What we need is a way to generate force without having to throw something the other direction. There is no known physics that works that way.

Without that, the center of mass of the rocket/fuel system remains on/near Earth no matter where it goes.

As to interstellar travel, there needs to be some radical new technology before that is even remotely possible. I suspect the physics simply won't allow it.

Mike C.

The time dilation thing has puzzled me WRT near lightspeed travel. If a target planet is 10 lightyears away and I went there on a ship that spent most of the journey at .99 C (let's ignore the acceleration and space debris collision issues) then turned around and came back to Earth would the trip take approximately 20 years of my time with someone remaining on Earth measuring the elapsed time as a much longer period? Or would my timekeeper on Earth be 20 years older while I had barely aged?

Intuitively is seems like it should appear to take 20 years to someone remaining on Earth since that's how long it would take light to travel the same route out and back. But that would mean that from my perspective it would hardly take any time at all (for .99 C I think that would be about 5 months).

Oh... like an Ion engine.

You are correct. The person traveling at .99 C will experience a much shorter time interval.

Proven with decay of muons created in the upper atmosphere by cosmic rays. They take much longer to decay (~50 us) than muons created by other processes on the surface (2.2 us).

Ion engines throw ions, which are charged atoms, using an electric field.

When you run out of atoms to throw, you are out of "propellant", and you had to take all that mass with you from launch.

Mike C.

Ion engines throw ions, which are charged atoms, using an electric field.

When you run out of atoms to throw, you are out of "propellant", and you had to take all that mass with you from launch.

Mike C.

If we ever want to expand humans into space we need to start somewhere. The first humans to cross a river on a floating log couldn't have imagined how to cross the pacific, but if they hadn't tried the log, we would never have figured to build ships that could colonize most of the islands in the pacific.

There is very little He3 on the moon. Its a lot compared to on earth, but you still need large scale mining technology to get economically useful amounts. We get He3 from tritium decay from nuclear reactors, and eventually (maybe) from fusion reactors.

They add a lot of risk. You need enough rockets ready for launch that you can get 6 successful launches in the fuel storage time window. All the docking maneuvers need to work.

Space is expensive because its to unforgiving of mistakes, and with so many systems that need to operate, each system needs incredible reliability. Needing more launches just pushes that further.

Not saying its impossible, but it will add cost

Compared to the expense of any other lunar mission, do all those launches represent a significant cost?

Putting on my "physicist" hat.

If you travel to another star and return, moving at near light speed, you will age much less than someone who stayed on earth. This is the "twin" paradox. The reason its not a paradox is that the person on earth moved in a straight line in space time, while the person on the rocket moved in an angled trajectory.

On a normal map, an angled trajectory is longer than a straight one, but in space time, its works backwards, the angled trajectory is shorter (in self-measured time) than the straight one.

Distance in space is distance = sqrt (X^2 + Y^2). but in space time (minkowski space the time measured by the moving clock (proper time) is tau = sqrt(T^2 - X^2).

Sorry if this is pretty obscure, trying to explain relativity in a post is tough.



There is still a big technology problem. Even fusion power only gets you to about 0.1 C (10% of speed of light) where this effect is very small.

Antimatter (and there is no known physics to efficiently make antimatter) gets you to maybe 0.5C which helps a little. If you try to go faster, you run into a version of the rocket equation. Getting to 0.99C would require your rocket to be almost entirely fuel, I can't imagine how this could be possible.







The time dilation thing has puzzled me WRT near lightspeed travel. If a target planet is 10 lightyears away and I went there on a ship that spent most of the journey at .99 C (let's ignore the acceleration and space debris collision issues) then turned around and came back to Earth would the trip take approximately 20 years of my time with someone remaining on Earth measuring the elapsed time as a much longer period? Or would my timekeeper on Earth be 20 years older while I had barely aged?

Intuitively is seems like it should appear to take 20 years to someone remaining on Earth since that's how long it would take light to travel the same route out and back. But that would mean that from my perspective it would hardly take any time at all (for .99 C I think that would be about 5 months).

Even if that works, you need to generate the power to do it, and that fuel (even if antimatter) ends up weighing too much to go much beyond 0.5C.

The best you can do is use matter / antimatter and then use the resulting high energy particles as exhaust. Even that is tricky though since a lot of energy ends up in neutrions which can't be directed by any physics we can imagine


I've seen proposed concepts that gather "space dust" to be used as reaction mass for propulsion. Of course any relative motion in the opposite direction of travel would oppose the thrust so that might not be very effective if you were moving fast.

Current cosmological theory posits a Dark Energy that's pushing the galaxies apart at high speed.

All we have to do is find it.

You've identified the major problem, the dust has velocity you have to absorb before you can throw it another direction.

I also wonder about flying near C and the density of the interstellar medium. The thermal energy form that might be immense because energy is velocity squared.

Interstellar space is about 1 H atom per cubic cm (estimates vary, but this seems typical). A 10 m diameter spaceship will hit 39 nanograms of H traveling at near C. If we assume the average particle is near stationary, each particle hits with 1/2mv^2 energy on the forward shield of the spaceship.

When you work it out, it comes to 1.7 MW that the forward heat shield has to withstand continuously. That seems like a lot and may be a reason why travel near light speeds won't work.

If at light speed you hit something more than an atom, say a 1 gram dust particle, that's roughly the same energy as the Hiroshima atomic bomb.

I think the universe has decided we can't leave the solar system. It either takes thousands of generations, or your speed will incinerate the ship, neither of which will actually work.

Disclaimer: my computations are unchecked and could be quite wrong.

Mike C.

Fast forward to 1:55 where Elon starts to talk.

"The Road to Making Life Multiplanetary: an update from @elonmusk on SpaceX's plan to reach Mars"

https://x.com/SpaceX/status/1928185351933239641

If you travel to BRO for a $100 hamburger check the website for Highway 4 being closed.

As we get closer to a launch date it gives you an idea of when the launch is going to occur.

SPACEX INFORMATION

PUBLIC NOTICE OF CAMERON COUNTY ORDER TO TEMPORARILY CLOSE STATE HIGHWAY 4 AND BOCA CHICA BEACH

https://www.cameroncountytx.gov/spacex/