Final answer:
To be 'beyond Earth's gravity', a spacecraft needs to achieve escape speed of about 11 kilometers per second. Once escaped, the spacecraft will coast to its target, following orbits around the Sun that are only modified when passing near planets.
Gravity-assisted encounters can redirect spacecraft to a second target, as seen with Voyager 2 visiting Jupiter, Saturn, Uranus, and Neptune.
Step-by-step explanation:
Beyond Earth's gravity, a spacecraft must achieve escape speed, which is about 11 kilometers per second (about 25,000 miles per hour). Once the spacecraft escapes Earth, it will coast to its target, following orbits around the Sun that are modified only when it passes near one of the planets.
In interplanetary flight, spacecraft are deflected by the gravitational force of a planet, either gaining or losing energy in the process. This can be used to redirect a spacecraft to a second target. For example, Voyager 2 used gravity-assisted encounters with Jupiter, Saturn, Uranus, and Neptune to reach its ultimate goal of exploring Jupiter.
To explore the outer solar system, spacecraft must be highly reliable and autonomous, as flight times are measured in years to decades. They also require their own power sources and heaters to stay operational in the extreme conditions of outer space.