By Nick Allen

As Nasa prepares to launch a mission to Pluto, the space tourism industry is gearing up for blast-off in 2009. But before you book a ticket, consider this: weightlessness is just falling with style, and as for a spaceship... Planet Earth is the best you'll find, and it's free! It is easy to forget that we are in space already. We are moving through it on a rocky globe that has sufficient gravity to hold us firmly to its surface. Indeed, Earth has so much gravitational attraction that - unlike the Moon - it can hold down individual gas molecules and provide us with an atmosphere. "Spaceship Earth" even rotates to give us a good view all round. Packed with food, minerals and natural resources, it is by far the best spaceship we have. But of course by space we normally mean "outer space" - the immense, cold, dark place up there, where "nobody can hear you scream". VOTE RESULTS Would you holiday in space? Yes 71.57%

No 23.59%

Don't know 4.84% 13674 Votes Cast Results are indicative and may not reflect public opinion The definition of outer space is straightforward - outer space begins where our atmosphere ends. Our atmosphere gradually peters out and gets thinner, so it doesn't have a clear boundary. However, at an altitude of 100km (62 miles) the atmosphere is so tenuous that for many purposes it can be regarded as absent. Accordingly, 100km is generally taken to be the limit of our atmosphere and therefore the edge of space. The 100km mark isn't totally arbitrary - it is also termed the Karman line after Theodore von Karman: a pioneer of aeronautics. Karman calculated that at this altitude the air is so thin that a conventional aircraft would need to be going so fast to get any lift under its wings that it might as well lose them, give up, and be a spacecraft instead. (This is something of an oversimplification - but that's the essence of it.) But what of gravity - does gravity end at the Karman line? The answer is a firm no. At 100km, an astronaut's weight has only fallen by about 3%. In higher Earth orbits, an astronaut's weight decreases further, but even at an altitude of 2,600km, it is still a half of its Earthbound value. High dives At great distances from Earth, gravity does eventually fall to almost zero - but orbiting astronauts remain, relatively speaking, close to our planet where gravity maintains high values. As an example, the space shuttle was designed to go no higher than 1,000km and, by all accounts, has never exceeded more than two-thirds of that. SPACE TOURISM PLANS Richard Branson is planning flights into space as early as 2009 He has teamed up with famed aircraft designer Burt Rutan In 2004 Rutan won the X-Prize for the first private piloted spaceflight Tickets on Branson's Virgin Galactic service are $200k (£114k) Test flights are due to start next year

Clear sky for Virgin spaceship Orbiting astronauts aren't weightless, they merely have the sensation of weightlessness - and you don't even need to be in outer space to experience this sensation. Olympic divers lose their sensation of weight every time they jump off a high board. Skydivers have the same sensation for a short time after they jump from their aircraft. These sensations are unfamiliar to most of us, because few of us are high divers or skydivers, but we have all felt something related to this whilst travelling in a lift. While you don't actually feel weightless in a lift, sometimes you do feel briefly heavier or lighter. To understand the feeling of absolute weightlessness we need to stretch our imaginations and take this lift example to its extreme. Should you ever be unlucky enough to be in a lift when its cable breaks, you would feel exactly the same sensation of weightlessness as a high diver, skydiver or indeed an astronaut in orbit. You, your fellow passengers and other loose items in the lift compartment would even appear to float around as you descend. Whenever you fall freely under the influence of gravity you feel weightless - in fact, gravity seems to disappear, but it hasn't. So orbiting astronauts aren't really weightless, they merely feel weightless because they are falling. This is a puzzle, because astronauts experience apparent weightlessness for hours, days, even months. How can they fall for so long without hitting the ground? The orbiting body is moving sideways so fast that, despite falling towards the Earth, it quite literally keeps missing it

