*Originally posted by twhitehead*

**I believe most plastic based 3D printers must heat the plastic while printing.
**

[b] .... if you are in a craft a billion miles from home or halfway between the stars, say 2 light years out going to Alpha Centauri, 4.3 ly away.

Then you should have nuclear power.[/b]

Yes you would have to, just showing how useless solar power would be in space between the stars. Halfway to Alpha Centauri the amount of energy you collect from the sun goes down by a factor of 256 million. Reaching that minimum if you turn your cells around you then start collecting about the same amount from AC, but the energy collected gradually goes up to about the same as you would have gotten from Sol since the two stars have similar energy outputs. So at Earth's distance, that array would collect 250 odd Gigawatts, a healthy chuck of energy and it would build up to something like that coming in to AC.

That energy could be the power needed to slow down the craft with some kind of rocket, like the Vasimir plasma rocket, low thrust but 24/7 operation. Besides the deceleration you would get just from the mirror reflecting the light from AC.

The problem there would be applied rocket thrust could distort the shape of the mirror so it would require stiffness able to withstand whatever the thrust levels you have.

Going to 0.5c at 1 g of acceleration would take about 6 months so doing the same at 0.1 g would take 60 months or 5 years. So if that was the thrust of the system, it would have to be able to take that amount of stress on the last leg of the journey. You would cover about 1/4 ly in that first 5 years, so that leaves about 4 ly to go and 3 3/4 ly at 0.5c or about 7.5 years plus ten years accelerating and decelerating so about 18 years for the whole journey. Say 2 years around AC and back, about 38 years total. At 0.5c there would not be much in the way of time dilution, about 15 % so for the portion spent at that velocity, about 15 years, the crew would be about 2 years younger than a twin, not much of a deal. So the whole trip would take 36 ship years but 38 years would have gone by on Earth.

You don't get serious with that time dilation till you get to about 0.9c which would be about 2.2 to 1, so if the trip was at 1 g, it would take say 11 months to get to 0.9c and you would have covered about 1/2 ly so 1 ly at about 0.5c average means two years of acceleration and deceleration and 3 ly of coasting at 0.9c which would account for about 1.3 years of ship time so now the whole trip takes about 11 years, with say, 1.5 years at AC studying the place.

Now you get about 2.3 to 1 ship time or roughly 5 years of ship time while 11 years goes by on Earth.

I would do such a journey!

It gets better both for longer trips at 0.9c and for more acceleration to get to say 0.999c where the ratio is 22 to 1, so a journey of 22 ly would mean 1 year of ship time so round trip, 2 years of ship time plus whatever time you spent at your goal, say 5 years, then Earth time would be 44 plus 5 years, or 49 years having gone by on Earth but the crew is only 7 years older. The world would be 42 years ahead of the crew, so if twins left, one on the journey and one left behind, at age 20, the crew member would be 27 coming back and the twin on Earth would be 62. Wouldn't that be something, to meet your twin brother or sister who is 42 years older.