To the stars
Ken Tapping, 9th February, 2016
Our most distant spacecraft has now penetrated nearly 20 billion kilometres out into interstellar space. However, getting that far into space has taken over 38 years. The nearest star after the Sun is about 2100 times further away, at a distance of 43,000,000,000,000 km. The fastest manmade spacecraft is moving at about 40 kilometres a second (km/s). So to get to the nearest star it would take roughly 34,000 years. The light from that star takes 4.3 years to get to us, so we say it is at a distance of 4.3 light years. Our galaxy is a disc of stars, dust, planets and other stuff 100,000 light years in diameter. We have achieved a lot in space, but we are a long way from being truly cosmic explorers. To get to other stars we will need to travel much faster.
Spacecraft engines work by ejecting material at high speed. The recoil pushes the vehicle in the opposite direction. We can get the same recoil, or thrust, by ejecting a large mass of material slowly, or a smaller mass more quickly. Because there are currently no filling stations in space, we have to carry all that propulsive material along with us. Since the amount we are carrying determines how fast and far we can go, the more efficiently we use it the better. We minimize the rate at which we use our irreplaceable propulsion material by using the highest possible exhaust speed.
Our current launch vehicles use chemical fuels that produce immense thrusts for short periods of time, by burning tonnes of fuel per second. We will never get to the stars in a reasonable time using them, even if we could build one big enough. Fortunately we now have an engine that offers a far better option. It generates small thrusts by ejecting ionized gas at extremely high speed. Its consumption of propulsive material is so low that it can run for months or years, slowly building up enormous speeds. Optimistic engineers suggest it might eventually be possible to make an ion drive powerful enough to accelerate a spacecraft at 1-g, that is, equal to the Earth’s gravity. In one year such an engine could accelerate a spacecraft to 270,000 km/s, 90% of the speed of light. However, that still means taking several years to get to the nearest star, and unacceptably longer to get further out. We need to go faster, a lot faster. Unfortunately, this is not possible.
The trouble is, we cannot exceed the speed of light (300,000 km/s), and as we approach to that speed, we move more slowly through time compared with how time is experienced by our friends and relatives who stayed home. This means we can get to nearby stars, but then come home to find that thousands of years have passed on Earth.
One other option is to build spaceships containing closed ecosystems, on which successive generations of passengers are born and die. After journeys of centuries or millennia, the distant descendants of those who embarked on Earth would arrive at the destination star system. So far, attempts to build even rudimentary closed ecosystems here on Earth have failed, so we are far from being able to launch such space missions.
Science fiction writers have used “warp drive” and other concepts to bypass these difficulties. These are fiction at the moment, but might not remain so indefinitely. They are currently being explored by physicists. Some seem possible but would require enormous amounts of energy. On the other hand, one thing we have found over and over again is that if we state categorically that something is “impossible”, we are usually wrong. The big issue is how much we want to do a thing. I believe that we will eventually get out there to explore our galaxy and possibly beyond, just not yet.
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