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The ISS weighs 450 tons and carries 7 people for three months without resupply.

Starship is supposed to carry 14 times as many people for more than twice the duration with one third the mass.

How?

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    $\begingroup$ Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on Space Exploration Meta, or in Space Exploration Chat. Comments continuing discussion may be removed. $\endgroup$
    – Rory Alsop ♦
    May 22 at 17:16
  • $\begingroup$ @armand please read the comment above. $\endgroup$
    – Tim
    May 23 at 20:10
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    $\begingroup$ There's a huge difference between "Carrying 100 people to Mars" and "Lifting 100 people off of Earth's surface". The ISS did not launch in one piece. It was assembled in orbit, requiring many launches containing each of its components and the crews necessary to put them together. Presumably a manned mission to Mars might start by launching a vessel into Earth orbit, possibly in multiple pieces like the ISS, and only then sending all of the passengers and supplies in many separate launches before beginning the transfer orbit to Mars. $\endgroup$
    – Darrel Hoffman
    May 23 at 20:55

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Have a look at this NASA document Astronaut Mass Balance for Long Duration Missions which lists a 5.74 kg mass balance (in and out) per person per day in Fig. 1. However, in Fig. 2 it shows that 4.53 kg of that or almost 80% is water in various forms.

For 9 months that becomes 1,500kg, for 100 people that's 150 tonnes. That is before adding the 100 humans for 10 tonnes more.

So doing it just by stacking the space full of food, air and water is marginal. However half of this mass is water, so even basic re-cycling equipment to allow water to be recycled two or three times would cut the required mass substantially.

So 100 passengers is not automatically implausible, but does depend a great deal on the details of the life support, and certainly would not be bringing much equipment for use on Mars onboard with them.

Edit: also changing the end result of this math is the possibility that Starship may trade reducing payload to allow higher energy departures to cut transfer times to as low as 80 days. The overall tradeoffs in reducing consumables load to allow quicker transfers are not clear as of 2023.

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    $\begingroup$ 900kg per berth is about the weight of a decent motorhome, depending on the trimmings. It should be possible to squeeze in livable crew quarters, not just people stacked in steerage. $\endgroup$
    – Cadence
    May 21 at 9:27
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    $\begingroup$ Volume also counts for rather more in microgravity, since all surfaces are available for use...someone can "stand" in the middle of what would be the ceiling under gravity just as easily as anywhere else. $\endgroup$
    – Christopher James Huff
    May 21 at 11:31
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    $\begingroup$ The 150t figure is the current Starship payload user's guide for a fully-reusable LEO launch without on-orbit refilling. As far as I know, there is no published number for Mars injection and insertion / braking / landing. I can't quite decide whether I expect that figure to be higher (due to on-orbit refilling) or lower (due to the fuel requirements for injection, insertion, and landing as well as just in-transit boiloff). $\endgroup$
    – Jörg W Mittag
    May 21 at 17:19
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    $\begingroup$ Yeah, there's a lot that depends on details of propellant storage, the effectiveness of aerodynamic braking, the particular transit trajectory, etc that aren't public knowledge or aren't even fully decided yet. There's no one obviously-right approach, and the overall architecture allows for a lot of variation...things like pre-entry braking burns, active braking during reentry, enlarging the flaps for Mars variants (or even using inflatable drag devices), jettisoning mass, trading transit time for reduced arrival velocity, etc. $\endgroup$
    – Christopher James Huff
    May 21 at 18:58
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    $\begingroup$ @Abdullah I said trade transit time for reduced arrival velocity. The lower limit for transit time for Starship is likely determined by braking in the atmosphere of Mars, not by delta-v, since, as you said, it has an abundance of delta-v for Mars missions. The payload is likely similarly limited by braking requirements, so you could (as an example) carry extra water for radiation shielding and dump it before atmospheric entry. You might carry extra propellant to brake from a faster transit on arrival, but it's not obvious how much difference that'd make. $\endgroup$
    – Christopher James Huff
    May 22 at 4:23
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Starship might be capable of carrying 100 people but that doesn't mean that 100 people is the goal for long term travel.

