I won’t talk about antimatter any more, except to say antiproton-proton reactions generate a lot more than a burst of gamma rays. Think: Fireball.
Anyhoo, no matter what propulsion system you use, you do have to slow down, because at those kinds of speeds, a useful orbital insertion is going to be pretty tough, if not impossible.
So you accelerate off for a while, turn the ship around, an decellerate. It’s actually not a bad thing, since you get artificial gravity that way for prolonged periods.
As for needing to find a habitable planet at the end of the trip, forget it. If you have a generation ship that will take decades to reach another star, the ship itself will have to be a self-sustaining habitat. Don’t think spaceship, think orbital habitat that just happens to be heading out into interstellar space. If your orbital habitat isn’t good enough to keep you going once you reach the other star, it certainly won’t be good enough to keep you going during the 100+ year trip.
So there won’t be generation ships until we have had a very long experience with people living their entire lives in space out on the edges of the solar system. The main difference between a generation ship and an orbital colony in the Kuiper belt would be that your internet connection speeds would get worse and and worse and worse until you wouldn’t bother any more. And you’d never ever get any visitors.
But of course, before we send out giant multiton manned ships we are going to send out unmanned probes, and do a lot of observations from our solar system. There’s a lot you can do with an optical interferometer if the distance between the sensors is the diameter of earth’s orbit. And we will probably be able to take spectra from the small extrasolar planets. If the spectra show free oxygen or methane it would be almost certainly caused by living organisms.
Of course, life forms on another planet are likely to have radically different biochemistries than earth life, so the environment of any lifebearing planet is likely to be highly toxic to a human. Or maybe not if panspermia happens.
Like a highly efficient H-bomb, yes. My point was that the fireball still doesn’t do anything unless it somehow gets some mass, which was at rest in relation to the ship, to move away from the ship very quickly. It might well throw off some particles at very high speed, but in terms of conservation of momentum that isn’t going to provide much force to accelerate the ship with.
Deep Space 1’s ion engine only had 81.5 kg of reaction mass, and yet took 20 months of continuous thrust to expend it. I imagine the exhaust velocity of an antimatter/matter reaction will be similarly tenuous but considerably faster than the 30 km/second speed of the ion engine.
It seemsclear that barring any revolutionary discoveries, we will have to travel at perhaps 0.1c, so the proposal I make is this:
WE located a 12-15mile diameter asteroid, and hollow it out.
The asteroid is set spiinning, so that ihabitants inside experience artificial gravity. In the center of the vod inside, we have an artificial sun-with light output approximating our sun. The interrior is them arranged with lakes, fields, and forests. Farms provide food, and a human population of 5-6,000 people makes this their home. Meanwhile, propulsion is provided by electric-ion drive, power source is a fusion reactor.
When the desired star system is reached, the asteroid goes into orbit around the star…then the colonists cab make their way to the palnet surface.
It’ll take thousands of years, but this is the only way I see it happening.
The planned Terrestrial Planet Finder mission will be capable of directly imaging Earth-like planets and spectroanalyzing them, with the hope of detecting free oxygen (either O[sub]2[/sub] or O[sub]3[/sub]), a certain tell-tale of existing life on the planet. Hopefully, the beaurocracy of NASA will change soon, so work can continue on this project.
And orbital insertion will, indeed, be impossible at interstellar speeds. If you can use orbital shenanigans to stop at your destination, then you could have used them to start on your voyage at this end, too. In fact, this is true for almost all propulsion technologies: If you can use it to stop, then you can use it to start, too. The two exceptions are ramscoop ships, which have nonnegligible drag from the tenuous not-quite-vacuum of space, and are therefore better suited for slowing down than for speeding up; and propulsion methods based on a laser which stays at the origin point and fire at a solar sail, which (failing cooperative aliens) wouldn’t work at the destination end.
Cool. Can you tell us how ion engines compare to conventional rockets? How fast could you reasonably expect a probe to go compared to the 17km/s that Squink mentioned for Voyager 1?
DAWN uses ion propulsion (IP) for one main reason: cost. A conventional chemical rocket was found to be too costly to justify as a line item in the federal budget, but, by going to an ion engine, it became a more desirable (and I might add, a little more sexy) for NASA to justify it before Congress.
One of the engineers supplied me with this little tidbit. From a rest state the DAWN spacecraft will top out at about 11.5 km/sec using about 150 kg of fuel (the fuel is my estimate, not the engineers). Mind you, I’ve not included the kick of chemical propulsion to allow the spacecraft to permanently escape from earth’s grip (~11.1 km/sec), so conceivably it could exceed Voyager One’s velocity. On the other hand, the DAWN the ion engine doesn’t remain on continuously–coasting is part of the mission’s trajectory profile. In addition, you’re going to need to slow down at some point to allow orbital insertion around each asteroid.
Most of Voyager’s speed is a result of gravitational assist (i.e., sucking off some of the momentum of the outer planets). which makes the IP system even more remarkable.
(Sort of a side note: The Saturn V rocket was capable of launching a 17 lbs payload to Alpha Centauri; I don’t have the cite, but recalled hearing about it on the Diane Rheem(?) show in which the author was discussing the capabilities of forementioned vehicle).
Yes and no. It will slow you down some. But if you’re launching using laser plus solar wind, then solar wind by itself won’t be enough to stop you.
And radiation pressure (from the photons themselves hitting you) is actually much more significant than solar wind (from non-photon particles blown off by the Sun), but the basic principle, and means of harnessing it, is the same.
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