Tuesday OpenThread | 152 comments (152 topical, editorial, 0 hidden)

Re: Is anybody out there? (4.00 / 2)

There's two barriers that have to be overcome:

1. Achieving 1G all the time.

Human biochemistry requires 1G for the chemical reactions to work correctly.

2. Constructing an electromagnetic shield

Sun produces a constant barrage of nasty particles, x-rays, etc. which does nasty things to human tissues. For long term space habitation being able to erect an artificial Van Allen Belt is a necessity.

She believed in nothing; only her skepticism kept her from being an atheist. -- Jean-Paul Sartre

by ATinNM on Tue Oct 29th, 2013 at 12:48:11 PM EST
[ Parent ]

Adding (none / 1)

If these aren't solved forget about human space exploration.

She believed in nothing; only her skepticism kept her from being an atheist. -- Jean-Paul Sartre

by ATinNM on Tue Oct 29th, 2013 at 12:49:03 PM EST
[ Parent ]

Re: Is anybody out there? (none / 1)

Sustainable self-contained life support through artificial eco-systems. I don't think a prototype has been achieved on Earth, even. I'd start with undersea structures to preclude cheating on containment.

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman

by Migeru (migeru at eurotrib dot com) on Tue Oct 29th, 2013 at 12:51:18 PM EST
[ Parent ]

Re: Is anybody out there? (none / 1)

The BioSphere Project was a success in proving we don't know enough about artificially closed eco-systems to do anything useful.

Which is, in my mind, a great step forward.

She believed in nothing; only her skepticism kept her from being an atheist. -- Jean-Paul Sartre

by ATinNM on Tue Oct 29th, 2013 at 02:02:16 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

Precisely, but the problem needs to be solved.

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman

by Migeru (migeru at eurotrib dot com) on Tue Oct 29th, 2013 at 02:36:06 PM EST
[ Parent ]

Re: Is anybody out there? (4.00 / 2)

And fortunately Biosphere II remains as a working lab. It is large enough to do a lot of valuable science and some of the engineering challenges it still faces are applicable to any prolonged occupation of other solar bodies, be they in Earth oribit, at LaGrange points, on the Moon, the Moons of Mars or on Mars itself. Especially useful would be working out some of the major parameters of the social aspects of long term confined isolation.

"It is not necessary to have hope in order to persevere."

by ARGeezer (ARGeezer a in a circle eurotrib daught com) on Tue Oct 29th, 2013 at 07:17:33 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

But physically Biosphere II looks like nothing you might build in orbit, except on a giant rotating structure at 1G as seen in Elysium.

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman

by Migeru (migeru at eurotrib dot com) on Wed Oct 30th, 2013 at 07:02:00 AM EST
[ Parent ]

1G (4.00 / 2)

Popular Science: Space-Born Jellyfish Hate Life On Earth (10.15.2013)

NASA first started sending jellyfish to space aboard the Columbia space shuttle during the early '90s to test how space flight would affect their development. As cool as being an astronaut baby sounds, the jellies didn't develop the same gravity-sensing capabilities as their Earthly relatives.
Jellyfish tell up from down through calcium sulfate crystals that ring the bottom edge of their mushroom-like bodies. The crystals are housed in little pockets lined with hair cells, and when the jellyfish moves, the crystals roll around, signaling to the brain which way is up by stimulating those hair cells. The pockets seemed to develop normally in space, but the astro-jellies later had trouble figuring out how to swim around in normal gravity. They had abnormal pulsing and movement when returned to Earth compared to non-astronaut jellyfish.
Humans sense gravity and acceleration using otoliths, calcium crystals in the inner ear (similar to those jellyfish have) which move sensitive hair cells to tell the brain which way gravity is pulling. So if the jellyfish had trouble developing their gravity senses in space, it's likely human space babies would get major vertigo too.

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman

by Migeru (migeru at eurotrib dot com) on Tue Oct 29th, 2013 at 12:52:50 PM EST
[ Parent ]

Re: Is anybody out there? (4.00 / 4)

You missed the main thing that needs to be overcome: the whole idea of human space travel is completely pointless.

by asdf on Tue Oct 29th, 2013 at 12:53:47 PM EST
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Re: Is anybody out there? (none / 0)

It's only pointless while we are trapped with chemical fuelled Newtonian machines which takes ages to get to the moon and a significant percentage of a human lifetime to reach Mars.

Orbiting the earth is fun, but it ain't going anywhere.

What we need is a new technology. Even a theory for one would be a start. Without that, I'd agree with you

keep to the Fen Causeway

by Helen (lareinagal at yahoo dot co dot uk) on Tue Oct 29th, 2013 at 01:02:36 PM EST
[ Parent ]

Re: Is anybody out there? (4.00 / 2)

Propulsion at 1G gets you to relativistic motion pretty quick, but even assuming optimal matter-to-energy conversion the payload-to-fuel ratio is lousy.

