BBC4’s Spectacular Vista of our Voyage to Neptune and Beyond.

For a long time in the 20th century it was widely believed that we would never be able to travel through space further than to our nearest neighbour, Mars. The fuel needed to counter the gravitational pull of the sun and planets would make that impossible.

But when a brilliant PhD student solved one of the great maths’ problems, the whole of the solar system suddenly opened up.

The problem being; how do you work out a space ship’s trajectory when its position is being constantly affected by the huge gravitational pull of the sun to one side, and an enormous planet to the other? Every new position will then be differently affected by both, and in constantly varying ways.

Once that had been solved however, they suddenly realized that you could use that massive gravitational pull as a lasso to fling your space craft off in any direction you liked. Furthermore, you’d be able to do so without using up any fuel whatsoever. Your momentum could propel you indefinitely.

Then another grad student spotted that the four biggest, outer planets, Jupiter, by far and away the biggest, Saturn, Uranus and Neptune (Pluto was re-classified as a dwarf planet in 2004) would all be aligned between 1975-7. We would have to wait another 200 years for the next chance.

So in 1977 the two Voyagers, I and II were launched. And over the next 12 years they sent back extraordinary data and photographs of our four biggest gas planets and their couple of hundred moons.

When Voyager II eventually arrived at Neptune, some 3 billion miles away, they needed to be able to calculate the precise moment it passed the planet’s North pole, to within one, single second! The photographs that resulted were spectacular.

And that it was thought was that. But then Carl Sagan, Nasa’s de facto spokesman had an idea. Why didn’t they get Voyager I, as it sped away from us, to turn around and take a photograph of us from the edge of our solar system?

The result, for my money, is the single most important photograph ever taken.

On the one hand, it’s a timely reminder of how insignificant we are in the grand scheme of things. But on the other, it’s a celebration of how extraordinary we are. We sent a machine nearly four billion miles and 13 years into the future to take a photograph and send the information back to us, so that all of us can have a look at it today.

Voyager I is 11 billion miles away as we speak and has just reached the outer reaches of our solar system. It’s still sending back data, which it does using a millionth of a billionth of a watt. And the data that it sends takes 15 hours for the speed of light to reach us.

And all of it built in 1977. That, by the way, was the year Apple was launched.

This was about as perfect a television programme as it’s possible to make. And it’s yet another in what is fast proving to be a golden age of science programming from the BBC (see also, for instance, my earlier review of the Antikythera mechanism here).

It struck exactly the right balance between calmly providing the facts, and quietly looking up in awe. And if at all you can, watch it.

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