Interstellar travel is extraordinarily difficult because of the enormous energy required to send spacecraft across the vast distance between stars in a reasonable amount of time. Current spacecraft leaving the solar system - Voyagers 1 & 2, Pioneer 10 & 11, and New Horizons - will take thousands of years to travel the distance to the nearest star. Solar sails can, on sunlight alone, accelerate out of the solar system. They could also be pushed out by giant solar powered lasers or microwave beams. Sail and laser/microwave technology will need to advance before high speed missions are possible, but the scientific and technological principles are well understood.