Tuesday, April 24, 2018

Armchair Space Travel

Yesterday I started my 64th orbit around the Sun, and I wondered, idly, how much distance that represented. Turns out it's quite a lot more than I thought, but compared to the outer limits of the solar system, let alone interstellar space, not very much at all.

The average radius of the Earth's orbit is about 150 million kilometres, handily defined as one astronomical unit, and the circumference of its slightly elliptical orbit is around 940 million kilometres, or around 6.27 AU. So even if you do nothing all year but sit in your armchair, your track around the Sun would, if unraveled and straightened out, reach somewhat beyond the orbit of Jupiter.* And by simply staying alive for 63 years, I've managed to travel 395 AU, or more than 59 billion kilometres. That's about ten times the average distance of Pluto from the Sun, and nearly three times the distance of Voyager 1 from Earth (currently 141.9 AU).

Voyager 1 was launched in 1977 and officially reached interstellar space in August 2012, after it escaped from the influence of the Sun's magnetic field, but it's still inside the region influenced by the Sun's gravity. Out there, far beyond the planets, are two clouds of icy planetesimals, the origin of comets that now and then fall on long, long orbits towards the Sun. The first is the Hills cloud, a disc-shaped belt extending 2000 -- 20,000 AU from the Sun, and beyond that is the spherical Oort cloud, which may reach out as far as 50,000 AU, a substantial fraction of a light year.

At its steady rate around the Sun, it would take 319 years for my armchair-based mode of space travel to clock up a distance equivalent to that of the inner edge of the Hills cloud, and almost 8000 years to pass through the Oort cloud. As for the nearest star, Proxima Centauri, that's 4.25 light years away, or around 268,700 AU; so far, my travel around the Sun amounts to just a tiny fraction -- 0.15% -- of that interstellar gulf. It would take 42,900 years to make a one-way trip to Proxima, and I'm already out of warranty. Space is big, and life is short. Yet still I move.

*Armchair space travel is more complicated that spinning around a fixed point. The Sun is orbiting the Milky Way galaxy at around 230 kilometres per second relative to the galactic centre; the Milky Way, along with the rest of the Local Group of galaxies, is plunging towards the Great Attractor at around 600 kilometres per second; and spacetime is expanding. But let's keep things simple.

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