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman

by Migeru (migeru at eurotrib dot com) on Tue Oct 29th, 2013 at 01:11:22 PM EST
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Re: Is anybody out there? (none / 1)

Assuming no math errors at 1G about a year would get you as close to the speed of light as you might want to go, but, while that might have certain advantages for life support it would require enormous thrusters to get the ship and the fuel moving. Part of the problem would be that, unless you plan to collect the matter which you plan to eject as you go, which has its own problems, especially as you approach relativistic velocities, you would have to start with twice the matter you need to get you close to C, presuming you intend to slow back down to something close to our current velocity. So it might be necessary to start off at a small acceleration, perhaps a tenth of a G, and let that acceleration increase as matter is ejected. And you would have to count on spending at least a year decelerating unless you wanted to exceed 1G rates, perhaps longer, depending on available thrust and remaining mass. For trips to nearby stars you would spend the majority of the trip accelerating and decelerating and would be lucky to average .4C for the trip. Thus a trip to a star 4 light years away would take 10 years or longer, depending on initial acceleration.

(Do you think I haven't thought at least a little about what it would take to get out of here? Dreams die hard.)

"It is not necessary to have hope in order to persevere."

by ARGeezer (ARGeezer a in a circle eurotrib daught com) on Tue Oct 29th, 2013 at 08:26:34 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

Part of the problem would be that, unless you plan to collect the matter which you plan to eject as you go

That negates the rocket effect. You can't collect what you eject.

In fact, the most efficient acceleration is achieved by ejecting light (i.e., shining it).

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman

by Migeru (migeru at eurotrib dot com) on Wed Oct 30th, 2013 at 02:19:12 AM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

No, but space isn't empty. I have seen proposals for a large scoop preceding the ship that would collect some of the few molecules per square meter ahead of the ship. This would become more problematic the greater the velocity of the ship, but it might enable the ship to start off with a significantly smaller total mass. And there are likely other serious problems, but they might not be unsolvable.

"It is not necessary to have hope in order to persevere."

by ARGeezer (ARGeezer a in a circle eurotrib daught com) on Wed Oct 30th, 2013 at 12:04:39 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

Ah, but collecting insterstellar matter from the front is not the same thing as collecting the ejecta you use for the rocket effect.

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman

by Migeru (migeru at eurotrib dot com) on Wed Oct 30th, 2013 at 12:16:16 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

Sorry if I was unclear. It never occurred to me to collect the ejecta, probably for the very reasons you mention.

"It is not necessary to have hope in order to persevere."

by ARGeezer (ARGeezer a in a circle eurotrib daught com) on Wed Oct 30th, 2013 at 12:24:27 PM EST
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Re: Is anybody out there? (none / 0)

Yes, this was ambiguous

unless you plan to collect the matter which you plan to eject

Collecting before ejecting, not after, of course.

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman

by Migeru (migeru at eurotrib dot com) on Wed Oct 30th, 2013 at 12:25:52 PM EST
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Re: Is anybody out there? (none / 1)

The concept would be similar to a magnetohydrodynamic drive for ships or submarines adapted to deep space operation. I realize that collection of these particles would produce a loss of momentum for the vessel, but at relatively low velocity it might be possible collect more material than is being ejected while accelerating the ejected particles to close to C.

"It is not necessary to have hope in order to persevere."

by ARGeezer (ARGeezer a in a circle eurotrib daught com) on Wed Oct 30th, 2013 at 12:20:21 PM EST
[ Parent ]

Magnetohydrodynamic drive (none / 0)

See upthread.

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman

by Migeru (migeru at eurotrib dot com) on Wed Oct 30th, 2013 at 12:28:38 PM EST
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Re: Is anybody out there? (none / 1)

Well, you probably want to plan for 12 ly of travel. That gets you to Tau Ceti e, which is pretty high on the list of habitable planets.

http://en.wikipedia.org/wiki/List_of_nearest_terrestrial_exoplanet_candidates

On the other hand, Venus is a better prospect for living on, and it's just a bit closer...
http://en.wikipedia.org/wiki/Thermoplanet#Habitable_class

In fact, this table sorted on ESI pretty much sums up the problem: You have to go 20 ly (or 1000+ ly, depending on details of how you read the numbers) to get to a planet that is marginally more suitable for life than Venus. And Venus is "somewhat challenging."

http://en.wikipedia.org/wiki/Standard_Primary_Habitability

by asdf on Wed Oct 30th, 2013 at 10:56:06 AM EST
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Re: Is anybody out there? (none / 0)

Looks like Gliese 667 Cc would be a better bet : 85% earth-like. Surely more habitable than Venus.

At 23.6 light-years, however, we're looking at multi-generation one-way travel times. One would want to know a bit more about the destination before committing oneself and one's posterity to such a trip.

And there are undoubtedly planets which are more suitable for life than Earth... but the nearest seem to be 500+ light-years away.

_{It is rightly acknowledged that people of faith have no monopoly of virtue
- Queen Elizabeth II}

by eurogreen on Wed Oct 30th, 2013 at 11:38:11 AM EST
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Re: Is anybody out there? (none / 1)

At 23.6 light-years, however, we're looking at multi-generation one-way travel times. One would want to know a bit more about the destination before committing oneself and one's posterity to such a trip.

Not at 1G acceleration in the direction of travel. But for that one needs a hell of a radiation shield, much better than the one ATinNM points out is necessary in any case even at nonrelativistic speeds.

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman

by Migeru (migeru at eurotrib dot com) on Wed Oct 30th, 2013 at 11:42:46 AM EST
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Re: Is anybody out there? (none / 1)

erm, are you planning on ignoring the speed limit?

This thing about using acceleration to get your physiological 1G. Doesn't work, does it?

