How do spacecrafts obtain the energy they need to operate?

Spacecraft that will operate within the inner solar system are generally equipped

with solar (photovoltaic) panels, which use sunlight to generate electric power.

Those that will spend most of their time in the outer solar system, or leave the

solar system entirely, will carry nuclear reactors to generate electric power.

The electric power runs the instruments, computers, and communications on the

spacecraft. But in most cases, electric power doesn't provide the forces needed for

navigation. Mid-course adjustments of attitude and trajectory are made the old-

fashioned way ... with rocket engines, or thrusters that poof out little puffs of gas.

Both of those run on fuel that's carried aboard the spacecraft at launch. When it's

gone, it's gone.