New Horizons is currently making 15.73 kilometers per second on its way to a Pluto/Charon flyby in July of 2015, impressive but not the kind of speed that would get us to interstellar probe territory. Interestingly, the fastest spacecraft ever built wasn’t headed out of the Solar System at all, but in toward the Sun.
End of quote. Now let’s analyze this in terms of interstellar travel. The best candidate for possible habitable planets we know nearby are part of the TRAPPIST-1 system, which has 3 planets squarely in “the habitable zone.”
This system is very, very close compared to the rest of Milky Way and the universe: Only a mere 40 light years.
Gravity swings work great inside a solar system. Out in deep space between stars with no other planets or bodies to gravity assist, you’re just going be traveling at the fastest gravity assist you could accomplish before leaving our solar system. That would be about 15.73 km/s.
Since a light year is about 5.88 trillion miles, or about 9.5 trillion killometers and one trillion equals one thousand billion, and one billion equals one thousand million, to travel 40 light years at 15.73 kilometers would take awhile. 15.73 km per second is 56,628 km per hour. So if we divide 9.5 trillion by 56,628 km it will tell us how many hours it would take to reach TRAPPIST-1. In this way, dividing that number by 24 will tell you that it would take 1,359,072 days to reach there, or about 3,723 years.
One way. To transmit back to Earth what you found would take another 40 years at the speed of light. Would anyone still be listening?
In other words, going as fast as we can, it would take three thousand seven-hundred twenty-three years to reach that system, what to speak of any other stars further away.
We’re not going to ever go there, and nobody from space is ever going to come here.