In my book 'Interstellar Travel:An astronomer's guide' I discuss some of the modern and planned developments in fast space travel. For the most part, it's all about speed!

Using the kinds of chemical-based propulsion systems we have today, and taking advantage of a 'gravitational slingshot' from Jupiter, we could probably get up to 150,000 miles per hour. The Galileo probe managed to get to about 106,000 miles per hour (18 km/sec) and currently holds the record for the fastest speed ever achieved by an artificial body. Alpha Centauri is about 4.3 light years from Earth or 40 trillion kilometers. At this speed it would take 72,000 years to get there, not including slowing down to entering the system.

Rocket designers have been studying ion propulsion since the 1950s, and mention of the technology often turns up in works of science fiction. Ion propulsion was featured in a September 1968 episode of Star Trek called "Spock's Brain," in which invaders steal Spock's brain and flee in an ion-powered spacecraft. The same technology is used intermittently for altitude control aboard 11 Hughes-built communications satellites in geosynchronous orbit 22,300 miles (35,885 kilometers) above Earth. The above photo shows the NASA NEXT ion engine firing at peak power during 2007 testing at NASA’s Glenn Research Center(Image: NASA).

Deep Space 1 is the first spacecraft to use it as a primary means of propulsion. Instead of the fiery thrust produced by typical rockets, an ion engine emits only an eerie blue glow as electrically charged atoms of xenon are pushed out of the engine. Xenon is the same gas found in photo flash bulbs and lighthouse search lamps. Acceleration with patience In the engine, each xenon atom is stripped of an electron, leaving an electrically charged particle called an ion. Those ions are then jolted by electricity that is produced by the probe's solar panels and accelerated at high speeds as they shoot out from the engine. That produces thrust for the probe. The ions travel out into space at 68,000 miles (109,430 kilometers) per hour. But Deep Space 1 doesn’t move that fast in the other direction because it is much heavier than the ions. Its cruising speed is closer to 33,000 miles (53,100 kilometers) per hour.

The thrust itself is amazingly light -- about the force felt by a sheet of paper on the palm of your hand. "If you want a mission in which you want to reach your destination in a hurry or accelerate quickly, ion propulsion's not for you," Rayman said. "It takes four days to go from zero to 60 (miles per hour). I like to say it's acceleration with patience."

But once ion propulsion gets going, nothing compares to its acceleration. Over the long haul, it can deliver 10 times as much thrust-per-pound of fuel as more traditional rockets. Each day the thrust adds 15 to 20 miles (25 to 32 kilometers) per hour to the spacecraft's speed. By the end of Deep Space 1's mission, the ion engine will have changed its speed by 6,800 miles (11,000 kilometers) per hour.

Using currention drive technology, with 10 times DS-1s acceleration of 25 km/hr per day, you you could reach 1/2 the speed of light (540 million km/hr) in about 4900 years. For our journey to Alpha Centauri, you would accelerate for half the trip, turn around and decellerate for the second half. A bit of math shows what you get for travel time:

Distance to turn around = 2.1 light years or 20 trillion km 10x DHS acceleration is 250 km/hr/day or 8x10-7 km/sec/sec. At this acceleration, distance = 1/2 acceleration x Time^2 so T = 228 years. You then turn the ship around and decelerate for another 228 years for a total trip time of 456 years! What is interesting is that this same ion rocket technology will let us travel to Mars in 270 days, so this isnt really pushing the technology envelope very hard! If we could get to mars in 30 days, then the same technology would get us to Alpha Centauri in about 50 years or a single human lifetime!

If you wanted to make the trip to Alpha Centauri in, say, 30 years, you would reach the half-way distance of 2.1 light years in 15 years. An acceleration rate, A, of 17 kms/sec per day would be needed. This is about 1000 times faster than Deep Space 1. At this pace, you would be traveling as fast as the solar wind (500 km/sec) in about one month, and would pass the orbit of pluto in 100 days traveling at a speed of 1700 km/sec.

Nuclear rocket technology can make the trip even faster because the exhaust speeds can approach 20% the speed of light or higher. At these speeds, a trip to Alpha Centauri would take less than 25 years! Here is one design for shch a nuclear-thermal rocket for interplanetary travel

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