Traveling to distant worlds


The discovery of an Earth-sized planet orbiting the closest star from our sun has already faded from the news cycle. That’s probably for the best.

The discovery immediately sparked premature speculation about the possibility of life on the planet, which has been dubbed Proxima Bb. The planet is, after all, just a bit larger than Earth. It orbits in the so-called habitable zone, the distance from the star in which water can remain a liquid, a necessity for life so far as our single specimen – our own planet – seems to tell us.

Never mind that Proxima Bb is only four million miles from its rather weak star. Never mind that the star is a flare star that would every few hours irradiate its planet with deadly doses of X-rays and perhaps blow any atmosphere, another condition for life, right off the planet. Never mind that Proxima Bb might not ever have an atmosphere and might not fulfill the many other conditions that produced life on Earth.

The star, Proxima Centauri, may be the closest star to our sun and Earth, but it will be years and perhaps decades before we find out if Proxima Bb has an atmosphere or liquid water or all the rest. In fact, the necessary conditions for life are the purest speculation on our part. Scientists, of course, do their best.

However, a single sample — our own planet Earth — doesn’t make for intelligent proof in the same way that a single hot summer doesn’t prove global warming. Climatologists must study many years of climatological before they can prove climate change. In the same way, Astro-biologists really ought to see at least one other sample of life on another planet before they can intelligently speculate.

Research is difficult at such extreme distances. Astronomers need gigantic telescopes, preferably orbiting outside of earth’s atmosphere, and observational technologies that have yet to be developed. All of that will be expensive and difficult to justify given the current needs of our nation and world. The launch in 2018 of the much-delayed Webb Space Telescope will be a step in the right direction, but it is only a step and an extraordinarily expensive one, at that.

However, telescopes can take us only so far. In effect, one has to go and look. Eventually, we will dig in the soil of Mars looking for some kind of life. And eventually, I suppose, we will send a robotic space probe to distant Proxima Bb.

That imperative has caused some people to suggest that we should at least start planning for an interstellar journey to the environs of Proxima Centauri. It is, after all, the closest star to us besides, of course, the sun.

To understand the difficulty of traveling even to the closest of extra-solar stars, we must imagine the unimaginable distance involved.

On the surface, things don’t seem so daunting. Proxima Centauri is only 4.25 light years away, a scant 25 trillion miles. However, consider that Pluto is only 12.6 light hours away, about 4.6 billion miles. Recently, the New Horizons space probe flew past Pluto and it took a bit under a decade to get there. Proxima is about 8,000 times farther away. Given the current state of the technology, the trip would not just take many years. It would take many lifetimes.

Consider, for example, the fastest outward-bound spacecraft humans have ever sent outside our solar system. Voyager 1 has covered 1/600th of a light-year in 30-year odyssey. It currently pokes along at 1/18,000 the speed of light, or over 37,000 miles per hour. At that rate, a journey to Proxima Centauri would take around 80,000 years – if it were pointed in that direction, which it isn’t.

Obviously, such a trip requires much faster velocities and an enormous amount of fuel to accelerate the spacecraft to ungodly speeds. But fuel adds mass, which requires even more fuel, which adds even more mass. It’s no accident that the Apollo spacecraft was a tiny capsule on top of an enormous set of fuel tanks.

Building such a spacecraft in orbit, where considerably less fuel will be required to lift the interstellar craft out of orbit, can solve some of the problem. However, the problem of acceleration to speeds of millions of miles per hour remains. Add to that problem the energy it would take to slow the spacecraft down to a velocity where it could study a star or extra-solar planet. There’s really no point in zipping past a star in the blink of an eye. Spend a century getting someplace, and you’d like to stick around a while.

Over the years, scientists and science fictionists have proposed various solutions to the fuel problem. Spacecraft could achieve velocities of perhaps 10 percent of the speed of light by controlled explosions of on-board nuclear bombs. Alternatively, they could deploy giant light sails powered by their own powerful lasers. Star-Trek style anti-matter drives have even been proposed. Using such methods, the journey might take only a century or so.

My personal favorite is the “ramjet” design proposed by Robert W. Bussard. He would use a giant scoop to collect stray hydrogen atoms from the interstellar medium. The hydrogen would be used to fuel an on-the-fly hydrogen-fusion reaction. In theory, velocities approaching the speed of light could be achieved, and the trip would be lengthened only a bit by the time it took to accelerate and decelerate to maximum velocity. We are therefore talking decades and not centuries for such a trip.

However, all the proposals require technology that doesn’t exist yet and will take a long, expensive while to develop.

Even if the fuel and velocity problems are solved, a more significant problem remains. Space may seem empty by our standards, but even in the vast emptiness of the space between the stars, a few specks of dust remain. From the point of view of a speck of dust, a spacecraft might be approaching it at several million miles per hour. From the point of view of the spacecraft, the dust particle is moving toward it with the power to rip a fist-sized hole all the way through it. Goodbye spaceship.

Thus, it’s probably a bit premature to expect such a trip in our – or our children’s or their children’s – lifetimes. Many more generations would pass before we saw any results from such a journey. But we can dream, can’t we?


Tom Burns is director of the Perkins Observatory in Delaware.

Tom Burns is director of the Perkins Observatory in Delaware.