Interplanetary spaceflight or interplanetary travel is travel between planets within a single planetary system. In practice, spaceflights of this type are confined to travel between the planets of the Solar System.
Current achievements in interplanetary travel
Remotely guided space probes have flown by all of the planets of the Solar system from Mercury to Neptune. The four most distant spacecraft (Pioneer 10, Pioneer 11, Voyager 1 and Voyager 2) are on course to leave the Solar System.
Space probes have also been inserted into orbit around the planets Venus, Mars, Jupiter, and Saturn, and have returned data about these bodies and their natural satellites. Further probes are currently en route to orbit Mercury, the dwarf planets Ceres and Pluto as well as the large asteroid Vesta.
Remotely controlled landers such as Viking, Pathfinder and the two Mars Exploration Rovers have landed on the surface of Mars and several Venera and Vega spacecraft have landed on the surface of Venus. The NEAR Shoemaker orbiter successfully landed on the asteroid 433 Eros, even though it was not designed with this maneuver in mind. The Huygens probe successfully landed on Saturn's moon, Titan.
Reasons for interplanetary travel
The costs and risks of interplanetary travel receive a lot of publicity — spectacular examples include the malfunctions or complete failures of unmanned probes such as Mars 96, Deep Space 2 and Beagle 2 (the article List of Solar System probes gives a full list).
Many astronomers, geologists and biologists believe that exploration of the solar system provides knowledge that could not be gained by observations from Earth's surface or from orbit around Earth. But they disagree about whether manned missions make a useful scientific contribution — some think robotic probes are cheaper and safer, while others argue that either astronauts advised by Earth-based scientists, or spacefaring scientists advised by Earth-based astronauts, can respond more flexibly and intelligently to new or unexpected features of the region they are exploring.
Those who pay for such missions (primarily in the public sector) are more likely to be interested in benefits for themselves or for the human race as a whole. So far the only benefits of this type have been "spin-off" technologies which were developed for space missions and then were found to be at least as useful in other activities (NASA publicizes spin-offs from its activities).
Other practical motivations for interplanetary travel are more speculative, because our current technologies are not yet advanced enough to support test projects. But science fiction writers have a fairly good track record in predicting future technologies — for example geosynchronous communications satellites (Arthur C. Clarke) and many aspects of computer technology (Mack Reynolds).
Many science fiction stories (notably Ben Bova's Grand Tour stories) feature detailed descriptions of how people could extract minerals from asteroids and energy from sources including orbital solar panels (unhampered by clouds) and the very strong magnetic field of Jupiter. Some point out that such techniques may be the only way to provide rising standards of living without being stopped by pollution or by depletion of Earth's resources (for example peak oil).
Finally, colonizing other parts of the solar system would prevent the whole human species from being exterminated by an asteroid impact like the one which may have resulted in the Cretaceous–Tertiary extinction event. Although various Spaceguard projects monitor the solar system for objects that might come dangerously close to Earth, current asteroid deflection strategies are crude and untested. To make the task more difficult, carbonaceous chondrites are rather sooty and therefore very hard to detect. Although carbonaceous chondrites are thought to be rare, some are very large and the suspected "dinosaur-killer" may have been a carbonaceous chondrite.
Some scientists, including members of the Space Studies Institute, argue that the vast majority of mankind eventually will live in space and will benefit from doing this.
Economical travel techniques
Interplanetary travel has to solve two problems, other than escaping from the planet of origin:
Doing this by brute force - accelerating in the shortest route to the destination and then, if it is farther from the sun, decelerating to match the planet's speed - would require an extremely large amount of fuel. And the fuel required for deceleration and velocity-matching has to be launched along with the payload, and therefore even more fuel is needed in the acceleration phase.
The change in speed (delta-v) required to match velocity with another planet is surprisingly large. For example Venus orbits about 5.2 km/s faster than Earth and Mars orbits about 5.7 km/s slower. To put these figures in perspective, Earth's escape velocity is about 11.2 km/second (it varies slightly depending on the launch direction). So matching a space shuttle's velocity with that of Venus or Mars would require a significant percentage of the energy which is used to launch a shuttle from Earth's surface.