Why light speed wouldn't matter

[Temstar]

2 hours ago, Shpaget said:

you believe that there will be no humans in 100 million years? As in an extinction, or that our descendants will not call themselves humans any more?

If you send a probe that will take 100 million years to get to Alpha Centauri, by the time it gets there it will probably find worlds teeming with humans.

[WestAir]

I don't use a transfer to get my Kerbal from an EVA back to the ISS. Just because two things are in orbit of the same body doesn't mean you need to transfer.

[Pds314]

When we start talking about things like the Valkerie, the correct answer is "screw orbital mechanics! I'm gonna point at the destination and go!"

 

The thing is, interstellar speed differences aren't really much more than interplanetary ones. The only time-practical way to do interstellar transfers is to get something going a good fraction of C, then apply reverse thrust for months or years at an ISP of millions.

Even RELATIVELY slow spacecraft like the voyager probes will get to other star systems, and it won't take 100 million years to do it either. (though it will take what humans call "too damn long.")

Basic thing is, most nearby stars move under 30 km/s relative to each other. When we're talking about practical interstellar starships that could transport humans without some sort of stasis or longevity improvement, their delta is necessarilly at least a thousand times that, if not ten thousand times that. Monster insertion burn just means reverse thrust to them.

[PB666]

16 hours ago, mjl1966 said:

So, when we think of going to some place like Alpha Centauri, we think "4 light years away".  Implementations such as the Valkyrie from Avatar where the ship spends half the trip accelerating to near light speed and the other half decelerating start to sound feasible.

But wouldn't we be facing the same problem we face with interplanetary travel?  If we want to go to Mars, we can't just wait for it to come by for a close pass and then hop over to it.  We *must* use a transfer orbit.  There's no way around it.  A transfer orbit is still going to be about half a full orbit around the sun.  We *may* be able to shorten that up by intercepting at some point along the transfer orbit than the aphelion, but that's going to require a monster burn for orbital insertion.  no free lunch.

Well, aren't both sol and Alpha Centauri orbiting the center of the galaxy?  Doesn't that mean you have to get there using a transfer orbit?  Doesn't that mean that  you're going to have to complete a large portion of an orbit around the center of the galaxy to get there?  If you use the short orbit method, then you have to do a really monster burn for orbital insertion.  I don't know what the numbers are, but I imagine the dV simply is not feasible.  (I'm guessing you would have to use about the same amount of energy as would be provided by the potential energy of a proper transfer orbit.  Math dudes, what say you?)

And here's the real middle finger from the universe: the speed at which you travel along that transfer orbit is not limited to the speed of light.  It is limited to the speeds dictated by the mechanics of the orbit itself, which are going to be much much slower than the speed of light.

The orbital period of the sun is 225x10(6) years.  This does not bode well for interstellar travel. 

Is there a way around this?

There are several issues here to start with sweep the garbage off the kitchen table alcubierre drivevis a fantasy, it is based on physics that does not exist and particles that have never been observed. 

Given this we revert to known physics which has an upper limit on the conversion of mass to energy of about 10% even if we could say package anti-matter. On any trip you have to speed up and slow down, which means you are limited to about 5%. This places the ceiling about 0.2c, 

This has implications, at 1 g it woukd take a 4/10ths of a year to accelerate, to decelerate, and 19.8 years to travel so just over 20 years. The speed of AC is immaterial. Time dilation is trivial. 

However AC is a non prime target, all of those are located more than 20ly from Earth, which means you have at least 100 years of non-dilating travel. 

Realistic 0.2c might be achieved with a highly efficient space probe, not with living sentients on board. For a colony ship 0.02c or less is more practical for several reasons. 

1st for a ship the size capable of sustaining a single human for 20 years at 0.2c is somewhere close to the energy output of our sun. 

At 0.2 c its basically a radioactive bath, every molecule in the ships path ........Extensive amount of weight would need to be devoted to shielding and exterior seals and such would degrade. The problem is still pretty bad at 0.01c but you could potential use laser and rf generators to nudge atoms out of the path, of course grains of sand ..... think of as nuclear bombs, you average piece of space dust, a bunker buster bomb. 

