. CASE FOR MOON FIRST - 11 This approach doesn't mean that humans can never land on Mars ever
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THIS APPROACH DOESN'T MEAN THAT HUMANS CAN NEVER LAND ON MARS EVER
The idea isn't at all to prohibit humans on Mars. The humans are not the problem; it's only their microbes that are. And the idea is to do it step by step and to make sure we understand Mars and understand the implications of our actions before making a decision about whether it is okay to have human boots on Mars.
I'm a spaceflight and science fiction enthusiast myself and I'd love to be able to cheer on humans on an expedition to Mars. Just for the childlike wonder of seeing humans doing things like that in space. So it would be fun to see humans go to Mars. And at least we can send them to Mars orbit whatever we might discover about the surface - so long as it is done with care to make sure that they can't crash on Mars.
SAFE WAYS TO GET HUMANS TO MARS ORBIT OR ITS MOONS TO AVOID ANY RISK OF CRASHES ON THE SURFACE
You couldn't do aerocapture in the Mars atmosphere as a way to get into orbit. It would be far too risky. Also Hohmann transfer with insertion burns are too risky also, as the insertion burn is done as close to Mars as possible to reduce the amount of fuel needed due to the Oberth effect. So you would need to be very sure that the insertion burn can't go on too long and end up on an impact trajectory with Mars.
I suggest ballistic capture is a far better method for human missions to Mars. The idea is that you launch the spacecraft to arrive ahead of Mars at just the right point for it to capture you as a temporary satellite. Once you leave Earth, you are already on a trajectory that ends up with your spaceship getting captured temporarily in a distant Mars orbit when it gets there, with no need for an insertion burn. Then once you are in that orbit, you use ion thrusters to spiral down to lower permanent orbits around Mars.
This is surely the safest of all the ways proposed to get into a Mars orbit, and the best way to prevent a crash of a human occupied spaceship on Mars.
Then you also have the flybys. Flybys are safe because although they involve precision targeting, you have months to set the target up. Also, the ones that are of most interest for Mars are free return, so even if your rocket fails, you are still on an orbit that will take you back to Earth again. You would use trajectory biasing of course, so that as you leave Earth you are biased away from Mars rather than towards it and use fine adjustment then to target the flyby orbit.
We have done many flybys, delicate ones, repeatedly for Saturn's moons with Cassini, and get them right every time, so it is obviously one thing we know how to do reliably. This has no time critical insertion burns. Just gentle thrusts nudging until you are in the right trajectory, which you set up long in advance of the actual flyby.
So, especially Robert Zubrin's double Athena flyby - a very interesting mission - is safe for humans to Mars. This has two flybys of Mars. The first diverts you into an orbit that closely parallels Mars for half of its year, so a full Earth year. The second flyby takes you back to Earth 700 days after the launch. It's free return - once you leave Earth you are already on a trajectory that will take you back to Earth 700 days later even if your rocket motors fail completely.
It's a great orbit for telerobotics as you spend several hours close enough to Mars for direct telepresence with each flyby, and days close enough for significant advantages relative to Earth, and over the entire one year period when you are almost paralleling Mars in its orbit, your crew are much closer to it for controlling robots on the surface than anyone on Earth.
TELEROBOTICS AS A FAST WAY FOR HUMANS TO EXPLORE MARS FROM ORBIT
Telerobotics lets us explore Mars much more quickly with humans in the loop. And you'd use an exciting and spectacular orbit for early stages of telerobotic exploration of Mars, following the HERRO plans. It comes in close to the poles of Mars, swings around over the sunny side in the equatorial regions and then out again close to the other pole, until Mars dwindles again into a small distant planet - and does this twice every day.
Imagine the view! From space Mars looks quite home-like, and the telerobotics will let you experience the Martian surface more directly than you could with spacecraft, actually touch and see things on the surface without the spacesuit in your way and with enhanced vision, blue sky also if you like. It's like being in the ISS, but orbiting another planet.
