The thing that makes it hard to go fast is not the energy needed but the mass you need to toss out the back. This is because you need to carry the mass you thro

The thing that makes it hard to go fast is not the energy needed but the mass you need to toss out the back. This is because you need to carry the mass you throw out the back. It adds to your total mass and slows you down. In space generally you carry all that toss out the back mass and once you use it, you can't get more. You can get more energy by using solar panels. Going the speed of light would require very little energy if you could just convert that energy directly into forward direction. Solution: If you are close enough to Earth that you experience drag from the atmosphere, use the atmosphere as the mass you toss. Now you have a really efficient long term way of going fast because you can always get more mass to toss from the atmosphere and you can always get more energy from solar panels. You no longer need to bring mass to toss to orbit and you never run out of mass to toss.