All objects in a star system are in orbit around the star; if they were not, they would have "left" the system or fallen into the star long ago. This implies that one cannot simply point oneself at another planet and fly in that direction, because upon arrival the planet will be moving at an inappropriate relative velocity or may have moved altogether. For instance, if a spacecraft were to start from the Earth and fly to Mars, its final velocity will be close to Earth's orbital velocity which is much higher than that of Mars. This is because any spacecraft starting on a planet is also in orbit around the sun, and a brief glance at the planetary speeds and distances demonstrates that the power of a rocket pales in comparison to the relative speeds of the planets. In order to make interplanetary travel possible, a reduction in the total amount of energy needed to do so is required.
For many years this meant useing the Hohmann transfer orbit. Hohmann demonstrated that the lowest energy transfer between any two orbits is to elongate the orbit so that the other focus lies over the orbit in question. Once the spacecraft arrives, a second application of thrust will re-circularize the orbit at the new location. In the case of planetary transfers this means adjusting the spacecraft, originally in an orbit almost identical to Earth's, such that the outer focus is on the far side of the sun near the orbit of the other planet. A spacecraft traveling from Earth to Mars via this method will arrive near Mars orbit in approximately 18 months, but because the orbital velocity is greater when closer to the center of orbit and slower when farther from the center, the spacecraft will be travelling quite slowly and a small application of thrust is all that is needed. If the maneuver is timed properly, Mars will be "arriving" under the spacecraft when this happens.
The Hohmann transfer applies to any two orbits, not just those with planets involved. For instance it is the most common way to transfer satellites into geostationary orbit, after first being "parked" in low earth orbit. However the Hohmann transfer takes an amount of time similar to 1/2 of the orbital period of the outer orbit, so in the case of the outer planets this is many years – too long to wait. It is also based on the assumption that the points at both ends are massless, as in the case when transferring between two orbits around Earth for instance. With a planet at the destination end of the transfer, calculations become considerably more difficult.
One technique, known as the gravitational slingshot, uses the gravity of the planets to modify the path of the spacecraft without using fuel. In typical example, a spacecraft is sent to a distant planet on a path that is much faster than what the Hohmann transfer would call for. This would typically mean that it would arrive at the planet's orbit and continue past it. However if there is a planet between the departure point and the target, it can be used to bend the path toward the target, and in many cases the overall travel time is greatly reduced. A prime example of this are the two craft of the Voyager program, which used slingshot effects to change trajectories several times in the outer solar system. This method is not easily applicable to Earth-Mars travel however, although it is possible to use other nearby planets such as Venus or even the Moon as slingshots.
Another technique uses the atmosphere of the target planet to slow down. In this case the spacecraft is sent on a high-speed transfer, which would normally mean it would go right past its target upon arrival. By passing into the atmosphere this extra speed is lost, and the amount of energy lost to transport the weight of the required heat shield is considerably less than the weight of the rocket fuel that would be needed to provide the same amount of energy. This concept, known as aerobraking, was first used on the Apollo program wherein the returning spacecraft did not bother to re-enter Earth orbit in a transfer, and instead re-entered immediately at the end of the journey. Similar systems are a included on most basic plans for a manned mission to Mars.
Recent advances in computing have allowed old mathematical solutions to be re-investigated, and have led to a new system for calculating even lower-cost transfers. Paths have been calculated which link the Lagrange points of the various planets into the so-called Interplanetary Superhighway. The transfers on this system are slower than Hohmann transfers, but use even less energy, and are particularily useful for sending spacecraft between the inner planets.
While manned interplanetary travel has not yet been achieved, a trip to Mars is quite feasible with present technologies and could probably be achieved within a decade (at most two) if the funds were made available. NASA's "Design Reference Mission" proposes a Mars exploration program costing $50 billion, but others have made detailed proposals with projected costs much less (see Mars Direct).
NASA's Apollo program landed twelve people on the Moon and returned them to Earth: Apollo 11-17, except 13, i.e. six missions, with each time three astronauts of which two landed on the Moon; Robot probes have been sent to fly past most of the major planets of the Solar system.
The most distant probe spacecrafts Pioneer 10, Pioneer 11, Voyager 1 and Voyager 2 are on course to leave the Solar system, but will cease to function long before reaching the Oort cloud.
Current achivements in interplanetary travel
NASA's Apollo program landed twelve people on the Moon and returned them to Earth: Apollo 11-17, except 13, i.e. six missions, with each time three astronauts of which two landed on the Moon; Robot probes have been sent to fly past most of the major planets of the Solar system. The most distant probe spacecrafts Pioneer 10, Pioneer 11, Voyager 1 and Voyager 2 are on course to leave the Solar system, but will cease to function long before reaching the Oort cloud.