I was shocked when I recently found out just how long it takes to get to Mars. I've been conducting a very informal survey amongst peers of how just how long people think it takes us to travel to Mars. My previous feelings were echoed by their answers: people think it takes us 3-5 years to travel to Mars. I feel like this has been reinforced by ideas like cryofreezing. I've seen several numbers put to it, but it looks like a 7month trip.

I was also shocked when I learned that people are ACTUALLY considering Mars to be a 1 way trip. This sounds asinine to me.

I was ALSO shocked to learn that Mars has 1/3 our gravity, making launch after landing seem really plausible to me.

Obviously I'm not working with the whole picture here. What is holding us back, besides funding?

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    $\begingroup$ "I was also shocked when I learned that people are ACTUALLY considering Mars to be a 1 way trip. This sounds asinine to me." It may well be the best way. Making the goal colonization, not mere visiting, simplifies a number of things. $\endgroup$
    – ceejayoz
    Commented Nov 18, 2014 at 21:57
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    $\begingroup$ The psychotype of people who sign up for a one-way permacolony trip to Mars (esp. one with no chance of return) will be very different to the psychotype for volunteers for a one/two-year research visit. Consider also how stable and dependable those people would need to be, and what happens if/when anything goes wrong? serious illness? depression? nervous breakdown? disability? death? You can't send them home, do you euthanase them? I think this is totally uncharted territory. $\endgroup$
    – smci
    Commented Nov 19, 2014 at 0:28
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    $\begingroup$ "1-way trip to Mars" and "2-way trip to Mars" are very, very different missions. It would be more useful to think of them as completely unrelated things rather than than "Going to Mars. Return ticket or one way?" options. A one-way ticket isn't half price, but rather 5%-10% price of a two-way mission, which may easily mean a choice of doing it that way or not doing it at all. Furthermore, even a two-way mission means a significant chance of noone getting home due to technical or social problems, and zero chance of getting home faster than scheduled. Illness? Nervous breakdown? Recover or die. $\endgroup$
    – Peteris
    Commented Nov 19, 2014 at 14:49
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    $\begingroup$ I disagree that one-way is that much cheaper. On one hand, you don't have to carry fuel for the return, but on the other hand, you must bring everything you need for a 40-to-40,000 year stay rather than everything you want for a 15-month stay. $\endgroup$ Commented Nov 19, 2014 at 16:37
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    $\begingroup$ I'm acting under the presumption that Mars One will wait until their technological promises can be kept at the expense of their scheduling promises; it appears you're acting under the presumption they will keep their scheduling promises at the expense of their technological promises. $\endgroup$ Commented Nov 19, 2014 at 21:02

4 Answers 4


The minimum-fuel (Hohmann transfer) travel time to Mars is about 8 months each way. It's possible to shave some off that time by using more fuel, but fuel-to-payload ratio is among the biggest engineering factors in ambitious space missions.

However, the alignment of the planets has to be just right to get that fuel-efficient course, and after reaching Mars the spacecraft would have to wait another year or so before heading home -- or spend vastly more fuel to make the trip. That makes the whole mission take about 32 months*.

Currently, our long-term space station missions have involved regular resupply from the ground; a round-trip Mars mission needs to go more than two and a half years on its own. That's a lot of supply cargo you have to carry and a lot of things that can break along the way, which means carrying a lot of weight in spare parts and/or people to maintain the equipment.

Mars's 1/3 g is about twice that of the moon, so you can imagine that a landing craft would need to pack a lot more engine and a lot more fuel than a lunar lander. Each ton of payload on a rocket incurs many tons of engine and fuel on the lower stages -- looking back to the moon missions, it's something like 75 tons of Saturn V paying for 1 ton of Apollo spacecraft.

Our moon landers only stayed on the surface for a couple of days. We'll be staying at Mars for more than a year, so we'll send a bunch of unmanned cargo flights in advance to drop off supplies and building materials for the surface station. That's more launches, but the payload and reliability requirements for those are relatively modest.

