The ideal interstellar propulsion system would be one that could get you to other stars as quickly and comfortably as envisioned in science fiction. Before this can become a reality, three scientific breakthroughs are needed: discovery of a means to exceed light speed, discovery of a means to propel a vehicle without propellant, and discovery of a means to power such devices. Why? - Because space is big, really, really, really big.
To help put this in perspective, consider that it takes light over 8 minutes to cover that 4 ft "Astronomical Unit" mentioned before. Light is the fastest thing that we know to exist! Just imagine...How long will take you to travel 210 miles if it takes you over 8 minutes to travel just 4 feet? Actually, the answer is 4.2 years. Our nearest neighboring star, Proxima Centauri, is 4.2 Light Years away.
The Voyager spacecraft left the solar system at 37,000 miles per hour. At that speed, it would take Voyager 80,000 years to reach Proxima Centauri.
The most obvious challenge to practical interstellar travel is speed. Our nearest neighboring star is 4.2 Light Years away. Trip times to reach our nearest neighboring star at conventional speeds would be prohibitively long. At 55 miles-per-hour for example, it would take over 50 million years to get there! I don’t think even the twinkies in the glove box would survive that long. At a more typical spacecraft speed, for example the 3-day trip time that it took the Apollo spacecraft to reach the moon, it would still take over 900 thousand years. I still don’t think the twinkies will make it. And even if we consider the staggering speed of 37-thousand miles-per-hour, which was the speed of the NASA Voyager spacecraft as it left our solar system years ago, the trip would still take 80,000 years. Maybe the twinkies would make it, but there would be nothing left on board to eat them. In conclusion, if we want to cruise to other stars within comfortable and fundable time spans (say, less than a term in Congress), we have to figure out a way to go faster than light.
This chart highlights two critical features of a rocket, Thrust and Specific Impulse. Thrust is how much push a rocket can give. The higher up on the chart, the greater the push.
Specific Impulse can be thought of as a kind of fuel efficiency for rocket engines, analogous to the miles-per-gallon for cars. The farther right on the chart, the less propellant you’ll need. It really has to do with how fast the fuel blasts out of the rocket.
What you should notice is the red region. This is the range of rocket performance we can conceivably create with what we know today. And what we need for interstellar travel is in that desired region or even more fuel efficient.
Here are four examples [large graphic] of what it would take to send a canister about the size of a Shuttle payload (or a school bus) past our nearest neighboring star...and allowing 900 years for it to make this journey.
Well....If you use chemical engines like those that are on the Shuttle, well..., sorry, there isn’t enough mass in the universe to supply the rocket propellant you’d need.
So let’s step up to next possibilities, nuclear rockets with a predicted performance that’s 10 to 20 times better!
Well...it’s still not looking all that good. For a fission rocket you would need a BILLION SUPERTANKER size propellant tanks to get you there, and even with fusion rockets you would still need a THOUSAND SUPERTANKERS!
Even if we look at the best conceivable performance that we could engineer based on today’s knowledge, say an Ion engine or an antimatter rocket whose performance was 100 times better that the shuttle engines, we would need about ten railway tanker sized propellant tanks.
That doesn’t sound too bad, until you consider that we didn’t bring along any propellant to let us stop when we get to the other star system...or if we want to get there quicker than 9 centuries.
Once you add the desire to actually stop at your destination, or if you want to get there sooner, you’re back at the incredible supertanker situation again, even for our best conceivable rockets.
In conclusion, we’d really like to have a form of propulsion that doesn’t need any propellant! This implies the need to find some way to modify gravitational or inertial forces or to find some means to push against the very structure of spacetime itself.