From my point of view the game is really to be played at 0.001c or about 1 million km/h, you still have lots of time to get to AC and stop before it reaches its closest approach and moves on. AC is a galactic wanderer, once it departs you wont be seeing those folks again, once their radio signal fades they will be a separate species. 

 

[Pds314]

16 hours ago, mjl1966 said:

So the question becomes, can we change the initial orbit sufficiently to make the trip to AC substantially shorter in time than it would be with a regular Hohmann transfer orbit?  What kind of dV are we talking about?  Is it feasible?

In short, the Hohman orbits around a galaxy to reach a nearby star require a few km/s to a few tens of km/s, and a hundred million years. They're perfectly practical today, but nobody has 100 million years, and it's unlikely that even machines would be able to tolerate 100 million years with no encounters with any matter or significant radiation. Your life support would end up costing obscene amounts of energy. A generational ship containing 10 humans would require about 100 megajoules per day of food. Multiplying that it, assuming the 10 humans don't evolve at all, it's the equivalent of 20 kg of matter and 20 of antimatter. You'd be far better off using that to boost your starship to get there in a few hundred or a few thousand years, since it your craft weighs 100 tonnes, and your fuel weighs 100 as well, you could be getting to a percent or so of the speed of light in delta-v.

1 minute ago, Pds314 said:

 

 

Edited January 24, 2016 by Pds314

[Green Baron]

15 hours ago, Vanamonde said:

One uses a hohman transfer to travel between bodies which will have moved a considerable proportion of their orbit during the flight time. But in the time it would take a spacecraft to travel between Sol and Alpha Centauri, the two stars will barely have moved, on galactic rotation time scales. Also, the distance between the two stars is so small in relation to their distance to the galactic center-of-mass that their orbital paths are nearly parallel to each other anyway. There's just no need to do anything fancier than fly straight there. 

 

Yep, i was surprised that "just fly straight there" is theoretically possible. Reading that galactic escape velocity at our "height" is just about 550km/s (http://arxiv.org/abs/1408.1787) would mean that a craft traveling at a speed of 150,000km/s travels practically on a straight line relative to the galactic center.

 

p.s.: Humans are per definition (oldowan stone tools) here since roughly 2.6 million years.

 

 

[Temstar]

1 hour ago, PB666 said:

On any trip you have to speed up and slow down, which means you are limited to about 5%. This places the ceiling about 0.2c, 

There might be ways around this. The ISV Venture Star from Avatar for example only carries anti-matter for two burns and hydrogen for one burn for a round trip. On the outward burn from earth it acts as a light ship with its photon sail extended and vast lasers from Earth propel the ship. Since there are no laser stations on Pandora it has to fire its anti-matter catalysed fusion engine to enter orbit, using up half of its anti-matter and all of its hydrogen propellant. It releases two SSTO Valkyries to the surface. The two Valkyries from previous ISV is then dispatched from the surface to the parent gas giant Polyphemus to scoop up hydrogen from the upper atmosphere to refuel Venture Star. When its time to leave the ISV Venture Star then fire its fusion engines again for Earth, using up all of its anti-matter and hydrogen. When it arrive back in the solar system it extends its photon sail again and Earth's laser station slows the ship into orbit.

It's quite a scary thought when you think about it, having a ship flying in Earth's general direction at 0.7c and relying on laser propulsion to stop. I suppose if something goes wrong with the photon sail and the ship looks like it will be unable to enter orbit the laser stations on Earth will then proceed to fry the ship for safety. You wouldn't want a mile long spacecraft travelling at some fraction of c slamming into your homeworld.

Edited January 24, 2016 by Temstar

[Bill Phil]

35 minutes ago, kemde said:

 

Yep, i was surprised that "just fly straight there" is theoretically possible. Reading that galactic escape velocity at our "height" is just about 550km/s (http://arxiv.org/abs/1408.1787) would mean that a craft traveling at a speed of 150,000km/s travels practically on a straight line relative to the galactic center.

 

p.s.: Humans are per definition (oldowan stone tools) here since roughly 2.6 million years.

 

 

Those weren't homo sapiens, nor homo sapiens sapiens. They were an ancestor. Albeit tool users.