_{It is rightly acknowledged that people of faith have no monopoly of virtue
- Queen Elizabeth II}

by eurogreen on Wed Oct 30th, 2013 at 12:17:28 PM EST
[ Parent ]

Also. (none / 0)

_{It is rightly acknowledged that people of faith have no monopoly of virtue
- Queen Elizabeth II}

by eurogreen on Wed Oct 30th, 2013 at 12:20:17 PM EST
[ Parent ]

Re: Is anybody out there? (4.00 / 2)

Time dilation!

I'm going to have to write a diary on relativistic rockets for the nonphsyicists among us.

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman

by Migeru (migeru at eurotrib dot com) on Wed Oct 30th, 2013 at 12:26:55 PM EST
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Re: Is anybody out there? (none / 1)

Good luck on the "what happens to the clock when you turn around to come back when you discover that the planet is worse than Venus" part...

by asdf on Wed Oct 30th, 2013 at 12:29:20 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

We're within 20 years of full spectroscopic characterization of atmospheric chemistry from Earth.

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman

by Migeru (migeru at eurotrib dot com) on Wed Oct 30th, 2013 at 12:32:32 PM EST
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Re: Is anybody out there? (none / 0)

Yeah, I was referring to the relativity thing about reversing direction.

by asdf on Wed Oct 30th, 2013 at 12:45:40 PM EST
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Re: Is anybody out there? (none / 0)

You never go on your trip if you discover that the planet is worse than Venus, though. You know that at the outset.

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman

by Migeru (migeru at eurotrib dot com) on Wed Oct 30th, 2013 at 01:04:59 PM EST
[ Parent ]

Re: Is anybody out there? (4.00 / 3)

Yeah, sounds good. Hope that detailed review of the planet 22 light years away is accurate!

I will work on my Venus floating city project in the meantime...which is about 99 times more practical than these interstellar spaceship dreams...

http://en.wikipedia.org/wiki/Colonization_of_Venus#Aerostat_habitats_and_floating_cities

by asdf on Wed Oct 30th, 2013 at 01:13:50 PM EST
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Re: Is anybody out there? (none / 0)

You might be surprised at the pace of progress in exoplanet spectroscopy. I'm pretty confident in a couple of decades we'll know the temperature and major chemical components of the atmospheres of most exoplanets within 50 light-years.

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman

by Migeru (migeru at eurotrib dot com) on Wed Oct 30th, 2013 at 07:32:58 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

Yep, and there's basically no limit to how big of a telescope you can make, so they will get more info as time goes on...

by asdf on Wed Oct 30th, 2013 at 09:35:19 PM EST
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Re: Is anybody out there? (none / 0)

Oh bugger. Of course. I'm forgetting my Heinlein.

But that doesn't solve the 1G problem. If it takes a year to get close to C with 1G acceleration, you need gravity while cruising for (Earth) years at that speed, yes? Or if you get close enough to C, does the relative time without acceleration become trivial?

_{It is rightly acknowledged that people of faith have no monopoly of virtue
- Queen Elizabeth II}

by eurogreen on Wed Oct 30th, 2013 at 12:32:35 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

You don't need gravity. Your 1G acceleration is the gravity.

Inertial forces!

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman

by Migeru (migeru at eurotrib dot com) on Wed Oct 30th, 2013 at 12:33:45 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

What happens when you try to flip the ship to start decelerating? That sounds like a fun engineering problem ...

by Colman (colman at eurotrib.com) on Wed Oct 30th, 2013 at 12:36:15 PM EST
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Re: Is anybody out there? (none / 1)

The only practical approach I can imagine is to reverse the direction of particle ejection. Nor do I know if it is practical to have an artificial gravity produced by rotation around the axis of travel at .8C cruise velocity, let alone while accelerating/decelerating. Rotating bodies proximate to large magnetic and static fields could be a problem, as could large static charge accumulation.

"It is not necessary to have hope in order to persevere."

by ARGeezer (ARGeezer a in a circle eurotrib daught com) on Wed Oct 30th, 2013 at 12:44:06 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

The problem is all the stuff plowing into the sides of the craft as you turn, I think: there's no physical problem turning - the laws of physics work just the same (we think ...)

I'm not sure how thick interstellar medium is when you're doing 90% of c or whatever and physically big.

by Colman (colman at eurotrib.com) on Wed Oct 30th, 2013 at 12:46:55 PM EST
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Re: Is anybody out there? (none / 0)

Not sure I see why it needs to be physically big. Because of time dilation, inter-generational communication is basically broken (ten generations from now, nobody on Earth will care about this dumb project), so there's no point in having living astronauts take this on.

Seems to me that it makes more sense to just freeze some eggs and sperm and have a robot that mixes them up in a test tube when you get there. If the planet is suitable for life, you get a few thousand Adams and Eves ready to start doing whatever it is that they are going to be doing. Much smaller, easier to shield, no need to worry about maintaining 1G, etc.

by asdf on Wed Oct 30th, 2013 at 01:00:31 PM EST
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Re: Is anybody out there? (none / 1)

What are you going to grow the eggs+sperm in?

Then again aggressive colonisation by relativistic techno-wombs from spaaaaace does have the makings of a cool SF story.

by ThatBritGuy (thatbritguy (at) googlemail.com) on Wed Oct 30th, 2013 at 01:24:25 PM EST
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Re: Is anybody out there? (4.00 / 3)

Well, this is a habitable planet we're going to, right? So you just take them test tubes and fill them up with the local pond water, leave them in the sun, and presto-chango, nine months later you have a bouncing little baby!

