5. The minimum orbital velocities required for a spacecraft to orbit the Earth, escape Earth's gravitational field, and leave the solar system, are respectively called: the first cosmic velocity is $\qquad$ kilometers/second; the second cosmic velocity is $\qquad$ kilometers/second; the third cosmic velocity is $\qquad$ kilometers/second.
A. $7.8 ; 11.3 ; 16.7$
B. $7.5 ; 11.1 ; 16.4$
C. $7.7 ; 11.4 ; 16.6$
D. $7.9 ; 11.2 ; 16.7$
【Answer】D
【Analysis】The cosmic velocities we often refer to mainly denote the three representative initial speeds of objects launched from Earth, moving within the gravitational fields of celestial bodies. Spacecraft need to achieve one of these three cosmic velocities depending on their mission. Let's look at each of them in turn.

One, First Cosmic Velocity: The first cosmic velocity is also known as the minimum launch speed of a spacecraft, the maximum operational speed of a spacecraft, and the orbital speed. It refers to the speed at which an object can move in a circular orbit just above the Earth's surface, and it is also the minimum launch speed for artificial Earth satellites. According to mechanical theory, the first cosmic velocity is calculated to be 7.9 kilometers per second. In reality, the Earth's surface has a dense atmosphere, so spacecraft cannot orbit the Earth at such a low altitude. They need to be at a flight altitude of 150 kilometers to orbit the Earth in a circular path. Here, it is particularly important to remember: the first cosmic velocity is approximately 7.9 kilometers per second.

Two, Second Cosmic Velocity: The minimum speed required for a man-made object to escape Earth's gravitational pull. When an object reaches a flight speed of 11.2 kilometers per second, it can break free from Earth's gravitational pull, leave Earth, and enter an orbit around the Sun, no longer orbiting Earth. This minimum speed to escape Earth's gravity is the second cosmic velocity. The initial flight speeds of various planetary probes are all higher than the second cosmic velocity. The second cosmic velocity is approximately 11.2 kilometers per second.

Three, Third Cosmic Velocity: The minimum speed required to launch a spacecraft from the Earth's surface to escape the solar system and venture into the vast Milky Way is called the third cosmic velocity. According to mechanical theory, the third cosmic velocity is calculated to be approximately 16.7 kilometers per second. It is important to note that this value is calculated assuming the spacecraft's entry speed is in the same direction as Earth's orbital speed; if the direction is different, the required speed will be greater than 16.7 kilometers per second. In short, the speed of the spacecraft is the only factor that can break free from the gravitational pull of Earth and even the Sun, and currently, only rockets can achieve this cosmic velocity.

Therefore, let's summarize the key points: The speed that allows an object to orbit the Earth in a circular path is called the first cosmic velocity, approximately 7.9 kilometers per second; the speed to escape Earth's gravitational pull and leave Earth is called the second cosmic velocity, approximately 11.2 kilometers per second; and the speed to escape the Sun's gravitational pull and leave the solar system is called the third cosmic velocity, approximately 16.7 kilometers per second.