In Greece around 100 BCE, Hero of Alexandria invented a rocket-like device called an aeolipile that used steam as a propulsive gas. Hero mounted a sphere on top of a water kettle. A fire below the kettle turned the water into steam, and the gas traveled through pipes to the sphere. Two L-shaped tubes on opposite sides of the sphere allowed the gas to escape, and in doing so gave a thrust to the sphere, causing it to rotate.

The first rockets to use combustible fuel were exploding bombs and fireworks used in Asia. Rockets were part of warfare in 1232 when the Chinese used rocket-powered arrows against the Mongols during the battle of Kai-Keng. These fire-arrows were a simple form of a solid-propellant rocket. A tube, capped at one end and left open at the other, was filled with gunpowder. The tube was attached to a long stick. When the powder was ignited, the rapid burning of the powder produced fire, smoke and gas that escaped out the open end, producing thrust. The stick acted as a simple guidance system that kept the rocket headed in the general direction of the enemy as it flew through the air.

Between 1609 and 1903, Johannes Kepler, Isaac Newton and Konstantin Tsiolkovsky made revolutionary discoveries that contributed immensely to rocketry and the future of space flight. Major contributions of each scientist are described below.

• In 1609 discovered that the paths of the planets around the Sun were elliptical.
• Determined the relative velocity of a planet as it travels around the Sun as well as a method for determining its period (the amount of time it takes to make one orbit).
• Proved that our Sun was the center of the solar system.

• In 1687 published a book that stated his basic laws of motion that not only govern the movement of objects on Earth, but also objects in space.
• Formulated a universal law of gravitation that explains the orbit of objects around stars and planets.
• Was a pioneer of calculus, which is a type of mathematics critical for space flight (as well as many other applications).

• In 1903 figured out all the basic equations of rocketry. He was a rocket visionary who thought space travel was possible and solved problems that would come up later.
• He drew pictures of rockets and predicted how the Saturn V rocket would operate for the first landing of humans on the moon 65 years later.

The father of modern rocketry is Robert Goddard. He designed, built and launched rockets. Goddard proved Tsiolkovsky's theories on rockets and in 1926 launched the first liquid fueled rocket. He received more than 200 patents for rocket components and design. While his rockets never reached space, his discoveries were used to design the first space-going rockets in the late 1950s.

Another pioneer was Hermann Oberth, who wrote a book, "The Rocket into Interplanetary Space," that influenced the modern world to take rocketry seriously and opened avenues for the exploration of space.

Wernher Von Braun and Oberth launched the first rocket capable of reaching space, called the German V2 rocket, and after World War II, worked in the US to develop the Saturn rockets. His developments enabled the US to eventually go to the moon.

The first spacecraft to leave Earth's atmosphere was the Soviet-built Sputnik. Sputnik 1 was launched on October 4, 1957. During its three-week mission it transmitted radio signals back to Earth that were used to determine the temperature of the spacecraft as well as measure the electron density of the ionosphere (a layer of the atmosphere). The first American spacecraft to reach orbit was the Explorer 1, which was launched on January 31, 1958. Explorer 1 measured the radiation in orbit.

Since the days of Sputnik 1 and Explorer 1, thousands of spacecraft have gone into space. Some of them have carried human passengers. The first person to orbit the Earth was the Russian Yuri Gagarin, on April 12, 1961. The first American to orbit was John Glenn, who completed three orbits on February 20, 1962. Glenn returned to space again at the age of 77 aboard the US space shuttle.

Glenn's flight was one of the manned Mercury Project missions. After Mercury came Gemini whose two-person capsules were launched by the more powerful Titan rocket. Following Gemini were the Apollo missions. On July 20, 1969, Apollo 11 was the first manned mission to the Moon. Altogether, six successful landings on the moon were achieved, all Apollo capsules on top of the giant (2,700 ton or 2,449 metric ton) Saturn V rocket. One such engine is displayed in the National Air and Space Museum of the Smithsonian Institution in Washington, DC, as is John Glenn's Mercury capsule, a moon lander and many other mementos from the early days of spaceflight.

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Today, American manned spaceflight is based on the space shuttle, a re-usable rocket vehicle with short wings that allow it to land on a runway like an airplane. The shuttle's rockets use about 700 tons (635 metric 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 (544 metric tons).

Manned spaceflight has also included various "space stations" intended for long-term residence. In the past, the US flew "Skylab," which was launched in 1973 and burned up during atmospheric re-entry in 1980. The USSR launched space stations of gradually increasing size, the Soyuz, Salyut and (in 1986) the Mir space station. Over the years, Mir was extended by additional modules and was still operating in 2000. It finally re-entered the atmosphere on March 23, 2001, near the Fiji islands in the Pacific Ocean. Its re-entry was observed from the ground and fragments that did not burn up splashed harmlessly into the ocean. Construction of the International Space Station began in November 1998. It was started with the Russian module "Zarya" being placed in orbit and was followed by NASA's "Unity" in December. At the end of 2000, a US-Russian crew was performing science experiments aboard the station.