We could send unmanned fuel tankers to rendezvous with the mission in Mars orbit; the total fuel requirement for the mission wouldn't change, but the primary spacecraft could be a little smaller this way, at the cost of a more complex (i.e. failure-prone) mission profile. Another option is ISRU, refining fuel on Mars itself for the return trip, but that's another fairly risky proposition, probably only suitable for the lander's ascent back to Mars orbit, if that.

The atmosphere of Mars is awkward; unlike Earth, it's not dense enough to give the lander free braking (once a returning spacecraft reaches Earth atmosphere, it doesn't need to use any more fuel to land safely, just drag and parachutes), but it is just dense enough that the landing craft has to be built with aerodynamic considerations (again a huge contrast to the lumpy Apollo LM). Which means, yep, more weight.

None of these problems are themselves insurmountable, but building and flying such a thing would be many times larger and more complex task than the Apollo flights. No nation presently has the will and budget surplus to make that happen.

*) If you can provide another 5km/s of ∆v -- requiring a launcher about 5 times as big -- you can cut the total mission duration in half, with a 30-day stay on Mars instead of a 336-day stay.

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    $\begingroup$ So, in the end, it IS a funding issue? I appreciate you taking the time to spell it out for me. $\endgroup$
    – Wutnaut
    Commented Nov 18, 2014 at 19:31
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    $\begingroup$ Depends on your terminology. I believe there are some not-yet-solved engineering issues involved as well as just funding/political will, but funding and political will could solve them -- there aren't any theoretical barriers that I know of. $\endgroup$ Commented Nov 18, 2014 at 19:37
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    $\begingroup$ Unsolved engineering problems include precision landing. We need that if people are to go; "precision" landing now means a one sigma ellipse of 20 km by 7 km, or a three sigma ellipse of 60 km by 21 km. That is not "precise" when it comes to picking up food. Providing food, water, and oxygen for that ~2 year interval is a big problem. Keeping people from losing muscle mass is a big problem. Keeping people from going stir-crazy during that interval is a non-technical problem, but it's a big one, too. $\endgroup$ Commented Nov 18, 2014 at 19:47
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    $\begingroup$ If you're willing to spend extra descent fuel on hover time, you can do precision landings LM style (cf Apollo 12's rendezvous with Surveyor 3). If you're willing to spend payload tonnage, you can just carry bulk food/water/O2. If you have unlimited $$$ & will, you can just brute force your way through a lot of engineering problems. :) $\endgroup$ Commented Nov 18, 2014 at 19:55
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    $\begingroup$ @Peteris: I would think people need far greater protection from acceleration forces than would food and supplies. Further, by my understanding (correct me if I'm wrong) a craft that needs to have at least a 99% chance of success can be a fair bit lighter than one which would need to have a 99.9999% chance of success, and much of the mass that would need to be delivered would be in the form of food and supplies. Additionally, I'm not sure whether a free-return trajectory from Mars would be practical in the way it was for the Moon, but if so I would expect using one would increase the delta-V. $\endgroup$
    – supercat
    Commented Nov 19, 2014 at 16:25

In many ways, the short answer is nothing is holding us back except funding. There are technological hurdles but nothing beyond our capabilities. The investment necessary to do this is very hard to estimate because it depends on many unknowns, but is definitely in the billions. For a government, there is no return on that other than the knowledge that you did it, so if the scientific merit of it and the national pride isn't enough for you, you aren't going to pay. The Apollo missions were ultimately justified by a military imperative. America wouldn't have done it if they didn't fear what Russian superiority in space might mean. For a private enterprise, there is no profit motive. Really none. SpaceX claims that eventually they can turn a profit by selling residency on Mars. That is a very shaky assertion.

There are no more Saturn V rockets. To make a trip to Mars, you'd need something at least that big. Bigger would be better. So first that rocket has to be designed and put into production, an undertaking of at least 10 years. SpaceX is currently in the process of beginning to test prototype engines for a rocket that is meant for this. The rest of the rocket exists only on drawing boards.

