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 MadaBidyoni September 18 2012 05:46 PM

Speed in space

Hi!
as i understand this - in space you have no friction, so you just gain more & more speed. so why can't we reach a near to light speed?
what is the problem?
a power source? the rockets ran out of fuel so the gaining of speed is slower?
(if i'm not wrong Voyager is the fastest so far :rolleyes:)

 Mars September 18 2012 05:54 PM

Re: Speed in space

^ Relativity. at all speeds the speed of light is always measured to be the same, all measures of time and space for the moving object distort so that measurements of the speed of light from their point of view, and that of some other observer moving at a different speed though space measure the speed of light as always the same.

This means that as a spaceship gets closer to the speed of light, the length of it in the direction of motion compresses relative to a stationary observer, and the time experienced by that spaceship slows down relative to that same observer. One can get as close to the speed of light, but any extra acceleration you apply to that spaceship will always come up short of the speed of light, you can never quite reach it, and for a person on the spaceship would would not appear to be anything close to the speed of light, as that speed will always be the same whether forwards backwards or sideways to the direction of motion.

 Christopher September 18 2012 06:04 PM

Re: Speed in space

Quote:
 MadaBidyoni wrote: (Post 6979945) Hi! as i understand this - in space you have no friction, so you just gain more & more speed. so why can't we reach a near to light speed? what is the problem? a power source? the rockets ran out of fuel so the gaining of speed is slower?
That's a large part of it. Acceleration requires thrust, thrust requires fuel, and you need to carry that fuel with you. The more you need to accelerate, the more fuel you need to carry, and that added mass makes it harder to accelerate. So it's a diminishing-returns situation; it's just not practical to carry enough fuel to accelerate you to a sizeable fraction of lightspeed. Especially since your effective mass would increase if you got really close to lightspeed, making it exponentially harder to accelerate further, though you'd have to get close to 90% of c for that to become a noticeable factor.

This is why many proposals for starships are for designs that don't carry fuel with them. The first such proposal was the interstellar ramjet, which collected hydrogen from the interstellar medium (ISM), refueling itself as it went. The problem with that, though, turned out to be that its magnetic fields would create drag against the ISM and slow it down. Now there's a lot of theoretical work done with sailships, such as lightsails accelerated by lasers back in the Sol system, or magnetic sails acelerated by particle beams. This way, the source of power is back home and the ship itself can be much lighter.

The other problem is that the closer you get to c, the more hazardous spaceflight gets. The starlight and background radiation from ahead is blueshifted to become far more intense and deadly, and oncoming specks of dust would hit you with the force of nuclear warheads. (Kinetic energy is mass times velocity squared, so as you go faster, the amount of energy something hits you with goes up considerably.)

 smiki September 18 2012 06:33 PM

Re: Speed in space

Explained in layman's terms:

The faster you go, the bigger your mass becomes. At everyday speeds this effect is absolutely negligible, but at the speeds closer to the speed of light, it becomes more and more pronounced. It's one of the aspects of Einstein's theories of relativity (general and special). So, if you were moving at a speed a fraction away from the speed of light your mass would be almost infinite. You can't move at the speed of light because then your mass would be infinite, and anything with infinite mass can't be moved. Light on the other hand has no mass, it's just energy (electromagnetic waves or photons) so it can and does move at the speed of light. Why are things this way? They just are, it's different from the things we see in everyday life, on the scales we live in, but it's true.

I'd just like to make it clear, that when I say "your mass increases", I don't mean just the feeling of weight (the way you feel heavier in an upwards accelerating elevator), but your actual mass increases (it becomes larger and larger, like the mass of a planet, and you become a stronger and stronger gravity source).

This is why spaceships in SF try to avoid this by using jump gates, warp, subspace or stuff like that.

Of course, if you know all of this yourself (you weren't clear on that), just ignore me, and read Christopher's post.

 Christopher September 18 2012 07:34 PM

Re: Speed in space

^Yeah... all the stuff about relativistic mass increase is true, but the thing I was trying to get at is that there are a lot of other problems you'd have to overcome before you could get anywhere near fast enough for that to become an issue at all. A lot of people seem to think that relativistic effects like time dilation and mass increase happen linearly with increasing speed, but really they're exponential -- you don't get your mass doubled until you get to 90 percent of lightspeed, and by 99 percent it's only increased seven times, and it's only in that last percentage point that the effects really skyrocket, approaching infinity as speed approaches c.

But if you look at the theoretical work that's being done on advanced starship design these days, people are talking about velocities of ten percent of lightspeed as a high-end goal. The speed of light is so incredibly huge that even ten percent is well beyond our current capabilities and would take vast amounts of energy to reach. The fastest speed a human-built spacecraft has ever achieved is about 70 kilometers per second, which is a measly fortieth of a percent of lightspeed. And that was a probe launched inward toward the Sun, so it got added acceleration from the Sun's gravity.

