Why does it take so long to get to another planet?
Without the magical propellants of sci fi space travel, we have to rely on chemical rockets to power our spacecraft. Whether their fuel is solid or liquid, the principle is the same: the space vehicle goes off like a firework rocket. Hot exhaust gases thrusting downwards blast the spacecraft beyond the pull of Earth’s gravity and towards its target.
Chemical rockets have limits practical and economic. Take speed: Voyager 2, the fastest space probe yet launched, is travelling at 18.5 km (11M miles) a second. By the hour- 66 600 km (41 400 miles) that sounds impressively fast, equal almost to doing a return journey from London to Australia twice in sixty minutes. But for space travel it’s a snail’s pace. Mars, which at times is the planet closest to us, is on average about 78 million km (48 million miles) distant. Even if a spaceship could travel to Mars at 18 km (11 miles) a second in a straight line, the journey would take seventy-nine days. American and Russian provisional plans for manned missions to Mars estimate that crews will spend two years on a return journey. Spacecraft have to follow a curved path made up of various orbits usually governed by the Sun’s gravity. And they need to aim at where their target will be, not where it is when they set off, a task requiring precise navigation to ensure that the vehicle doesn’t zoom past its goal and fly for ever into space.
It’s reasonable to expect that propulsion systems will improve. But even if we take everything at its best boundless energy, a spaceship with ultimate powers of acceleration and the ability to fly in a straight line nobody knows the limits of human endurance in space. To travel faster requires a faster breakout from the constraints of Earth’s gravity. All motorists know that fast acceleration from a standing start thrusts them back in their seats. Similarly, a spacecraft’s rapid lift off creates within the vehicle an artificial gravity that presses its occupants fiercely downwards. High speed over a long journey would make limbs feel useless, and possibly damage the heart.
Without air resistance, an object falling from a height accelerates at a rate of 9.8 m (32 ft) per second every second. (A falling apple, for example, will reach a speed of 9.8 m per second after one second, 19 6 m per second after two seconds, and so on.) Physicists express that acceleration as 1 g one times the force of gravity. Space scientists say that a journey in which the craft accelerates at 1 g is possibly the limit of human endurance. To guarantee a landing on Mars, the brakes would have to be applied, and the spaceship would need to decelerate at 1 g for the second half of the trip. At that rate, the journey would take only forty-nine and a half hours.
But what about trips to more distant planets? If you are planning to go aboard, you’d better take something to read. The closest that Neptune comes to Earth is 4300 million km (2672 million miles) fifty-five times as far as Mars. At 1 g and in a straight line, the journey would take fifteen and a half days. When your skipper or ground control applied the brakes at the hallway mark, you'd be travelling at a sizzling 6529 km (4057 miles) a second.
Journeys to the stars would be more formidable, because a new speed limit would come into force the speed of light. As Einstein demonstrated, nothing can travel faster than 300 000 km (186 000 miles) a second. Even if our spaceship could accelerate at 1 g until it reached 99 per cent of the speed of light, then decelerated at 1 g in the final stretch, a trip to Proxima Centauri, our nearest star, would take five years and four months.
Five years plus is how we on Earth would time the journey. But, strangely, the astronauts would find the trip much faster. As Einstein predicted in his theory of relativity, the spaceship’s clocks would slow down compared with those on Earth.
A voyage across our whole galaxy one that takes light 100 000 years to make might happen while the astronauts had their morning coffee. Even journeys across the immense distances between galaxies, reckoned in thousands of millions of years at the speed of light, might be possible. Those left on Earth would age at the normal rate. When the astronauts returned from the stars after a five year trip, by their reckoning, they would land in a world that had aged by several million years.
It’s reasonable to assume that one day spacecraft will travel at 30 000 km (18 600 miles) a second, one tenth the speed of light. At that speed, a trip to the nearest star would occupy about half a lifetime. Space scientists have considered what to do if nobody applies for that kind of holiday. They have seriously discussed sending teams of robots into space on a kind of planet hopping mission.
The first team would arrive in, say, forty years. On landing, it would set up a factory to build two replica spaceships, mining the materials from an asteroid. Each new craft would head off to another target, and repeat the operation, while the parent robots transmitted data to Earth about their landing site and its surrounding system.