Edited January 24, 2016 by Bill Phil

[PB666]

1 hour ago, Temstar said:

There might be ways around this. The ISV Venture Star from Avatar for example only carries anti-matter for two burns and hydrogen for one burn for a round trip. On the outward burn from earth it acts as a light ship with its photon sail extended and vast lasers from Earth propel the ship. Since there are no laser stations on Pandora it has to fire its anti-matter catalysed fusion engine to enter orbit, using up half of its anti-matter and all of its hydrogen propellant. It releases two SSTO Valkyries to the surface. The two Valkyries from previous ISV is then dispatched from the surface to the parent gas giant Polyphemus to scoop up hydrogen from the upper atmosphere to refuel Venture Star. When its time to leave the ISV Venture Star then fire its fusion engines again for Earth, using up all of its anti-matter and hydrogen. When it arrive back in the solar system it extends its photon sail again and Earth's laser station slows the ship into orbit.

It's quite a scary thought when you think about it, having a ship flying in Earth's general direction at 0.7c and relying on laser propulsion to stop. I suppose if something goes wrong with the photon sail and the ship looks like it will be unable to enter orbit the laser stations on Earth will then proceed to fry the ship for safety. You wouldn't want a mile long spacecraft travelling at some fraction of c slamming into your homeworld.

Im not going to waste my time replying to fantasies. What hollywood creates for drive physics is all fantasy. 

They have a plot, they need humans (or star warsians, or battlestar galactians, whatever) to go from point x to point y.......so they look at all the known physics and then they say if we bend the energy equation say 1000, 1000000, or is some cases like star trek, infinity, and they come up with drive system.

Just remember sci fantasy is about action and plot, its not about physics reality, never was meant to be. If there is physics reality in science fantasy there is no genre.  If you want to get a good perspective on the field see hhgttg. The whole infinity improbability drive is a play on all the crap the entertainment industry sell. 

[Green Baron]

1 hour ago, Bill Phil said:

Those weren't homo sapiens, nor homo sapiens sapiens. They were an ancestor. Albeit tool users.

Hello Bill Phil,

i didn't say that. I said humans, which, when used by prehistorians, include all species and subspecies of genus homo. Which exists per definition since 2.6Ma, since the first stone tools because they define humans (which is just a convention). And i did so because further up in the thread someone mentioned that humans are a million years on earth.

There are other views on the topic by creationists etc., but that's not "my style".

No offence meant to anyone

k

Edited January 24, 2016 by kemde

[GoSlash27]

18 hours ago, Vanamonde said:

One uses a hohman transfer to travel between bodies which will have moved a considerable proportion of their orbit during the flight time. But in the time it would take a spacecraft to travel between Sol and Alpha Centauri, the two stars will barely have moved, on galactic rotation time scales. Also, the distance between the two stars is so small in relation to their distance to the galactic center-of-mass that their orbital paths are nearly parallel to each other anyway. There's just no need to do anything fancier than fly straight there. 

^This. the distance to Alpha Centauri is so tiny and the orbits are so identical that a Hohmann transfer wouldn't save any DV.

Best,

-Slashy

[Bill Phil]

11 minutes ago, kemde said:

Hello Bill Phil,

i didn't say that. I said humans, which, when used by prehistorians, include all species and subspecies of genus homo. Which exists per definition since 2.6Ma, since the first stone tools because they define humans (which is just a convention). And i did so because further up in the thread someone mentioned that humans are a million years on earth.

There are other views on the topic by creationists etc., but that's not "my style".

No offence meant to anyone

k

I'm referring to the fact that homo sapiens sapiens is the modern human. There's another "possible" subspecies, but it's premodern human, and the evidence is shaky.

And the wikipedia article (I know, how reliable *sarcasm) of human is about modern humans, or homo sapiens sapiens (primarily).

Scientifically, human is/can be the genus homo, but what's oft considered human by most people is the species homo sapiens. Most people aren't prehistorians. And the earlier species would then be prehuman. The word human is derived from words that only meant "man", as in an example of one of our species. Zoologically, it's "belonging to the genus Homo," but only in that context, which wasn't quite established.

