### Earth to Mars vs. Earth to Venus

Consider three orbits:

- the orbit of Venus about the sun
- the orbit of Earth about the sun
- the orbit of Mars about the sun

Must a spaceship gain energy or lose energy in order to move from an orbit like that of Earth to one like that of Mars?

Must a spaceship gain energy or lose energy in order to move from an orbit like that of Earth to one like that of Venus?

The key here is to think about the fact that the orbits are all about the Sun. To go from Earth to Mars requires an increase in total energy, while to go from Earth to Venus requires a
decrease in total energy.