High School Earth Science/Early Space Exploration

Humans have long dreamed of traveling into space. Greek mythology tells of Daedelus and Icarus, a father and son who took flight using wings made of feathers and wax. Daedelus warned his son not to fly too close to the sun, but Icarus, thrilled with the feel of flying, drifted higher and higher. When he got too close to the Sun, the wax melted, and Icarus fell into the sea. This myth is often interpreted to be about foolishness or excessive pride, but we can also relate to the excitement Icarus would have felt. Much later, science fiction writers, such as Jules Verne (1828–1905) and H.G. Wells (1866–1946), wrote about technologies that might make the dream of traveling beyond Earth into space possible.

Lesson Objectives

  • Explain how a rocket works.
  • Describe different types of satellites.
  • Outline major events in early space exploration, including the Space Race.



Humans did not reach space until the second half of the 20th century. However, the main technology that makes space exploration possible, the rocket has been around for a long time. A rocket is a device propelled by particles flying out of it at high speed. We do not know exactly who built the first rocket, or when, but there are records of the Chinese using rockets in war against the Mongols as early as the 13th century. The Mongols, in turn, spread rocket technology in their attacks on Eastern Europe. Early rockets were also used to launch fireworks and for other ceremonial purposes.

How Rockets Work

Figure 23.16: When the skateboarder pushes against the wall, the wall exerts an equal force on the skateboarder in the opposite direction.

Rockets were used for centuries before anyone could explain exactly how they work. The theory to explain this did not arrive until 1687, when Isaac Newton (1643–1727) described three basic laws of motion, now referred to as Newton's Laws of Motion:

  1. An object in motion will remain in motion unless acted upon by a net force.
  2. Force equals mass multiplied by acceleration.
  3. To every action, there is an equal and opposite reaction.

Newton's third law of motion is particularly useful in explaining how a rocket works. To better understand this law, consider the skateboarder in Figure 23.16. When the skateboarder pushes the wall, the skater's force—the "action"—is matched by an equal force by the wall on the skateboarder in the opposite direction—the reaction.

Once the skateboarder is moving, however, he has nothing to push against. Imagine now that the skateboarder is holding a fire extinguisher. When he pulls the trigger on the extinguisher, a fluid or powder flies out of the extinguisher, and he moves backward. In this case, the action force is the pressure pushing the material out of the extinguisher. The reaction force of the material against the extinguisher pushes the skateboarder backward.

Figure 23.17: Explosions in a chamber create pressure that pushes gases out of a rocket. This in turn produces thrust that pushes the rocket forward. The rocket shown here is a Saturn V rocket, used for the Apollo 11 mission––the first to carry humans to the Moon.

For a long time, many believed that a rocket wouldn't work in space because there would be nothing for the rocket to push against. However, a rocket in space moves like the skateboarder holding the fire extinguisher. Fuel is ignited in a chamber, which causes an explosion of gases. The explosion creates pressure that forces the gases out of the rocket. As these gases rush out the end, the rocket moves in the opposite direction, as predicted by Newton's Third Law of Motion. The reaction force of the gases on the rocket pushes the rocket forward, as shown in Figure 23.17. The force pushing the rocket is also called thrust.

A Rocket Revolution


For centuries, rockets were powered by gunpowder or other solid fuels. These rockets could travel only fairly short distances. At the end of the 19th century and the beginning of the 20th century, several breakthroughs in rocketry would lead to rockets that were powerful enough to carry rockets—and humans—beyond Earth. During this period, three people independently came up with similar ideas for improving rocket design.

The first person to establish many of the main ideas of modern rocketry was a Russian schoolteacher, named Konstantin Tsiolkovsky (1857–1935). Most of his work was done even before the first airplane flight, which took place in 1903. Tsiolkovsky realized that in order for rockets to have enough power to escape Earth's gravity, they would need liquid fuel instead of solid fuel. He also realized that it was important to find the right balance between the amount of fuel a rocket uses and how heavy the rocket is. He came up with the idea of using multiple stages when launching rockets, so that empty fuel containers would drop away to reduce mass. Tsiolkovsky had many great ideas and designed many rockets, but he never built one.

The second great rocket pioneer was an American, named Robert Goddard (1882–1945). He independently came up with some of the same ideas as Tsiolkovsky, such as using liquid fuel and using multiple stages. He also designed a system for cooling the gases escaping from a rocket, which made the rocket much more efficient. Goddard was more practical than Tsiolkovsky and built rockets to test his ideas. Figure 23.18 shows Goddard with the first rocket to use liquid fuel. This rocket was launched on March 16, 1926 in Massachusetts. Over a lifetime of research, Goddard came up with many innovations that are still used in rockets today.

