Basic physics question

[rivan]

So I was reading an XKCD what-if and wondered - is the amount of energy required to turn Voyager around the same in all scenarios?

He seems to describe applying force pointing more or less directly back to earth, eventually stopping and re-accelerating Voyager back this way.

Would it take more, less or the same energy, to slowly steer it into a U-turn?

 

[Krazy4Real]

I would think it should take less energy, but I am not a rocket scientist.

 

[Hayabusa Rider]

So I was reading an XKCD what-if and wondered - is the amount of energy required to turn Voyager around the same in all scenarios?

He seems to describe applying force pointing more or less directly back to earth, eventually stopping and re-accelerating Voyager back this way.

Would it take more, less or the same energy, to slowly steer it into a U-turn?

The amount of energy to reverse course is the same, however different approaches have different efficiencies, consequently the total amount of energy would be that to reverse course plus any wasted by the selected technology. Short answer is no for practical purposes.

 

[Midwayman]

To steer it in a u turn you need to overcome momentum away from Earth plus some additional to push it sideways. Then you eventually need to counter that sideways motion as well. This is space. You don't have friction to help you turn.

 

[Possessed Freak]

Might be less force to just let voyager and the mating machine (to steer it) go towards the next celestial body and slingshot around that back towards Earth.

But I am sure that would take a few more years...

 

[Daverino]

I would guess, gravity being what it is, you would only need to nearly stop Voyager to 'turn it around.' The Sun would still exert the dominant gravitational force in Voyager's neighborhood and eventually pull it in towards the solar system.

I understand that any little drift would probably just put Voyager into an orbit around the Sun, similar to a comet. But it would certainly get it headed back towards Earth.

EDIT:
Having now read the article, I see they reached the same conclusion that I did

 

[Jaepheth]

you can't U-turn in space. There's no atmosphere or medium to push against.

If you're traveling along X direction and you apply a perpendicular force in the Y direction, you'll just start moving at some other angle, but the speed in the X direction would not change. If the force is applied not through the center of mass then you'll also introduce a spin to the object, but it'll continue traveling in the X direction.

The only way to get moving in the -X direction is to apply a force in the -X direction. Which means bringing it to a stop and then accelerating back the way you came. Any force in the Y or Z directions would just be wasted energy.

 

[Fritzo]

It would take Superman.

 

[SphinxnihpS]

So I was reading an XKCD what-if and wondered - is the amount of energy required to turn Voyager around the same in all scenarios?

He seems to describe applying force pointing more or less directly back to earth, eventually stopping and re-accelerating Voyager back this way.

Would it take more, less or the same energy, to slowly steer it into a U-turn?

The only relevant variable I see is the time it takes to get to Voyager. The longer it takes, the further it is from the Sun, and the longer the Sun will take to pull it back.

 

[rivan]

To steer it in a u turn you need to overcome momentum away from Earth plus some additional to push it sideways. Then you eventually need to counter that sideways motion as well. This is space. You don't have friction to help you turn.

you can't U-turn in space. There's no atmosphere or medium to push against.

If you're traveling along X direction and you apply a perpendicular force in the Y direction, you'll just start moving at some other angle, but the speed in the X direction would not change. If the force is applied not through the center of mass then you'll also introduce a spin to the object, but it'll continue traveling in the X direction.

The only way to get moving in the -X direction is to apply a force in the -X direction. Which means bringing it to a stop and then accelerating back the way you came. Any force in the Y or Z directions would just be wasted energy.

Durr, you're right. I was thinking about it as if there's continued thrust keeping it going.

 

[phucheneh]

Durr, you're right. I was thinking about it as if there's continued thrust keeping it going.

You could 'U-turn' off of the gravity of something large enough, though; same as they did with the moon missions. That basically 'bends' your forward momentum and directs it in a circle, assuming the object is captured and put into an orbit. If you can make that orbit a very skewed ellipse, you don't even have to deal with breaking free of it on the other side.

In short: slingshot, engage.

 

[superccs]

Why would you want to aim a satellite back towards us?

 

[Howard]

you can't U-turn in space. There's no atmosphere or medium to push against.

Yes you can, but it's silly unless you're Han Solo.

 

[TheVrolok]

I go through phases every few years where I read XKCD and then completely forget about it. But man, that's good stuff.

 

[Jeff7]

"The tyranny of the rocket equation."

To get the rocket into space, you need fuel. To get that fuel into space, you need more fuel. To get that fuel into space, you need more fuel. And so on.

So, to get to Voyager, you've got to go faster than Voyager. Voyager's had the benefit of some gravity assists from planetary flybys, so it's moving at a pretty nice speed. Then, once you've gotten to it, you need to have enough fuel to be able to cancel out all that hard work you've done just to get the rocket out that far. Now you've got to slow down the rocket and Voyager and all the fuel that you had to bring along in order to accomplish that very task. Then you also had to bring along enough fuel to get Voyager moving back toward Earth, along with the fuel that had to bring that along. That amount of fuel is dependent in part on how quickly you want it to get back.





* Record for most legitimate uses of "fuel" in a single ATOT post.

 

[Paratus]

you can't U-turn in space. There's no atmosphere or medium to push against.

If you're traveling along X direction and you apply a perpendicular force in the Y direction, you'll just start moving at some other angle, but the speed in the X direction would not change. If the force is applied not through the center of mass then you'll also introduce a spin to the object, but it'll continue traveling in the X direction.

The only way to get moving in the -X direction is to apply a force in the -X direction. Which means bringing it to a stop and then accelerating back the way you came. Any force in the Y or Z directions would just be wasted energy.

That may not be entirely accurate. We have some folks at work researching this:

Quantum Vacuum Plasma Thruster

A Q-thruster will utilize quantum vacuum fluctuations as its propellant. The quantum mechanical Casimir effect has demonstrated that quantum vacuum fluctuations do exist.[1]

So no propellant would be needed. You push against the vacuum. Hypothetically of course.

 

[Jeff7]

That may not be entirely accurate. We have some folks at work researching this:

http://en.m.wikipedia.org/wiki/Quantum_vacuum_plasma_thruster



So no propellant would be needed. You push against the vacuum.

This sort of thing always makes me think of this. One of these days, we're really gonna piss off physics.

 

[BladeVenom]

Why would you want to aim a satellite back towards us?

That's the same question I had. What's the point?

 

[Paratus]

This sort of thing always makes me think of this. One of these days, we're really gonna piss off physics.

Nice


You think Q-thrusters are bad, check out this other thing they're investigating:

Warp Field Interferometry