Jewels of the Internet: 2006

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News Article:

Tips for space tourists

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".

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.

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?

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.


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