Escape velocity for solar system and galaxy

[Baby Astronaut]

It takes a lot of power for rockets to leave Earth, so wouldn't it take more power for a spacecraft to leave the galaxy? I ask because in our solar system, it's puzzling that we'd be able to escape a star whose gravity holds in large planets.

 

Along that reasoning, how can we escape a galaxy that holds in

?

 

Unrelated, but...I'm also thinking, if the ride into our solar system's edge has a nasty bump near the heliosheath, then perhaps the Milky Way has its own analogous "galaxy wind" that ends up in a galaxial-sheath with a devastating bumpiness.

[Klaynos]

Yes it would take more energy.

 

A quick note, space craft leaving the earth do not get to earths escape velocity, the escape velocity of a body is the velocity required so that without any other force acting on the object other than gravity the craft will reach infinite distance at 0 velocity. That's not a good way of explaining it.

 

It doesn't take much velocity change to change the orbital position of something, it's just that planets are so big it's hard to give them enough energy to change velocity...

[Sisyphus]

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

 

Apparently, the escape velocity of the Milky Way Galaxy at about the radius of our sun is about 1000km/s. The location is important, since escape velocity is just when an object already has enough kinetic energy to escape the gravity well, which obviously if you're already "higher up" would be less. You wouldn't ever have to actually reach that velocity if you were going to leave, just add the equivalent amount of kinetic energy over all. You can do that all at once, like with being fired out of a cannon (at which point the muzzle velocity would have to be at least escape velocity), or slowly "climbing" out with continuous thrust, which you can do at any speed. Since the galaxy's gravitational gradient is so low, you could probably do that with only a very small amount of thrust over a very long long period of time.

[Airbrush]

If a spaceship from Earth could accelerate at exactly one G on its' way to the nearest stars, that would give the crew exactly the gravity they need to stay healthy over the long term. You can get up to very high speed this way, before you know it. Then half way to the destination, the spacecraft turns around 180 degrees and begins deccelerating at exactly one G until they reach the destination. That will solve the problem of zero gravity during long starship voyages!

[Sisyphus]

If a spaceship from Earth could accelerate at exactly one G on its' way to the nearest stars, that would give the crew exactly the gravity they need to stay healthy over the long term. You can get up to very high speed this way, before you know it. Then half way to the destination, the spacecraft turns around 180 degrees and begins deccelerating at exactly one G until they reach the destination. That will solve the problem of zero gravity during long starship voyages!

 

That would also give you craaaazy relativistic time compression, exponentially more the longer the trip. Like, a few decades travel, ship time, to other galaxies. The problem is that one G of thrust is actually a whole lot to sustain for very long. You'd need ridiculous amounts of energy.

[Airbrush]

Yes, if the starship gets near light speed it will become so massive that it will take more and more energy to maintain a constant one G acceleration. But on trips to the nearest stars, it would be possible to reach maybe a modest 10%C and then turn around and deccelerate the second half of the voyage there. It will take many years anyhow. I would never volunteer for the mission, because it will probably be a one-way trip and space is a dangerous place. They would have to make the trip there a lot of fun to interest anyone qualified. Maybe virtual reality games to keep the space travelors occupied. They will never see Earth again.

[Martin]

It takes a lot of power for rockets to leave Earth, so wouldn't it take more power for a spacecraft to leave the galaxy? I ask because in our solar system, it's puzzling that we'd be able to escape a star whose gravity holds in large planets.

 

Along that reasoning, how can we escape a galaxy that holds in

?

...

 

This is already answered by the others but I think you should know this handy approximate rule of thumb.

 

Multiply the circular orbit speed by the square root of two. That works for a central body setup, where most of the mass is inside the orbit.

 

The earth's orbit speed is about 30 clicks (30 km per second). So multiply that by sqrt 2, or 1.4. That will give you the speed you would need, at this distance from sun, to coast out of the system.

 

The sun's orbit speed around galactic center is, if I remember, around 220 clicks. So you might think something going 40 percent faster than the sun would be destined to leave the galaxy. That would indeed be true if most of the galaxy's mass was concentrated at the central bulge. But it is not, so as the probe gets farther and farther from center he sees more and more mass pulling him back. Because of that, multiplying by sqrt 2 only gives a very rough ballpark. You actually need more of a kick than that to escape from Milky Way

 

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

 

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

(this says roughly about 1000 km/s, but I don't think we actually know, because of dark matter, estimates of total mass of galaxy not all that precise)

Edited May 6, 2009 by Martin

[Sisyphus]

Yes, if the starship gets near light speed it will become so massive that it will take more and more energy to maintain a constant one G acceleration. But on trips to the nearest stars, it would be possible to reach maybe a modest 10%C and then turn around and deccelerate the second half of the voyage there. It will take many years anyhow. I would never volunteer for the mission, because it will probably be a one-way trip and space is a dangerous place.

 

That's not the reason why. It would not take more and more energy to accelerate as you go along, first of all, and you would never ever get any closer to C, from your own perspective. Remember, in your own reference frame, you always have zero velocity. From someone watching on Earth, your acceleration would taper off as your velocity approached C, but that's from Earth. It would take ridiculous amounts of energy because 1 G is actually a lot of acceleration. Think about it. After 1 minute at 1G acceleration, you're going 1300 miles an hour, literally faster than a speeding bullet. And you want to maintain that amount of thrust for years?

[Airbrush]

Oh well I didn't realize that maintaining one G acceleration would get you going so fast so quickly. How long would it take to reach a modest 10%C, which I heard is quite possible in the near future, at a constant one G acceleration? I don't know how to do that math.

[Royston]

Oh well I didn't realize that maintaining one G acceleration would get you going so fast so quickly. How long would it take to reach a modest 10%C, which I heard is quite possible in the near future, at a constant one G acceleration? I don't know how to do that math.

 

It would take very roughly 5 weeks (just using basic kinematics). [math]t = \frac{v_x - u_x}{a_x}[/math] So reaching, 0.1 C is not modest, as for travelling at that speed, are you referring to this ?

[Airbrush]

Certainly 10%C is not a modest speed. Thanks for calculating how long it takes at 1 G. Wow, great story about antigravity for masses moving near light speed. Does that also create a "tractor-beam" like effect that sweeps the path in front of the speeding spacecraft to remove objects that could destroy it? Even a grain of sand impacting at such speed would be disaster.

 

However, I decline traveling thru space that fast. People are not meant for interstellar space travel. Just send probes that can sense everything around so well, the controllers at home will feel like they are THERE.

[J.C.MacSwell]

It would take very roughly 5 weeks (just using basic kinematics). [math]t = \frac{v_x - u_x}{a_x}[/math] So reaching, 0.1 C is not modest, as for travelling at that speed, are you referring to this ?

 

That looks like a legitimate site (is it?), but the article sounds like something out of "The Onion".

[NowThatWeKnow]

Certainly 10%C is not a modest speed. Thanks for calculating how long it takes at 1 G...

I thought you had read about the relativistic rocket at

http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html

The Andromeda galaxy is only 28 years away at 1G acceleration.

One year at 1G will put you at .77c.

And you can let the calculators do all the math for you.

http://www.cthreepo.com/math1.shtml

[k nester]

if the fact that the universe is ment to be 150 billion light years across & only 15billion years old then it expands 5 times quicker then light so forget the g force how long would it take to accelerate to 5x speed of light to be able to pass the expansion and reach another galaxy. youve got to reach that speed pass it and keep it up to the nearest galaxy