When it comes to interplanetary travel, it's essential to minimize the mass of propellant needed by the spacecraft and its launch vehicle. Interplanetary space flight, or interplanetary travel, is the journey between stars and planets, usually within a single planetary system. In practice, this type of space flight is limited to traveling between the planets of the Solar System. Unmanned probes have been sent to all the observed planets in the Solar System, as well as to dwarf planets Pluto and Ceres, and to several asteroids.
Orbiters and landers provide more information than flyby missions. Manned flights have landed on the Moon and have been planned for Mars, Venus and Mercury. While many scientists recognize the value of unmanned flights, the value of manned missions is more controversial. Science fiction writers suggest a number of benefits, such as extracting asteroids, accessing solar energy and space for colonization in case of a terrestrial disaster. To understand why hydraulic electric rockets offer revolutionary prospects for space travel, let's consider the challenge faced by the designer of a 500-day round trip to the Martian moons.
To go back and forth, every kilogram of mass - including the crew, their life support and their spaceship - needs many kilograms of rocket fuel. Therefore, mission planners must reduce mass as much as possible, often compromising the robustness of life support systems and minimizing the amount of drinking water and food. Cut too much and you'll endanger your crew; cut too little and you'll weigh too much to go anywhere. For a 500-day mission to Mars, each astronaut can have only about 5 tons of water for drinking, bathing and cleaning.
A very short shower can be a weekly luxury. So, great; we currently have working interstellar space probes. Except they're not going anywhere very fast. Each of these intrepid interstellar explorers is traveling tens of thousands of miles per hour - which sounds pretty fast - but they are not headed in the direction of any particular star because their missions were designed to explore planets within the solar system. If any of these spaceships were to head to our closest neighbor Proxima Centauri, just 4 light-years away, they would reach it in about 80,000 years. Scientists expect to find a wide range of organic compounds in some of the planets, moons and comets in the outer Solar System, with a wide range of possible uses.
At the dawn of the space age, many experts predicted that robust and durable spaceships like Ernst Stuhlinger's Cosmic Butterfly would one day travel between planets like any other maritime vessel. However, if there is a planet between the starting point and the goal, it can be used to bend the path to the goal and in many cases reduce total travel time. Travel techniques must take into account changes in speed necessary to travel from one body to another in the Solar System. The Sun's gravity affects speed; when closer to its center it is faster (n) and slower when farther away. This means that when approaching Mars, a spacecraft will travel quite slowly and only a small push application will be needed to put it into a circular orbit around Mars. Fusion rockets are considered a likely source of interplanetary transport for a planetary civilization (see article on spacecraft propulsion).
These thrusters could be stored in orbit in a fuel tank or transported to orbit in a fuel tanker truck for direct transfer to an interplanetary spacecraft. In addition to making travel faster or cheaper, these improvements could also allow for greater safety margins in design by reducing the need to make spaceships lighter. In the case of planetary transfers, this means directing the spacecraft originally in an orbit almost identical to that of Earth so that its aphelion is on the opposite side of Sun close to orbit of other planet. Water exists in even greater abundance farther from Sun making up much of crusts of many dwarf planets, moons and asteroids and even occasionally forming underground oceans. In general, planetary orbiters and landers return much more detailed and complete information than flyby missions.