Figure 23.18: (Left) Robert Goddard launched the first liquid-fueled rocket on March 16, 1926; (Right) This schematic shows details of Goddard's rocket.

The third great pioneer of rocket science was a Romanian-born German, named Hermann Oberth (1894–1989). In the early 1920s, Oberth came up with many of the same ideas as Tsiolkovsky and Goddard. His early work was not taken seriously by most scientists. Nonetheless, Oberth built a liquid-fueled rocket, which he launched in 1929. Later, he joined a team of scientists that designed the rocket shown in Figure 23.19 for the German military. This rocket, first called the A-4 and later the V-2, played a major role in World War II. The Germans used the V-2 as a missile to bomb numerous targets in Belgium, England, and France. In 1942, the V-2 was launched to an altitude of 176 km (109 miles), making it the first human-made object to travel into space. An altitude of 100 km (62 miles) is generally considered to be the dividing line between Earth's atmosphere and space.

Figure 23.19: V-2 Rocket: Explosions in a chamber create pressure that pushes gases out of a rocket. This in turn produces thrust that pushes the rocket forward.

The leader of the team that built the V-2 rocket was a German scientist, named Wernher von Braun. von Braun later fled Germany and came to the United States, where he helped the United States develop missile weapons and then joined the National Aeronautics and Space Administration (NASA) to design rockets for space travel. At NASA, von Braun designed the Saturn V rocket (Figure 23.17), which was eventually used to send the first humans to the Moon.



One of the first uses of rockets in space was to launch satellites. A satellite is an object that orbits a larger object. To orbit something just means to travel in a circular or elliptical path around it. This path is also called an orbit. When you think of a satellite, you probably picture some kind of metallic spacecraft orbiting Earth, but the Moon is also a satellite. Human-made objects put into orbit are called artificial satellites. Natural objects in orbit, such as moons, are called natural satellites.

Newton's Law of Universal Gravitation


Isaac Newton, whose third law of motion explains how rockets work, also came up with the theory that explains why satellites stay in orbit. Newton's law of universal gravitation describes how every object in the universe is attracted to every other object. The same gravity that makes an apple fall to the ground, and keeps you from floating away into the sky, also holds the Moon in orbit around Earth, and Earth in orbit around the Sun.

Newton used the following example to explain how gravity makes orbits possible. Consider a cannonball launched from a high mountain, as shown in Figure 23.20. If the cannonball is launched at a slow speed, it will fall back to Earth, as in paths A and B in the figure. However, if it is launched at a fast enough speed, the Earth below will curve away at the same rate that the cannonball falls, and the cannonball will go into a circular orbit, as in path C. If the cannonball is launched even faster, it could go into an elliptical orbit (D) or leave Earth's gravity altogether (E).

Figure 23.20: Isaac Newton explained how a cannonball fired from a high point with enough speed could orbit Earth.

Note that Newton's idea would not actually work in real life; a cannonball launched from Mt. Everest, the highest mountain on Earth, would burn up in the atmosphere if launched at the speed required to put the cannonball into orbit. However, a rocket can launch straight up, then steer into an orbit. A rocket can also carry a satellite above the atmosphere and then release the satellite into orbit.

Types of Satellites


Since the launch of the first satellite over 50 years ago, thousands of artificial satellites have been put into orbit around Earth. We have even put satellites into orbit around the Moon, the Sun, Venus, Mars, Jupiter, and Saturn. Imaging satellites are designed for taking pictures of Earth's surface. The images can be used by the military, when taken by spy satellites or for scientific purposes, such as meteorology, if taken by weather satellites. Astronomers use imaging satellites to study and make maps of the Moon and other planets. Communications satellites, such as the one in Figure 23.21, are designed to receive and send signals for telephone, television, or other types of communications. Navigational satellites are used for navigation systems, such as the Global Positioning System (GPS). The largest artificial satellite is the International Space Station, designed for humans to live in space while conducting scientific research.

Figure 23.21: This is a Milstar communications satellite used by the U.S. military. The long, flat solar panels provide power for the satellite. Most of the other instruments you can see are antennas for sending or receiving signals.

