# What would be required to fly four astronauts at constant 1G to Mars? [closed]

Is it technically possible to build a ship that could keep a constant 1G acceleration / deceleration on a flight from Earth to Mars? What could provide energy? What could provide accelerant? (I am not asking if it would be affordable, though a guess at the cost would be fun to see )

• Is your goal here to keep the astronauts under a constant 1g force for their comfort and well-being, or to keep the ship on a constant 1g acceleration to achieve a 4-day flight time? Brian Lynch's answer assumes the former. – Russell Borogove Jan 19 '16 at 19:09
• Even assuming that answers about requirements for constant acceleration transfers between the Earth and Mars are expected (with half point brachistochrone turn), this is too broad to answer. There are many possibilities for high thrust high mass efficiency propulsion if we can completely neglect energy efficiency or how ludicrously expensive it would be to build it. Short answer: Build a phased array laser system with ~ 1.5 GW power for every kg of the transfer vehicle and its cargo (at 100% spot coverage and sail reflectance). Long answer: Too long, no time, gave up. – TildalWave Jan 19 '16 at 22:13
• Related? (not a dupe) How fast will 1g get you there? – Dan Pichelman Jan 20 '16 at 16:57
• related - fuel needed if a rocket space.stackexchange.com/questions/4001/… – mart Jan 21 '16 at 7:59

There are simply two options: either you provide 1 g of acceleration on the spacecraft the entire flight (from thrust) or you use a rotating habitat to provide centrifugal acceleration. Applying 1 g of acceleration is probably not possible given the fact your trajectory will have to be planned to accommodate that (i.e., you will be expending a ridiculous amount of propellant to achieve this), so a rotating habitat would be the most feasible option.

In that case, the energy required is minimal -- either just enough to overcome friction between the rotating and non-rotating components of the spacecraft, or essentially zero if the entire spacecraft itself rotates.

But... Let's consider some ballpark estimates if you still insist on trying to achieve a 1 g continuous acceleration from thrust...

If you want to do this with chemical rocket motors, you'll be looking at a manoeuvre that needs so much fuel my calculator simply says "error" (basically you'll need a $\Delta v$ of over 2500 km/s, assuming a 3 day flight).

If you want to use ion thrusters then you'll still need to achieve the same $\Delta v$ but your specific impulse will be much higher (say 6,000 s instead of 350 s). This leads to a mass ratio of about 6E18, or rather 6 billion billion (so you will need over 6,000,000,000,000,000,000 kg of propellant for every 1 kg of empty mass). Not to mention the assumptions made here in terms of how you will power this and provide the thrust on that huge mass to get 1 g of acceleration.

Note that I'm approaching this question in terms of accessible technology. You could look into what kind of specific thrust would make this achievable for a more acceptable mass ratio, or some kind of external source of propulsion, but at that point you are making up propulsion technology that does not exist!

• Didn't anybody notice that a 1g artificial gravity by centrifuge would get people violently sick if they tried to move even slightly. The laws of angular motion dictate that any attempt to walk the longitude around the rim of the centrifuge would have you staggering at right angles to where you thought you were going. I suggest zero gravity with all of its physiological longterm problems might be preferable to a centrifuge gravity that doesn't work as we expect, with major disorientation. And how could you play a game of tennis? Hit a ball straight at your opponent & it zooms sideways – Stan H Oct 17 '17 at 7:45
• It depends on the centrifuge size. What about 1km centrifuge? Twin ships connected by a rope or something. – nponeccop Mar 15 '19 at 2:16