Nearby planets - travel to and colonisation of

[eltelioni]

Continuing here because colinha's question is closed. (Why?)

I don't agree that this has been done to death. Although there are practical difficulties with interstellar travel I see no objection to doing a thought experiment, though we must obey the laws of physics. A constant 1g acceleration is impossible because, as factor30 discovered, the ship would eventually exceed the speed of light. Perhaps we should consider a constant thrust giving 1g at the start of the journey, but with the acceleration reducing as we gain relativistic mass.

Time dilation WILL reduce the journey time in the frame of reference of the crew. There is a parallel with muons created in the upper atmosphere by cosmic radiation. The muons travel at relativistic speeds. They have a half-life of 2usec which suggests they should only go about 600m before half of them decay. In practice a lot more than half make it to the Earth's surface because, in their own reference frame, the journey time is shorter.

So, can anyone do the maths for our thought experiment?

[beso]

To produce thrust, something must be expelled from the vehicle. This material will have to be carried from the start making the initial mass enormous. In addition the fuel irequirements would be phenominal since there is very little energy available in interstellar space.

While the relativistic effects will make time pass more slowly for the travellers the rest of the Universe continues as normal. By the time the destination star system is reached it will have already burnt out. Indeed the Universe may have already expanded to oblivion.

[rov1200]

They say if you travel far enough in one direction you will return to the Earth. Maybe the way to see these far off galaxies is to travel in the opposite direction and we will be there in no time.

[eltelioni]

There were people who thought the human body could not withstand speeds greater than 10 mph. And the Wright brothers were told it was impossible to build an engine powerful enough for manned flight. There are people thinking about interstellar drives. Bussard ramjets collect the reaction mass from interstellar space. Orion drives use a series of fusion bombs. Anti-matter drives completely annihilate matter & anti-matter to produce vast amounts of energy. Stephen Hawking advocates the use of Hawking radiation from a mini black hole. Others are wondering if we can do anything with dark matter. But this question is not about interstellar drives.

As for journey times, even at the Voyager's coasting velocity, you would reach the nearest star in 17000 years. I am not suggesting that is a viable method, just pointing out that the journey can be completed long before the universe ends.

Can no-one answer the question? How long would it take in the frame of reference of the travelers, to go 40 light years using a constant thrust providing 1g at the start of the journey?

[eltelioni]

colinha - I found the answer! 20 years.

There is an applet on http://www.astrophysi.../TravelEarthTime.html which calculates the time for you. You can set the acceleration (default is 1g) and the distance. I plugged in 12000 parsecs which corresponds to 40 light years. The journey time in the reference frame of the travelers is 20 years. I need to read the accompanying article more closely before I will understand what the elapsed time on Earth will be.

[eltelioni]

The elapsed time on Earth is about 50000 years.

[vascop]

That site looks dodgy to me - why? Because Special Relativity only applies when there are no accelerations.

[vascop]

I take back my last post after reading an explanation of this in Roger Penrose's book The Road to Reality.

[vascop]

According to Penrose, Special Relativity can be used as long as gravitational effects are negligible.

[eltelioni]

The author (Dr Jerome James Brainerd) seems to know what he is talking about. He has been working on a NASA project to build an electromagnetic propulsion system for interplanetary travel.

[R1Geezer]

1 parsec = 3.26 light years, 40 light years = 12.27 Parsecs not sure where 12000 comes from!

[jake-the-peg]

perceived time= earth time / sqrt(1-(v²/c²))

So at 99% the speed of light it's about a seventh.

Electromagnetic propulsion systems use magnetic systems to eject an ion beam out of the back. These ions are of very low mass but can eject at very high speeds over a significantly long period of time.

This means they will accelerate at very low g forces but over a very long period of time quite high velocities can be reached.

This makes them very unsuitable for manned space flight but potentially quite good for space probes.

So far one has made it to the moon.

NASA is working on a new one http://en.wikipedia.org/wiki/HiPEP

Note that this can produce 670mN of thrust ( that's about the equivilent of a 67g weight ) so its a very small thrust but it only uses 5.6mg of fuel a second

Brainerd's is a little different (http://www.reiszengineers.com/togobeyond.pdf
)
but the key figure is the thrust about 70mN in a Saturn V rocket the Thrust was about 33MN thats Mega as opposed to milli Newtons or nearly half a billion times as much.

If we assume that your spacecraft is a petite thousand tonnes (and that's probably several orders of magnetude too light) you'll accelerate at 0.00000007m/s and reach a relativistic 0.9c in about 120 million years

[colinha]

I never believed that slow multi generation / large Ark ships were ever an option for sociological reasons.
Since interstellar voyages can be made in a reasonable fraction of a short human lifetime, it seems inevitable that this will happen metime in the future once a suitable drive technology is found.

[vascop]

Yes, you're right geezer, 12000 parsecs are equal to about 36,000 light years, not 40 light years.

[R1Geezer]

" Since interstellar voyages can be made in a reasonable fraction of a short human lifetime, it seems inevitable that this will happen metime in the future once a suitable drive technology is found. "

did you not read the above (especially jake)? Interstellar flight is probably impossible, even at light speed it takes 8 years to say hello and get a response from the closest star. It won't happen in a thousand life times.

[Old_Geezer]

Not knocking your thought experiment, but the ideas of these being nearby planets is somewhat amusing.

It's been 4 decades since mankind first stepped on our satellite moon. And there hasn't been the ability or the will to even set up a rudimentary base there.

Despite enthusiasm from the Mars Society and plans from Robert Zubrin, it seems as if the radiation to get humans to that nearby planet is still considered too risky.

Personally I don't anticipate any further manned mission projects during my lifetime. Which I find sad as I will miss the events if they ever happen. And as time goes on I wonder if humans will ever have the will to do so, or whether they'll just sit here until it is all too late.