire, loss of pressure, and releases of toxic gases within a small enclosed environment. Health in zero g is a bit of a red herring, I think, as probably they will use artificial gravity. All that research into how to survive for years in zero g, exercising for two hours a day and using drugs, may be just not needed at all for interplanetary flight.
As for cosmic radiation protection and solar storm protection, I think a shelter sufficient to protect during a solar storm doesn’t seem too hard to do. For the more penetrating cosmic radiation protection then it involvees decisions about where to place the fuel and water. For instance if you shield sleeping quarters and the places where astronauts spend most of the time, that deals with most of the issue. Also, it is basically a matter of how much mass you can carry, and it seems likely that we will be able to build more and more powerful rockets to solve such issues.
But powerful rockets will not solve the problem of reliable life support. We have to tackle that directly.
WHY ZERO G IS A RED HERRING
At the moment NASA, ESA, Russia are all devoting lots of research into ways to survive for years at zero g, exercising two hours a day. But there have been almost no experiments on use of short arm centrifuges or long arm tethers - and what experiments there were done show that humans can tolerate spinning motions in space conditions far more easily than on Earth.
Here is Tim Peake spinning at about 60 rpm in the ISS. for a couple of minutes, no nausea, only momentary dizziness when he stops.
He says he is pretty sure he couldn't tolerate that on Earth. So anecdotally it suggests that we can tolerate very high spin rates in zero g. Taking the radius as 0.25 meters at a guess, his head and feet will be both under full g, his torso around zero g as he spins. Could he spin like this indefinitely? If so, it's very promising I think for the use of a short arm centrifuge to counteract health issues of humans in zero g.
For more about this see my Small centrifuge based artificial gravity experiments in LEO in Case For Moon First
Also for longer tethers, see Joe Carroll's tether experiments in artificial gravity - which we could do right now
So I think myself that probably the challenge of zero g will be solved rather easily as soon as we do a few experiments in artificial gravity in space.
CLOSED SYSTEMS FOR INTERPLANETARY TRAVEL
The next bit though is far tougher. We have nuclear submarines that in theory can stay submerged for twenty years - but they need resupply of food from the surface (don’t grow their own food) and they get all their water from the sea and have hundred megawatt scale nuclear power plants (Nuclear marine propulsion). And if there is an emergency, they just need to surface to escape to small boats floating on the sea in a habitable breathable atmosphere.
Going on an interplanetary mission is far far harder to make safe than that.
First, I used to think that such a mission would need to use biological closed systems. But if you do the calculation, then actually a system like the one used for the ISS works better up to around 500 days. Though there is more supply needed for a mechanical system, you don’t have the overhead of the greenhouses and other extras needed for a biological system. The biological approach pays for itself many times over once you get to missions well over two years. You’d never use an ISS type system if you wanted to be away from Earth without resupply for a decade.
But even with triple redundancy, the ISS system scores over the biological one for shorter missions. Though some hybrid system involving algae, especially as a backup, may be useful, and of course it is likely to help with crew morale to have some fresh food they grow in a small on board greenhouse.
You could use a biological system for a two year mission, as it more or less breaks even with the ISS system, but at present the ISS approach is more thoroughly tested. Longer term I think the biological system may be more robust, because you can grow a plant from a seed, and algae from just a single microbe. There would be no diseases of plants in space, easy to eliminate them as for aeroponic and hydroponic farming on Earth which doesn’t have any problem with plant diseases. You could never replicate a mechanical life support system from a seed like that. So long term I think the biological approach is likely to be more robust and used as the main system with mechanical systems just as backup, but for now, mechanical systems are the best tested.
For details, see my:
- Sending humans to Mars for flyby or orbital missions - comparison of biologically closed systems with ISS type mechanical recycling (also relevant to long duration lunar missions)
- Conclusions and implications of these calculations - an ISS style mission to Mars seems possible
- Hybrid system - adding algae to an ISS type system
So you might think “Okay, zero g is probably not a problem, we just need to do some experiments in artificial gravity and we may sort it out - and we know how to do life support for the ISS long enough for a mission of two years or so - so we are good to go are we not?”
Well in principle, yes. But in practice, no. The problem is that the ISS has not been as reliable as you might hope, and it would also be a totally new spaceship, and would need to be tested a fair bit before you know it is reliable.
