Interplanetary space flight, or interplanetary travel, is the journey with or without crew between stars and planets, generally within a single planetary system. In practice, space flights of this type are limited to traveling between the planets of the Solar System. Unmanned space probes have explored all the observed planets in the Solar System, as well as to the dwarf planets Pluto and Ceres, and to several asteroids. Orbiters and landers return more information than flyby missions.
Manned flights have landed on the Moon and have been planned, from time to time, for Mars, Venus and Mercury. The most efficient way to send spaceships between planets is called a Hohmann transfer orbit, or lowest energy orbit. This type of orbit is placed in an irregular orbit around the Sun, with one part that is lower or closer to the Sun than the other. To enter orbit around a planet, the spacecraft needs to reach it traveling slowly enough relative to its speed to be captured by gravity.
To slow down from its original speed in the vacuum of space is quite complicated. Long-range exploratory spacecraft, such as Voyager 2, use rocket force and gravity to reach their destinations. By using only rockets, spaceships are limited to visiting planets close to Earth (such as Mars or Jupiter) due to fuel limitations. To visit more distant planets, spaceships use the gravity assistance method to accelerate and change their course.
Voyager 2 used the energy of a rocket to reach Jupiter, but then explored Saturn, Uranus and Neptune using the gravity of each planet it visited to launch it to the next planet and beyond. The gravitational slingshot technique uses the gravity of planets and moons to change the speed and direction of a spacecraft without using fuel. This allows us to give or receive orbital energy to or from the planet and change the speed of the spaceship without using fuel. By passing through Mercury several times, in addition to other flybys of the inner planets, the spacecraft lost enough energy to enter orbit around the planet, which allowed scientific studies that would otherwise be impossible to carry out scientific studies. Even today, there is still no perfect formula for finding the best way for a spaceship to reach a planet through flybys.
Several technologies have been proposed that save fuel and allow you to travel significantly faster than the traditional methodology of using Hohmann transfers. If a spaceship sent from Earth is aimed at planets in the outer Solar System, such as Jupiter or Saturn, it needs to increase its distance from the Sun. At the dawn of the space age, many experts predicted that robust and durable spaceships, such as Ernst Stuhlinger's Cosmic Butterfly, would one day travel between planets like any maritime ship. Unlike what happens with interstellar travel, the barriers that prevent rapid interplanetary travel have to do with engineering and economics, rather than with basic physics. Earth is on average ten times closer to Mercury than to Jupiter, but missions to both planets can take a similar amount of time to reach their scientific orbits. The fuel needed to produce these changes in speed must be launched together with the payload and, therefore, even more fuel is needed to put both the spacecraft and the fuel needed for its interplanetary journey into orbit.
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