If all went well, the number of space probes would double every forty years. In 1000 years’ time, some thirty million space laboratories and their robots would be doing field work among the stars.
Chemical rockets have limits practical and economic. Take speed: Voyager 2, the fastest space probe yet launched, is travelling at 18.5 km (11M miles) a second. By the hour- 66 600 km (41 400 miles) that sounds impressively fast, equal almost to doing a return journey from London to Australia twice in sixty minutes. But for space travel it’s a snail’s pace. Mars, which at times is the planet closest to us, is on average about 78 million km (48 million miles) distant. Even if a spaceship could travel to Mars at 18 km (11 miles) a second in a straight line, the journey would take seventy-nine days. American and Russian provisional plans for manned missions to Mars estimate that crews will spend two years on a return journey. Spacecraft have to follow a curved path made up of various orbits usually governed by the Sun’s gravity. And they need to aim at where their target will be, not where it is when they set off, a task requiring precise navigation to ensure that the vehicle doesn’t zoom past its goal and fly for ever into space.
It’s reasonable to expect that propulsion systems will improve. But even if we take everything at its best boundless energy, a spaceship with ultimate powers of acceleration and the ability to fly in a straight line nobody knows the limits of human endurance in space. To travel faster requires a faster breakout from the constraints of Earth’s gravity. All motorists know that fast acceleration from a standing start thrusts them back in their seats. Similarly, a spacecraft’s rapid lift off creates within the vehicle an artificial gravity that presses its occupants fiercely downwards. High speed over a long journey would make limbs feel useless, and possibly damage the heart.
Without air resistance, an object falling from a height accelerates at a rate of 9.8 m (32 ft) per second every second. (A falling apple, for example, will reach a speed of 9.8 m per second after one second, 19 6 m per second after two seconds, and so on.) Physicists express that acceleration as 1 g one times the force of gravity. Space scientists say that a journey in which the craft accelerates at 1 g is possibly the limit of human endurance. To guarantee a landing on Mars, the brakes would have to be applied, and the spaceship would need to decelerate at 1 g for the second half of the trip. At that rate, the journey would take only forty-nine and a half hours.
But what about trips to more distant planets? If you are planning to go aboard, you’d better take something to read. The closest that Neptune comes to Earth is 4300 million km (2672 million miles) fifty-five times as far as Mars. At 1 g and in a straight line, the journey would take fifteen and a half days. When your skipper or ground control applied the brakes at the hallway mark, you'd be travelling at a sizzling 6529 km (4057 miles) a second.
Journeys to the stars would be more formidable, because a new speed limit would come into force the speed of light. As Einstein demonstrated, nothing can travel faster than 300 000 km (186 000 miles) a second. Even if our spaceship could accelerate at 1 g until it reached 99 per cent of the speed of light, then decelerated at 1 g in the final stretch, a trip to Proxima Centauri, our nearest star, would take five years and four months.
Five years plus is how we on Earth would time the journey. But, strangely, the astronauts would find the trip much faster. As Einstein predicted in his theory of relativity, the spaceship’s clocks would slow down compared with those on Earth.
A voyage across our whole galaxy one that takes light 100 000 years to make might happen while the astronauts had their morning coffee. Even journeys across the immense distances between galaxies, reckoned in thousands of millions of years at the speed of light, might be possible. Those left on Earth would age at the normal rate. When the astronauts returned from the stars after a five year trip, by their reckoning, they would land in a world that had aged by several million years.
It’s reasonable to assume that one day spacecraft will travel at 30 000 km (18 600 miles) a second, one tenth the speed of light. At that speed, a trip to the nearest star would occupy about half a lifetime. Space scientists have considered what to do if nobody applies for that kind of holiday. They have seriously discussed sending teams of robots into space on a kind of planet hopping mission.
The first team would arrive in, say, forty years. On landing, it would set up a factory to build two replica spaceships, mining the materials from an asteroid. Each new craft would head off to another target, and repeat the operation, while the parent robots transmitted data to Earth about their landing site and its surrounding system.
If all went well, the number of space probes would double every forty years. In 1000 years’ time, some thirty million space laboratories and their robots would be doing field work among the stars.
Comments
Post a Comment