The ISS was a collaboration of ROSCOSMOS and NASA both with several decades of experience with life support in space and space stations. Yet many things have gone wrong with life support on the ISS, some of which would undoubtedly have been fatal on an interplanetary mission.
FAILURES OF LIFE SUPPORT AND CRITICAL SYSTEMS ON THE ISS
The ISS has had many failures of its life support systems and other systems. And the events like a fire, depressurization, or toxic gases which on the ISS would lead to an abort to Earth in contingency planning and the reason they have to have lifeboat Soyuz TMA attached at all times sufficient for all the crew would be end of mission death of all the crew for an interplanetary mission.
Soyuz TMA-19 (emergency lifeboat for three crew) docked to the Rassvet Mini-Research Module with Progress resupply mission in the background. The ISS is required to have sufficient “lifeboats” for its entire crew attached at all time. The crew can board this and return to the Earth in an emergency, and the lifeboat has sufficient supplies for the short journey back.
The most serious is the Elektron failure for the ISS in 2004 and 2005, with the second one requiring the crew to breath oxygen from a docked progress resupply mission. Bear in mind that this is a system devised by the Russians who had a lot of prior experience in life support on board MIR - arguably more experience than NASA had with Skylab.
Then there's the failure of urine recovery system in 2008 and 2009
Lots of other equipment failures over the years that were not critical but you can imagine some of them could have been in a multi-year mission.
International Space Station maintenance - Wikipedia
Those include computer failures, and toxic gas releases, e.g. an ammonia leak.
This is another issue from 2011: Astronauts service station's air purifier, oxygen generator
Here is a more recent issue from 2013 for the ISS
"Minor issues with elements of the International Space Station (ISS) Life Support hardware continue to show the need for highly robust systems on spacecraft bound for Beyond Earth Orbit (BEO) missions. A problem with a valve on the Carbon Dioxide Removal Assembly (CDRA) is continuing to require mitigation from ground crews."
ISS hardware issues providing lessons to be learned for BEO missions
An issue like that is not life threatening on the ISS. The space station is required to have “lifeboats” attached to it at any time, sufficient to fly all the crew back to Earth. At present these are the Soyuz TMA spacecraft. In the event of a serious risk of any sort, the first thing the crew will do, if they can’t fix it right away, is to retreat to the Soyuz TMA. They are then safe, can return to Earth if needs be, and then can work on trouble shooting the problem without putting their own lives at risk.
On an interplanetary spacecraft, an issue that would be minor on the ISS, like a partial failure of life support, could be a disaster. Once an interplanetary spacecraft has left Earth on a Hohmann transfer, there is no way to go back again. Even a failure of life support on the very day they leave Earth orbit would mean they have to come back to Earth via Mars, or Venus or wherever they are going, just as the Apollo 13 astronauts returned back to Earth via the Moon.
It’s one thing to last for a few days with a damaged, only partially functional life support system. The Apollo 13 astronauts survived by using the oxygen designed to refill the lunar module after each EVA, as well as the CO2 scrubbers routed through the lunar module systems through McGyver style ingenious use of duct tape. It’s another matter altogether to somehow survive like that for two years.
If they can’t fix it in situ, enough to last out for two years, they will be doomed. There will be nothing anyone on Earth can do to help.
EMERGENCY SITUATIONS
The ISS EMERGENCY OPERATIONS paper covers emergencies that could arise on the ISS requiring immediate evacuation.
The main categories are: depressurization, fire, and toxic gas release.
On the ISS, if any of those things happen, then you retreat to the Soyuz TMA as usual. On an interplanetary cruise, there is nowhere to retreat to.
Coast Guard Lifeboat practicing in the big surf just outside and south of the Morro Bay harbor mouth, California. Photo © 2012 “Mike” Michael L. Baird
We can equip a habitat on the Moon with lifeboats for the entire crew, supplied with provisions for two days, to get back to Earth in an emergency.
For a typical Hohmann transfer orbit, the crew are on their own, even one hour after their spaceship sets off for Mars. It won't have enough delta v to reverse course and return to Earth at that point, and there is no other way to get the crew back quickly with present day technology either. They would have to come back via Mars.
This makes the Moon far far safer than Mars or any other interplanetary flight for a human crew in the near future.
I think there might be something you could do about that. That is to travel in convoys, of three or more spacecraft at a time. Then if there is a disaster in one, the crew can transfer to the other two. Make sure that each spacecraft has enough supplies for all three of them. So triple triple redundancy - each spacecraft triply redundant, then three of them fly at once.
