The voyage of light

Space is big. You just won’t believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it’s a long way down the road to the drug store, but that’s just peanuts to space.

-Douglas Adams

Space. Once again, space. I like space. But this time, here is some non-speculative space. Non-futuristic, non-optimist, just plain, regular, utterly non-fictional space.

Although The Martian is an excellent film and you should all go and watch it right now.

But that is not the point. The point is this.

The point!

That point of blue-white light is a picture of the Earth, taken by the Voyager 1 interstellar probe from about 6 billion kilometres away. That was, and remains, the furthest picture of our planet ever taken. And I think that it gives the solar system a magnificent sense of scale.

So I thought that I would do my own attempt at giving our local system a sense of scale. With a voyage. Not the Voyager probes, beautiful though they are. The voyage of a beam of light, travelling from the sun outwards.

So let’s begin

Sol. Or the Sun. Either one works.

Sol is the centre of our Solar system. It is bloody massive. It is more than 100 times the width of the Earth, with a mass 330,000 times that of Earth, It is also very, very hot. Think 1.57 million degrees Celsius. Not pleasant.

Our photon is emitted from the very core of the sun, which makes my calculations easier. It then proceeds to head outwards at the speed of light, directly on the orbital plane of the planets. 2.32 seconds later, it leaves the surface of the sun. From here, there is no going back.

281521main flyby2 20081007 226.jpg


Mercury is the innermost planet, and is slightly smaller than our moon. It is also a nasty place. Surface temperatures range from -173°C at night to 427°C during the day. It is also blasted by solar radiation, having no atmosphere worth mentioning.

3 minutes and 52 seconds later, our photon arrives at Mercury. 0.016 seconds later, the photon leaves Mercury. It’s not a big place.

Venus in approximately true colour, a nearly uniform pale cream, although the image has been processed to bring out details.[1] The planet's disc is about three-quarters illuminated. Almost no variation or detail can be seen in the clouds.


Venus is, according to several experts, ‘a hellhole’. The atmospheric pressure is 90 times that of Earth, the daytime temperature is around 460°C, hot enough to melt lead. It also rains sulfuric acid. At night, it is exactly the same only darker. Do not go to Venus.

150 seconds later, our photon arrives at the surface of Venus. It decides that it doesn’t like it and leaves 0.04 seconds later.

Sol 3! Home!

This is Earth, You might know a thing or two about it. It’s quite nice.

148.4 seconds after Venus, the photon passes the Moon. 1.31 seconds later, it zooms past the Earth. Narrowly missing some solar panels, it leaves. It spent 2.66 seconds inside the orbit of the moon. Bye now.

The planet Mars


Mars is the fourth planet, and the most Earth-like in our solar system. The temperature varies from around -50°C and 20°C. The atmosphere is sparse, about 1% of that of Earth’s. Mark Watney No one lives there.

1173 seconds after leaving Earth, our photon passes Deimos, the outermost moon of Mars. The Martian system is much smaller than that of Earth, so the photon speeds past the other side of Deimos’ orbit in just 0.15 seconds.

Now, so far, it’s only taken us 13 minutes to tick off all the planets. That’s because the inner solar system is really titchy. Timescales are about to get bigger.

An image of Jupiter taken by New Horizons.

Jupiter. Not to scale.

Jupiter is the biggest planet in our solar system. It is bloody massive. I mean, the barycentre of the Sun and Jupiter is actually above the Sun’s surface, if that means anything to you. Jupiter, big, red. Pretty. Also, fierce levels of radiation.

Jupiter is also far away. It takes a full 30 minutes for our photon to reach the planet, passing all 67 of Jupiter’s natural satellites. It takes a whole 0.4 seconds to pass the body of the planet. It takes 12 seconds to travel through the Jovian system. Big planet.

Saturn at its equinox.

Saturn. Check out some of the images that Cassini is sending back, they are amazing.

Saturn is the second biggest planet in our solar system, and has the biggest ring system. It has a lot of moons and a very pretty colour.

The outer solar system is big. It takes 35 minutes for the photon to get from Jupiter to Saturn. It takes 23 seconds for the photon to pass both sides of the orbit of Iapetus, Saturn’s outermost large moon.

Uranus as seen by Voyager 2

Uranus, not looking very interesting.

Uranus (heh heh heh) is a fairly uninteresting planet. That’s probably because we’ve never sent a mission to study it. NASA, get on that.

1 hour and 20 minutes after leaving Saturn, we arrive at the frozen outpost of Uranus. There are only five major moons of Uranus, all fairly close to the planet. It takes 3.8 seconds to enter and exit the system at the speed of light. Farewell, hilariously named planet.

Neptune from Voyager 2.

Neptune, a pretty planet.

Neptune is the furthest planet of the solar system. It is a very pretty blue colour, and has 15 moons that we know of. Interestingly, it is home to a very unusual moon – Triton. It is the only large body in the solar system which orbits in the opposite direction to the rotation of the planet, and consists of 99.5% of the mass orbiting Neptune.

Neptune is a very, very long way away. One and a half hours away. And it’s outermost moon, Neso, is very far away from the planet. It takes almost five and a half minutes to enter and exit the orbit of Neso.

And not our lonely photon leaves the planets of the Solar system. After four hours of speed-of-light travel, we are done with the Solar system. There’s only a few more landmarks to point out. All following times are given from the Sun, not from Neptune’s edge.

After 5 hours and 28 minutes, we pass Pluto.

5 hours and 36 minutes: New Horizons! Keep going there buddy!

18 hours and 24 minutes: Voyager 1. Still going, still awesome.

5 days, 18 hours: The inner edge of the Oort cloud. This is a massive, massive field of comets that surrounds the Sun in a mighty sphere that extends a really long way out. We assume that it’s there, but we’re not quite sure.

577 days: The outer edge of the Oort cloud. When I said it was really big, it was really big,

About 4 years: the lonely photon reaches Alpha Centauri. Finally, the photon is absorbed into another ball of hydrogen fusion and taken in by the glow of another star.

The end.


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