Once you have the rockets, you will need at least 3, maybe 4. The Mars Direct proposal explains one approach to this, or you can look at NASA's mission plan, which is considerably more infrastructure intensive. Both plans largely depend on using self-contained chemical plants to produce some of the fuel needed to return to Earth, and also to produce things needed for survival on Mars - water and oxygen. Without the weight-savings in what you have to launch from Earth this provides, a Mars mission would be several times more expensive. You launch the first few rockets as cargo missions that stockpile needed supplies on Mars. Once you have confirmed those missions have successfully landed the cargo on Mars, you can send a crew.

Landing on Mars is a big challenge. You need to do a powered landing because the atmosphere is too thin to use aerobraking like a spacecraft reentering the atmosphere of Earth does, but the atmosphere is still enough to cause your craft to heat up tremendously due to friction with the atmosphere, so you need a heat shield. Powered landings - meaning you fire your engines to brake - were done on the moon, but because it has no atmosphere, this was much simpler. Engineering a lander capable of taking several tons of payload to the surface is probably the biggest technical challenge of a Mars mission.

  • $\begingroup$ There is one profit move for a mars trip: Advertisement. How much would e.g. Nike pay for having the first human shoe on Mars? $\endgroup$ Commented Nov 20, 2014 at 0:54
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    $\begingroup$ Not enough, by a very, very wide margin. Really, we are talking about an investment of tens of billions of dollars. You can do a reality tv show and all that, but it will barely put a dint in your costs. $\endgroup$
    – kim holder
    Commented Nov 20, 2014 at 1:04
  • $\begingroup$ A very important profit is eduction. Exposing the young generation to such an amazing achievement is a tremendous opportunity to reach out to those kids and boost STEM (Science, Technology, Engineering and Math) education that will later profit the society and feed future discoveries. $\endgroup$ Commented Nov 21, 2014 at 8:19

Having discussed this with a couple of people who know it well, seems to me the limiting issue will be human psychology, specifically confined environment psychology, compounded by gerontology and serious communication timelag:

  • the psychotype of people who sign up for a one-way permacolony trip to Mars (esp. one with no chance of return, under any circumstances) will be utterly different to the psychotype for volunteers for a one/two-year research visit.

For a few who didn't read through, the following is primarily about the specific never-returning permacolony scenario:

  • how many is the minimum number needed for a colony, such that you don't go nuts? 8? 20? 100? 1000? How many of you could live in a glasshouse with the same set of people for the rest of your natural life? (Compare to McMurdo, Antarctica, which even with its winter population of 250, is practically like Club Med, and it's not the same 250 permanently, some new people rotate in/out every summer.)

  • at what point do you get to childbirth on the colony? How do you manage that? Birth complications? Death in childbirth? How seriously weird does the kid grow up with no contact with any other children? Unless you send a batch of prospective parents. (How do you even toddler-proof an entire biosphere, anyway?)

  • how would you even get young, fertile couples to go there - don't forget the 7% per-person cancer risk per trip, due to gamma ray exposure.

  • consider also how stable and dependable each of those people would need to be, and what happens if/when anything goes wrong? serious illness? depression? nervous breakdown? boredom? loneliness? decrepitude? disability? death? You can't send them home, do you euthanase them? How do you do elderly care on Mars? How many infirm people can the others support, before you have to euthanase people? [or, per PeterMasiar, actively encourage them to commit suicide]

  • what is the governing law jurisdiction on Mars anyway, and is euthanasia allowed? (Do we need to create a new jurisdiction just for this?)

  • I think this is all totally uncharted territory. It's boldly going where no gerontology study has gone before.

  • consider also the very unnerving 3-21 minute communication lag. Ok let's assume AI technology advances so you can have a chatterbot and upload it daily (or hourly) updates, at least you could talk to it in real-time, like the Kevin Spacey robot in Moon.

  • See confined environment psychology, and the negative outcome of the two New Mexico Biosphere 2 closure experiments, 1991-1994

  • so I think the one-way permanent-colony scenario with no chance of return is further out than people want to believe. Till that day it will be one/two-year scientific visit type trips.