Any manned mission faces the problem of a safe return to Earth. This requires getting rid of the huge amount of energy associated with orbital motion. A spacecraft in low Earth orbit moves at about 8 km per second. To slow down, the spacecraft uses the friction of the atmosphere. The air hitting the spacecraft slows it down, but the friction also creates an immense amount of heat. Enough heat is generated to melt or even evaporate the re-entering material, even if it is a tough metal. To eliminate this heat, the vehicle re-enters the atmosphere at a shallow angle. 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 that gradually were worn down (ablated), sparing the spacecraft, which then landed by parachute. The shuttle's bottom is lined with lightweight heat-resistant tiles of a special ceramic.

So great is the variety of unmanned spacecraft that (as with manned spaceflight) it is impossible to describe all of it here. Unmanned spacecraft can be divided into five types:

1. Satellites that observe our Sun, the solar system or the universe (such as the Japanese Yohkoh that observes the Sun, or the Hubble orbiting telescope). Orbiting observatories can sense wavelengths that the atmosphere does not let pass through, such as ultra-violet light or X-rays.
2. Satellites that observe the Earth from above, for scientific, military and commercial purposes — such as those that provide global images of cloud patterns for weather reports.
3. Satellites that sample their local environment, for instance, those that monitor the radiation belts and the solar wind.
4. Satellites used for the benefit of humanity, such as communication satellites and the GPS system used for determining position.
5. Spacecraft that are not tied to Earth's gravitational field, but explore other planets and the far reaches of the solar system.

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Rockets are essentially a massive sustained explosion in which the hot gases from the combustion process are directed to create thrust. Of course, it is a very carefully controlled and directed explosion — no small engineering feat! Engineers must design fuel tanks and nozzles that are just the right size, shape and thickness to withstand the pressure of the explosion and accelerate the fuel away. Engineers also design materials and components that can handle the heat and vibrations generated. If their design is too heavy, the rocket may be strong but will not fly; if it is too light, the rocket structure may be too weak and explode.

Rockets move according to the natural behavior described in Newton's third law of motion: for every action there is an equal and opposite reaction. This means that a rocket moves forward by pushing hot gas backwards. The hot gas moving backward is the action, while the rocket moving forward is the equal and opposite reaction. How much power the rocket provides is called "thrust." To create thrust, volatile fuel is placed in a combustion chamber and ignited. When the fuel is ignited, it creates super-heated gas that expands, creating incredible pressure in the combustion chamber. A small opening in the combustion chamber lets the high pressure gasses escape, and a nozzle directs the gas into the desired direction. This is why we see fire and smoke coming from a rocket as it lifts off.

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Other common rocket elements include fins, fuel tanks and nose cones. The fins are small wings that keep a rocket moving straight ahead. The great amount of energy required to leave the Earth's surface comes from fuel stored in huge fuel tanks. The nose cone on the tip of the rocket helps the rocket slice through the air. Without the nose cone, it would be much harder for the rocket to move though the atmosphere on the way to space.

Main components of the space shuttle. click for copyright

On a typical spacecraft, we might also see heat shields, antennae, parachutes, computers, scientific instruments, solar panels and thrusters. The heat shield protects the spacecraft from the heat of re-entry, while parachutes are often employed to ensure a safe landing. Antennae are important for communicating with the ground and sending scientific data. Computers monitor the spacecraft and help the astronauts operate the craft and perform experiments. Thrusters are small rockets that maneuver the spacecraft while it is in space. Solar panels help provide the craft with power; they turn the Sun's rays into electrical power.

The US space shuttle is designed to transport people and cargo to and from orbit around Earth. The main space shuttle components are two solid rocket boosters (contains solid fuel used to push the shuttle into the air), an external tank (holds liquid fuel) and the orbiter (includes a crew cabin for up to seven people; a cargo bay for supplies and equipment, and three engines). A great amount of the space shuttle is devoted to providing fuel. After liftoff, when the solid rocket fuel is gone, the boosters fall back down to Earth. Next, when empty, the external tank comes off. Then, the orbiter reaches space; it is able to return to Earth and land like an airplane. Everything except the external fuel tank is used for another shuttle flight.

Engineers are responsible for all these rocket, spacecraft and shuttle elements. Without them, space travel would not be possible. It is when all these carefully-designed elements work together that we have a successful space mission.