# How could Earth be propelled into a further orbit?

In my story, technologically advanced aliens create a "white hole" at the center of the sun that constantly spits out matter (hydrogen) from a parallel universe, increasing the star's mass quickly. Then, they leave without a trace. The sun is now destined to turn into a red giant in the matter of millenia.

My question is:

How could Earth's orbit radius be increased slowly (several million kilometers per century) using futuristic yet realistic technology?

I have thought of creating massive caves 50-100 kilometers under the surface of the Earth and turning them into fusion chambers, building giant nozzles that end above the atmosphere to propel Earth. Or vacating a large area near the equator and detonating gigaton-magnitude bombs there regularly.

• To put things in perspective, if we say 5 million km/century, that's 5.787 km/h. That's a brisk walk. – F1Krazy Sep 15 '17 at 16:30
• The solution is simple, just always launch spacecrafts off the earth in the direction of earth's motion. You just have to launch enough to match the rate you want earth to lose angular momentum. – A. C. A. C. Sep 15 '17 at 16:38
• The premise however is scientifically absurd even if you ignore the white hole. Even massive stars 50 solar masses live for hundreds of millions of years. Such a star would burn the surface of the earth even if you put earth at jupiter's orbit because they shine a million times brighter than the sun. Also for a star to grow that fast, it's very likely extremely unstable as it grows, any planet trying to keep themselves alive at human timescales will have to deal with extremely unstable solar weather which can fluctuate in output far faster than you could ever move a planet. – A. C. A. C. Sep 15 '17 at 16:44
• If in the event that our sun has put on so much weight that its schwarzschild radius exceeds 3km, does it form a black hole or grey hole or ying-yang hole(whatever that is)? – user6760 Sep 16 '17 at 4:07

Gravitational tractors, or gravitational assists.

Building giant fusion rocket engines into the ground is a bit absurd. With the right sort of active support structures, you could do it, but there's no reason to deal with all that complexity.

If you want to do it with rocket engines, put them in space, in orbit. You want to arrange it so that reaction mass from the engines escapes, but the engine itself remains gravitationally bound to Earth, acting as a tractor, attached to its "trailer" by gravity rather than by a material tether. There are numerous ways to do this. This most straightforward to explain is to have a satellite that constantly fires its engines at 90 degrees to the plane of its orbit; this will effectively shift the orbital plane slightly off-center, so it does not pass directly through the center of gravity of the Earth anymore, but as long as the engines are firing the shape of the orbit remains static, and the satellite never escapes Earth. It would be fairly simple to vary the thrust slightly to make the orbit precess such that the net thrust is always tangential to the Earth's orbit around the sun.

A gravitational tractor is a good way to move asteroids, but for moving the entire Earth, there is a better option: build a few trillion small satellites out of asteroidal material, equipped with light sails. Each satellite is designed to perform a gravitational assist maneuver around Earth to reduce its solar orbit, then use its light sail to re-boost its solar orbit and move back into position for another assist. This gradually transfers energy and orbital angular momentum to the Earth from the sun, via the satellites. Any particular satellite may take decades to complete a single cycle, and only adds a teeny-tiny bit of momentum on each pass, but with trillions of them you can get a near-continuous mass stream bending around the Earth and steadily pulling it outward. If you need to go faster, just add more units to the stream.

Moving planets around is actually possible by scaling up the process of momentum exchange. When a spaceship is in a slingshot orbit around Jupiter, it takes some of Jupiter's orbital momentum to change direction and speed up. Jupiter loses the same amount of momentum, but because the ratio between the spaceship's mass and Jupiter's mass is so lopsided, we cannot measure the imperceptible loss of Jupiter's orbital momentum.

So to move the Earth into a larger orbit, we would need to slingshot thousands or millions of small masses past the Earth is such a manner that the objects lose momentum while Earth gains momentum. Since the supply of asteroids or small spacecraft is finite and we need to use potentially millions of flypasts to move the Earth, each object needs to be fitted with a solar sail and a sophisticated guidance system. After the object passes the Earth, it unfurls the sail and uses solar energy to pick up energy and momentum so it can be reused in another flypast of Earth.

Naturally, this process would take a long time and there would need to be some pretty tight controls to ensure the objects don't intersect another planet. moon or passing spacecraft.

Paul Birch took this idea up to "11" by substituting very small, high speed pellets accelerated around the sun by a "Solar Windmill". By tapping @ 2% of the luminosity of the sun, he estimated that Venus could be moved into an orbit similar to Earth's in just 30 years read "How to move a planet".

Fast moving pellet stream to move a planet

Of course a dynamic pellet stream moving at insane velocities like this would be a grave danger to anything crossing paths, so the calculations and safety precautions needed to make this work would be staggering to say the least.

The sun's larger mass makes it much brighter and also causes the earth to spiral inwards. In short you don't have that long to move away. The largest stars, those around 100 solar masses, can last as little as several hundred thousand years. https://en.wikipedia.org/wiki/Supergiant_star

At this mass, the earth will be scorched in days as it falls inwards. If the whitehole can't add mass at a rate of 100 solar masses per day, then its rate is the main factor. In short you don't move the earth, most of it is useless rock and the energy needed would melt the surface. Much better to pack up the important bits, and leave, now. (the important bits could be all buildings, all life and most of the soil or just a hundred humans with the bare minimum tech needed to live)