How do spacecrafts obtain the energy they need to operate as they travel through the solar system?

Answer 1

The most common way is to use a Radioactive Thermoelectric Generator (RTG). As you may know, thermoelectric devices allow a small amount of electric power to be produced using thermal energy from a source of energy. In fact they are just developments of the simple thermocouple used to measure temperature electronically. For space use, where the device has to operate perhaps for some years without any fuel being supplied, radioisotopes offer one way to provide this heat source. The most commonly used source is plutonium 238, which gives off alpha radiation and is therefore easy to shield. The following extract from Wikipedia gives a summary: the full article is given as a link below.

A common application of RTGs is as power sources on spacecraft. Systems Nuclear Auxiliary Power Program (SNAP) units were used especially for probes that travel far enough from the Sun that solar panels are no longer viable. As such they are used with Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, Galileo, Ulysses, Cassini and New Horizons. In addition, RTGs were used to power the two Viking landers and for the scientific experiments left on the Moon by the crews of Apollo 12 through 17 (SNAP 27s), excluding Apollo 13 because that mission never reached the Moon's surface. RTGs were also used for the Nimbus, Transit and Les satellites. There have also been some nuclear reactors put into space, both US (SNAP-10A) and Russian (RORSAT), but these can now be considered historical, I don't think there is any current use. Links to articles are given below.

Answer 2

Mainly solar I think, but also fuel cells.

Answer 3

The spacecraft have solar panels to use the Sun's energy. Spacecraft may also get energy from small nuclear powered generators. These are used when the spacecraft has to go far from the Sun.