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Comment by mtgx | Hacker News Reader

Why haven't we solved the gravitation issue for these sort of spacecrafts yet? Can't we get gravitation if we do a spinning part around the ship like suggested here?

http://youtu.be/3kuPkc7D35U?t=1m10s

How long would it take us to go to Mars at current speeds? A year? What would be the ideal travel time till there so we don't get affected by space radiation too much?

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My impression is that while emulating gravity with a rotating reference frame is certainly possible, the things holding that approach back are engineering realities. In order to minimize the "gravity" gradient felt over the length of a human standing up, the radius of your spacecraft needs to be quite large.

If you are lifting that sort of mass already, your time is probably better spent lifting several tons or water that would be used to fill a double hull, providing protection from radiation. Then you can give your crew a few rubber bands and count on some minimal muscle loss. (We have kept people in space and reasonably healthy for the several months getting to Mars would take, but that is in LEO where radiation is much less of an issue).

Just have two capsules separated by a tether. No need for a torus of huge radius.

NASA has a deathly fear of tethers for some reason, but I suppose that wouldn't stop Musk.

Or even better, a rigid tunnel. That would allow people to travel between the two bodies, which would significantly improve their happiness.

We don't need to solve the gravitation issue. People are spending long term time in the ISS and with proper training it's not bad.

The long-term effects of zero or low gravity on human health are very serious - muscle, bone and eyesight loss and a host of other problems. Current mechanisms of dealing with the problem (exercise and special suits) do not cater adequately to long-term space journeys.

They're doing 6-12 month missions on the ISS these days, which would be roughly similar to a Mars transit.

The gravitational field in the low earth orbit is quite large compared to what you would encounter in outer space.

The ISS is at a distance less than 5% the distance between us and the moon.

isn't the problem weightlessness, not zero gravity?

ie, centrifugal acceleration canceling out low gravity in LEO

No, weight is just mass plus some sort of acceleration, if there's something to substitute gravity everything would be well, the main problem is with body fluids, we evolved to deal with earth surface level of gravity, in the interplanetary space I do not see how centrifugal acceleration is possible unless it's something that the spaceship do to generate it, from what I know this is hard and do not see solved in the foreseeable future. I would love to be wrong.

The gravitational field you're in doesn't matter if you're in free-fall.

You're probably right, I am not a physicist, but this is the only when you consider rigid bodies right?

Nope, this is actually just a restatement of one aspect of Einstein's Theory of General Relativity, which applies to everything.

A trip to Mars at modern speeds is about 7-8 months, but this is quite variable. You could get there in about 2 months if you went really fast, but no such propulsion system currently exists to my knowledge. However, we are talking about propulsion systems as they will exist in 15-20 years, so perhaps something like 4-6 months is a reasonable estimate of trip duration.

Edit: There's that famous "40 days to Mars" study from a while ago about theoretical new rockets: http://www.space.com/8009-rocket-engine-reach-mars-40-days.h...

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