So I just learned what Terminal Velocity is, that gravity pushes down as much as the air molecules are pushing up, meaning that it reaches as fast as it can when it reaches that point (terminal velocity) and will not go any faster.
With no air in space to stop the “pushing up” can things increasingly build up speed with not cap point?
I tried googling and it says this “The only terminal velocity in space is the speed of light. For anything moving more slowly than the speed of light the limiting factors are the specific impulse (the force applied multiplied by the time it acts) and the mass of the object, which together determine the acceleration and the time that acceleration acts.”
But I don’t really understand that, I don’t understand “specific impulse (the price applied multiplied by the time it acts)” what does “acts” mean?
Also I understand there is also no gravity in space, but I know that planets can exert gravity without actually pulling the object into its atmosphere…..
Can someone explain this to me in layman’s terms??
In: Physics
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Firstly, space IS NOT empty. There are particles floating around everywhere. So, you will always encounter resistance.
But, let’s assume that isn’t true, and that space is truly a vacuum. In that case, you can in theory keep accelerating upto the speed of light… except you can’t. The faster you go, the heavier you become (and the more time slows down, but that doesn’t really matter here). So as you accelerate towards the speed of light, it requires more and more energy to go just a little bit faster.
Only particles without mass can travel at the speed of light.
Terminal velocity just means that there is a speed reached by an accelerating object at which the force accelerating the object is counteracted by some other force. In case of terminal velocity in atmosphere, you have the force from gravitational acceleration being matched by the force acting the opposite direction due to air resistance. In space, you can define a terminal velocity of an object if a force other than the one accelerating it acts on it; if there isn’t such a force, there’s “no terminal velocity” as defined in classical terms. But, there is a speed limit for the whole universe and that is the speed of light as you mentioned.
“Terminal velocity” specifically refers to objects free-falling through air under the effects of gravity.
In deep space where there’s neither air nor a nearby gravitational object, the term doesn’t really apply.
You can continue accelerating under your own power until you hit “relativistic” speeds near the speed of light. Once your velocity starts to become an appreciable fraction of light speed, the energy required to further accelerate you starts to increase dramatically. It will require infinite energy for an object with non-zero rest mass to reach the speed of light.
Impulse is just the power of your engines – how much force they’re applying and for how long. Since there’s no friction in space, only the power of your engines and your fuel supply matter.
The sentence about the specific impulse basically just translates to “How fast you can go in space depends on your rocket engine and on how much fuel you have, compared to the total mass of the ship”. It’s just formulated in an overly weird, hard-to-understand way. What he’s trying to say is that, while there is nothing in space to slow you down, you first have to accelerate, and that is still hard, even in space.
The thing about “gravity in space” is often misunderstood. In theory, gravity has infinite reach, just getting weaker with distance. In practice, for example, the Earth’s gravity field reaches at least to the moon, otherwise the moon could not circle around Earth. Similarly, the Sun’s gravity encompasses at least the entire solar system.
When you see astronauts being “weightless” on the international space station, what is actually happening is that they are pulled down by gravity at the exact same rate as the space station itself. If you were in an elevator cabin, and someone cut the cable, sending the cabininto freefall, you would also begin to float in the cabin, appearing weightless, in the same way as the astronauts (With the difference that your weightlessness ends once you reach the basement, of course).
The ISS is still well within Earth’s gravity field. The strenght of the gravirty up there is ca. 90% of what it is down here, not much of a difference. The astroauts only appear weightless in reference to the space station itself.
> With no air in space to stop the “pushing up” can things increasingly build up speed with not cap point?
Other than the speed of light being an absolute limit, yes. Spacecraft travel at speeds very much higher than any Earth based vehicles could achieve; the ISS is orbiting at about 7.6 km/s or about 17000 mph.
> the specific impulse (the force applied multiplied by the time it acts)
That is the definition of *impulse*, not specific impulse. The specific impulse is the impulse per unit mass of fuel/propellant burned. Another way of putting this is that the propellant burn rate multiplied by the specific impulse gives you the thrust created by a rocket. Specific impulse is therefore a measure of the fuel efficiency of a rocket engine.
The specific impulse doesn’t provide an absolute limit on how fast a spacecraft can go but it does impose a practical limit. The reason is that the amount of fuel required [goes up exponentially](https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation) with the desired velocity. Therefore, for a given rocket engine, there’s only so fast you can make it go before the rocket becomes impracticably large.
“Terminal velocity” implies some speed at which forces balance out. In falling, it’s the force of gravity speed you up vs. the friction of the air molecules slowing you down. Your speed of falling increases until these two forces balance out.
This may not be the question you’re asking, but maybe it is?
There is a similar concept for space travel. One of the realistic concepts for interstellar travel is called a “ram scoop” or “Bussard ram jet” . (There are a couple of realistic concepts, but this one is maybe the most realistic.)
Space isn’t empty. There’s atoms floating around – mostly hydrogen atoms. If you want to go fast, you have to account for the slowing force due to hitting those atoms. A ram-scoop cleverly would then use the atoms that it hits as fuel, like a turbocharger on a fast car (hey, you hit that atom and it already slowed you down, why not burn it in your fusion reactor, blow the exhaust out the back, and use it speed back up?)
The physics isn’t trivial. It depends on details like the elemental fractions of the gas in space. But it’s an interesting topic.
See “Bussard Ramjet” for details.
Yes.
Terminal velocity exits only with respect to the gravity of something causing your acceleration so most of these answers fail to answer your question. Terminal velocity does not apply to powered flight like rockets and airplanes so “impulse” is irrelevant. Yes, even if there is no atmosphere a planet will have a “terminal velocity”. If you start “stationary” at some distance from such a planet the maximum speed you can (and will) reach before hitting the surface will be equal to the escape velocity of that planet.
For example, if you fell towards the moon with no initial velocity or extra rockets you would reach 2.38 km/s
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