I have to admit, I’m a complete space geek. That’s one reason why I was thrilled to read this week about the Defense Advanced Research Projects Agency’s grand initiative to seek the fundamental science of interstellar travel, with the goal of developing the enabling technologies by 2111, one hundred years from now. This is good stuff. We need it, by God. In the middle of the last century John Kennedy challenged us to land a man on the moon, and that vision sustained America, in many ways, through some dark and divisive days. In this time of credit downgrades, defaults, unemployment, inflation, foreclosures, the snickering of our enemies, and the bickering of our politicians, an initiative that calls us to envision the greatness we might achieve in a century is more than welcome.
That said, I think I would have suggested a more reasonable target than the stars. I grant that “The Stars!” is a far more potent rallying cry than “A One Gravity Constant Acceleration Propulsion System!” Nevertheless, that’s what I would have suggested, and that’s where I believe our energy and resources ought to be focused. The stars may someday be accessible to us, but a high specific impulse one gravity drive would open up the entire solar system as soon as it was available. The math of constant acceleration is a wonderful thing to behold.
Start from a dead stop and begin accelerating at one gravity, or 9.8 meters per second, squared. In 10 seconds you are travelling 98 meters per second. In 30 seconds you’re moving at 294 meters per second, or 17 kilometers per minute. By the end of your first hour of one-gravity acceleration you’ve reached the blistering speed of 35,280 meters per second. That’s more than 126,000 kilometers per hour. If you can keep this rate of acceleration up for a week you’ll be moving at more than 5000 kilometers per second. In about 354 days you’ll hit C, the speed of light in a vacuum. Well, actually you won’t, or maybe you will; we really don’t have any idea. But with a one-g drive we could find out in a year. A few years more could put a probe powered by such a drive in the Alpha Centauri system.
As cool as that sounds, the benefits of a one gravity drive don’t derive from streaking across interstellar space at the speed of light. The benefits lie right here in the solar system. Currently getting around the local neighborhood is extremely difficult for us. The reaction engines we have available produce many times one gravity of acceleration, but can only operate for very short periods at high rates of fuel consumption. That’s good, because we need such engines to get off the ground. Obviously if you light off a one-gravity drive at ground level the thing will just sit there and look imposing, and perhaps hum a bit. But once in space these low specific impulse (fuel inefficient) reaction motors become a liability.
Everything in the galaxy is in orbit around something else, riding around and around the inside of a deep well of space created by the gravitational fields of massive objects. The moon orbits Earth, which orbits the Sun, which orbits the galactic center, which orbits something much vaster that I am unfamiliar with. The center of a cluster or something. A spacecraft, once it is free of Earth, is another orbiting object in this vast dance. But, because it has power, it can do something no other object in the universe can do: it can decide to change its orbit. It can do this by applying forces to add to or subtract from its velocity. To put it simply, if you are in an orbit, and you add velocity, you move to a higher orbit. If you subtract velocity you move to a lower.
Because we can only burn our reaction motors for very short times, a human spacecraft that wants to travel from Earth to Mars, for example, has to use something called a Hohmann Transfer Orbit. To put it simply, because I am incapable of putting it other than simply: the spacecraft begins at Earth, sharing Earth’s solar orbit. It is then accelerated until it is in a higher solar orbit that meets with the orbit of Mars at some point in the future. It then coasts until it reaches the orbit of Mars, and decelerates to match Mars’ orbital velocity. Voila! And it only took five years! Yeah, Hohmann transfers are slow. First, you have to delay even starting until the objects are in relative positions that make the voyage practical. Then you have to boost yourself into a high orbit, switch off the motors, and just ghost along for years.
With a one-gravity constant acceleration drive the trip would be slightly more expeditious. You could calculate a trajectory through the solar gravity field that intercepted the object you wished to reach, taking into account deflection from other gravity fields, and then point yourself in the right direction and light off the drive. You accelerate at 1 G until you’re halfway there, and then turn around and decelerate at 1 G for the remainder of the voyage. The moon would be hours away. Earth to Mars? Something like three days. Mars to Jupiter? Maybe ten. Now that’s cool. Such a drive would instantaneously render the entire solar system open to human exploration and exploitation, and would lead to further advances that might one day actually take us to another star.
Exciting, for sure. Achievable? Make no mistake: a one-gravity constant acceleration drive of high specific impulse is magical fantasy. Perhaps it is slightly less so than a lightspeed drive, a warp drive, a transporter, or a holodeck, but it is fantasy just the same. Still, if we’re going to set a very high, very long term goal, I think this is the one to shoot for. We may very well never get to another star. Even if the challenges of distance, time, and velocity can be overcome, the remaining technical challenges of fuel consumption, heat dissipation, hull integrity, radiation, communication, and a hundred other problems might very well be enough to keep us home.
By contrast, we already know pretty well how to survive and navigate our own solar system and return home safely. We simply lack the power to do it in a reasonable time. If we were to gain that power, it would be of far more immediate benefit to the human race than reaching another star system. It would open the vast resources of every corner of the solar system, and if there is other life to be found in the galaxy, mastery of our own system would set the stage for someday encountering them on more equal terms. The stars might make a more compelling objective, but a one-gravity drive is really the key to a human future in space.