Or, your robot goes out and catches some of the female humanoids that are wandering around (or their ape ancestors, depending on alignment of evolutionary timescales of Earth and this Paradise we're talking about) and sets them up as surrogates. Hopefully you won't screw up and go to the planet after they have pulled the same sort of environmental catastrophe that we are here. (Possible problem with remote sensing of planet's conditions before spaceship launch: what if the natives screw it up in the 200 or so years it takes you to get there?)

Or, in parallel with the development of this spaceship's propulsion system, you have also developed an in-vitro baby growing machine. Which seems imminently more practical that the spaceship itself.

Other possibilities come to mind, none of which approach the suspension of disbelief required by the spaceship itself...

by asdf on Wed Oct 30th, 2013 at 02:06:46 PM EST
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Re: Is anybody out there? (4.00 / 2)

Well shit, all these scenarii and more were played out by the mid 1970s when I stopped reading science fiction.

Forward into the past!

_{It is rightly acknowledged that people of faith have no monopoly of virtue
- Queen Elizabeth II}

by eurogreen on Wed Oct 30th, 2013 at 06:21:46 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

asdf:

Hopefully you won't screw up

hollow laugh...

we are not jellyfish, we are..... DEVO

'The history of public debt is full of irony. It rarely follows our ideas of order and justice.' Thomas Piketty

by melo (melometa4(at)gmail.com) on Thu Oct 31st, 2013 at 04:49:34 AM EST
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Re: Is anybody out there? (none / 0)

In this context a sperm is physically big.

by Colman (colman at eurotrib.com) on Wed Oct 30th, 2013 at 01:26:29 PM EST
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Re: Is anybody out there? (none / 0)

Well, if the proposal is to get the 1G of gravitation from rotating the spaceship, not the acceleration from the motor, then you're talking about something that's like a km in diameter, right? To avoid dizziness.

But then I suppose these astronauts will have had their dizziness-sensing mechanisms removed as part of the general bionic preparation that they will want in any case...given that they are expected to build a civilization using nothing but what they bring along with them...

by asdf on Wed Oct 30th, 2013 at 02:09:48 PM EST
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Re: Is anybody out there? (none / 1)

given that they are expected to build a civilization using nothing but what they bring along with them...

3D printing will save the day!

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman

by Migeru (migeru at eurotrib dot com) on Wed Oct 30th, 2013 at 06:44:42 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

The spaceship is supposed to come near rest at the end of its journey so it has to be able to get its 1G from a combination of rotation and axle-wise linear acceleration. And it has to be able to get 100% of it from rotation, so yes...

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman

by Migeru (migeru at eurotrib dot com) on Wed Oct 30th, 2013 at 06:47:01 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

Also, if you're using a ramjet type design, your ram isn't pointing the right way any more.

by Colman (colman at eurotrib.com) on Wed Oct 30th, 2013 at 12:48:11 PM EST
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Re: Is anybody out there? (none / 0)

You would only reverse the direction of thrust, so the particles would now be ejected in the direction of travel. Hope nobody ahead minds.

"It is not necessary to have hope in order to persevere."

by ARGeezer (ARGeezer a in a circle eurotrib daught com) on Wed Oct 30th, 2013 at 12:57:48 PM EST
[ Parent ]

Ejected particles (none / 1)

Optimal relativistic rocket thrust is through a beam of light (or electromagnetic radiation generally).

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman

by Migeru (migeru at eurotrib dot com) on Wed Oct 30th, 2013 at 06:49:00 PM EST
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Re: Ejected particles (none / 0)

I had assumed protons or beta particles. I guess it would be a giant laser instead and the fuel would likely be deturium which would be converted to helium with the energy generated being used to drive the laser. But that would drastically reduce the efficiency of collecting 'fuel' via a scoop. So perhaps a large linear accelerator might be used instead to enable collection of fuel en route. It should be possible to design a linear accelerator so that it can work in either direction.

"It is not necessary to have hope in order to persevere."

by ARGeezer (ARGeezer a in a circle eurotrib daught com) on Thu Oct 31st, 2013 at 10:35:06 AM EST
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Re: Ejected particles (none / 0)

If you eject massive particles your payload to fuel ratio deteriorates: you need the least fuel if you can just shine light behind you to accelerate (yeah, I know...)

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman

by Migeru (migeru at eurotrib dot com) on Thu Oct 31st, 2013 at 11:52:25 AM EST
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Re: Ejected particles (none / 0)

Don't think of an on-board light source: that still requires you to carry around the energy = mass. It would be best to use starlight. You'll need enormous solar sails, though, and an enormous electromagnetic shield to protect them from ionised radiation (which still leaves the problem of neutral atoms hitting at relativistic speeds).

_{*Lunatic*, n.

One whose delusions are out of fashion.}

by DoDo on Fri Nov 1st, 2013 at 06:37:31 AM EST
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Re: Ejected particles (none / 1)

Light sails are brilliant...

Is light-sails vs. magnetohydrodynamic drive the analogue of wind vs. nuclear?