That could be combined with tethers for artificial gravity - the spacecraft and their final stages could be tethered together around a common hub, maybe with air beam type passage ways joining them together, so making one big habitat they can all share.
So, I think this is addressable, but it does need careful thought. In the near future, there is no realistic way that the crew could evacuate back to Earth or anywhere else habitable in the case of fire, depressurization or toxic gas release.
HUMAN FACTORS ON INTERPLANETARY CRUISE WITH ORDINARY FOLK ON BOARD
There’s one other issue that I think might arise especially if we have a spaceship with a hundred ordinary folk in it as for Elon Musk’s ideas. Astronauts for the ISS are comfortable with a clear chain of command, are prompt at reacting in an emergency, and will do exactly what mission control or the station commander say. When astronauts are told by mission control to cut short the EVA, they don’t say “Oh, please let me stay a bit longer, I’m sure it is fine” or disobeying orders. They just do exactly what they are told to do with no questions at all. They may give more information to help ground control or the commander to evaluate the situation, but they then just do as they are told. It is one of the things astronauts have to be able to do to be accepted for the ISS - that they must be able to respond to a chain of command like this.
If we have ordinary folk not used to that sort of thing, they may well behave in strange ways in an emergency. They may cause problems and make it harder for the trained astronauts to resolve the situation. And they may also actually cause the emergency, press the wrong button, find themselves somewhere where they shouldn’t, do something that they don’t realize is dangerous.
Another issue that could arise in a long cruise with lots of people is sleep walking. It’s rare but sometimes people will do long complex actions, even drive a car, while fast asleep. Anyone prone to sleep walking like that might well be a danger on an interplanetary cruise.
NEED FOR SHAKE DOWN CRUISE - SUGGESTION OF L2
For all these reasons I think it is vital that the systems are tested closer to Earth first, in a situation where the spaceship can be supplied with lifeboats sufficient for the entire crew to get them back to Earth in an emergency.
This might not be as expensive as you might think. After all, if the whole thing can be a single mission, with no resupply, for interplanetary missions - well it could be a single mission with no resupply for the Earth Moon system too.
Imagine if we had a space stations just above the near and far sides of the Moon? Those are the L1 and L2 positions, where the gravity of Earth and Moon balance to keep a space station poised in equilibrium, needing just minor adjustments to keep in position.
Astronauts there would be ideally situated to explore the surface of the Moon by telepresence. They could assist surface operations - explore the ice at the poles, the vast lunar caves, the lunar far side. They could build radio telescopes using robots that trail wires across the surface. They could prospect for ice, for metal deposits such as platinum, for thorium and uranium etc. They could search for meteorites from early Earth - an estimated 200 kilograms of meteorites per square kilometer of the lunar surface, which might also include meteorites from Mars, Venus and Mercury and not just recently ejected meteorites but ones that left those planets billions of years ago and have just been sitting on or near the surface of the Moon ever since. They could find ones that preserve organics too in the ice deposits at the lunar poles.
This would be a very low cost mission as, to simulate an interplanetary mission, you send them as a single mission, launched from Earth, then leave them there without resupply of any sort for a couple of years.
I would predict that the first attempt to do this is likely to turn up many problems, just as with the ISS. If so, you resupply from Earth, if necessary you return the astronauts to Earth in an emergency, and work on improving the design.
This step by step approach was the key to the success of Apollo. Even Apollo 10, which went right down nearly to the surface and returned to EArth without landing, turned up issues that might well have doomed Apollo 11 astronauts if they hadn’t found them in Apollo 10.
In the same way, we need to do many precursors for interplanetary missions. But these precursors will each take several years, rather than several days. For that reason, I think you are talking about probably a decade or two before we are ready for safe interplanetary flight. Of course if we are lucky and the first spacecraft are just flawless, everything works perfectly, then it might be faster than that. But I think safest to assume that things turn up that need to he fixed in the shake up cruises.
Meanwhile though, we are exploring the Moon, a fascinating place, even biologically. And it’s also a great place to try our first experiments in life support on the surface. It may be the easiest place of all for astronaut gardening. See my
- An astronaut gardener on the Moon - summits of sunlight and vast lunar caves in low gravity
For more on all this see my
and my two books
MOON FIRST Why Humans on Mars Right Now Are Bad for Science
and
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