[Jweb_Guru]

Or at least, highly energy-intensive and fragile (due to problems like cosmic radiation) to the point of impracticality, particularly when it comes to something like panspermia (or, for that matter, sending high-energy signals directly at a planet that doesn't detectably have life from the perspective of the originating species). This possibility is not weighted nearly highly enough in most analyses.

[TheOtherHobbes]

Interstellar travel is perfectly possible if you're not in a hurry.

Assuming there's no way to sidestep GR, the real problem is the human lifespan (and attention span.)

If our culture moved at 0.1% of the current rate and we lived a thousand times as long, a 1400 year round trip wouldn't be problematic.

It's possible to imagine low-energy lifeforms that move that slowly. But they wouldn't be looking for Earth-like planets to colonise - they'd be looking for much colder and more stable locations.

And we wouldn't be looking for the right spectroscopic signals to give them away, because we don't know what they are.

[Jweb_Guru]

The problem isn't just sending something (or someone) to another star (though it would take far longer than 1400 years with any currently plausible technology and energy expenditure). It's sending something accurately to another star (can't use too much energy adjusting course, since fractional additions to weight require tremendously more power) with a payload that's still operational by the time it gets there. Think about the myriad problems NASA probes have after just a relative handful of years or so in space.

Also worth noting is that all deep space missions thus far have had to rely on nuclear power, usually using 238-Pu with a half life of less than 90 years. With such technology, a well-shielded, self-correcting computer system traveling at reasonable speeds and energies could not survive too long because it would simply run out of power. AFAIK, workarounds for this rely on exotic power sources and unproven physics--it's entirely possible that these don't pan out, and this provides our "Great Filter."

[stcredzero]

There's been some nifty work done on the practicality of electrodynamic tethers and using them to turn a starship against the galactic magnetic field. You won't get sci-fi ship maneuverability out of this, but it's useful for course corrections without having to carry or expend reaction mass.

As far as onboard power sources go, fission seems perfectly cromulent. You just have to protect those radiators.

[Jweb_Guru]

I can only find one source on using tethers in interstellar travel: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/2005021.... To quote the article, for energy production:

> Since the tether current is 1,333 amps, ne = 3 X 10‘‘ m-3 and the ship’s velocity is 900 km/sec, the effective electron-collector radius is approximately 3 13 km.

This appears to have a toroidal-field ramscoop as a prerequisite.

For thrustless turning, a 0.06 degree turn requires a tether 10^3 km long:

> To obtain a six-degree trajectory modification during a 1,400-year journey with the tether current assumed, the tether length must be increased by a factor of lOOX to equal l0^5 km. This would increase tether mass to 2.7 X lo5 kg.

And this is using a reference ship many scientists are skeptical about:

> ...it is assumed here that the primary propulsion for these ships is the ultra-thin, space manufactured solar sail unfurled as close to the Sun as possible at the perihelion of a parabolic or hyperbolic solar orbit. After acceleration to interstellar cruise velocity, it is also assumed that sail and cables are wound around the habitat section to provide extra cosmic ray shielding. The sail is unfurled again for deceleration at the destination star.... since the baseline sailcraft for this analysis is somewhat faster, either more advanced sail/cable materials are required or the pre-perihelion orbit is hyperbolic.

I'd definitely categorize that under "experimental power sources and unproven physics." Perhaps you are thinking of a different article?

[stcredzero]

I got that from a Robert Forward nonfiction book. The craft he described didn't have ramscoops, but were lightsail craft.

[camikazeg]

1,400 light years/~45,000mph (the speed of New Horizons) = 23,844,146.8 years of travel time. I guess that wouldn't be impossible, but if you've got a ship that can last that long in interstellar space, then what do you need a planet for? Or for that matter, a star?

[danielweber]

You wouldn't try to make a 1400-ly trip, and you would try for a much faster departure than NH did.

There are about 10 stars we know about within 11-ly of earth. A serious attempt to reach one would involve doing slingshots around many bodies in the solar system, probably culminating with a dive to the Sun in order where the major rockets would be fired. (The Oberth effect says you get the most ∆v boost at the point closest to the orbiting body. You cannot logically use the Sun as a passive slingshot because you are trying to leave the solar system.) Getting the speed to 1% of c this way looks entirely reasonable, which gives about a century of travel time.

It would be very expensive, but in a few more centuries society will be a lot richer. This is also something that would only occur after significant colonization within the solar system, and mapping of distant star systems to find good candidate systems.