When will interstellar travel be possible?

You will never visit another solar system. Nobody alive today will visit another solar system, or even see a probe reach another solar system, for that matter. To claim that this will soon be possible is preposterous; to attempt to create a hard target for true interstellar travel, futile.

But also fun! And possibly illuminating.

M. G. Millis of the Tau Zero Foundation (and formerly of NASA's Breakthrough Propulsion Physics Project) has done a bit of napkin-scribbling on the subject, and has come up with an estimate: A human-carrying probe could conceivably embark on a 75-year, 4-light-year journey to Alpha Centauri as soon as 2196.

Millis' calculations probably aren't likely to be borne out, because they're too loose, too abstract and include a great many wildly uncertain variables. Gauging the future of humanity's forays into space is nearly impossible on a scale of a few decades, much less centuries.

That said, the substance of his calculations is fascinating--and taken as a exploration of how to talk about the future, rather than a piece of hazy prognostication, actually quite valuable. Here's the rough methodology:

Projections for the earliest interstellar mission possibilities are calculated based on  27 years of data on historic energy trends, societal priorities, required mission energy, and the implications of the Incessant Obsolescence Postulate (Where newer probes pass prior probes). Two sample missions are considered: launching a minimal colony ship where destination is irrelevant, and sending a minimal probe to Alpha Centauri with a 75 year mission duration.

In order to approach a problem like this, Millis had to take a wide view. The resulting estimate wasn't the result of a tenuous map of contingent human accomplishments eventually leading to the launch of a specific interstellar people-mover, but rather broad extrapolations about the future of mankind in general. (The process is a bit like a mathematical version of what sci-fi authors do when they're trying to establish a plausible fictional universe.)

As factors that could inhibit interstellar flight, Millis examines the following:

• Energy resources available
• Energy requirements
• Propulsion options
• Human motivation

The most rigorous part of the paper, by far, is the consideration of energy availability and use:

When estimating the amount of energy that humanity can devote to interstellar missions, two factors are considered: the total amount of energy produced by all humanity and the proportion of that energy which is devoted to space endeavors.

Millis assumes an annual growth in total energy available to humanity at a rate of about 1.9%, based on 27 years of recorded data. He then assumes that the proportion of humankind's available energy devoted to spaceflight--something he calls the "Space Devotion Ratio"--to be about 1.3 x10-6, and works from there.

In any case, the paper is an interesting read, if not as a glimpse of the future of space travel then as a exercise in talking about the (nearly) impossible.

(Hat tip to TR's Arxiv blog.)

This post was originally published on Smartplanet.com