Friar Bellows

I wrote a rather tedious and amateurish paper on this subject a couple of years ago. The example I considered was that of a rocket which travels to the centre of our galaxy and back. There are four legs in such a journey. The first leg involves accelerating at 1g to a point halfway between Earth and Galactic centre. The second leg involves decelerating at 1g until we reach Galactic centre. The third and fourth legs are the same as the first and second legs except that the rocket is now heading back to Earth.

First, some numbers. In the rocket frame, the time elapsed for this entire journey is just under 40 years. In the Earth frame, the time elapsed is 52,187 years. If the mass of the rocket -- not including fuel -- was only 10^5 kg, or 100 tons, the rocket carried its own fuel, and the exhaust speed was 70% the speed of light, then the amount of fuel needed at the start of the journey would be a staggering 2 x 10^30 kg, which is the mass of the Sun. Increase the exhaust speed to 90% the speed of light, and you would need the mass of the Earth in fuel. Increase the exhaust speed to 99% the speed of light and you would need the mass of the Moon in fuel. If you could convert your fuel to photons and use the photons as exhaust material then you'd need about two thirds the mass of the Moon in fuel. All this assumes 100% efficiency in every process involved, including some magic engine which can expel matter or energy out at relativistic speeds at a rate which would make a supergiant star feel proud.

The figure below shows how Earth time varies with rocket time during the course of the journey. The flat regions aren't actually flat -- they just look flat because of the different scales of the two axes.



The next figure shows how the rocket velocity varies with rocket time. Notice how quickly the rocket reaches relativistic speeds.



The next figure shows the distance travelled by the rocket (according to Earth observers) vs rocket time. Notice how much of its time the rocket spends apparently dawdling along. It's only at relativistic speeds that the rocket makes significant headway.



If you're feeling masochistic, the entire paper can be read or downloaded here:

http://home.iprimus.com.au/cragwolf/rocket.pdf (26 pages, 277 kb, includes a discussion of interstellar ramjet rockets).

Answerer

Apparently, I was never as sarcastic as you and, of course, by your own definition, you were never civilized and your topics were never interesting. Please use your brain more.


:boohoo: :boohoo: :boohoo:

Jutsuka

Answerer: That was uncalled for, if you do not have anything constructive to bring to this thread then please leave and amuse yourself elswhere.

Jesse

Answerer, no more personal or off-topic comments to Magus55, please. Any further such comments will be edited or deleted.

Answerer

Okok, that was my last post to him in this thread anyway. I was preparing to ignore him.

Bialar Crais

The term "relativistic rocket" comes from having the space ship accelerate at a constant rate till it reaches relativistic speeds. See this for more info... 1g is basically for comfort. Not to low, not to high, and very much like old mother Earth.

American Agnostic

I would say that the first manned interstellar mission will take place in the next 500 years (provided we last that long). The main reason to do it is survival. To spread ourselves out so we're not sure an easy target for nature (or ourselves ). It's only a matter of time before we get the necessary technology to build a starship and the manned exploration and colonization of our own solar system will provide invaluable experiance. The human need to see what's over the next hill has goten us this far, why shouldn't it get us to the stars?


Isn't that also why god allegedly prevented humans from building the Tower of Babel. to attempt to contain human ambition. A futile attempt.

Infidelettante

God even voices his fear in Gen.11:6 "...and now nothing will be restrained from them which they have imagined to do."

God is afraid that we will "imagine" him right out of existence.

JT

Unbeliever

This is interesting, in that it brings another possibility into play. Apart from the God part, that is. Another way of reaching the stars in a single human life-time is to drastically lenghthen the human life span. Not only do I think this can be done, but I think it must eventually be done. I have little doubt that genetic science will accomplish this task sometime in the coming decades, though I hesitate to make a precise prediction as to the time-frame.

Ovazor

But if you wanted to be able to stop and say, land at the other end you'd have to spend massive amounts of energy to slow down again.