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman

by Migeru (migeru at eurotrib dot com) on Fri Nov 1st, 2013 at 06:49:13 AM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

When you turn off your thrusters you're in free fall... but you're being fried by a beam of cosmic rays coming from your destination. So you need a full-sphere particle shield if you want to turn. Or you can switch to a front-mounted rocket.

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman

by Migeru (migeru at eurotrib dot com) on Wed Oct 30th, 2013 at 01:04:05 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

As I said, fun engineering problems.

by Colman (colman at eurotrib.com) on Wed Oct 30th, 2013 at 01:27:37 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

I can understand encountering relatively stationary particles at .8C would require that they be collected or deflected but it would seem to me that comic rays would be incoming from the same directional distribution as at rest. If all electromagnetic radiation would be measured at C from the ship's perspective why would cosmic rays from ahead of you be worse than any others? If they are highly energetic charged particles I can see that they could easily overcome the power of a field designed to deflect/collect interstellar matter and that this would require serious additional shielding.

"It is not necessary to have hope in order to persevere."

by ARGeezer (ARGeezer a in a circle eurotrib daught com) on Wed Oct 30th, 2013 at 01:29:58 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

it would seem to me that comic rays would be incoming from the same directional distribution as at rest

Wrong at relativistic speeds relative to the medium. More details in the diar{y|ies}.

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman

by Migeru (migeru at eurotrib dot com) on Wed Oct 30th, 2013 at 06:50:14 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

Wrong at relativistic speeds relative to the medium

What medium, ether?

Does the 1G acceleration mean in the velocity (v) or impulse (p)? Doesn't the acceleration change under Lorenz transformations?

by das monde on Thu Oct 31st, 2013 at 12:36:32 PM EST
[ Parent ]

Aether? (none / 0)

Interstellar medium, which is an actual, material, thin plasma with a notion of average speed and a temperature.

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman

by Migeru (migeru at eurotrib dot com) on Thu Oct 31st, 2013 at 12:44:19 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

Does the 1G acceleration mean in the velocity (v) or impulse (p)? Doesn't the acceleration change under Lorenz transformations?

1g acceleration is as measured in the rest frame of the accelerating particle. Which is the only thing that makes sense.

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman

by Migeru (migeru at eurotrib dot com) on Thu Oct 31st, 2013 at 01:24:48 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

And the amazing thing is that the 1 G is so hard to overcome that all terrestrial life is bound to a layer only around a dozen meters thick.

by asdf on Thu Oct 31st, 2013 at 01:53:28 PM EST
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Re: Is anybody out there? (none / 0)

So the acceleration decreases in any inertia frame, as the accelerating particle leaves its rest frames very fast?

by das monde on Thu Oct 31st, 2013 at 02:12:01 PM EST
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Re: Is anybody out there? (none / 0)

I don't understand the question.

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman

by Migeru (migeru at eurotrib dot com) on Fri Nov 1st, 2013 at 09:22:17 AM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

The particle "rest frame" is not accelerating together with the particle, right? If he have to do standard computations in the same rest frame, the acceleration must be decreasing in any such frame.

by das monde on Fri Nov 1st, 2013 at 01:01:38 PM EST
[ Parent ]

Re: Is anybody out there? (none / 1)

"Rest frame" is the frame in which the particle is at rest, thus it is accelerating, too. But yes, observed from inertial frames, the particle's acceleration would appear to be decreasing.

_{*Lunatic*, n.

One whose delusions are out of fashion.}

by DoDo on Fri Nov 1st, 2013 at 06:51:35 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

And yes, acceleration is the same as seen from two reference frames which are related by a Lorentz transformation.

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman

by Migeru (migeru at eurotrib dot com) on Thu Oct 31st, 2013 at 05:30:35 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

How is the acceleration an invariant of Lorentz transformations when the velocity is not? The particle mass is not an invariant either, thus the same forcing (that should be a Lorenz invariant) should cause different acceleration in different frames.

by das monde on Fri Nov 1st, 2013 at 01:02:26 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

If you're coasting, you're coasting, regardless of how you measure it.

by asdf on Fri Nov 1st, 2013 at 01:29:58 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

Right, acceleration is not invariant. And neither is the force, by the way, because the force is a component of a four-vector that Lorentz-transforms. But acceleration is also component of a 4-vector that transforms appropriately.

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman

by Migeru (migeru at eurotrib dot com) on Fri Nov 1st, 2013 at 07:39:47 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

Interesting video on this topic. There are probably better ones out there, but I don't see basic flaws in this one...

by asdf on Thu Oct 31st, 2013 at 02:10:14 PM EST
[ Parent ]

Re: Is anybody out there? (none / 1)

it would seem to me that comic rays would be incoming from the same directional distribution as at rest

That's wrong, too. This is what happens to the incoming directions as you accelerate (see the "boost" in the upper right)

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman

by Migeru (migeru at eurotrib dot com) on Fri Nov 1st, 2013 at 06:10:12 AM EST
[ Parent ]

Re: Is anybody out there? (none / 1)

At 0.8 c relative speed, a particle's kinetic energy corresponds to exactly ⅔ times its rest energy. For comparison, for an average solar wind particle hitting an interplanetary spacecraft, the relative speed is 0.002 c and the kinetic energy is 0.000002 times the rest energy. In other words, 0.8 c is pretty bad already. To boot, most cosmic rays are charged particles which can be deflected by a shield of electromagnetic field, but neutral atoms and molecules in the interstellar medium won't be stopped by such a shield. Maybe that can be solved if you use a multi-wavelength laser beam from Earth to ionise everything in your path.