Humans, in the everyday context, haven't existed for millions of years, merely a few hundred thousand. The genus homo has existed for millions of years.

But even now there's some debate about what the genus homo even is. Linnaeus made it to put humans into a taxon, but then there's other species, which he hadn't known about at the time, such as the Neanderthals, and of course homo erectus.

[Green Baron]

Seeing homo sapiens as the sole representant of "human" is a creationists view. Which is ok for me, it's just not what i as a geoscientist and prehistorian have learnt.

The other subspecies of homo sapiens is probably homo (sapiens) neanderthalensis and the genetic evidence is not that bad.

Again, no offence meant. Your right, there is debate about anthropoligists. Lumpers and splitters create and delete species sometimes on fragments of bones. But, hey, let's get back into space :-)

 

Does anyone know where AC is in it's orbit relative to our system ? Ahead/behind or above/below ? While AC is quite near it's also probably quite uninteresting, having seemingly no planets. How about traveling to a 1400ly distant systems (just because of the distance :-)) ? How would the gravitational field of the galaxy effect the course of a ship traveling 0.1c, 0.5c ?

Not easy to get real figures on that or does anyone know about papers/info ...

 

Edited January 24, 2016 by kemde
Inserted probably and brackets cause things change in months ...

[KSK]

1 hour ago, PB666 said:

Im not going to waste my time replying to fantasies. What hollywood creates for drive physics is all fantasy. 

They have a plot, they need humans (or star warsians, or battlestar galactians, whatever) to go from point x to point y.......so they look at all the known physics and then they say if we bend the energy equation say 1000, 1000000, or is some cases like star trek, infinity, and they come up with drive system.

Just remember sci fantasy is about action and plot, its not about physics reality, never was meant to be. If there is physics reality in science fantasy there is no genre.  If you want to get a good perspective on the field see hhgttg. The whole infinity improbability drive is a play on all the crap the entertainment industry sell. 

For somebody who wasn't going to waste their time replying to a fantasy, you seem quite willing to waste your time being rude. 

Besides, even if we exclude any sort of FTL drive, we're going to need some sort of exotic propulsion system for interstellar travel. Avatar was certainly exaggerated (accelerating to 0.7c using solar sails seems unlikely to me) but at least it made a reasonable attempt at a sensible (for want of a better word) interstellar propulsion system, with some thought given to the challenges involved. Need for a extremely efficient source of energy - check. Logistical challenges involved in using actual propellants - check. Use of ISRU to get around some of those challenges - check.

Even the antimatter catalysed fusion drive is at least rooted in actual science even if the details and the actual engineering are necessarily hand-waved away. Muon catalysed fusion has been proposed and tried, although it didn't break even. Using the muons created from matter-anti-matter annihilation might be one method of producing a sufficient muon flux whilst providing a way of actually harnessing some of the energy released from that annihilation.

 

[Vanamonde]

The longevity of species and whether or not there would be humans to react to such a voyage are not relevant to the subject of this thread. That's an interesting discussion to have, but if you're going to, please take it to its own thread. 

And as always, please take a moment to calm down and remember that your fellow forum members are not your enemies. 

[Green Baron]

So Alpha Centauri is in rendezvous distance regarding galactic measures.

How about star systems 1500ly away ? Assumption: no wormholes, no warpdrive, max speed .999c. If the speed is high enough the ship would probably still travel in a quasi-straight line ?  There'd be a correction angle for gravity and travel of destination object, right ? How about relativistic effects ? Time at destination is different from time on board, what kind of correction would apply ? I can imagine there is quite a difference in target position during travel time (3000yrs @ 0.5c as measured on earth). Galactic time ? That means the target moves faster when observed from board of the ship, correct ?

Man, that's early 20th century stuff :-) Anyone firm in Relativity ?

 

[magnemoe]

4 hours ago, PB666 said:

Im not going to waste my time replying to fantasies. What hollywood creates for drive physics is all fantasy. 

They have a plot, they need humans (or star warsians, or battlestar galactians, whatever) to go from point x to point y.......so they look at all the known physics and then they say if we bend the energy equation say 1000, 1000000, or is some cases like star trek, infinity, and they come up with drive system.

