Interstellar Tours (eBook)

WELCOME ONBOARD 1

One thing that the classic TV show Star Trek (mostly) got right is that starships don’t land on planets. It’s easy to underestimate just how difficult it is to get a massive object off the surface of a planet and into outer space. The problem lies in escaping the gravity well. An object as big and heavy as the Earth – which has a mass of around 6x1024 kilograms (13x1024 pounds) – holds on to objects on its surface with an iron grip. Even when birds or planes do make it into the sky, they soon have to return to the surface. What goes up really does usually come down.

Are you massive?

Because we are going to spend our time during the journey out in space, it’s worth quickly clearing up the distinction between mass and weight, because the difference matters very much when you are away from the surface of the Earth. These terms tend to be used interchangeably back home, but they are very different things, and in space this will become obvious.

Mass is an intrinsic property of an object, which is measured in kilograms (officially, the traditional mass unit is called a slug (14.59 kilograms), although the pound tends to be used more often). It doesn’t matter where an object is, it will always have the same mass, unless bits are removed from it or added to it. You could see mass (a concept introduced to the world by Isaac Newton back in 1687) as a measure of the amount of stuff in an object – whether that object is you, a starship or something as large as the Earth.

Weight, by contrast, is the force that is felt by an object under the gravitational pull of a body such as a planet. When we talk about the weight of something, we really mean ‘its weight when it is on the surface of the Earth’, though we tend to omit the last bit. Your weight would be totally different if you were on the surface of the Moon, for example – about a sixth of what it is on Earth. In space, your weight could be zero, though, as we will discover, it certainly doesn’t have to be, and it will only be zero on the Endurance when in a special, low-gravity entertainment area. Having weight makes doing many things much easier – from eating to visiting the toilet.

Although your bathroom scales will give you your weight in kilograms or pounds or stones, this is a cheat. Strictly speaking, weight is the force due to gravity acting on the mass of an object. Scientifically, this should be measured in units called newtons (foot-poundals for traditional unit fans), but in practice we tend to fudge it and still use the mass based on what’s measured on the Earth’s surface. So, when we say that that on the Moon you will weigh one-sixth of what you do on Earth, what this means is that you will feel the force pulling you down that you would experience if you had one-sixth of your mass on the Earth’s surface.

Whether we talk of mass or weight, the first stage of taking our interstellar tour is getting off the Earth. And here things have not moved on as much since the earliest days of space flight as 21st-century people might have expected. The old rockets were extremely unsafe and scary. Nonetheless, we are still using the equivalent of rockets, although with far less risky sources of propulsion, and the ability to make the journey into orbit under a level of acceleration that won’t put the stresses on the body experienced by early astronauts. For us, it’s no different from taking a plane trip. We have moved on from the first journeys into space, but not as much as science fiction writers might have hoped for.

As for the Endurance itself, nothing as massive as a starship could survive landing or take-off from a planet. The ship was assembled in space with materials originating from Earth, the Moon and mined asteroids. The Endurance is a native of space itself.

No easy getaway

Using a traditionally fuelled rocket to get off the Earth was both expensive and risky. There are two ways to get an object into space. You can throw it, or you can push it. In practice, we usually go for the latter, but first it’s worth taking a look at the former.

If you can throw something faster than ‘escape velocity’, it will get away from the Earth’s gravitational pull and not return. If you had a suitable superhero to help you out, they would have to throw a ball straight upwards at 11.2 kilometres (seven miles) per second for it to reach this speed. That’s extremely nippy. The fastest fighter jets of the early 21st-century flew at around three times the speed of sound, but the ball would need to travel eleven times faster than this.

There is a way to cheat a little, because helpfully the Earth is rotating and we can make use of that. Something that is shot off the Earth in the right direction does not have a standing start, because it is already travelling at the speed of rotation of the Earth’s surface. By piggybacking on the Earth’s movement, we can get escape velocity down to around 10.8 kilometres (6.7 miles) per second – but that is still ridiculously fast. This, incidentally, was the approach taken by one of the first science fiction space flight stories, Jules Verne’s From the Earth to the Moon (De la Terre à la Lune)*.

In his novel, Verne’s adventurers were shot from the Earth using a 274-metre (900-foot) long cannon called Columbiad. The distinction between a cannon and a rocket is that the projectile in a cannon is only being accelerated while it is in the barrel. As soon as it leaves, it can only get slower. Unfortunately, to get a capsule up to escape velocity by the time it had traversed the Columbiad barrel would have required so much acceleration that the astronauts would have been mashed into jelly. Even if Verne had stretched Columbiad to ten kilometres (6.2 miles) in length, those on board would have suffered 600 times the force of the Earth’s gravity. The acceleration they endured would be vastly more than the around 9g* that is about the most a human can survive.

Yet we’ve all seen video of rockets taking off and getting away from the Earth. They seem to climb very ponderously into space. Although some of the slowness is illusory, they don’t fly at anywhere near escape velocity. The reason they can travel relatively slowly and still get up into space is that they aren’t thrown like a projectile from a cannon. All the time that the rocket motor is active, the capsule is being pushed. And as long as the force of that push is bigger than the force of gravity pulling the spaceship down, you can travel as slowly as you like away from the Earth. It’s the same as riding up in a lift. We don’t need to go quickly; we just need to have enough upward force to overcome the downward pull of the Earth.

The catch, though, with rockets is that the more mass the object has, the more fuel it takes to keep it moving long enough to escape the Earth’s gravity well. And every drop of fuel you have onboard adds to the mass. So that takes even more fuel. This is why rockets that carried any sizeable payload used to have multiple stages. (This was before the use of modern nuclear or antimatter-powered orbital shuttles.) That way, once the fuel in one section is mostly used up, a large chunk of the mass could be dropped off in the form of a stage, leaving far less mass for the remaining fuel to propel. Add in the fact that traditional rocket fuel is highly inflammable and potentially explosive and it can be seen that using a straightforward rocket to get into space was always a last resort.

That stages would be necessary for manned flight was publicised by Russian rocket theorist Konstantin Tsiolkovsky as early as 1903, the year that the Wright Brothers first flew an aircraft. While we’re dealing with rocketry, it’s also worth mentioning that to begin with, a number of respectable (if scientifically ignorant) figures doubted that rockets could work at all in space. Back in 1920, US rocket pioneer Robert Goddard published a paper entitled ‘A Method of Reaching Extreme Altitudes’ that suggested a rocket might be used...