Realistic Interstellar Travel

Background: Earlier this week we had a visit from (possibly) the world expert on interstellar travel, Les Johnson. Johnson is doubly qualified to speak on the subject since he is

i) the Chief Technical Officer at NASA's Marshall Space Flight Center and an expert on solar sails

ii) a published science fiction and nonfiction author (see some of his books listed at the end of this post), and a technical consultant for movies like Lost in Space

In his talk, he laid out the basic calculations on realistic interstellar travel, recognizing that the audience was the College of Science, but without overwhelming technical details.

Motivation: It seems like life is rare in the universe, said Johnson, therefore we should spread it. Traveling to other places would ensure volcanoes/asteroids/plagues/climate change do not end life as we know it.

Observation:  He started out by giving the audience some sense of how large distances in the universe are, especially for stellar travel: the closest star system is about 4 light years away (Alpha Centauri).

Trip Time: How long should the round-trip take? He picked 1000 years as an acceptable time - judging by our monuments that's the amount of time we seem to have a cultural memory for. He would really prefer 100 years or so. For example: traveling at about 10 percent of the speed of light would take 40 years to get to Alpha Centauri.

Transportation Choices: First he mentioned the options that are not good: Chemical rockets (too fuel inefficient, the trip would take about 50,000 years), electric rockets (10,000 years), nuclear fission rockets (less efficient than electric).

Then the options that are more realistic: nuclear fusion rockets (1000 years; but we cannot yet control fusion), nuclear pulse rockets (400 years; this would require a sequence of nuclear fission explosions, each uncontrolled), and antimatter rockets (100 years; Star Trek gave the original inspiration; but this would need a metric ton of antimatter- right now we produce nanograms per year).

Fun fact: Bananas are rich in potassium, which contains a small number of radioactive isotopes with a lifetime of 75 minutes, after which it releases a positron (in your stomach).

The most promising transportation technology for interstellar travel, according to Johnson, then, is solar sails.

Solar sails: These are light sheets of reflective material which use the momentum transferred by photons (from the Sun) to the sail for propulsion. Fuel never runs out - since the Sun is always shining - and doesn't add to the mass of the spacecraft.

Some sails have been tested, though smaller than required (a kilometer square) for interstellar travel, and heavier than feasible (eventually we need to learn to build them from graphene, the strongest material known).

More details can be seen in the video here. You may want to view it to learn about 'Les's law of solar sails' (analogous to Moore's Law for microchips); to learn whether we should send robots or people on interstellar journeys; to learn about the speculative Alcubierre warp drive.

Special thanks: To my colleague Prof. Grover Swartzlander, an expert on solar sails, for informing me about the talk. Some books by Johnson:

[1] Interstellar Travel

[2] A Traveler's Guide to the Stars

[3] Stellaris: People of the Stars