# Is the Andromeda Milky Way collision the most likely form of intergalactic space travel?

Assuming that some form of sentient organization gradually spreads within the Milky Way through technology predicated on currently known physics, is the Andromeda-Milky Way collision in 4 billion years the earliest/most feasible option for intergalactic space faring before the Local Group coalesces into a single galaxy in 450 billion years?

This question results from two perhaps simpler sub-questions :

• is the gravitational pull of the galaxy impossible to overcome by a space craft?

• will the ejection of star systems during the collision result in a vanishing chance for these systems to join other galaxies within 450 billion years?

• Based on our current technology, I would say that intergalactic gravitational pull would be the fasters/most efficient from of intergalactic travel. Nevertheless, this is assuming we won't ever develop some superluminal technology (see, for example: Alcubierre's warp drive) that could potentially reduce even intergalactic travel times. However, this is purely speculation, so don't take my comment at face value. – Charlie Oct 8 '18 at 20:53
• According to space.stackexchange.com/questions/3948/…, the escape velocity of the Galaxy is around 550 km/s. This does not seem impossible to overcome. – Stéphane Rollandin Oct 8 '18 at 21:08

As Stéphane pointed out, the escape velocity of the Milky Way is around 550 km/s - far higher than any current rockets and well above anything plausible for chemical rockets (since their exhaust velocity is on the order of km/s, the rocket equation requires a vast amount of reaction mass - for a 1 kg payload with a 4.4 km/s H-O rocket we need $$\approx 10^{54}$$ kg reaction mass). Using a VASIMR wiht 100 km/s gives a mass ratio of 244.69, and were we to use a photon rocket the mass ratio is just a few percent (but now we need some awesome energy source to power the laser or photons).
Even if one doesn't build a rocket one can leave the galaxy. The galaxy emits hypervelocity stars, likely due to binary encounters with the central supermassive black hole. They have velocities above galactic escape velocity. In another paper I sketch out an argument that one can nudge stellar orbits to direct streams of hypervelocity stars to move galaxies (I have updated calculations in greater detail for a book project, finding a $$\Delta v$$ of about 48 km/s for the Milky Way if we use all stars); while moving entire galaxies is very cumbersome one could certainly use the stars and their solar systems as spacecraft. The speed is not great, it will take hundreds of million years to get to Andromeda.