Wandering Earth

[Sword7]

I watched "Wandering Earth" on Netflix and "Wandering Earth 2" at cinema. Very interesting!! Scientists built giant thrusters on earth and move earth out of solar system for Alpha Centauri system before our sun is dying (expanding into giant). It looks impossible to me.

Is that possible to move any planet with giant thrusters?

 

[Max-Q]

Short answer: No.

Longer answer: Definitely no! It took a Saturn V worth of energy to send a little spacecraft to the moon. To escape the sun's gravity well, it would take many orders of magnitude more energy. Plus enough energy to slow down and insert Erath into a circular orbit once you got to Alpha Centauri... So basically double the energy required.
It would be next to impossible to even eject a near Earth asteroid from the solar system due to the energy required.
That aside, there are other issues. The Earth rotates, so you'd have to stop it's rotation to keep the thrusters pointed in one direction. Also, Earth would rapidly turn into a frozen chunk of ice long before it ever passed the orbit of Jupiter.

The other thing is, if you put these thrusters on the surface, they would be dumping a bunch of energy into the atmosphere, would cause many other problems, like heating the air to a plasma. Putting them above the atmosphere would require at least a 50 mile high tower strong enough to handle enough thrust to move a planet.

These are just the things I came up with off the top of my head, there are many, many more issues with this... questionable... plan.

In short, please don't try this! If you really want to see if it would work, either find an Earth no-one else is using at the moment, of build a craft in Orbiter with the size, mass, and inertia of a planet.

 

[Arvil]

You would have to convert the Earth itself to fuel, and I suspect you would run out of Earth long before you got far.

 

[Max-Q]

Yeah, I was ignoring that part... I was assuming that is this scenario, the fictional engines ran on something super-high energy, like matter/antimatter propulsion.
For conventional chemical engines, you'd need many times more mass of fuel than mass of planet. Here are some rough numbers:

Assuming: Accelerate Earth to 5% of the speed of light for an 80 year trip.

- Matter/Antimatter drive: Assuming enough thrust and an ISP of 10 million seconds.
You'd need 35.8% of the Earth's total mass, of course this means that 17.9% of the Earth's mass would need to be converted to anitmatter and stored.
That's [imath]1.069 * 10\overset{21}{}[/imath] metric tons of antimatter!
That much antimatter would have as much energy as [imath]8.27406 * 10\overset{42}{}[/imath] kilotons of TNT!

- LOX/LH2: Downright impossible! All of Jupiter's hydrogen wouldn't be enough, not to mention LOX!

Did I mention this is probably a bad idea?

 

[jedidia]

Is that possible to move any planet with giant thrusters?

Specifically like this, no. You already run into almost unsolvable issues when just trying to stop the earths rotation without melting its surface. That's a lot of energy that has to go somewhere...

There are more "practical" solutions to the problem than thrusters though... For certain measures of "practicality", anyways. This usually involves moving the entire solar system rather than individual planets, which actually has less problems, as it effectively just uses the star itself as a photon drive, pulling the planets along with it without much hassle.
The engineering- and timescales involved in such an operation are well beyond current human capability and collective attention span, though... See stellar engine for more information, class A specifically (Class B is only used for energy extraction, not for propulsion).

Also, this makes for a much more boring movie, since the issue of accidentally bumping into jupiter pretty much goes away...

 

[DaveS]

Here's a Kyle Hill video on this exact subject:

 

[Urwumpe]

For more fun: The exhaust velocity for moving Earth would be the hyperbolic excess velocity of every exhaust particle that these thrusters produce. if the specific impulse / average exhaust velocity is at the escape velocity (11.7 km/s) or less, you produce no thrust at all, you only move Earth around the barycenter of it and the exhaust cloud.

For having the effective specific impulse of a SSME (4500 m/s), you would need an specific impulse of the thruster near Earth of:

[math]\sqrt{V_{esc}^2 + V_{exc}^2} = \sqrt{11.7^2 + 4.5^2} = 12.1[/math]
So, you need something like an electromagnetic or electrostatic (ion) thruster (installed on a huge rigging that puts it outside the atmosphere), to move Earth. And still require as much fuel as if you would break down Earth into small chunks and strap an SSME to each.

Better solution: Move the moon and use it as gravity tractor.

 

[Sword7]

Very interesting! Thanks for replies.