interstellar travel
We already took a glimpse at the vast distances between stars: to reach the nearest stars - the Centauri system - light needs about 4.3 years. Light from the Orion Nebula needs 1,344 years to get to us. The spectacular Andromeda nebula is one of the closest galaxies, yet light needs 2.5 million years to bridge the largely empty space between them. “Nothing moves faster than the vacuum speed of light!” - do you remember from my previous posts? It seems that the chances are bleak that we can reach any interstellar object, even with advanced technology.
On a planet in Andromeda
And yet, it is entirely possible to set foot on a planet in the Andromeda galaxy within a human lifespan! How?
We borrow technology from some distant future and assume that we can build a spacecraft with a propulsion system that can sustain an acceleration of 9.81 m/s/s for centuries if we should need it. This acceleration would benefit the astronauts feeling normal gravity onboard since 1g = 9.81 m/s/s is also the gravitational acceleration on Earth’s surface. Solid fuel rockets can easily create an acceleration of a multiple of that but only for a few minutes. The most modern workable concept is perhaps the field-emission electric propulsion, which can sustain an acceleration of months. Still, the force of the thruster is only the equivalent of a few grams weight on Earth. It is also improbable that our spacecraft can carry the fuel with it for centuries to come. Quite likely, this technology would harness energy from the gravitational network or the quantum realm.
Why can we reach the Andromeda galaxy within a few decades?
The critical observation is that after some months of acceleration with 1g, we reach velocities that become comparable with the speed of light. Time flow will slow down for us (see my blog “flow of time”). Rather than speculate about the wonders of future technology, I would like to explore what we can reach with a propulsion system like this.
If we want to visit a distant star, we cannot arrive there with almost the speed of light. This would be a short visit. Rather than, we would like to bring the spacecraft to a halt. It is most effective regarding our time, when we accelerate half the distance, turn the spaceship around and reaccelerate the other half.
In the following, I will show distance, which is the entire distance to the object of interest, maximal velocity reached after half the journey, total time passed on Earth and full time for us in the spacecraft.
Journey to Jupiter
Distance: 588 million kilometres (closest approach)
Max. speed: 0.8% speed of light
Earth time: 5.6 days
time onboard: 5.6 days
Mars seems to be in reach with modern space technology, but the journey would take about six months or more. Our futuristic spacecraft would allow us to zip to Jupiter in just five days. The speed reached is still tiny compared with the speed of light: the clocks on Earth and onboard are still (almost) in sync. Things change when we venture on an interstellar journey.
Journey to Alpha Centauri
Distance: 4.3 lightyears
Max. speed: 95% speed of light
Earth time: 5.6 years
time onboard: 3.6 years
Half-way to Alpha Centauri (when we turn around the spacecraft) we already reach 95% of the speed of light. Relativistic effects are significant, and the journey takes for us 3.6 years while on Earths pass 5.6 years.
Journey to M35 - Cradle of Light
Distance: 3,870 lightyears
Max. speed: 99.9999874% speed of light
Earth time: 3,872 years
time onboard: 16 years
If we try to reach a cluster of stars nearby, we already come very close to the speed of light. Time dilation is strong: while the journey takes just 16 years for us, millennia pass on Earth.
Journey to Andromeda:
Distance: 2.5 million lightyears
Max. speed: 99.999999999972% speed of light
Earth time: 2.6 million years
time onboard: 28.7 years
Within less than 30 years onboard the spacecraft, we can set foot on a planet in Andromeda! For most of the journey, we fly with speed very close to that of light. This explains while roughly 2.6 million years pass on Earth while we cross 2.6 million lightyears.
Should we decide to have a flyby of the Andromeda galaxy, we would not turn around and start de-accelerating halfway. In this case, we would reach Andromeda already in 15 years (while still on Earth have passed 2.6 million years). We then turn off the propulsion engine, float freely in the spacecraft and enjoy the view.
Flying through Andromeda
We would reach Andromeda with a velocity of 99.99999999999305% speed of light. The flow of our time is much slower than that of the stars around us: 1 year around us would only last 11.8 seconds in the spacecraft. If stars are spaced on average by about ten lightyears (near the centre), we will pass a star every two minutes. Our instruments would show a picture similar to the adjacent BBC animation (by Giphy).