"James A. Robbins"  wrote:
> "Doug Freyburger"  wrote:
>
> Have you ever looked at the problem of fast interstellar travel?

Sure.  I'm enough of a space program buff that I selected
my college based on its chance of getting me a job in the
space program and I started my career at JPL.  I've since
spent most of my career in commercial bit I keep an eye
on space issues.

> If you consider purely Newtonian physics for the moment (which
> holds pretty good for anything traveling less than .9c). =A0If you
> accelerate at 1 g you can achieve a velocity approaching c
> in about 1 year. =A0Given a fusion or total conversion drive this
> may not be too hard to do.

Poul Anderson's Tau Zero.  It had not yet been figured out
that a ramscoop generates drag by created a cyclone of
magnetically accelerated hydrogen.  But that would actually
be a good thing for issues you describe below.

> When you reach approximately .9c
> weird things start to happen.

Special relativity - Second half of freshman year physics.
Gotta learn that to know that all magnetism is is the
Lorenz-Fitzgerald contraction of moving charge, right?
By the time I was hip deep in tat stuff I realised I had
probably not chosen the right college ...

> To an observer stationary to your
> course line it would appear that your acceleration will asymptotically
> approach zero as you asymptotically approach c. =A0Your subjective
> experience will be that your acceleration will remain constant
> and the distance to your destination will begin to decrease (as
> if it were accelerating toward you).

If only it worked out that it sorta looked like you'd just
kept accelerating past C ...

> The down side is that

A ship big enough to colonize would need unbelievable
energy to accelerate that much.  One of the reasons I
did not address very high speeds.  Even 0.1 C is
unreasonable with a fusion torch ship that's big enough to
hold generations.

> if and when you returned to the earth, it
> will have been a very long time you will have been gone. =A0Depending
> on the length of your journey, years, 100s of years, or even millions
> of years may have passed since you left (which could be very
> inconvenient if you left the water running).

Not an issue for colonists who chose it.  Interesting
implications to how that effects the gene pool remaining ...

> The biggest problem, though, is due to a different kind of relativity.
> As your speed increases, in your subjective experience, the speed
> of all the hydrogen, cosmic dust, grains of sand, etc. coming
> directly at you also increases. =A0As you approach c this stuff
> appears to be bearing down on you at a speed approaching c.
> You get a first had experience of what it is like to be sitting at
> the business end of an active particle accelerator! =A0Something
> has to be done to prevent you from being fried to a crispy critter!
> No known physical shielding can protect you from this rain of
> death. =A0Thus the need for postulating force fields.

Which Arthur C Clarke novel was the one about the sun
going nova and Earth having 2-3 centuries to launch colony
ships?  In it he used comets converted to giant icebergs as
shields and stayed slow enough for them to last light years.
I'd use nickle-iron asteriods as shielding to last longer but
the trafe off is where to store propellant.

> So to summarize, to be able to move between the stars at anything
> approaching a good speed you need:
>
> 1) A power source capable of accelerating you to high velocities
> =A0 =A0(doable within the bounds of known science and technological
> =A0 =A0 extrapolation at least as far as fusion and antimatter goes.
> =A0 =A0 Total conversion reactors are fantasy at this point).

Or very low total mass - Seed ships not colony ships.

> 2) A force field to protect you from the radiation storm you will
> =A0 =A0experience (not doable within known science).

Or a way to deal with an asteriod as a shield.

> or, ...
>
> Simply by-pass the problem by warping space-time, creating a
> worm hole, or bypassing normal space-time by traveling through
> hyperspace (all great devices to use to move a story along but
> lacking any scientific basis for existence).

Stephen Hawking, go go go!

> Or we just plod along at low velocities for years to get to the
> next star. =A0So it goes.

Stay at well under 0.1 C and the shielding is easier.

Thing is my shoot from the hip 0.1 C didn't mean that speed
for most of the journey.  To me it actually means a slow
acceleration to well above that then a slow deceleration to the
new system - So the average speed for the entire trip ends up
near 0.1 C.  That's a problem for shielding in the middle of the
journey.

The radiation problem must be solved to colonize Mars and
the asteriods, though.  Solar flares and solar wind is just as
bad given long enough years.
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