[geijoenr]

Definitely a common sense step in the right direction. Humans going anywhere beyond the moon using chemical propulsion seems quite problematic due to all the unsolved issues related to radiation and micro-gravity.

Now talking speculative fiction, the real breakthrough will come if we figure out a way to induce acceleration without an action-reaction process. Just an energy source, and no propellant. Being able to sustain 1g for a few months on a heavy spacecraft means interstellar travel (proxima centaury) would be within grasp.

[Kim_Bruning]

Meanwhile, having engines that have higher specific impulse helps quite a bit as well. That is to say: a nuclear engine needs less propellant to provide the same amount of delta V (actual work). Since the majority of a rocket's mass is in the propellant, this scales non-linearly.

A nuclear rocket is a big improvement over chemical rockets already.

[eddsh1994]

How does your initial point of 1g for a few months on a heavy spacecraft lead to traveling 4.246 light years? Genuine question!

[gpm]

It would take 6 years to get there (earth time) or 3.6 years (spacecraft time) if you could have constant 1G acceleration (in the opposite direction for the second half of the journey).

https://cosmicreflections.skythisweek.info/2019/09/04/space-...

[geijoenr]

With a spacecraft big enough to live in, we are talking of a time frame not too different from the one from the age of exploration by sea. I have no doubt many people would want to take such a trip even if it takes 10 years to get back to earth.

Unmanned spacecrafts would be able to go much faster, sustaining more dramatic accelerations. So getting cargo and robots there for support would be way faster.

[eddsh1994]

This takes you to 0.99C, how feasible is that (thinking materials more than anything)

[nine_k]

Sadly, not very feasible because of the hydrogen atoms floating around in the empty space, about 1 per cubic meter.

At speeds about 0.5C a collision with such an atom produces X-rays, and harder gamma rays at higher speeds. They are pretty hard to insulate against, and are actively harmful. For a spaceship of a considerable size, enough collisions would occur to be dangerous. The paper: https://www.scirp.org/journal/PaperInformation.aspx?paperID=...

[gigaparsec]

To make it even worse, that's based on an average density 1.8 atoms/c3. The real values would probably vary wildly, and you won't be able to "break" in advance in order to go through a high density area.

And then there's the problem of hitting an interstellar grain of sand at 0.5c.

[mr_toad]

I wonder what the odds of survival are compared to sailing around the world in a Galleon.

[gpm]

Only 0.95 per the source linked above. I don't know.

Speculating you'll substantially blue shift the light coming from in front of you, and I suppose that can't be good for materials (or people), but I'm not sure if there is enough to matter. Any dust you collide with is also going to have ridiculous amounts of energy, but you'll be in interstellar space when you're at high speeds so there shouldn't be much of it (even for space) either.

[credit_guy]

It's not feasible. It would require more fuel than the Earth is heavy, even if we had nuclear propulsion.

[geijoenr]

It depends on the mass of the ship, but yes; quite a bit.

https://www.quora.com/How-much-energy-does-it-take-to-accele...

I don't think that means is unfeasible if the energy source allows for that.

[credit_guy]

No, actually it does not depend.

The rocket equation says that the fuel mass of a rocketship is higher than the cargo mass by exp(delta_v/v_exhaust).

When the final velocity is relativistic, delta_v should be replaced with delta_rapidity. In our case this would introduce a factor of 2.65, but the results are so ridiculous that we can ignore that.

So, let's simply say that delta_v is the speed of light, or 300000 km/s.

The exhaust velocity for a nuclear thermal rocket is about 9 km/s.

The ratio between delta_v and the exhaust velocity is about 33000. The exponential of that is roughly speaking 1 followed by 15000 zeros.

There are less than 10^100 atoms in the known universe.

So, even if you want to accelerate just one single atom to 99% of the speed of light, you would need more fuel than the entire universe. Many, many, many times more.

[geijoenr]

Yeah well, we don't really know what happens at high speeds. Probably biochemistry stops working at 0.3c? It doesn't seem structural materials would remain solid at 0.95c... who knows.

[inasio]

High speeds relative to what? The earth orbits at 30 km/s, our galaxy arm is at over 200 km/s (say 0.1% c), I'm not sure relative speed matters at all

[monkeydreams]

I am not a physicist but I suspect that we are already moving at significant speed relative to other bodies. We have no special frame of reference that defines our "real" speed.

[gpm]

That's not quite right. There's the cosmic background radiation "rest" frame, which can reasonably be taken as 0 velocity.

The laws of physics don't change in different frames (Einstein's assumption), but that doesn't mean that all frames are equivalent in other respects.

[geijoenr]

The speed of light is the frame of reference. It remains constant for any observer no matter at which speed is moving.

[brnt]

In fact, you could get anywhere in the universe in your lifetime/timeframe, if you could maintain the 1g acceleration.

[mr_toad]

Depending on the nature of the expansion of the universe, that might not be true.

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

[dmitrybrant]

> without an action-reaction process. Just an energy source, and no propellant.

Unfortunately, in our universe, momentum is conserved.

[mr_toad]

If you could somehow convert mass into a beam of photons then the mass loss would be tiny compared to the momentum.