Since the days of Sputnik and Explorer 1, thousands of spacecraft have gone into space, some of them carrying human passengers. The first person to orbit the Earth was the Russian Yuri Gagarin, on April 12, 1961. The first American to do so was John Glenn, who completed three orbits on February 20, 1962; more than 26 years later, at age 77, Glenn returned to space aboard the space shuttle.
Glenn's flight was the first in "Project Mercury" which adapted the Atlas missile. It was followed (in the US) by "Gemini" whose two-man capsules were orbited by the more powerful Titan rocket. Then came the three-man "Apollo" missions, first flying around the Moon and then landing on its surface, on July 20, 1969. Altogether 6 successful landings on the moon were achieved, all using the giant (2700 ton) Saturn V rocket, powered by five huge F-1 rocket engines (and one more on the second stage). One such engine is displayed in the National Air and Space Museum (NASM) of the Smithsonian Institution in Washington, as is John Glenn's capsule, a Moon lander and many other mementos from the early days of spaceflight.
The story of manned spaceflight is fascinating and long, and it still continues to unfold. This short account cannot possibly do it justice, and those who wish to know more are directed to other resources available. Today manned spaceflight is based on the space shuttle, a reusable rocket vehicle with short wings, which allow it to land on a runway like an airplane. The shuttle's rockets use about 700 tons of liquid hydrogen and oxygen, a most effective fuel; in addition it carries two strap-on solid fuel rockets, each weighing about 600 tons.
Reentry
Any manned mission faces the problem of safe return to Earth, which requires getting rid of the huge amount of energy associated with orbital motion. A spacecraft in low Earth orbit moves at about 24 times the velocity of sound. Since the energy of motion ("kinetic energy") is proportional to the square of the velocity v, gram for gram (or ounce for ounce) such a spacecraft carries 242 = 576 times more energy than an object moving at the velocity of sound (in air), e.g. a typical pistol bullet.
Atmospheric friction converts this energy into heat, enough heat than to melt or even evaporate the re-entering material, even if it is a tough metal. To get rid of this heat, the vehicle re-enters the atmosphere at a shallow angle, stretching out the time spent in the more rarefied upper layers. It is also useful to present a blunt obstacle to the air, creating a strong shock front well ahead of the reentry vehicle, which dissipates much of the heat. For this reason the space shuttle starts re-entering the atmosphere bottom-first, and only after almost all its velocity is lost does it turn around to the nose-first attitude of an airplane.
Still, a great amount of heat reaches the spacecraft, requiring its forward-facing part to be lined with heat-resistant material. Mercury, Gemini and Apollo used shields which gradually wore down (ablated), sparing the spacecraft which then landed by parachute. The shuttle's bottom is lined with lightweight heat-resistant tiles of a special material. The Soviet Union built and in 1988 flew its own space shuttle, the Buran, but though its tests were successful, it was not used afterwards.
Picture on right: Wernher Von Braun and one of the F-1 rocket engines, the type that powered the Apollo moonflights. Such an engine is on display at the national Air and Space Museum of the Smithsonian, in Washington, DC.
Unmanned Spacecraft
So great is the variety of unmanned spacecraft that (as with manned spaceflight) it is impossible to cover all of it here. They can be divided into five groups, described separately in files linked to the list below:
- Satellites that observe the Sun, the solar system or the universe (such as the Japanese Yohkoh which observes the Sun, or the Hubble orbiting telescope). Orbiting observatories can cover wavelengths which the atmosphere does not let pass through, such as ultra-violet light or X-rays.
- Satellites which observe the Earth from above, for scientific, military and commercial purposes--such as the ones providing the global pictures of cloud patterns, shown in weather reports on TV.
- Satellites which sample their local environment, for instance, the ones that monitor the radiation belts and the solar wind.
- Satellites used for the benefit of humanity, such as communication satellites and the GPS system used in determining position.
- Spacecraft which are not tied to Earth, but explore other planets and the far reaches of the solar system.
