Cosmic rays – which are usually protons – typically arrive at enormous velocities. You could imagined one arriving at a fairly typical 200,000 kilometres per second, about two-thirds of the Speed of Light. (Which is exactly 299,792.458 kps: the second and the metre being now defined in terms of light-speed.) This is gigantic compared to speeds in the solar system, where the Earth orbits at just under 30 kps. The comet that the Rosetta space probe attached itself to in August 2014 was travelling at just over 15 kps: it speeded up as it approached the sun. Jupiter’s average orbital speed is just over 13 kps, while Neptune is much slower at 5.43 kps. The universe beyond our Earth is often counter-intuitive: planets further from the sun move more slowly, even though they have much further to go. Mercury, right next to the sun, orbits at more than 47 kps.
All of these speeds are way outside human experience. A typical passenger aircraft will fly at about 900 kilometres per hour. Concorde had a maximum speed of just over 2000 kph. The fastest regular aircraft so far produced, the Lockheed SR-71 Blackbird, set a record with 3,529.6 kph, just short of 3600 kph/1 kps. The fastest-ever manned vehicles were the Apollo missions to the moon, which returned to Earth at just over 11 kps. This was needed to get there and back in sensible times: the moon’s own speed relative to the Earth is just over 1 kps. The Space Shuttles were not as fast, moving at less than 8 kps.
We are also very small, in cosmic terms.
The average distance of the Earth from the sun is known as the Astronomical Unit. It is about 8.32 light-minutes. The Inner Solar System could be sensibly defined by the maximum distance from the sun achieved by the planet Mars: this is 1.666 AU or 13.86 light-minutes. So we might consider the Inner Solar System as a circular target of 603.67 square light-minutes.
And interstellar space? There are 525,960 minutes in a Julian Year – that’s a year of 365.25 days, the unit used for light-years. Just now the nearest star is Proxima Centauri at 4.26 light-years: but stars in a galaxy have individual speeds and directions within a general rotation, so our neighbours vary. Think of our neighbourhood before you get to the next stars as a circular target with a radius of two light-years: anything beyond that is the neighbourhood of some other star. That comes to 3,467,000,000 square light-minutes. The Inner Solar System is about 0.0000000174% of the stellar neighbourhood, or about 1 in 5,760 million.
The Earth as a target is also very small. The Inner Solar System viewed as a circular target is more than 19 trillion square kilometres. The Earth as a circular target is just over 127 million square kilometres. That’s a mere 0.000000000659 %, or less than 1 in 157 billion. But free protons exist in enormous numbers, despite space being almost a vacuum from a human viewpoint. So we get hit by cosmic rays.
Long Revolution 