单项选择题
| Placing a satellite into orbit requires
a tremendous amount of energy, which is achieved from multistage rockets burning
chemical fuels. The first stage of a multistage rocket consists of rocket engines that provide a huge amount of force. The first stage lifts the entire launch vehicle--with its load of fuel, the rocket body, and the satellite--off the launch pad (发射台) and into the first part of the flight. After its engines use all their fuel, the first stage portion of the rocket separates from the rest of the launch vehicle and falls to the earth. The second stage then ignites, providing the energy necessary to lift the satellite into orbit. It, too, then separates from the satellite and any remaining rocket stages. The rest of the launch depends on the satellite’s mission. And a third rocket stage provides the energy to lift the satellite to its final orbital altitude. After the satellite has reached the final altitude, another rocket engine fires and gives the satellite a circular orbit. All rocket-engine burns occur at a precise moment and last for a precise amount of time so that the satellite achieves its proper position in space. In 1990 the United States began launching some satellites from aircraft flying at high altitudes. This method still requires a rocket-powered launch vehicle, but because the vehicle does not have to overcome friction with the thick atmosphere found at low altitudes, much less fuel is needed. However, the size of the rocket is limited by the size and strength of the aircraft, so only smaller satellites can be launched this way. |