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Interplanetary spaceflight

By Wikipedia,
the free encyclopedia,

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.

No manned missions have been sent to any other planet of the Solar System. NASA's Apollo program, however, landed twelve people on the Moon and returned them to Earth.

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:

  • The planet from which the spaceship starts is moving round the sun at a different speed than the planet to which the spaceship is traveling, because the two planets are at different distances from the sun. So as it approaches its destination, the spaceship must decrease its speed if the destination is closer to the sun, or increase its speed if the destination is further away (assuming a Hohmann transfer orbit).
  • If the destination is farther away, the spaceship must lift itself "up" against the force of the sun's gravity.

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.

Hohmann transfers

For many years economical interplanetary travel meant using the Hohmann transfer orbit. Hohmann demonstrated that the lowest energy route between any two orbits is an elliptical "orbit" which forms a tangent to the starting and destination orbits. 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 directing the spacecraft, originally in an orbit almost identical to Earth's, so that the apogee of the transfer orbit 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 mass (i.e. the Sun) and slower when farther from the center, the spacecraft will be traveling quite slowly and a small application of thrust is all that is needed to put it into a circular obit around Mars. If the manoeuver 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 ½ 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.

Gravitational slingshot

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Published - July 2009

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