So it's really kind of misleading to focus on the relativistic aspects here. There are plenty of simpler reasons why it's prohibitively hard to get to the point where relativity would even begin to have a noticeable effect.

 smiki September 18 2012 08:39 PM

Re: Speed in space

^That is all true. You're right.

Anyway, when it comes to laser/particle-beam sail technology, there's a significant problem with one thing. If it is to be used as a viable method of transportation from point A to B, you're gonna have to have the same kind of technology at your destination, as a means of decelerating from the speeds you reach, otherwise you'd just zoom pass the destination and keep on going... And of course, to do that, you'd first have to get to your destination by some other way, and then build the laser/particle cannon there. Which kinda defeats the purpose of a sail-ship. Maybe if it proves to be more efficient it could be used as a type of a "interstellar third rail railway" that has its power source located at the stations/planets once space travel becomes somewhat common, but it's definitely not a means of making a breakthrough. It could be used for research, like a probe, but not an actual spaceship.

Unless we make cold fusion work, or some other form of highly efficient energy production and propulsion, no luck for Newtonian spaceships nor for interstellar tourists.

 MadaBidyoni September 18 2012 09:22 PM

Re: Speed in space

A.
ok so not near to light speed & not 90%.
70-80% :)
& lets say we will find a way to do it without a fuel tank & get the energy from outside in some way...
will we get to... say... 70%?

B. lets make it simpler. why a comet can't get to 70%? it just accelerating & accelerating (no friction)...

Quote:
 The fastest speed a human-built spacecraft has ever achieved is about 70 kilometers per second, which is a measly fortieth of a percent of lightspeed. And that was a probe launched inward toward the Sun, so it got added acceleration from the Sun's gravity.
oh... wasn't voyager the fastest?

 gturner September 18 2012 09:35 PM

Re: Speed in space

Quote:
 smiki wrote: (Post 6980782) ^That is all true. You're right. Anyway, when it comes to laser/particle-beam sail technology, there's a significant problem with one thing. If it is to be used as a viable method of transportation from point A to B, you're gonna have to have the same kind of technology at your destination, as a means of decelerating from the speeds you reach, otherwise you'd just zoom pass the destination and keep on going... And of course, to do that, you'd first have to get to your destination by some other way, and then build the laser/particle cannon there. Which kinda defeats the purpose of a sail-ship.
There's a proposed braking trick where you use an electromagnetic field to slam into the solar wind of the target star, a bit like re-entering a planet's atmosphere, except you're starting the deceleration out about where our Voyager probes are now. The technique should produce detectable Bremmstrahlung radiation, which is something to listen for in other star systems.

 sojourner September 19 2012 12:27 AM

Re: Speed in space

Quote:
 MadaBidyoni wrote: (Post 6981034) B. lets make it simpler. why a comet can't get to 70%? it just accelerating & accelerating (no friction)...
No, comets are not accelerating and accelerating. neither is anything else in an orbit. objects in orbit accelerate as they approach perigee and slow as they approach apogee.

 Christopher September 19 2012 12:43 AM

Re: Speed in space

Quote:
 smiki wrote: (Post 6980782) Anyway, when it comes to laser/particle-beam sail technology, there's a significant problem with one thing. If it is to be used as a viable method of transportation from point A to B, you're gonna have to have the same kind of technology at your destination, as a means of decelerating from the speeds you reach, otherwise you'd just zoom pass the destination and keep on going...
Not really. For lasers, Robert L. Forward proposed a compound sail that could split apart so that the outer portion could then reflect the laser backward onto the inner portion and decelerate it; see his paper here. And for magnetic sails, remember what I said about a ramjet's magnetic field creating drag against the interstellar medium? A magsail ship would have a built-in braking system, and could use the destination star's magnetosphere to brake against.

Quote:
 MadaBidyoni wrote: (Post 6981034) A. ok so not near to light speed & not 90%. 70-80% :) & lets say we will find a way to do it without a fuel tank & get the energy from outside in some way... will we get to... say... 70%?
Depends on what technology you use and whether it's a crewed craft or a robot probe. Most cutting-edge propulsion proposals are looking at 10-30% of lightspeed as an optimistic goal. However, a particle beam could potentially accelerate a lightweight sail probe to a much higher percentage of c.

I recommend you track down the book Centauri Dreams by Paul Gilster, or do an archive crawl on his blog of the same name. They have material on advanced starship design concepts.

Quote:
 B. lets make it simpler. why a comet can't get to 70%? it just accelerating & accelerating (no friction)...
Comets are not under thrust. They're big chunks of rock and ice orbiting the Sun on eccentric orbits. When they come close to the Sun, the heat evaporates their volatiles, which are blown outward in a tail by the solar wind and radiation pressure. This leads a lot of people to the false conclusion that comets are some kind of fireballs roaring through space like rockets and leaving fiery exhaust trails behind them, but that's about as close to reality as the Moon being made of green cheese.

