If a certain 1960s sci-fi series which may or may not have anything to do with The Swiss Family Robinson is to be believed, we should have space travel sorted by now. Instead of contenting ourselves with GPS satellites and Mars rovers, we should have sent the first deep space colonisation mission on its way over a decade ago.
“In that case, where’s my low budget spaceship complete with skin tight ski-spacesuits and an evil, moustache-twirling doctor?” I hear you demand. Rest assured, the world of interstellar space exploration is not as Lost in Space would have you believe.
The Robinsons’ destination and the nearest star to our solar system visible to the naked eye is Alpha Centauri, located a distance of roughly 4.3 light years (the distance traversed by a beam of light in one year) from Earth.
If we were all ants, and the distance between Earth and the Sun were about the width of a doorframe, then the distance between our solar system and Alpha Centauri would be the equivalent of crawling all the way from Dublin to Manchester. No mean feat for an ant.
Assuming you’re not an ant, but rather a human in a spacecraft hurtling though space at a velocity equivalent, for example, to one tenth of the speed of light, it would take almost 80 Earth years to travel to Alpha Centauri and back. This illustrates one of the major snags when it comes to interstellar travel: distance and time. Things in space are simply very, very far away from each other. Getting anywhere is going to either take a long time, or require a very speedy spacecraft and vast amounts of energy.
One tenth of the speed of light was an arbitrary number, mind. At the moment, most rocket-based crafts cannot travel anything as fast. The main limiting factor in rocket propulsion is the fact that you need fuel to go fast. But, the more fuel you carry, the heavier your ship and the more fuel is needed to shift the mass and so on; a kind of catch 22 situation ensues.
One of the solutions to this problem is to build a vehicle that is more efficient at burning fuel, which exploits nuclear fusion or fission to create thrust. A ‘nuclear rocket’ would get more bang for its buck fuel-wise, but there would still be an upper limit to its speed.
“But wait,” you cry. “Travelling through space means travelling through a vacuum, which has the advantage of virtually zero friction. Despite how slowly my rocket is accelerating, it could eventually reach quite high speeds, right?” True. However, the downside of a frictionless travel medium is that there is also nothing to slow you down once you get to your destination. So, you’re going need to bring with you double the amount of fuel you need to accelerate, if you expect to slow yourself down in time to reach your destination and not pass it by. This means carrying even more mass, and so the idea of using a rocket becomes increasingly less appealing.
A light-sail might be worthy alternative. This is an extremely thin membrane (perhaps one atom thick) that can be set up at the head of the ship and propelled along by high-intensity laser beams directed towards the ship across interstellar distances from the planet of origin, in the same way that wind drives a sailboat.
When the ship needs to decelerate, the sail can be detached and a second rear sail can be erected. The free-flying front sail can then act as a mirror to reflect the laser beam back onto the rear sail and slow down the ship.
However, regardless of your means of travel, speed will always be an issue. The next obvious problem is travel time. At the moment, the average human lives for less than 100 years. Even in a very fast ship, a round-trip to the nearest star would lasts far longer than the average human lifespan.
Certain solutions have been offered to the human problem, such as ‘generation ships’, which carry whole families who work, live and die on board, though this only adds to the mass problem. Another idea is that of a kind of robot-operated space-ark, which carries frozen human embryos that undergo artificial gestation upon arrival.
These solutions aren’t exactly the definition of feasible. Therefore, many believe that the future of space exploration lies in the use of robotic probes rather than manned star ships.
A robotic pilot does not grow old and die. It would also weigh a lot less than a human pilot, and that’s without factoring in everything necessary to keep a human being alive in space such as water, air, a facility to grow food etc. For example, in the early Russian missions, Sputnik 2 that carried Laika the dog into space, weighed about six times more than the unmanned Sputnik.
Such an approach could permit increased travel speeds and allow for missions of indefinite duration. In fact, researchers at the University of Michigan are currently developing nano-particle thruster technology that would allow flotillas of microscopically-sized robots of negligible mass to travel through space at very high speeds, and interact together to act as a kind of data-gathering super-computer network.
So, do robotic probes spell the end for manned space flight? Speaking at a recent guest lecture here in UCD, Astronomer Royal Martin Rees was both optimistic and realistic about the future: “The practical case for sending people into space gets ever-weaker with each advance in robots and miniaturisation. Indeed as a scientist or practical man, I see little purpose in sending people into space at all. But as a human being, I’m an enthusiast for manned missions. I hope some people now living will walk on Mars.”