12th April 2011: International Space Station astronaut Cady Coleman takes pictures of the Earth from inside the cupola viewing window.- I've "photoshopped" in Hubble's photograph of Mars from 2003 to give an impression of the view of an astronaut exploring Mars from orbit.
This is a video I did which simulates the orbit they would use - in orbiter. I use a futuristic spacecraft as that was the easiest way to do it. Apart from that, it is the same as the orbit suggested for HERRO.
It would be a spectacular orbit and a tremendously humanly interesting and exciting mission to explore Mars this way. The study for HERRO found that a single mission to explore Mars by telepresence from orbit would achieve more science return than three missions by the same number of crew to the surface - which of course would cost vastly more. Here is a powerpoint presentation from the HERRO team, with details of the comparison.
Then, you'd also have broadband streaming from Mars. As well as being very safe, also comfortable for the crew, you'd also have wide-field 3D binocular vision. It's amazing what a difference this makes, I recently tried out the HT Vive 3D recreation of Apollo 11. We'd have similar 3D virtual reality experience of the Mars surface.
Also, it would actually be a much clearer vision than you'd have from the surface in spacesuits, digitally enhanced to make it easier to distinguish colours (without white balancing the Mars surface is an almost uniform reddish grayish brown to human eyes)|.
Here is this hololens vision again, which though it's not telepresence, I think gives a good idea of what it might be like for those operating rovers on Mars in real time from orbit, some time in the future with this vision.
See Scientists Can Virtually Wander Around Mars for Miles with HoloLens .
It's safer too. No need to suit up. No risk from solar storms - at worst you have to go to a storm shelter in your spaceship, not rush back to your habitat as fast as you can to get out of the storm in time. No risk of falling over and damaging your spacesuit. And when you need to take a break, have your lunch, or whatever, you can just take it up again where you left off, indeed leave the robot doing some task while you have your lunch or sleep.
TELEROBOTICS WITH HUMANS IN ORBIT COMPARED TO ROBOTS CONTROLLED FROM EARTH
That's not to say that humans to orbit controlling robots on the surface would be better than robots controlled from Earth, bearing in mind the costs of the two types of mission. I don't know if anyone has done a comparison study there.
You might be able to compensate for the advantage of humans in orbit by having many more robots on the surface for the same cost, especially if broadband communication is possible, better robotic autonomy, and techniques from gaming such as artificial real time (building up a copy of the Mars surface explored by your robot in your computer on Earth and navigating that to help speed up movement from a to b on Mars).
But a human expedition might well capture the public imagination and so permit a much faster exploration of Mars from orbit. And would be an exciting and fun expedition to follow, and interesting for the crew too.
As a later mission you could then go on to explore Phobos and Deimos. They have many advantages for exploration. For instance Phobos has meteorites and micrometeorites throughout its surface layer of regolith, from the entire history of Mars, back to when Phobos first formed or was captured. This probably includes meteorites from the time when Mars had global oceans and then later on, lakes. Our Mars meteorites on Earth all left Mars no more than twenty million years ago (because the terrestrial planets clear their orbits so NEOs have to be replenished over a twenty million year time period).
Deimos also has a Mars facing crater which helps protect it from cosmic radiation, and solar storms - Mars obscures it from the sun in its local daytime, except for a few hours a day. Deimos may well have ice too, as it is related to a type of asteroid that often does have ice in its constitution.
There are many other advantages and points of interest of Mars' two moons.
For more on this, see my:
Exploring Mars By Telepresence From Orbit Or Phobos And Deimos
So, how soon can we do such a mission? I suggested that while we explore the Moon robotically, we work on closed systems research, and also artificial gravity in LEO. That makes sense for a Moon base which you plan to keep occupied for years on end. But what about a first flyby of Mars? When could we try that?
NEED FOR NEW COMPARISON STUDIES OF THE VARIOUS WAYS OF EXPLORING MARS
The HERRO comparison was just a small scale study, done several years ago. But I don't know of any other. It's surely high time that we had a much more thorough and detailed comparison study of the various possible ways of exploring Mars.