_{*Lunatic*, n.

One whose delusions are out of fashion.}

by DoDo on Fri Nov 1st, 2013 at 07:02:50 AM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

Let's start again : For a 23 light-year trip,

you accelerate at 1G for about a year (earth time), getting close to C
???
Approaching destination, you decelerate at 1G for about a year (earth time).

What do you do for gravity during (2), and how long is this phase from the point of view of the travellers?

_{It is rightly acknowledged that people of faith have no monopoly of virtue
- Queen Elizabeth II}

by eurogreen on Wed Oct 30th, 2013 at 12:41:01 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

Why do you stop accelerating?

by Colman (colman at eurotrib.com) on Wed Oct 30th, 2013 at 12:42:16 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

Dread of approaching C?

"It is not necessary to have hope in order to persevere."

by ARGeezer (ARGeezer a in a circle eurotrib daught com) on Wed Oct 30th, 2013 at 12:53:16 PM EST
[ Parent ]

Re: Is anybody out there? (none / 1)

You're always at the same distance from c, since the speed of light is always c from your point of view.

Dread of being fried by cosmic rays as you accelerate the interstellar medium to close to c relative to you.

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman

by Migeru (migeru at eurotrib dot com) on Wed Oct 30th, 2013 at 01:01:34 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

Or dread of losing the field that would collect/deflect interstellar matter that you would be encountering at around .8c.

"It is not necessary to have hope in order to persevere."

by ARGeezer (ARGeezer a in a circle eurotrib daught com) on Wed Oct 30th, 2013 at 01:18:50 PM EST
[ Parent ]

Re: Is anybody out there? (4.00 / 2)

In the rest frame of the ship the field always looks the same. What happens is that the interstellar medium becomes a narrow jet of energetic cosmic rays coming from the direction of travel.

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman

by Migeru (migeru at eurotrib dot com) on Wed Oct 30th, 2013 at 06:51:56 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

Of course! If you are traveling towards them the at .8C the two or three molecules/cubic meter, which have random, low velocities WRT the interstellar medium, appear to be coming at you at .8C and thus appear to be identical to what we would normally call 'cosmic rays' which are particles ejected from stellar objects at high velocities. And the ejecta from your ion drive would appear, to anyone who encountered it, similar to the effects of having the rotational axis of a neutron star or magnetar pointed directly at you.

"It is not necessary to have hope in order to persevere."

by ARGeezer (ARGeezer a in a circle eurotrib daught com) on Thu Oct 31st, 2013 at 10:17:20 AM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

well, I suppose you don't, if you have an adequate/infinite supply of energy and propulsion mass.

But that's just an engineering problem. Perhaps.

Question : is the mass and energy required to keep accelerating at 1G proportional to on-board relative time (as opposed to externally observed time)? That would certainly improve the miles per gallon with increasing speed.

_{It is rightly acknowledged that people of faith have no monopoly of virtue
- Queen Elizabeth II}

by eurogreen on Wed Oct 30th, 2013 at 06:55:57 PM EST
[ Parent ]

Re: Is anybody out there? (none / 1)

You need to eject a constant fraction of your mass-energy per unit time so your total mass (fuel + payload) decreases exponentially with on-board time.

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman

by Migeru (migeru at eurotrib dot com) on Wed Oct 30th, 2013 at 07:09:03 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

Well, each time your answer seems irrelevant to my question, it turns out I asked the wrong question.

So I guess I'll wait for the diary.

_{It is rightly acknowledged that people of faith have no monopoly of virtue
- Queen Elizabeth II}

by eurogreen on Thu Oct 31st, 2013 at 04:39:41 AM EST
[ Parent ]

Re: Is anybody out there? (none / 1)

The question that matters is: what speed do you want to attain (relative to the Sun). The energy/mass expended will be the same, regardless of the acceleration you use to reach it resp. the length of time (in whichever reference frame) you expend for acceleration.

_{*Lunatic*, n.

One whose delusions are out of fashion.}

by DoDo on Fri Nov 1st, 2013 at 07:13:25 AM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

In my original calculation of time I had allowed an early period of low or no acceleration above .1C specifically to allow the collection of additional interstellar matter to be used later for ejecta. But that might be unnecessary if you can rely entirely on just accelerating the matter which your scoop funnels into your engine. Or there might be engineering reasons to do so that would relate to the difference between collecting and storing matter as opposed to funneling it through the engine.

But AT is probably right and the answers to these questions are well understood by some. So, no matter how much fun the babbling might be I will try to await Mig's diary.

"It is not necessary to have hope in order to persevere."

by ARGeezer (ARGeezer a in a circle eurotrib daught com) on Thu Oct 31st, 2013 at 10:51:10 AM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

Well, I was thinking of starting with the simple kinematic problem of constant acceleration followed by constant deceleration. Because you surely cannot do the trip faster than that. (Though "faster" in Earth time actually means "slower in ship time, due to relativistic time dilation)

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman

by Migeru (migeru at eurotrib dot com) on Thu Oct 31st, 2013 at 11:48:22 AM EST
[ Parent ]

Re: Is anybody out there? (none / 1)

You don't want to collect the material at the start: then you have to accelerate even more mass. Either collect it continuously, or before and after the switch-over for the deceleration.

