Scanning through the headlines on The Engineer’s website recently, I noticed that UK aerospace company Pulsar Fusion has revealed plans to build a nuclear fusion rocket engine capable of producing temperatures hotter than the Sun. They aim to use this rocket to halve the mission times to Mars and facilitate journeys to Saturn. However, the sentence that really caught my attention is the last, in which Pulsar Fusion’s CFO, James Lambert, is quoted as saying, “To the fusion community, AI truly does have the potential to allow us to achieve engines capable of interstellar space travel.”
Obviously, as a science fiction writer, I am intensely aware of the massive distances involved in interstellar travel, and the enormous difficulties in designing anything capable of bridging them in a realistic human timeframe. Alpha Centauri, our nearest neighbouring solar system, is so far away that it takes light, moving at approximately 300,000 kilometres per second, four years to cross the gulf between us.
To put this in some kind of perspective, our fastest-moving probes, Voyager 1 and Voyager 2, were launched in 1977, and only reached the edge of our own solar system in 2012 and 2018 respectively. At that speed, it would take them about 75,000 years to reach Alpha Centauri even if they were headed in the right direction, which they are not.
Obviously, 75,000 years isn’t a feasible timeframe if you want to include a crew or make a return journey. To speed the process up, we would want to give our craft some sort of propulsion. Unfortunately, the speeds and distances required may make this harder than it sounds.
If you want your starship to be able to slow down and stop when it reaches its destination, you will need to equip it with enough fuel to match the amount used to accelerate it in the first place. And if you want it to be able to return, you also need to factor that in. But the more fuel you carry, the heavier your craft will be, and hence the more fuel it will need to move it.
This becomes a particular problem if you start to approach respectable fractions of the speed of light. According to Einstein, the closer you get to light speed, the more massive your craft becomes, and hence the more energy it needs to accelerate.
A possible solution, and one that is being investigated by NASA and other organisations, involves the deployment of a reflective light sail. In simple terms, this sail utilises a large surface area to harness radiation pressure from sunlight, using it to accelerate the craft in an analogous way to a sailboat using wind to propel it. Although this acceleration would be very low, it would also be constant and not require onboard fuel. Several test sails have been deployed in Earth orbit, and current estimates are that using this method, a probe could be sent to Alpha Centauri in as little as half a century.
Unsurprisingly, the poetic aspect of ‘sailing to the stars’ hasn’t been lost on science fiction authors, and they turn up in many stories. Possibly the most well-known of these are the 1974 novel, The Mote in God’s Eye, in which an alien species arrives in human space by means of a light sail, and the stories of Cordwainer Smith, which chart a future in which humanity initially moves out into the universe using light sail technology.
However, if Lambert is right, and an increasing use of AI makes stable nuclear fusion engines a reality, it seems within the bounds of possibility that a small craft could be sent to the Alpha Centauri system in around 40 years, at a velocity of around ten percent of the speed of light. Equipped with a large enough radio transmitter or signalling laser, such a craft would then be able to return data to Earth within four years of arrival, with the 44 year interval between launch and first signal roughly equal to the time it took the Voyager probes to travel from Earth to the heliopause at the edge of our solar system.
Constructing an interstellar probe would be a massive, and expensive, engineering project, with propulsion accounting for just one small piece of the jigsaw. Payload, shielding, and communications will all be equally vital, and all face unprecedented challenges. But then, we have already overcome similar challenges before. Nobody knew with a hundred percent certainty whether a man could survive a spaceflight until Gagarin took his historic voyage; and nobody knew for sure that we could land on the moon, until we did. At the end of the day, as our technologies become better and cheaper, I think it’s inevitable that should our species survive the next century, we will find a way to send missions to our nearest stellar neighbours.