Isaac Newton provided a solution a long time ago. Newton reasoned that, because the Earth has a curved surface, the ground gradually curves downwards and falls in height as you move across it. He proposed that, if a falling object could move across the Earth's surface rapidly enough as it fell, the surface of the Earth might fall away under it at the same rate. In this way, a falling object could maintain the same height above the ground, despite falling. By travelling fast enough to follow the Earth's curvature, it could literally "fall around" it. What Newton was describing was a low-Earth orbit. There are other ways of explaining how this works. Another, perhaps simplistic, way is to argue that the orbiting body is moving sideways so fast that, despite falling towards the Earth, it quite literally keeps missing it. 8km/second For future space-tourists and the unconvinced, we can put some figures into Newton's example to illustrate. At an altitude of 100km, your spaceship will fall vertically by about 4.75m every second. Flying start: Space tourism flight trials are due to start next year However, calculations show that the Earth's curved surface also drops in height by 4.75m when you travel across it for about 8,000m. This means that as long as your spacecraft is travelling across the Earth's surface at about 8,000m per second as it falls, you should maintain the same height. You are in orbit. While the principle of a low-Earth orbit is straightforward, it poses many challenges. Eight kilometres per second is about 29,000km per hour or 18,000mph - you simply can't go this fast in the atmosphere due to the immense drag forces produced. Even if the necessary power were somehow available, the heat generated by the friction would cause your vehicle to burn up. To go this fast, you need to get above the restrictive atmosphere - above the Karman line - into space. This is the real reason why astronautics is commonly associated with weightlessness. It really has little to do with escaping gravity, especially in the vicinity of Earth. It is to do with escaping the atmosphere. Only above the atmosphere can astronauts attain the immense speeds necessary for orbit. Once in orbit they are constantly falling and this gives them the appearance of being weightless. Sounds fun. But I for one don't intend to become a space tourist. Spaceship Earth is fine by me - I don't need expensive tickets to travel on it, and I can also see outer space from it. It's the best spaceship we have. Rather than paying to leave it, maybe we should concentrate on looking after it - and all its passengers. Nick Allen is a Master of Science in astrophysics and a fellow of the Royal Astronomical Society. Your comments: Good article...I would have to agree that we should be concentrating on taking care of Earth rather than trying to leave it. Apparently, this opinion is not the popular opinion.

Anica Brandt, West Vancouver, Canada Three hundred years ago, when Captain Cook sailed, home was just little England. Now many of us, including Mr. Allen, think of the entire earth as our home because of explorers like Cook. One day because of space exploration, the definition of "home" will be further broadened. Perhaps in one hundred years, you will see an article written on the BBC by a similarly minded "homebody" like Mr. Allen advocating that we humans should not venture beyond Pluto and the Kuiper belt, and instead focus on exploring and improving our home in the solar sytem within the orbits of Neptune and Pluto.

Mike Dwyer, Tokyo, Japan Excellent piece of scientific writing. I would have never known the fact about 'Karman Line'. It has motivated me enough to do a bit of my own study on weightlessness.

Reynel Castelino, Norfolk,VA,USA I disagree - I don't feel weightless when I skydive. When you leave the plane you quickly accelerate to about 120mph, and the wind resistance gives the impression of lying on something soft. Your weight feels firmly supported by it, although obviously it isn't. I for one (were I rich) would love to travel into space, purely to see the Earth from that unique viewpoint. It would be a colossal waste of money though, when you consider how many people your money could help instead.

Rob Low, Tunbridge Wells, Kent, England RESPONSE FROM ARTICLE'S AUTHOR: Hi Rob, I totally agree - you certainly won't feel weightless when you reach your terminal velocity at about 120mph. You can only have the sensation of weightlessness when you are freely accelerating. That's why I was very careful to write that skydivers only have this sensation "for a short time after they leave their aircraft." BTW, thanks for all the nice comments.

Nick Allen All very interesting Nick, and I know your article is somewhat tongue-in-cheek, but I think you are missing the point of space tourism. For me the experience would not be about pseudo weightlessness but rather the thrill of the ride and most of all the view out the porthole. Looking down at the distant earth as it passes beneath me and marvelling at the unhindered clarity of the sky would surely be worth the expense.

Duncan I, Worthing This was the first time I was able to understand the whole concept of low earth orbit fully. Thank you very much. Now can you please write something simple like this about catapulting satellites, probes using the gravity of planets. I think I will understand that concept with some more reflection, but just in case... Keep writing stuff like this, please.

clifford, Montreal, Canada After a 3 year physics course with space science modules over the final two years, I can quite definitely say I wouldn't go up into orbit in a spacecraft in this age. The amount of dangers posed by space debris, the risks from the spacecraft launch, and above all the fact that the protection from cosmic rays and particles emitted by the sun is vastly reduced in orbit make me very comfortable studying space with my feet on the ground.

Alex Vukmirovic, Southampton Thanks for an informative and readable article on the difference between weightlessness and free fall. I'd just like to take issue with you on one point though. Free fall isn't quite like falling, as we usually experience it. It's a lot more like floating underwater. Describing it as falling doesn't do justice to the wonderful feeling of freedom and security that comes with it. As a nervous airline passenger I was worried that a parabolic flight would feel like really bad turbulence. Instead it feels like the best scuba diving ever, only better.

Nick Colford, Madrid, Spain. "Planet Earth is the best you'll find, and it's free!" Where do you live? I want to live there!

Richard Hodges, United Kingdom Nice to see someone finally trying to put an end to the popular misconception that there's no gravity in space and that being in orbit is merely perpetual free-fall! However I'd jump at the chance to go see space ship earth from space (wherever that is!?)!

Mike, Leeds



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