_{*Lunatic*, n.

One whose delusions are out of fashion.}

by DoDo on Fri Nov 1st, 2013 at 07:15:08 AM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

I haven't done even a crude a quantitative analysis but my thought was to get a ship with a much smaller fuel load up to some reasonable velocity more quickly and then to accumulate the fuel to accelerate to .8G and back to rest. Then I don't know how the density of space might vary from within to without the Ort cloud and overshooting the target due to lack of fuel could be awkward. And .8G was chosen as the cruise velocity as it seemed like the additional time and fuel to get to .9C or higher might not be worth the time lost & fuel saved by remaining at .8C.

"It is not necessary to have hope in order to persevere."

by ARGeezer (ARGeezer a in a circle eurotrib daught com) on Fri Nov 1st, 2013 at 11:59:39 AM EST
[ Parent ]

Re: Is anybody out there? (none / 1)

If you reduce the thrust time with on-board fuel just by a small bit, you do reduce the total mass needed, but not by orders of magnitude as is desirable. You would also need to spend a lot of time collecting the material.

The density of interstellar gas in the Oort cloud is not expected to differ from that beyond: outside the heliosphere, the influence of the solar system is marginal, compared to supernova shock waves, for example.

_{*Lunatic*, n.

One whose delusions are out of fashion.}

by DoDo on Fri Nov 1st, 2013 at 07:41:22 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

You accelerate continuously at 1G
???
You decelerate continuously at 1G

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman

by Migeru (migeru at eurotrib dot com) on Wed Oct 30th, 2013 at 12:58:27 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

The thought has occurred to me that the limits on absolute velocity relative to C might be a function of the density of space. If so it may be possible to travel faster than C as determined on Earth. After all we do know that light travels slower than C in dense liquids. Space based experiments could give some information on that hypothesis, especially if a vehicle were launched close to perpendicular to the orbits of the planets in the solar system and with a net velocity away from the sun's axis of rotation. And beyond the Ort cloud I expect density will drop again.

"It is not necessary to have hope in order to persevere."

by ARGeezer (ARGeezer a in a circle eurotrib daught com) on Wed Oct 30th, 2013 at 12:52:35 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

c is speed of light in a vacuum.

Light travels as fast as is possible, so its speed is c, I think.

by Colman (colman at eurotrib.com) on Wed Oct 30th, 2013 at 12:56:08 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

c is speed of massless objects in vacuum. Nothing goes faster than c in any medium. "Light" is a red herring.

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman

by Migeru (migeru at eurotrib dot com) on Wed Oct 30th, 2013 at 12:59:27 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

Well, that is why I was only planning on accelerating to .8C to begin with.

"It is not necessary to have hope in order to persevere."

by ARGeezer (ARGeezer a in a circle eurotrib daught com) on Wed Oct 30th, 2013 at 01:05:37 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

That's what I said.

by Colman (colman at eurotrib.com) on Wed Oct 30th, 2013 at 01:20:37 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

<nitpick>
c is the speed of objects without rest mass in vacuum. They do have dynamic mass = energy or they would be nothing.
</nitpick>

_{*Lunatic*, n.

One whose delusions are out of fashion.}

by DoDo on Fri Nov 1st, 2013 at 07:27:40 AM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

<nitpick squared>I don't like \[E = mc^2\] where momentum is \(p = mv\) and \(m\) depends on the state of motion. I prefer \[E^2 - (p c)^2 = (m c^2)^2\] where \(m\) is rest mass independent of the state of motion and \(pc^2 = Ev\). It makes the algebra (and the physics) so much simpler...</nitpick>

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman

by Migeru (migeru at eurotrib dot com) on Fri Nov 1st, 2013 at 08:20:32 AM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

Whether you write m or m₀ for the rest mass in your algebra, I don't care. But what about gravity, especially in the first few thousand years of the Universe?

_{*Lunatic*, n.

One whose delusions are out of fashion.}

by DoDo on Fri Nov 1st, 2013 at 08:38:05 AM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

Yes, the source of gravity is (E), not (m).

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman

by Migeru (migeru at eurotrib dot com) on Fri Nov 1st, 2013 at 08:58:03 AM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

On Earth. I don't know how well C has been measured in deep space. But, if I recall correctly, if you measured C in a vacuum while traveling in a ship moving at .8C it would not be .2C from your ship-based reference point, it would be C.

"It is not necessary to have hope in order to persevere."

by ARGeezer (ARGeezer a in a circle eurotrib daught com) on Wed Oct 30th, 2013 at 01:03:40 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

Gravity itself does not have mass but yet light is bent by gravity. But it was only a conjecture to begin with and I wouldn't expect any effect of gravitational fields to be very significant.

"It is not necessary to have hope in order to persevere."

by ARGeezer (ARGeezer a in a circle eurotrib daught com) on Wed Oct 30th, 2013 at 01:08:40 PM EST
[ Parent ]

Re: Is anybody out there? (none / 1)

Getting off-track here...

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman

by Migeru (migeru at eurotrib dot com) on Wed Oct 30th, 2013 at 01:09:56 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

Sez who? I didn't know there was a "track." :-)

by asdf on Wed Oct 30th, 2013 at 01:14:54 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

Not at 1G acceleration in the direction of travel.