We may get practical experience of telerobotics in space with lunar missions in the near future. When that happens I think we'll find that machines are far more capable than they were in the days of lunakhod, operated from Earth most of the time, semi-autonomous, route finding on their own, able to do many things just by themselves with occasional help from Earth.
In a situation like that - operated remotely from Earth, or semi-autonomous, doing a lot of their own driving from place to place and then the crew in orbit around Mars step in to control robots that need particular help. I think that it would be much more than a 3 to 1 ratio compared with them working directly on the surface in spacesuits.
And everything they saw would be streamed back to Earth in HD meaning that after an astronaut has just walked past a place and maybe glanced at a rock via telerobotics, amateurs and experts back on Earth can explore that footage with the same direct telepresence, binocular vision etc. experience, and maybe alert them to something they missed.
I think a proper comparison study has to take all of this into account. I think a proper comparison study is probably best done by neutral parties or best perhaps, a workshop / panel that includes proponents of both sides in the debate as well as neutral parties. The cost of such a panel or workshop would be peanuts compared to the costs of the missions that we might commit to in the future for the exploration of Mars.
SENDING HUMANS TO MARS FOR FLYBY OR ORBITAL MISSIONS - COMPARISON OF BIOLOGICALLY CLOSED SYSTEMS WITH ISS TYPE MECHANICAL RECYCLING (ALSO RELEVANT FOR LONG DURATION LUNAR MISSIONS)
We don't need biological closed systems or efficient mechanical recycling for short duration missions into space. For very short duration spaceflight, such as the Apollo missions to the Moon, we can use open systems with no recycling at all.
So when does the break-even point come, where it's worth the extra mass to send a system with better recycling into space? Maria Johansson worked it out for us in her thesis from 2006, which I summarize in Could Astronauts Get All Their Oxygen From Algae Or Plants? And Their Food Also?
The area you need to grow all the food for the astronauts is far less than you might expect from field agriculture which typically requires about an acre (4,000 square meters) per person. If you use hydroponics or aeroponics, and rapidly growing crops, this can be reduced hugely. In the BIOS-3 experiments, the Russians needed only 30 square meters of growing area per crew. The crops were grown on a culture conveyor with 2 to 10 plantings of different ages simultaneously. They grew wheat, sedge-nut, beet, carrots, and other crops, ten crops in total. With those 30 square meters per person, they produced 95% of the daily requirement for oxygen, water, food etc (by weight), reducing the supplies needed per day for the crew of three from 13.01 kg to 0.5982 kg. The remaining 5% consisted of animal products, salt for the humans, nutrients for plants and personal hygiene supplies. They could produce 45% of the food and nearly all the oxygen and water with only 13 square meters.
This section is mainly calculations. Skip to the next section if you just want to read the conclusions. I've done the best here using the materials available, but there are gaps in the calculations as you'll see. Maybe it gives a good first idea however, a rough picture to get started and highlight some of the main questions.
The three systems she looked at are BIOS-3 developed by the Russians, MELiSSA which is under development by the ESA and the system used currently on ISS. The main difference between BIOS-3 and MELiSSA is that in BIOS-3 the plant wastes are burnt, while with MELiSSA they are composted (which adds a fair bit to the mass of the MELiSSA system). Burning the plant wastes is not as wasteful as you would think as it just completes the cycle with the CO2 produced incorporated in the next generation of plant growth. Both BIOS-3 and MELiSSA are closed systems which recycle just about all the carbon dioxide, oxygen, and water.
The MELiSSA system is not in the picture for the shorter duration missions, with a startup mass of 15,711 kg for four people - it is a hydroponics system and the water takes up most of the mass at 8.89 tons, and much of the rest is taken by the centrifuges at 2.8 tons.
The ISS used 6.5 kg per day for four people. Or 1.635 kg per person. With a startup mass of 1773 kg. The BIOS 3 system uses 0.5 kg per day for four people, so a tiny amount. But that's with a startup mass of 6250 kg for BIOS-3 .
