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科技英语阅读文章:星系旅行

2011年12月01日  所属:科技英语  来源:外语教育网  作者:佚名

科技英语阅读文章练习:星系旅行是指在给定的行星系统内在不同星体间的旅行。

星系旅行是指在给定的行星系统内在不同星体间的旅行。

By definition, interplanetary travel is travel between bodies in a given star system.

Current achievements 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, each with 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 spacecraft 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.

Robot landers such as Viking and Pathfinder have already 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.

Orbital mechanics of interplanetary travel

To date, the only form of spacecraft propulsion used for interplanetary missions is the chemical rocket engine. The limitations of this engine dictate the trajectories and travel times required for interplanetary travel.

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 chemical 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 using the Hohmann transfer orbit. Hohmann demonstrated that the lowest energy transfer between any two orbits is to elongate the orbit so that its apogee 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 apogee 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 (ie. the Sun) 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.

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