I had figured on perhaps ten years to get to cruise velocity at around .8C with enough fuel to decelerate and at least a year to decelerate at -1G, although it may be necessary to spend longer at .1C just to collect matter - depending. So we would go about 4 light years the first ten years and 0.4 light years the last year. At .8C the remaining 19.2 light years would take 24 years. So the departing generation would arrive alive with adult children and grandchildren.

"It is not necessary to have hope in order to persevere."

by ARGeezer (ARGeezer a in a circle eurotrib daught com) on Wed Oct 30th, 2013 at 12:36:24 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

Um, ship time or earth time?

by Colman (colman at eurotrib.com) on Wed Oct 30th, 2013 at 12:37:23 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

Ship time. And hope the assumptions hold.

"It is not necessary to have hope in order to persevere."

by ARGeezer (ARGeezer a in a circle eurotrib daught com) on Wed Oct 30th, 2013 at 12:54:17 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

Slightly more than one year to get to 0.8c, ship time at constant 1g acceleration. 1.2 years Earth time. 2 ships years to get to 97% of c. 4 earth years.

24 light years takes about six years, ship time, about 30 earth time, so it's a 12/60 round trip, I think.

Numbers from here, but they seem sane.

by Colman (colman at eurotrib.com) on Wed Oct 30th, 2013 at 01:19:14 PM EST
[ Parent ]

Re: Is anybody out there? (none / 1)

When I first became seriously interested in this subject in the mid 80s as the possible basis for an SF piece there was no internet and so I purchased all three volumes of Burnham's Celestial Handbook and dug out the nearest stars from that. Of course we had very little information then about possible planets.

"It is not necessary to have hope in order to persevere."

by ARGeezer (ARGeezer a in a circle eurotrib daught com) on Wed Oct 30th, 2013 at 02:20:15 PM EST
[ Parent ]

Re: Is anybody out there? (none / 1)

And just in time, earth's twin discovered... sort of.

Astronomen Entdecken Erd Zwilling (auf Deutsch)

A bit too close to it's sun, with a surface temp of 2200 to 2800 grad celcius. But hey, fits the thread.

"Life shrinks or expands in proportion to one's courage." - Anaïs Nin

by Crazy Horse on Wed Oct 30th, 2013 at 06:11:19 PM EST
[ Parent ]

Re: Is anybody out there? (none / 1)

Yep. Also dark matter experiment was in the news. See, ET gets you the news before it happens!

I will still take Venus, though...

by asdf on Thu Oct 31st, 2013 at 02:15:02 PM EST
[ Parent ]

Re: Is anybody out there? (4.00 / 3)

At least for a species which shits its own bed.

"Life shrinks or expands in proportion to one's courage." - Anaïs Nin

by Crazy Horse on Tue Oct 29th, 2013 at 01:04:55 PM EST
[ Parent ]

Re: Is anybody out there? (none / 1)

Well, there's that too

keep to the Fen Causeway

by Helen (lareinagal at yahoo dot co dot uk) on Tue Oct 29th, 2013 at 01:27:20 PM EST
[ Parent ]

Re: Is anybody out there? (none / 0)

Ok I've done my Twitter duty.

by asdf on Tue Oct 29th, 2013 at 01:09:54 PM EST
[ Parent ]

Re: Is anybody out there? (4.00 / 2)

The point was to get away from Earth before it's taken over by deadly jellyfish. I guess even that won't work any more.

by gk (gk (gk quattro due due sette @gmail.com)) on Tue Oct 29th, 2013 at 02:55:18 PM EST
[ Parent ]

Re: Is anybody out there? (4.00 / 2)

Can't you solve the two problems at once? A rotating ring mounted on a counter-rotating axis, with a powerful magnet on the axis mimicking the way the Earth's core generates a magnetic field.

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman

by Migeru (migeru at eurotrib dot com) on Tue Oct 29th, 2013 at 12:54:37 PM EST
[ Parent ]

Re: Is anybody out there? (4.00 / 2)

In addition, the magnetic field would route incoming protons (cosmic rays!) to the rotating axis, where they could be collected as fuel for fusion power, or jetted through for propulsion.

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman

by Migeru (migeru at eurotrib dot com) on Tue Oct 29th, 2013 at 12:57:45 PM EST
[ Parent ]

Re: Is anybody out there? (none / 1)

Well, when you get the EM hydrodynamics figured out to make that all work, you will probably have also figured out how to make the fusion power system that will get you to wherever it is you're going.

by asdf on Tue Oct 29th, 2013 at 01:55:44 PM EST
[ Parent ]

Re: Is anybody out there? (none / 1)

Fusion power is the only possibility I can see that would make such a thing possible.

"It is not necessary to have hope in order to persevere."

by ARGeezer (ARGeezer a in a circle eurotrib daught com) on Wed Oct 30th, 2013 at 01:11:57 PM EST
[ Parent ]

Re: Is anybody out there? (none / 1)

No idea. Space Engineering isn't my bailiwick and I know enough to know I don't know (Known-Unknown) and I'd rather not babble-n-blather.

:-)

She believed in nothing; only her skepticism kept her from being an atheist. -- Jean-Paul Sartre

by ATinNM on Tue Oct 29th, 2013 at 01:54:51 PM EST
[ Parent ]

Tuesday OpenThread | 152 comments (152 topical, 0 editorial, 0 hidden)

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