Cosmic!

Tim Radford

  • Korolev: How One Man Masterminded the Soviet Drive to Beat America to the Moon by James Harford
    Wiley, 392 pp, £24.95, June 1997, ISBN 0 471 14853 9
  • Countdown: A History of Space Flight by T.A. Heppenheimer
    Wiley, 398 pp, £24.95, June 1997, ISBN 0 471 14439 8
  • Something New under the Sun: Satellites and the Beginning of the Space Age by Helen Gavaghan
    Copernicus, 300 pp, £15.00, December 1997, ISBN 0 387 94914 3
  • Space and the American Imagination by Howard McCurdy
    Smithsonian, 294 pp, £19.95, November 1997, ISBN 1 56098 764 2

For every action, there is an equal and opposite reaction. When you light the blue touchpaper on Guy Fawkes night, the force goes downwards and the rocket goes upwards. But gravity tugs rockets, apples and everything else downwards at a rate of roughly thirty feet per second per second, so to climb out of gravity’s well, and make it into freefall in orbit, a rocket has to get up to a speed of more than five miles per second – ‘escape velocity’, it’s called.

Another of Newton’s laws of motion says that a body subjected to a constant force constantly accelerates. To remain upwardly mobile, a rocket carries an engine to go on burning fuel during the climb. But to accelerate to a speed of five miles a second takes sustained and colossal thrust: in effect, a controlled bomb within the rocket motor. To get that kind of burn, a rocket has to carry its own oxygen as well as its own kerosene or liquid methane or hydrogen, and it has to carry enough to go on burning for more than a hundred miles. So most of the rocket’s weight is fuel. There are two problems: one is that the heat inside the motor becomes so fierce that engineers have to devise ways to stop it incinerating before it gets to the stratosphere. The other is that although the mass of the rocket decreases with each second of the burn – the Saturn rocket that lifted Apollo to the Moon burned 3500 gallons of fuel every second – the machine’s centre of gravity shifts so that the rocket is likely to become unstable. This means that the engineers have to build in guidance systems and nozzle controls to keep it on course.

Once the rocket gets up to a height of about two hundred miles and a speed of five miles a second, it is safe: it will go into orbit and sail round the planet every ninety minutes or so, before gradually falling back to Earth. But there is no point in just sending up a rocket. What matters is the payload. With a bit more thrust, and some fuel aboard the payload, you can send that into even higher orbit, or despatch it towards another planet. This is both easy and difficult. It is easy because another restatement of Newton’s laws says that once set in motion a body will go on at its existing speed until something stops it. In space, there is no friction, no air resistance. You could aim your little spaceprobe at the Moon or Mars like a snooker ball at the end of a cue and it would roll perfectly across the true cloth of the void to the pocket you intended. But the Earth revolves at 1000 mph at the Equator, and at the same time travels through space round the Sun at about twenty miles a second. The Moon is moving at a different speed, so is Mars, so are all the planets. And the spacecraft, which has slipped the foolish bonds of Earth, has become in effect a little planet itself, which could be described by its mass and its initial velocity and is in orbit around the Sun. A spacecraft bound for Mars won’t therefore point itself towards the Red Planet and beetle there directly: it will arc in a swinging path from Earth to a rendezvous point at which the two heavenly bodies will converge on their different orbits. To get this right, the rocket engineers either have to know the exact time and inclination at which to launch the rocket, or they have to supply the payload with even more fuel to be able to correct its course en route, or (usually) both. A heavier payload means even more demand on the rocket, so the sums get tricky.

Late last year, just after the 40th anniversary of Sputnik 1 – the first craft into space – the US and European Space Agencies launched the Cassini-Huygens mission to the planet Saturn. Cassini is the size of a 30-seater bus, and it is heading away from Earth at 67,800 mph. It has already travelled more than 120 million miles, but its journey has barely begun. It is heading for Venus, will swing round it and steal some extra speed from its gravitational force, then fly by the planet Earth, do the same thing, and be hurled back to Venus for another gravity assist. Only then will it have the oomph to sail across space and rendezvous, in 2004, with the mysterious giant Saturn on its long journey round the Sun.

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