New energy sources are needed, when going to outer space. Solar panels are not that effective at Mars as they are on Earth. At Jupiter it's even worse: you almost can't use them there at all. You will also need a stronger source of energy, when you want to go faster. So increasing speed will most likeley need better energy sources. And when you want to go with a manned spacecraft, you will even need more energy, as you will have life support, etc. So the point is: we need new, stronger sources of energy - maybe independent from the sunlight.