Interplanetary travel would not be using a mass passenger transport variant of starship, it'd be using a more spacious variant with fewer people. Regardless of mass constraints you'd really not want to be crammed airline style into a restricted space with 100 other people.

The 100 people variant would be used for shorter trips to earth orbit or maybe lunar transfers (though even that might use fewer people).

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  • $\begingroup$ correct, the 100 person cabin was projected for the point to point suborbital transport version which was never going to go anywhere (literally and figuratively). Cramming 100 people in starship for a flight from New York to Tokyo in 2-3 hours is feasible, 2-3 days to the moon would be extremely uncomfortable. $\endgroup$
    – jwenting
    May 23 at 7:15
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    $\begingroup$ @jwenting their website still says "As the most powerful launch system ever developed, Starship will be able to carry up to 100 people on long-duration, interplanetary flights. Starship will also help enable satellite delivery, the development of a Moon base, and point-to-point transport here on Earth." though there's a key "up to" in there $\endgroup$
    – Alan Birtles
    May 23 at 7:18
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    $\begingroup$ the key words here are UP TO. Yes, it can carry them, but it's not practical and they know it. Just as an A380 was designed to carry UP TO 800+ passengers, but none in service are configured to do so. $\endgroup$
    – jwenting
    May 23 at 7:22
  • $\begingroup$ While it's correct that capacity will vary depending on mission, putting 100 people on a Starship really doesn't require "airline like" accommodations. It won't be a luxury cruise, and might be roughly the reasonable limit for Mars (depending on recycling system efficiency and mass/volume requirements), but it's ample room for a trip to the moon. Airliner-like passenger densities would give a much larger number. $\endgroup$
    – Christopher James Huff
    May 23 at 12:31
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    $\begingroup$ @ChristopherJamesHuff with an unpressurized volume similar to the size of a 747: secure.boeingimages.com/archive/… I imagine the pressurised volume minus space for life support, cargo etc. will be significantly smaller. Whilst it might still be a little less dense than a typical airliner I doubt there'll be much more space per person than a chair/bed $\endgroup$
    – Alan Birtles
    May 23 at 13:05
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As mentioned by other answers, Starship may not carry 100 people to Mars; however, it's also worth noting that there are various Starship variants, including crewed and cargo variants. It's very likely that a manned mission to Mars would have some uncrewed cargo Starships to carry all the supplies needed to keep a surface base running; in that case, it would be trivial to use the cargo ships as supply depots for the crewed ships while in interplanetary space.

Edit: to be clear, I'm not saying that this is the best way to transport crew to Mars; it's just one option if you want to put 100 crew in one ship and send them interplanetary.

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  • $\begingroup$ To the individual who downvoted: I'm curious why you don't think this is a good idea. Possibly you are concerned about the process of transferring cargo from one ship to another? $\endgroup$
    – LorenDB
    May 23 at 18:08
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    $\begingroup$ While I personally somewhat agree with you, the downvoter might not. "Trivial" might not be the best word to use for the situation :) $\endgroup$
    – Mr47
    May 24 at 8:28
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    $\begingroup$ @LorenDB Doing a mid space intercept has a bunch of interesting and potentially lethal complications, starting with unmanned supply ship having some sort of failure AFTER manned ship commits to Mars departure. there was a Question around here somewhere on the topic, will see if I can find it. $\endgroup$
    – GremlinWranger
    May 24 at 10:28
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    $\begingroup$ It may be routine, but docking spacecraft and especially doing a spacewalk is certainly not trivial. $\endgroup$
    – Christopher James Huff
    May 24 at 14:03
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    $\begingroup$ If you were going to do this, wouldn't it make sense to just put 50 people and supplies in each Starship? Then no rendesvous required. The reality is that we won't be sending 50 or 100 people to Mars at one time for a long, long time if ever. We first have to get all the support equipment to Mars to care for those people. By the time we're ready to send large numbers of people to Mars, we'll probably be two or more generations of spaceships from where we are now. $\endgroup$
    – Dan Hanson
    May 25 at 17:48

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