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Science lab · Physics game

Newton's Cannon

What if you fired a cannonball so fast it never came down? Launch probes from real spaceports, fight the jet stream and hurricanes, reach orbit, hit the Moon, and escape the Earth for good.

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What Newton imagined

In A Treatise of the System of the World, published in 1728, Isaac Newton described a thought experiment. Put a cannon on top of a very high mountain and fire it sideways. The ball curves down and lands some distance away. Fire it faster and it lands further away. Now keep going: at some speed the ground curves away beneath the ball exactly as fast as the ball falls towards it. The ball never lands. It is in orbit, and in Newton's drawing it comes all the way round to the mountain again.

That is all an orbit is: falling, while moving sideways fast enough to keep missing the ground. Near the surface the speed you need is about 7.9 km/s, or roughly 28,000 km/h. The International Space Station, about 435 km up, moves at more than 27,000 km/h and goes round the planet about every 90 minutes.

Why 45° is not always best

On flat ground and without air, a throw at 45° goes furthest. On a round planet the ground drops away under a long shot, so for long distances the best angle is lower. Mission 2 lets you feel this: a lower angle reaches Chile from Florida with less speed. At orbital speed the best angle is completely flat.

Wind and storms

Real jet streams blow from west to east about 9 to 10 km up, at at least 108 km/h and up to about 400 km/h. Hurricanes spin anticlockwise north of the equator and clockwise south of it; at 252 km/h or more they are Category 5. In the game the wind reaches much higher than in reality (real jet streams stay below about 15 km), otherwise a probe flying hundreds of kilometres up would never feel it.

A free push from the spinning Earth

The Earth turns from west to east, and at the equator the ground moves at about 465 m/s. A rocket fired eastwards near the equator gets that speed for free. That is why Europe launches from Kourou in French Guiana, just over 500 km from the equator: the European Space Agency says the spin adds about 460 m/s. In mission 6 the planet turns under your probe, so its path on the map bends.

The Moon, and leaving for good

The first human-made object to reach the Moon was the Soviet probe Luna 2, which struck it on 13 September 1959 after launching from Baikonur. The Moon itself probably formed in a far bigger crash: a Mars-sized body called Theia hit the young Earth, and the debris came together into the Moon. Fire faster still, at about 11.2 km/s from the surface, and a probe escapes the Earth's gravity altogether and never comes back.

How the game works

  • Gravity is real Newtonian gravity, worked out step by step in your browser. Speeds and distances are real: 1 km/s in the game is 1 km/s on Earth.
  • There is no air resistance. Real rockets climb out of the thick lower atmosphere before they speed up sideways, which is why the orbit missions start 200 km up.
  • The wind is exaggerated in height, as explained above.
  • The Moon is drawn 27 times closer than it really is (about 384,000 km away), so it fits on the screen. Because it is closer, it also goes round faster than the real Moon's 27 days.
  • Country borders come from Natural Earth (public domain).

More stories where physics meets history: the volcano heard 4,800 km away, the summer nights you could read by and the day the Sun set off a minefield.

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