Note though, that this is before the new approach of recovering the oxygen from exhaled carbon dioxide using the Sabatier reaction which reduces the daily mass requirement but increases the startup mass. It also omits some the mass required for resupply of parts, and spares.
A more recent calculation for the ISS (or as a pdf) makes the startup mass (upper bound) 2,563 kg/person or 10,252 kg startup mass for a crew of four and supplies of 467.1 kg/person/year or 5.12 kg per day for a crew of four.
So, in this section I'll do my best to work it out from those figures, but it probably needs to be looked into in more detail. I can't find a more recent comparison of the mass needed for mechanical recycling with that needed for biological recycling in the literature, Maria Johansson's thesis seems to be the only source to attempt this (do let me know if you know of any other material on the subject).
I think the ongoing logistics figures are probably reasonably accurate here, and the 0.5 kg / day of BIOS 3 for a crew of 4 is a lot less than the 5.12 kg day for an ISS type system. I'm not so sure about the startup masses. She has a lower figure for the ISS than the more recent figures. And for BIOS-3 her figure does not include the mass for the habitats used for growing the plants.
The paper just cited suggests that the BEAM module for the ISS would supply food for a crew of 2, with mass of 1,360 kg for the module, which seems reasonable. So for a crew of four, that would be 2720 kg, so I'll add that to Maria Johansson's figures, making the startup mass 2720 + 6250 or 9320 kg. If these figures are right, then a BIOS-3 type system would score over an ISS type system from the get go with less startup mass as well as providing a somewhat more spacious spaceship with two BEAM modules added to it.
Both are much better than open supply even for short duration flights, as those require 12,775 kg per person per year when you include everything. That's about 140 kg / day for a crew of four. Another source though puts it at 38.1 kg / day for a crew of four. I'll go with the lower estimate here.
So, assuming a crew of four, which I think sounds about right for a very minimal interplanetary mission, then for Dennis Tito style 589 day flyby, that's
589×5.12 +10,252 kg = 13,267 kg for the ISS type system
589×0.5+ 9,320 kg = 9,614.5 kg for BIOS-3
(589×140 = 82,460 kg with no recycling - larger estimate)
589×38.1 = 22,460 kg with no recycling
For a 700 day mission such as Robert Zubrin's double Athena:
700×5.12 +10,252 kg = 13,836 kg for the ISS type system
700×0.5+9320 kg = 9,670 kg for BIOS-3
(700×140 = 98,000 kg with no recycling - larger estimate)
700×38.1 = 26,670 kg with no recycling
If you know of good sources to use to refine these calculations or any other comparisons of the biological systems with the ISS type system since Maria Johansson's thesis, do say!
CONCLUSIONS AND IMPLICATIONS OF THESE CALCULATIONS - AN ISS STYLE MISSION TO MARS SEEMS POSSIBLE
If those figures are approximately right, then biological recycling would save on the ISS system for all durations of mission. Break even for BIOS-3 over a mission with no recycling would happen after = 9320/(140-0.5) or 67 days and for the ISS type system at 10252/(140-5.12) = 76 days. So those forms of recycling are worth doing for missions of more than a couple of months or so, and biological recycling such as BIOS-3 works much better than the current ISS for very long duration missions.
However we know how to do the ISS system right now, while BIOS-3 hasn't been tested in space. Also, BIOS-3 probably wouldn't work so well in zero g as plants behave differently in zero g. If combined with artificial gravity, it looks promising to be better than the ISS system. So if the astronauts were in artificial gravity for health, and also grew crops using aeroponics and hydroponics, then the BIOS-3 system seems likely to work well.
So far we have no information about how well the crop plants of BIOS-3 would do in lunar gravity. They might do fine, or we might be able to breed plants or use genetic manipulation to get plants that grow well in lunar g. Also, just as for interplanetary and orbital missions, we could have artificial gravity to augment lunar gravity if necessary, centrifuges around a vertical axis for the plants, wouldn't have to rotate that quickly, for instance a 1 meter radius centrifuge, 30 rpm, tangential velocity 3.13 meters per second (seven miles per hour) produces full gravity, which seems practical enough for plants in a lunar greenhouse.