Just remember sci fantasy is about action and plot, its not about physics reality, never was meant to be. If there is physics reality in science fantasy there is no genre.  If you want to get a good perspective on the field see hhgttg. The whole infinity improbability drive is a play on all the crap the entertainment industry sell. 

The avatar spaceship is pretty plausible compared to most scifi ships, yes it would require energy proportional to the energy earth get from sun to power it in other word an class 1+ civilization. 
This however is far more plausible than an probe or spaceship surviving 100 million years. 

Its the problem with generation ships but thousands of times worse. The problem with an generation ship using thousands of year going between stars is not primary power but spare parts, or raw materials and facilities to build everything, you will also need spare air to replace the one lost, large enough living quarters who will need more of everything. This will bring up the mass a lot but also add loads of new challenges. 
 

[pincushionman]

You're not going to be using transfer orbits of any kind in interstellar travel. While the forces underlying the system are the same, the galactic environment is far different from a star system. One is a collection of small discrete point sources of mass dominated by a large central body. The other is a more or less continuous distribution of mass (in the large scale) from the center to far beyond the visible edge.

As a result, stars don't "orbit" the galactic center in clearly-defined Keplerian ellipses, and the speeds of individual stars don't vary very much, especially between bodies that are close together. Instead, you end up with families of stars coursing through space at pretty much the same velocity (speed and direction).

Plus, any "small" differences wll be dwarfed by the enormous speeds you'd need to attain to make these journeys in any kind of "reasonable" time frame, or even to escape from your first star.

[Nothalogh]

12 hours ago, Shpaget said:

There are living fossils such as Queensland lungfish

I read that as "There are living fossils in Queensland", at first

[fredinno]

5 hours ago, magnemoe said:

The avatar spaceship is pretty plausible compared to most scifi ships, yes it would require energy proportional to the energy earth get from sun to power it in other word an class 1+ civilization. 
This however is far more plausible than an probe or spaceship surviving 100 million years. 

Its the problem with generation ships but thousands of times worse. The problem with an generation ship using thousands of year going between stars is not primary power but spare parts, or raw materials and facilities to build everything, you will also need spare air to replace the one lost, large enough living quarters who will need more of everything. This will bring up the mass a lot but also add loads of new challenges. 
 

You might be able to intercept with rouge planets for resupply, but that would be a pain, and you would waste time decelerating.

[Shpaget]

6 hours ago, Nothalogh said:

I read that as "There are living fossils in Queensland", at first

Well, they're called Queensland lungfish because they live in Queensland.

[Elrond Cupboard]

4 hours ago, fredinno said:

You might be able to intercept with rouge planets for resupply, but that would be a pain, and you would waste time decelerating.

Sorry to pick nits, but that's "rogue"; "rouge" is French for "red".

 

Sorry.

[fredinno]

47 minutes ago, Elrond Cupboard said:

Sorry to pick nits, but that's "rogue"; "rouge" is French for "red".

 

Sorry.

Does it matter? 

[magnemoe]

1 hour ago, fredinno said:

Does it matter? 

No, and yes jumping rogue or even rouge planets might be an way this can work out, starting and stopping is not an major issue, you are moving slow and refueling is the easy part. 
More of an issue to find other stuff, planets between stars are likely to have thick ice layers, too thick to drill trough as pressure at depth will collapse anything.  
Smaller objects are probably better. 
And yes this will require an self replicating system, either an machine or colony ship both being able to build copies of itself in practice.

[11of10]

On 24. januar 2016 at 3:17 PM, Temstar said:

It's quite a scary thought when you think about it, having a ship flying in Earth's general direction at 0.7c and relying on laser propulsion to stop. I suppose if something goes wrong with the photon sail and the ship looks like it will be unable to enter orbit the laser stations on Earth will then proceed to fry the ship for safety. You wouldn't want a mile long spacecraft travelling at some fraction of c slamming into your homeworld.

I imagine it might actually miss the planet in that case, what with it being faster than it's supposed to be, and just flying straight past us.

However, this now occured to me: Can ground based laser stations actually make that lightsail ship enter a stable orbit around Earth?