Also, as I said, the amount of fuel involved is prohibitive. If you did mount an engine on a comet, feed in all its ice, and blast it out the back of the engine -- heck, even if you extracted the hydrogen and used it to fuel a fusion rocket -- you'd run out of fuel long, long before you reached any significant fraction of c. Again, it's very difficult for the human mind to comprehend just what a huge velocity the speed of light is. It's fast enough to cross the North American continent in less than the blink of an eye, or to go around the entire Earth eight times in one second.

Quote:

Quote:
 The fastest speed a human-built spacecraft has ever achieved is about 70 kilometers per second, which is a measly fortieth of a percent of lightspeed. And that was a probe launched inward toward the Sun, so it got added acceleration from the Sun's gravity.
oh... wasn't voyager the fastest?
Apparently not. Here's a Centauri Dreams post about it.

 JustAFriend September 19 2012 01:31 AM

Re: Speed in space

Quote:
 MadaBidyoni wrote: (Post 6979945) ...in space you have no friction, so you just gain more & more speed. so why can't we reach a near to light speed?
Simpler answer : We have yet to build a rocket that can burn for more than about 15minutes (not counting the couple of probes using ion engines... their thrust is rated in micrograms. But even they could build up significant velocity given enough time and fuel.)

 Mars September 19 2012 01:12 PM

Re: Speed in space

Quote:
 smiki wrote: (Post 6980782) ^That is all true. You're right. Anyway, when it comes to laser/particle-beam sail technology, there's a significant problem with one thing. If it is to be used as a viable method of transportation from point A to B, you're gonna have to have the same kind of technology at your destination, as a means of decelerating from the speeds you reach, otherwise you'd just zoom pass the destination and keep on going... And of course, to do that, you'd first have to get to your destination by some other way, and then build the laser/particle cannon there. Which kinda defeats the purpose of a sail-ship. Maybe if it proves to be more efficient it could be used as a type of a "interstellar third rail railway" that has its power source located at the stations/planets once space travel becomes somewhat common, but it's definitely not a means of making a breakthrough. It could be used for research, like a probe, but not an actual spaceship. Unless we make cold fusion work, or some other form of highly efficient energy production and propulsion, no luck for Newtonian spaceships nor for interstellar tourists.
Actually there is solar wind, charged particles emitted by our sun. Other stars probably have stellar wind too, and while our solar wind is not fast enough to boost us to interstellar velocities, the relative velocities of other star's stellar wind compared to an incoming spaceship moving toward it at an appreciable fraction of the speed of light will be quite high relative to the ship. A mag sail could then act as a parachute to slow the spaceship down, all with the help of the star that its approaching. The methods for slowing down a starship are more numerous than those for speeding it up.

 Edit_XYZ September 23 2012 08:07 AM

Re: Speed in space

For a rocket:
The problem with gaining high velocities is the rocket equation - the amount of fuel you need to carry grows exponentially (that's really, REALLY fast) for an increase in delta v (that's the difference between final and initial velocity).

With nuclear fuel you need millions of tonnes of fuel to reach a delta v of 0,2c (acceleration to 0,1c and deceleration to 0).
You can reach a delta v of 0,1c (final speed 0,05c) with a reasonable quantity of nuclear fuel, though - without a genius to change the paradigm, that's the maximum velocity theoretically possible with today's physics.
With chemical fuel - :guffaw:.

Note: 0,1c is not even close to 0,7c , where relativistic mas increase actually becomes a problem.
We should be so lucky as to have only this problem.

For a laser/particle-beam sail technology:
The laser emitter must be the size of Jupiter to counter diffraction. And it consumes an amount of energy comparable to burning through millions of tonnes of nuclear fuel in order to create enough photons for a delta v of 0,2c (photons are REALLY energy hungry momentum transfer instruments).

For warp drive:
Call me when you get exotic matter - and there are about half a dozen other unsolvable (currently) problems with it.

 publiusr September 28 2012 11:17 PM

Re: Speed in space

There are different ways to determine speed. A probe sent to the sun has to go to Jupiter to null out the angular momentum it got from Earth. The solar Probe + will tear past Sol at 200 km/sec. Now you have Voyager, which did gravity assists on each of our four gas/ice giants and completed the grand tour after all now that pluto isn't a planet anymore. New Horizons started out more quickly than the Voyagers, but will never catch up due to only passing near Jupiter. So it all depends on what you mean by speed. A comet can pass fast near the sun--but it will slow down if in an orbit as if moves farther from the sun. Russia uses similar Molniya orbiting craft due to being too far north for many geosynch sats to service.

I would consider something fast if it can go extra-solar, and not count whizzing about in the inner solar system.

 Christopher September 29 2012 03:12 AM

Re: Speed in space

Quote:
 publiusr wrote: (Post 7028167) I would consider something fast if it can go extra-solar, and not count whizzing about in the inner solar system.
Although sometimes going slowly is a better way to get out of the Solar system...

http://www.centauri-dreams.org/?p=24...auri+Dreams%29

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