Space Train

Lets follow the irritation Nicole Stenson feels as she realizes travel in space is less cruise ship, and more subway. It certainly is a far cry from travel by car or air where direction and speed changes are virtually on-demand.

If you are just joining us, what you are about to read is a fictional account of what real humans will likely experience at some point in our second century in space. I follow the fictional story with an explanation of the tech, science or other principle which makes a story like this virtually assured.

So lets check out what Nicole is going through today…

The Fun Part

December 13, 2071
Approximately 5 million kilometers from Mars

“This is absurd” Nicole said, “what do you mean we don’t have a choice but to continue on to the asteroid?”

“Just that ma’am,” replied the somewhat cowed first officer of the space liner WindStar. “We are relying on the gravity of Mars to slow us as we approach, and have only enough fuel storage for emergency maneuvers en-route.”

“But James…aren’t you the one who gave us the announcement that we will be unable to land on the resort because of the…what was it you called it? A bacteriological contamination of the transit tubes?”

“Yes ma’am” James replied, “I realize the inconvenience this is causing you and the other passengers, and we have already sent a message to corporate back on Earth, and will let you know immediately what remedy they have”

“Inconvenience is having to shit in a zero-G James…Inconvenience is having a water ration for showering…Inconvenience is having exactly NO variation in the planned menu.” Nicole was on the verge of losing what vestiges of control she had. Despite growing up in a family where outbursts were anathema, she was on the verge of breaking. “James…inconvenience is not traveling three months with 20 strangers, my future ex-husband, and my aging dog, only to find out we did it all for nothing.”

“This happening on a cruise ship on Earth is understandable, but there we just divert to another port. Are you suggesting the two million dollars I paid for this trip wasn’t enough for a little extra gas? This is starting to feel more like a subway trip rather than a solar cruise.”

“That is actually a good analogy Mrs Stenson” James replied. “Within a few days of setting out from Earth orbit we were basically committed to this trajectory. By the end of the first week had already burned nearly 40% of our fuel, and our emergency plan consisted of a slingshot maneuver around Mars for a return home.”

Nicole deflated a little at that explanation. Intellectually she knew he was right…but instinctively she felt there must be another way. They were in space after all…how easily did ships alter their vector in the movies? As Nicole returned to her cabin she figured if she wasn’t going to get a chance to lay on the virtual beaches of the martian resort, she could afford to take a night eating and drinking as much as she wanted. What use was a beach body up here anyway…

The Real Deal

Six weeks after launching from Cape Canaveral, an Israeli-built probe funded through private donations arrived in orbit around the moon. The Beresheet spacecraft carried enough fuel for a single maneuver to steer it into orbit around the moon. If the probe misfired, Israeli officials said the spacecraft would have continued on into deep space, bringing the mission to an end.

The Beresheet spacecraft’s six-minute deceleration burn Thursday steered the probe into orbit around the moon. Credit: SpaceI

While this was the first privately funded ship to enter lunar orbit, the mechanics of how it decelerated are not unique to any spacecraft.

On earth we have the delightful benefit of friction to slow anything we happen to be traveling in. This allows us to have our vehicles ‘push’ their way along. When it comes time to stop the friction of the ground, brakes, air, or water is used to slow the vehicle, rather than reversing the engine’s output (yes most oceangoing ships do have the ability to do this because ‘braking’ at sea is tricky).

In space there is no friction…at least none that will have a measurable impact on the velocity of an object. So in order to stop, you have two options. First, get yourself captured by a gravity well like the Beresheet probe did. This involves adjusting your vector and speed slightly, and then entering what is called a degrading, elliptical orbit. This is just a fancy way of saying the orbit looks like a spiral oval.

The second option is to reverse your thrust. This could be accomplished by having an engine output on either side of your ship. But an easier way would be using small thrusters positioned along the hull to flip the ship around until the main engine is facing your direction of travel. The space equivalent of pulling the ‘e-brake’, spinning around, and gunning your car’s engine.

This second option requires a ship to carry sufficient fuel to both get up to speed AND decelerate. This would have the practical result of the ship going slower to conserve the amount of fuel needed to brake.

But what about changing direction?

So this explanation has largely focused on stopping at your destination, rather than changing direction. This is purposeful, because the principle is the same.

In space, changing direction means changing your velocity, which requires some sort of energy expenditure. So instead of being able to rely on the friction of air over wings, or the ground on your tires, in space there is no such option. You either use your engine or a gravity well.

Not great options…but we chose to use trains and subways to get around here on earth because those were faster and more convenient ways to haul people and cargo than the alternatives. It looks like until we get some sweet new tech to take us around the stars without the need for limited fuel, we are stuck riding space subways.

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