Is Humanity a Spacefaring Race?

By Seth Shostak

We're destined to go to the stars. That's the assumption we've been making for a century, and I daresay most readers believe this as surely as they believe we'll eventually cure dandruff.

Our anticipation of an interstellar destiny is not merely the consequence of too many couch-hours spent watching Spandex-suitedastronauts in Star Trek, Star Wars, or Futurama. It's been a subtext of our space program. You might recall a low-grade, 1960 biopic about Wernher von Braun entitled I Aim at the Stars. Or perhaps you know the sunny motto of the National Space Society: "ad astra" ("to the stars"). Boldly sending our descendants into the galaxy's stellar realms seems as inevitable as teen sex.

I like the idea of becoming a spacefaring society. It's just a matter of how far we'll fare. Yes, we'll colonize our cosmic backyard; after all, if our species has not spread beyond Earth within a century, then we're headed for eternal internal conflict and armageddon@home. In the words of rocket pioneer Konstantin Tsiolkovsky, humans can't stay in the cradle forever.

I trust we'll soon be landing people on Mars, and eventually the satellites of the outer planets. I anticipate the construction of space habitats on the moon, the asteroids, and in orbit around Earth.

But colonizing the vicinities of other stars is enormously harder than settling the solar system. Our fastest rockets can take us to Neptune in less than a decade - a long ride, but not inconceivable.  On the other hand, a trip to the closest star system, Alpha Centauri, would take 75,000 years.

Since our lifetimes are typically a century or less, practical travel to the stars requires relativistic velocities. Sending a 100-ton habitat, stuffed with a small crew, to Alpha Centauri at a speed that will deliver them before they turn up their toes requires as much energy as the United States consumes in a year. At standard utility rates, that will cost you about $3 trillion, just for the fuel.

Now this is a familiar discourse, and the usual rejoinder is to appeal to the inevitable development of faster rockets. We'll still go to the stars; we just need better transport.

But permit me to point out something both relevant and important: our remote sensing technology is improving much faster than our rocket technology. And that is a game-changing circumstance.

Let's look at some numbers. Von Braun's V-2 rockets crossed the English Channel at 1 mile per second. NASA's New Horizons mission to Pluto - the fastest spacecraft ever launched - is headed to this erstwhile planet at 10 miles per second. That's an order-of-magnitude improvement after 70 years.

Now consider one component of our remote sensing capabilities - our ability to "see" what we're exploring. The Mariner 4 spacecraft - the first to snap decent photos of Mars - was fitted with a monochrome TV camera having a resolution of 40 thousand pixels. In the summer of 1965, it sailed by the red planet while imaging craters as small as a few miles across.

Today, the HiRISE camera on NASA's Mars Reconnaissance Orbiter boasts a resolution of 200 million pixels (and shoots in color). It can discern items on the surface as small as a horse. 

In other words, in seven decades our rockets sped up by a factor of ten, but in little more than half that time our cameras improved by a factor of five thousand. There's no comparison: probe technology is marching to the beat of a faster drummer.

Probes have always offered the advantage of lower cost and minimal risk. For interstellar travel, their smaller size makes them especially practical. For the same energy bill, you could propel a one-ton reconnaissance craft to another star in one-tenth the time of sending even a small clutch of humans. 

Now you might argue that human exploration is qualitatively different than sending mechanical proxies. We humans want to experience the frontier, not just watch it come up on our computer screen. We want to smell it, feel it, and look around.

OK, but what if we could send back all those sensations with a fidelity as good as being there?  That's becoming more and more practical. The bandwidth of a single human eye, recently measured at the University of Pennsylvania medical school, is roughly 2 megabytes per second.  The bandwidth of your ears is much smaller - no more than a few hundred kilobytes per second.  Your fingertips and other parts of your anatomy require even less of a data pipe.

In other words, we could send back everything a human could sense with a telemetry channel of, say, 10 megabytes per second. This is roughly the data rate you'll soon be getting off a blue-ray disk.  It's not trivial to send data at this rate from star to star, but it's a lot easier than sending ourselves.

The technology that can propel us virtually into deep space is quickly outstripping the technology that can propel our protoplasm there. So while we envision our 23rd century descendants cruising the Milky Way in search of other beings or fresh real estate, my guess is that we'll send small robots instead - extensions to our neurons that can be both cheap and expendable. 

To spread our descendants among the nearby worlds of our solar system is more than our destiny: it's an imperative for our future. But galactic exploration will be different: that's something we'll do from the comfort of our own homes. And we won't need the Spandex.