Where much more efficient recycling really scores is for long duration missions. That could also make a big difference for safety factors. Suppose you want to add a safety factor of two years to a mission to Mars orbit, in case they are stranded ed in Mars orbit, and need resupply from Earth, only possible every two years. Then the figures for the extra mass needed for a two year extension for four people are:
730×5.12 +10,252 kg = 13,989.6 kg for the ISS type system
730×0.5+ 9,320 kg = 9,685 kg for BIOS-3
730×140 = 102,200 kg with no recycling (using larger estimate of 140 kg / day for a crew of four
730×38.1 = 27,813 kg with no recycling (using lower estimate of 38.1 kg / day for a crew of four).
If you have a longer, ten year mission, it's going to make a huge difference to have biological recycling. This also makes a big difference for the lunar base - once you have people living there for years on end.
These figures change a bit if you also add in the cost for developing new systems. Resupply without recycling has the least development costs.
For a long mission such as a mission to Mars with no "lifeboats" to get you back to Earth, then you also have to factor in the need for redundancy. Adding triple redundancy for everything for the start up mass, then for a 700 day mission for four, you get
700×5.12 + 3×10,252 kg = 34,340 kg for the ISS type system
700×0.5 + 3×9,320 kg = 28,310 kg for BIOS-3
700×140 = 98,000 kg with no recycling (using larger estimate of 140 kg / day for a crew of four)
700×38.1 = 26,670 kg with no recycling (using lower estimate of 38.1 kg / day for a crew of four).
The no recycling option still requires a lot more mass. But with triple redundancy, it can actually be less mass than the recycling options. Then as well as that, you can throw away the mass as it is used up, so by the time you get to Mars, the amount you need to boost back to Earth is reduced, so less fuel is needed to accelerate your spacecraft if you need an extra burn to get back to Earth (this will make no difference for free return flyby missions however as all the acceleration is done at the outset as you leave Earth).
For a Mars base, if we did send humans to the surface, resupply works better than recycling for short missions of 500 days or so. Here is a breakdown of the costs for the various styles of mission, including cost break downs for lunar missions: The Life Cycle Cost (LCC) of Life Support Recycling and Resupply.
HYBRID SYSTEM - ADDING ALGAE TO AN ISS TYPE SYSTEM
You could also use a hybrid mission, with some biological and some mechanical components, which you could do because the ISS type system is the one most thoroughly tested to date, but it could be worth adding some easily testable biological systems. A simple form of redundancy would be to add algae to a mechanical system like the ISS.
Algae are extremely low mass for the amount of recycling they do. By the figures for Bios-1, a Russian experiment carried out in 1965, you require 18 liters of algal culture per person to scrub the atmosphere and provide all the oxygen they need. So around 18 kg per person, or just 54 kg for the entire crew of six. You also need containers, and solar collectors. It needs to have a surface of eight square meters exposed to light per person which you achieve by filling the container with light pipes to pipe light into its interior. So you also need light tubes to pipe light into the middle of the mixture. For the algae, they used chlorella which is toxic and inedible by humans, but does a great job of producing oxygen.
This shows the chlorella cultivar used in the originally rather secretive Russian BIOS-1 experiment. Image from this paper. I can't find much by way of details of its construction so far and how it worked. But the principle was simple - supply lots of light to Chlorella algae and it photosynthesizes, absorbs carbon dioxide and produces oxygen. In modern designs, they do this by piping light into the container using light tubes, which gives you a compact design.
In the first experiment it was in a separate room, tended from outside, and supplied all the oxygen for a single volunteer. In BIOS-2 they put the cultivar inside the habitat, recycled other wastes as well, and produced some crops. In BIOS-3 they made the habitat larger, with a crew of 3, and changed to growing crops as their sole source of oxygen.
I can't find a total mass for the equipment as the BIOS-1 summary doesn't go into a great deal of detail, but it doesn't require much space to install it and it can't be that massive.