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    $\begingroup$ Very interesting considerations, thank you for them. Seems that basically volunteers for 1-way trip will need to create different society with very different rules: extremely harsh to those who do not contribute to the survival of the tribe. And probably should just start small "country" with such constitution here on Earth and try it. $\endgroup$ Commented Nov 20, 2014 at 1:02
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    $\begingroup$ @PeterMasiar: Not extremely different rules. Similar 'rules' were followed back when our ancestors set out by sea to discover lands. I may be wrong but I understand the Vikings typically did a crew-only exploratory trip , then returned to take whole families to create a settlement. Either the settlement survived on it's own, or it did not; the original village had no way to know. Contemporary technology makes things a little easier on the communications score ... $\endgroup$
    – Everyone
    Commented Nov 20, 2014 at 2:48
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    $\begingroup$ "Eskimo" (people living near the arctic) societies might be the closest thing we get to having any experience the martian situation. Survival of the tribe was/is far more important than modern considerations, such as, the right to life. Infanticide was fairly common, but only when required. In particular girls where killed more often than boys (for good reasons). Old and infirm where left in the cold to die, when they became to large a burden. If these things where not done, the tribe would starve and everybody would die. The martian societies would have to adopt similar customs. $\endgroup$
    – Clearer
    Commented Nov 20, 2014 at 21:12
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    $\begingroup$ I read (and it might be romantic writing) that old and infirm was not let to die - they actively left village and went to wilderness to die if they felt that they do not contribute to the survival of the tribe. $\endgroup$ Commented Nov 20, 2014 at 23:37
  • $\begingroup$ @PeterM.-standsforMonica: yes a very remote self-sufficient society would have to create a very different set of rules to work on consensus, but note the negative outcomes I cited of multiple confined environment psychology/biosphere tests specifically designed to test the Mars-colony scenario. And if people refuse to self-euthanase/suicide for the good of the colony, then order breaks down.... $\endgroup$
    – smci
    Commented Jul 14, 2020 at 21:37

What is the largest hurdle of [a manned] mission to Mars?


  • The daily dosage received by a human on earth is 0.00001 sieverts.
  • The shortest round-trip to Mars involves a 0.66 sievert dose.
  • You get a 5.5% increased chance of cancer from 1 sievert.
  • The Earths magnetic field and atmosphere offer considerable protection against radiation.
  • Mars doesn't. You might have to live underground.

It's a problem on the moon and would be a larger problem on Mars and in transit.

One of the largest recorded [Solar Particle Events] occurred in August 1972, between the Apollo 16 and Apollo 17 missions. The August 1972 event is one of the largest recorded SPEs in flux density and contained more high-energy (10-200 MeV) protons than most other historic events. For this event, astronauts who were thinly shielded on the Lunar surface (for example, astronauts conducting extra vehicular activities [EVAs] such as a spacewalk) could have received fatal radiation doses.

From Health Physics Society

enter image description here
From Spaceflight now

Note that the scale is logarithmic. A 1-year round trip to Mars (if possible) would involve receiving maybe 14 x the maximum permitted radiation dose of a worker at a nuclear power plant and several thousand times the dose of people living at sea level.

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    $\begingroup$ But is that really a hurdle? People (including me) are very keen on taking small risks, like that of a slight increase of cancer. I don't think you will find it difficult to recruit well informed astronauts, even given your data. $\endgroup$
    – LocalFluff
    Commented Nov 20, 2014 at 12:24
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    $\begingroup$ How hard is it to shield against that sort of radiation during the trip? Is it more complicated than just trading off shielding weight around the habitation module against cancer risk? $\endgroup$ Commented Nov 20, 2014 at 19:58
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    $\begingroup$ Interestingly, that increase in cancer rate is just about the fatality rate of the STS. NASA should have no problem with that. $\endgroup$
    – dotancohen
    Commented Nov 21, 2014 at 17:01
  • $\begingroup$ @LocalFluff: you call a 1 in 18 chance of cancer, each way, i.e. in 1 in 9, a "slight risk"? Consider if you get cancer on the outbound trip, you'll probably die. $\endgroup$
    – smci
    Commented Dec 10, 2014 at 23:35
  • $\begingroup$ @smci Getting cancer any time later in life, maybe 15 maybe 50 years later. $\endgroup$
    – LocalFluff
    Commented Dec 11, 2014 at 0:28

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