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Comment by ck2 | Hacker News Reader

We already know that low-gravity causes all kinds of bone and muscle weakness and even messes with eyesight in a dramatic manner.

The scary part is while the bones/muscles can be healed over time, the eyesight problems persist and appear to be irreversible.

They have to solve this before trips to Mars will be possible, unless they are going to simulate gravity by constantly (slowly) accelerating half-way there and then slowly decelerating. It's technically plausible but a million other little problems have to be solved too. Won't happen in the lifetime of anyone alive right now.

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A constant-acceleration flight profile would also be a lot faster for interplanetary travel, but is impractical for any known or currently plausible reaction engine: the rocket equation strikes again. Ion or Hall-effect thrusters require only electrical power and so can more or less continuously accelerate, but they produce far too little thrust to provide meaningful pseudogravity.

Classically, the way to mitigate the medical problems of long microgravity exposure is by giving the spacecraft a rotating section, which produces enough centrifugal "down" to mostly satisfy human physiology. I understand the engineering problem involved there to be fairly severe in its own right, but most likely soluble given an established orbital manufacturing industry amply supplied by heavy lift from Earthside.

Another option might be to cable a pair of spacecraft together and spin them about their common barycenter, as I believe has been done with satellites in a few cases. Perhaps this will serve to improve the economics of Martian convict labor by increasing survival rates of transportees, while the English travel first class aboard luxury liners with spin gravity and nightly zero-G cabaret.

It's really interesting/odd that spin gravity stations have not been tested out in practice yet. Everyone just wants to do zero G experiments on their station and nothing else.

I think the dual spacecraft with a long cable is really the only practical option, since you get far too much coriolis for it to be bearable otherwise. Remains to be seen if they can make it work with two spacex starships though.

New scifi story idea: two space ships tethered to spin together, Mars-bound on a multi-month trajectory, both carrying crew[1], and one of them has the cliched space horror encounter with nasty alien life -- tell the story mostly from the point of view of the other crew. What to do?

[1]: Cargo can fly cheaper & slower, not worry about radiation exposure.

Well fusion would be too exotic to be developed in this century unless there is some unbelievable breakthrough but it is very realistic that all the micro-nuclear-reactor research & development being invested into by datacenter giants now might power a spaceship someday a decade down the road?

(the engine development would also have to advance obviously but not implausible)

It's been studied in the past (NERVA, et al.), and does seem to offer a specific-impulse advantage in the extremely high temperatures achievable by routing propellant through the reactor core. (So-called "closed-cycle" systems heat reaction mass outside a fairly conventional primary coolant loop; "open-cycle" doesn't have a coolant loop, but rather ejects hot coolant directly as reaction mass. There was also a nuclear-powered cruise missile design, I think called "Project Pluto," on this theme, although not directly related and so far as I know only studied in theory.)

It does of course still require reaction mass, but theoretical efficiency seems higher than other feasible systems; it wasn't really explored much in the old days due to the general post-Apollo builddown, and also due to the concerns of radioactive debris in case of mishap and radioactive exhaust if the system were used as a launch motor. (If not used to launch, it would need some orbital assembly and a lot of heavy lift, given not least the massive shielding required for remotely survivable crewed use.)

Another fission-based propulsion system, not relying on a steady-state reaction, was also studied under the name "Project Orion." I'll leave that one as a diversion for the interested reader, mainly because it seems unlikely anyone not already familiar would believe a word of any description I could give.

Astronauts have survived in the ISS for longer than it would take to make the trip to Mars. I don't think slightly damaging your eyesight would be a deal breaker if you're willing to go to Mars, a radioactive, unbreathable, freezing, near-vacuum and sterile desert.

Yeah but it's gonna be OUR radioactive, unbreathable, freezing, near-vacuum and sterile desert!!

NASA always manipulates news coverage… I recall the days 20 years ago whenever a politician got serious about cutting their budget, NASA would run a “water on the Moon!” story. If it comes out that long term space travel is going to cause brain damage (my guess here) their golden goose is cooked.

But long term, that's good: we have only one spaceship (earth) in the end, that's the one we have to take extreme care of.

NASA is more than manned space travel with lots of unmanned missions to other planets and moons. If space travel causes brain damage, it's more like Elon will have problems.

so he'll be fine then?

I looked into manned Mars missions in a fit of enthusiasm 30 years or so, and I concluded that we couldn't do it because landing a human + life support on Mars requires deceleration technology we just don't have. Basically the speed of a Martian orbiter needs to be reduced to 0 or so to avoid splatting the human, but Mars' atmosphere is so thin that parachutes just don't provide the required break.

Has that now changed or is it still just fantasy?

Um, the moon also has no atmosphere. We were able to land on that in the 60's.

https://en.wikipedia.org/wiki/VTVL.

Lots of things to be worried about for a Mars trip, landing probably isn't really one of them.

But the moon is 1/9th the mass of Mars (approx)

The moon has an average orbital velocity of 1km per sec (approx)

Mars is 24km per sec (approx) Terminal velocity is 600ms, on earth it's 54ms

So we are talking about something that is an order of magnitude more difficult. I think if retro rockets are to be used then far more fuel will be needed. But that means far more mass, which means more fuel - how much fuel would be needed to slow something like Starship for example?

Many of the landers we've already put on Mars are heavier & less tolerant of G forces than a human, so it's clearly not an insurmountable hard limit. They use a combination of parachutes, retro rockets & airbags usually. But you're right that the lander stage has to be a small fraction of the size of the interplanetary stage, & the economics aren't very good. Maybe you'd need to send several Starships to Mars orbit to refuel & support a single one landing &/or returning to Earth -- at least until refueling facilities exist on the surface, & assuming Starship is actually capable of this mission.

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