Celestia: Across the universe... for free

It's the ultimate ride: tour the entire galaxy from your own computer. And what's more, it's free, says Charles Arthur

Since it was hard to breathe last week without being reminded that another Star Wars episode had come out, it seemed appropriate to try out a space travel program that gives you the real feeling of space: that is, mostly very empty, with some damn big stuff lurking around there in the darkness.

Since it was hard to breathe last week without being reminded that another Star Wars episode had come out, it seemed appropriate to try out a space travel program that gives you the real feeling of space: that is, mostly very empty, with some damn big stuff lurking around there in the darkness.

But while it may have cost Mark Shuttleworth $20m (£13.3m) to become only the second space tourist, heading out to space will cost you and me absolutely nothing – apart from the patience required to download an 11Mb file.

The program that has been amazing me is Celestia, available from www.shatters.net/celestia. It runs on Windows (98 to XP), Linux and Mac OS X (though not the older Mac OS). Ideally, you'll want a powerful new machine with a good graphics card – something from the past three years should do the job. Celestia lets you take yourself on a tour of the solar system, including comets and moons, and get nearly as close as you like to the Earth; near enough to see the lights on the night side of the globe (which turns, of course).

There are distant stars to visit (it goes up to galaxy scale and has data on 100,000 stars). You can cruise the constellations (helpfully outlined for you). And when you're tired of the vast emptiness, you can just set the controls for the heart of the Sun and head home.

The detail is stunning, because the surfaces of the planets and moon, and even asteroids, come directly from the US space agency Nasa's photo archives, taken by passing spacecraft such as Voyager. The program itself is open-source, meaning that if you're competent in C you can tinker with it for yourself; in fact its inventor, Chris Laurel, positively encourages that, because it gets more people thinking about innovations and improvements. Judging by the forum boards at the site, plenty of people are.

It would be easy to dismiss this as just another orrery (a solar system simulator, in plain language) similar to 1st Light from Vibrant 3D (though that costs $20). But that would be to overlook the fact that as well as a fantastic simulator, it is built with a modular approach that means you can add planets and detail as much as your graphics card will bear. Thus people have devised real "models" of spacecraft such as Mir and the Space Shuttle (though I had some trouble locating them – possibly I needed to reverse time to themoments they were actually aloft). Someone with an active imagination has devised some "2001" additions – a black monolith and an empty pod from a spaceship, somewhere near Jupiter. (See www.shatters.net/celestia/images/monolith.jpg – though shouldn't there be a floating astronaut somewhere too?)

That is the power of open-source development writ large. For anyone who finds the talk about Linux and its open-source basis tedious, this is an example of how things can progress once there is a solid base to build from. And it's hard to argue with the price. It's worth noting that it's only possible because of Nasa's own open-source approach to its photographic data; who would embark on writing such a program if they had to pay for raw data like that, with no idea what return there would be? Somewhere in there is a lesson for our Government, especially Ordnance Survey, which guards its map data as though it had some inherent value. It doesn't; it's only when you build things with it that value emerges.

What is thrilling in this program will surely be different for everyone. Perhaps it'll be the ability to see the orbits of the planets, and how bunched up the inner ones are. Perhaps it'll be the sheer beauty and loneliness of our planet compared to the brisk ugliness of the others.

For me the key was simply discovering how incredibly difficult it is to find your way around in space – and how much space there is, so to speak. You may be just above the Earth, but do you know how to find the Moon, a tiny dot 400,000km away? Do you know where in the sky to look? There's a lot of it, and everything is a long way off – a fact that's made starkly clear when you decide to accelerate to light speed and head to the nearest star, Alpha Centauri. If you don't artificially speed time up, then you'll be sitting there for more than four years. Be thankful that in Star Wars they invented jumping into hyperspace. Otherwise you'd still be watching Han Solo plod around the galaxy far, far away.

For a free Celestia download for Windows, Linux and Mac OS X, go to www.shatters.net/celestia.