So how long does a mission have to be, for it to be worthwhile to include green algae as a way to reduce the amount of mass supplied to the mission? You are saving 0.84 kg of oxygen a day per astronaut over a system with no recycling (from table 2 of this paper). This makes, the time for payback of the mass of the algae mixture alone, through oxygen savings, 18 / 0.84 or 21 days. So, in terms of payload mass, there is no point in doing this for missions less than three weeks. The exact timescale for payback would depend on how much mass of extra equipment you need. For instance if it is roughly half in half algae and equipment, the payback period is 42 days.
When you take account of the light collectors to collect sunlight from outside the spacecraft and pipe it in, and the light pipes that must pervade the solution, it is probably going to be a lot more mass. I don't know of any detailed figures for the mass of those, I've searched but can't find any figures. If anyone knows of any do let me know. But the payback time is surely going to be of the order of months rather than years.
This approach really scores long term, as a low mass system. It would only be a modest amount of mass compared to the payload of a three year mission to Mars orbit and back, say, and the algae give you a way to buy extra time in a situation where something goes wrong with the carbon dioxide scrubbing and oxygen recovery. Even if the mechanical life support system fails when you leave Earth, with the prospect of having to come back to Earth via Mars before you can repair it, even then, you have a chance that the algae could save the life of the crew.
I'd say it is more reliable too. There is very little by way of equipment to go wrong, and if anything happens to the algae, then so long as you have a few viable cells left, you can just grow a new batch. Try doing that with machines. Also, it would last indefinitely, with no mass supply per day needed. The algae grow on the carbon dioxide exhaled by the astronauts. All that is necessary is for the astronauts to have enough food to eat, taken along as provisions. So long as the astronauts eat well, they will produce more than enough carbon dioxide for the algae to grow, and so it will continue to produce more than enough oxygen, with no need for any other supplies.
This idea of using algae for oxygen is being explored on the ISS with occasional experiments there. It's part of the MELiSSA project.
They use Spirulina which is better than the Chlorella used in BIOS-3 because it is edible, nutritious and safe to eat with no toxic byproducts. It contains 60% protein and has all the amino acids though with less methionine, cysteine and lysine compared with meat and milk. It's a decent source of protein but has to be supplemented with vitamin B12. It's not such a good source of carbohydrates however, so though it can supply all the oxygen needed in a small space and with not much mass, and can provide some of the protein the crew need, it can't supply all of their food.
It could replace part of the mechanical systems, or double up with it. It worked fine for BIOS-1 and the only reason they went on to crops later, was because they wanted to see if they could grow all the food as well. The spirulina algae used in modern algae recycling systems are also edible, though they can't provide a complete diet, so some of it would be used to supplement food.
Some worry about reliability of these biological systems. But they don't have soil borne pests, and as for airborne pests, only whatever you might have in the habitat (e.g. mold). The BIOS-3 system was very reliable. And as for algae, then if you get some disaster that kills nearly all the culture, then as long as you have a few cells left, then it will recover quickly. It's more reliable than a mechanical system in some ways - you don't need spare parts for the vital components, can just grow more of them in the presence of light and water.
It could be a multi-redundant system for extra safety. Choose whichever works best, the algae or the mechanical method as your main system and the other as backup, or both at once, but with capacity enough so either can also work on its own. That's something you could test quite easily at an early stage in LEO also.
3D PRINTED FOOD IN SPACE
Also another technology that could be useful in a hybrid system is the food printer. This reduces waste, and deals with issues of storage and food going off / getting stale.
And here is a commercial 3D food printer which is going to be available in market.
