George Greenstein

Astrophysicist / Educator / Writer

Sidney Dillon Professor of Astronomy Emeritus at Amherst College

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A Trip to Alpha Centauri

Conventional rocketry cannot do the trick. We need new technology


They’ve discovered a planet orbiting Alpha Centauri. “We should drop everything and send off a mission” enthused a local “expert.” But I’d like to inject a note of caution. We tend to forget how truly gigantic are the distances to the stars. Alpha Centauri, for instance, is tens of thousands of times farther away than our local planets. To get there in a reasonable amount of time requires a technology that would put Star Trek to shame.

Maybe an analogy will explain why. Going to a star is like driving your car off on a trip – a long trip, very long: all the way across the country perhaps. How much fuel will you need? Well, you know how far you are planning to drive and you know how many miles per gallon your car gets, so you might just divide miles by miles per gallon.

But suddenly you realize that things are more complicated than that – because you just happen to know that there are no gas stations along the way. So you will have to carry along all the gas that you are ever going to need from the very start. Of course, the car’s fuel tank isn’t big enough for all that gas. So you’ll have to tow a trailer-full of the stuff along behind.

But then you realize that all this fuel weighs something! And then you realize something more: that, hauling such a heavy load, your car doesn’t get its usual mileage. Now its mileage is worse. You’re going to need even more gas than you thought since you’ll be dragging along that extra weight. So your initial calculation of how much fuel you need was too low.

And so you realize that you will need an even bigger trailer than you had thought, all filled with even more fuel than you had thought. . . . and then you realize that your car’s mileage is even worse than you had thought.

And so on and so on.

The math tells us that, if we use a conventional rocket -- the Space Shuttle main engine for instance – and if we wish our spacecraft to reach Alpha Centauri in a century, we would need more fuel than the proven oil reserves of the entire Earth to send off a single atom.

Let’s turn the problem around: if we do use the entire world oil reserves, what sort of engine would suffice to send a one-ounce payload on that century-long voyage to Alpha Centauri? The answer works out to an engine that spits out its exhaust tens of thousands of times more rapidly than the Space Shuttle’s. This is more than any technology anyone has ever discussed. Indeed, it is nearly one-third the velocity of light.

A friend once asked me how Starship Enterprise ever managed it’s faster-than-light travel. I’d ask the same of this. I’m not saying it’s impossible: I’m saying that conventional rocketry cannot do the trick.

What we need is brand-new technology.