Challenge 965 Response

Interstellar Speeds

with L. L. Richardson

[LLR] Hello, Don... You pose an interesting and fun question regarding space travel: how fast would a spaceship need to travel to reach the star Proxima Centauri if the craft took two hundred years?

[Don W.] Thank you, L. L. You restate accurately the question 4c as posed in Challenge 965:

The frozen-sleep spaceship is programmed for a voyage of 200 years. At what fraction of light-speed would it be traveling if it were going to Proxima Centauri, the star system nearest to Earth, in that time?

[LLR] If my math is correct, the ship would need to race along at an average of 2% of the speed of light. In my reading, I have found many experts who suppose a future a propulsion system might get a space vehicle to a speed of 10% light speed. There are many technical problems associated with space propulsion; 10% light speed just might be do-able. So, 2% light speed might seem relatively easy.

Two percent light speed is still very fast. Currently, our fastest spacecraft, the Parker Solar Probe, is expected to eventually reach 450,000 mph. The craft would need to go about 30 times faster to get to 2% light speed. Like I said, there are lots of technical challenges to that.

[DW] You’re right to state that a spaceship would have to average about 2 percent of the speed of light in order to reach Proxima Centauri in 200 years. After all, we presume that the ship will glide to a halt at its destination rather than zoom past at full tilt. And that means the ship’s top speed will be more than 0.02c before it begins to decelerate.

[LLW] Thankfully, a sci-fi story doesn’t need to explain the “hows” of space travel. Yet, science fiction has inspired scientists, engineers, and theorists to reach for the seemingly unreachable. It is these experts who will make possible a future trip to Proxima Centauri; we will leave it to them to explain the “hows.”

[DW] Quite so, LL! Writers know first off not to explain “how” to do something in a science fiction story. It’s a pretty sure thing that neither the Patent Office nor science fiction readers will be interested in anything impossible, such as generating cold fusion by using the element “handwavium.”

A classic example: Nobody knows how Star Trek’s transporter might work. And nobody really cares. In fact, it malfunctions so often that television viewers might wonder why anyone would use such unreliable technology unless it’s to generate interesting plot twists!

As for a cold-sleep ship or generation ship traveling to another solar system, the prime question is not how but why. As one of our essays points out, “why” calls for some tall explaining; see Space Colonies: the Dark Side.