MASS REQUIREMENTS FOR HUMANS TO MARS (FLYBY OR ORBIT) - AND SHAKEDOWN CRUISE SUGGESTION
The logistic figures support the idea that you could send humans to Mars at an early stage with life support based on the systems already used in the ISS. The ISS systems have failed several times, but what we have now may be reliable enough for a Mars mission. It's hard to be sure though, with a sample size of 1. In a recent evaluation the conclusion is:
"With several readily apparent exceptions, WRS [water recovery system] and OGS [oxygen generation system] equipment has been shown to be capable of achieving operational lifetimes on the order of those needed to support such missions. It is important to note, however, that the sample size represented by the fleet of WRS and OGS ORUs (Orbital Replacement Units) that have been used in operational service remains very small (sample size of 1 in most cases) and that statistical reliability predictions cannot be supported by this data alone. Furthermore, other challenges likely to be faced in developing Mars transit and surface vehicles, such as mass and volume constraints, water and oxygen loop closure needed to support mission scenarios, dormancy management, equipment reparability, etc., also will need to be considered as part of an integrated Mars exploration mission and vehicle design. But in terms of highlighting first-order trends and focus areas needing improvements, the daily operation of the ISS WRS and OGS is providing an invaluable first step towards human Mars exploration. "
So how would that work out? If your main aim is to send humans to Mars, I think you would need to start with a "shake down" cruise in LEO testing the actual systems you would use for the mission for a similar duration mission in LEO with no resupply from Earth.
You wouldn't need to go to the Moon for this. However, I have a suggestion here for a shakedown cruise. Of all the interesting missions we could do in the Earth Moon system, the one most like a Mars flyby or orbital mission might be a mission to the Earth Moon L2. They'd have the psychological isolation of living for several years with Earth not visible at all, hovering over an alien landscape (in some ways more isolated than a Mars trip with the Earth visible as a star in the distance if you know where to look).
It would also help with lunar exploration and you could do teleoperated exploration of the far side of the Moon which would prepare you for telerobotic exploration of Mars. And it would be a similar cosmic radiation environment to the mission to Mars. It would be an interesting mission, with the crew doing many tasks of value so not just a "makework" mission, with many of the challenges of a mission to Mars. Yet it's safe also, permitting "lifeboats" to get you back to Earth within three days, or resupply in an emergency if the equipment fails.
As for the mission to Mars, flyby or orbit, itself, the calculations suggest that you could do it with only the systems we already use in the ISS. I wonder if it might help to look into the option of adding algae recycling as that's low mass and a simple system to test, and could give more time to solve problems in an emergency.
So, to sum it all up, if we have developed closed system biological recycling by then, it has the advantage that you don't need to add much mass to the system to allow for unexpected extensions, even of several years. I think the biological one is safest for that reason, if it turns out to be equally reliable. But if you want to go to Mars at an early stage, the ISS approach is better tested and lower mass. Perhaps a hybrid system with algae for oxygen generation might be worth looking into because even though it would add to the mass, the extra mass is low and it would add a safety margin for oxygen generation and carbon dioxide scrubbing.
WHAT ABOUT HUMANS ON MARS LATER ON?
We could decide what to do later on, based on what we find out. If we find that there is some vulnerable early RNA based life on Mars for instance, I think that public opinion might well swing in the direction of saying we need to go slow here, and study it first before doing anything that could make it extinct on Mars. The scientists would be on the TV talking about how exciting it is, and I think nearly everyone would soon understand the importance of what we had found.
In the other direction, there might be other findings that show that microbes would have minimal impact on Mars. For one example, suppose that none of the proposed habitats turn out to be habitable for Earth life? I think that's an unlikely scenario myself, and it would be a disappointment for exobiologists, but it's a possible future as of writing this.
Or maybe new technology gives us the capability to send humans to Mars in a biologically reversible way. Again, it's hard to see that with present day technology, at least not for an interesting mission for the humans involved. But the human in a metal sphere idea shows that it is at least possible in a minimal rather uninteresting way.
Could there be some other more flexible and more interesting ways to achieve the same thing? I can't imagine how that would happen but there are many technologies today that I couldn't have imagined in the 1960s when I watched the Apollo landing on the moon on TV as a child of 14. Indeed right up to not long before the landing, the science fiction writers never imagined that it would be watched on global TV as it happened. So sometimes your ideas about the future can be upturned like that, suddenly, in just a couple of years.
If you have any other ideas for biologically reversible human exploration of Mars, do share in the comments!
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