Types of Orbits


The speed of a satellite depends on how high it is above Earth or whatever object it is orbiting. Satellites that are relatively close to Earth are said to be in low Earth orbit (LEO). Satellites in LEO are also often in polar orbit, which means they orbit over the North and South Poles, perpendicular to Earth's spin. Because Earth rotates underneath the orbiting satellite, a satellite in polar orbit is over a different part of Earth's surface each time it circles. Imaging satellites and weather satellites are often put in low-Earth, polar orbits.

A satellite placed at just the right distance above Earth–35,786 km (22,240 miles)—orbits at the same rate that Earth spins. As a result, the satellite is always in the same position over Earth’s surface. This type of orbit is called a geostationary orbit (GEO). Many communications satellites are put in geostationary orbits.

The Space Race


From the end of World War II in 1945 to the breakup of the Soviet Union (USSR) in 1991, the Soviet Union and the United States were in military, social, and political conflict. This period is known as the Cold War. While there were very few actual military confrontations, the two countries were in an arms race—continually developing new and more powerful weapons as each country tried to have more powerful weapons than the other. While this competition had many social and political consequences, it did also help to drive technology. The development of missiles for war significantly sped up the development of rocket technologies.



On October 4, 1957, the Soviet Union launched Sputnik 1, the first artificial satellite ever put into orbit. Sputnik 1, shown in Figure 23.22, was 58 cm in diameter and weighed 84 kg (184 lb). Antennas trailing behind the satellite sent out radio signals, which were detected by scientists and amateur radio operators around the world. Sputnik 1 orbited Earth in low Earth orbit on an elliptical path every 96 minutes. It stayed in orbit for about 3 months, until it slowed down enough to descend into Earth's atmosphere, where it burned up as a result of friction with Earth's atmosphere.

Figure 23.22: The Soviet Union launched Sputnik 1, the first artificial satellite, on October 4, 1957.

The launch of Sputnik 1 started the Space Race between the Soviet Union and the United States. Many people in the U.S. were shocked that the Soviets had the technology to put the satellite in orbit, and they worried that the Soviets might also be winning the arms race. On November 3, 1957, the Soviets launched Sputnik 2, which carried the first animal to go into orbit—a dog named Laika.

The Race Is On


In response to the Sputnik program, the U.S. launched their own satellite, Explorer I, on January 31, 1958. Shortly after that—March 17 1958—the U.S. launched another satellite, Vanguard 1. Later that year, the U.S. Congress and President Eisenhower established the National Aeronautics and Space Administration (NASA).

The Soviets still managed to stay ahead of the United States for many notable "firsts". On April 12, 1961, Soviet cosmonaut Yuri Gagarin became both the first human in space and the first human in orbit. Less than one month later—May 5, 1961—the U.S. sent their first astronaut into space: Alan Shepherd. The first American to orbit Earth was John Glenn, in February 1962. The first woman in space was a Soviet: Valentina Tereshkova, in June 1963. The timeline in Table 23.1 shows many other Space Race firsts.

Table 23.1: Space Race Timeline
Date Accomplished Country Name of Mission
October 4, 1957 First artificial satellite, first signals from space USSR Sputnik 1
November 3, 1957 First animal in orbit (the dog Laika) USSR Sputnik 2
January 31, 1958 USA's first artificial satellite USA Explorer 1
January 4, 1959 First human-made object to orbit the Sun USSR Luna 1
September 13, 1959 First impact into another planet or moon (the Moon) USSR Luna 2
April 12, 1961 First manned spaceflight and first manned orbital flight (Yuri Gagarin) USSR Vostok 1
May 5, 1961 USA's first spaceflight with humans (Alan Shepherd) USA Mercury-Redstone 3 (Freedom 7)
February 20, 1962 USA's first orbital flight with humans (John Glenn) USA Mercury-Atlas 6 (Friendship 7)
December 14, 1962 First planetary flyby (Venus) USA Mariner 2
June 16, 1963 First woman in space, first woman in orbit (Valentina Tereshkova) USSR Vostok 6
March 18, 1965 First extra-vehicular activity ("spacewalk") (Aleksei Leonov) USSR Voskhod 2
February 3, 1966 First soft landing on another planet or moon (the Moon), first photos from another world USSR Luna 9
March 1, 1966 First impact into another planet (Venus) USSR Venera 3
April 3, 1966 First artificial satellite around another world (the Moon) USSR Luna 10
June 2, 1966 USA's first soft landing on the Moon, USA's first photos from the Moon USA Surveyor 1
December 21, 1968 First humans to orbit another world (the Moon) (James Lovell, Frank Borman, Bill Anders) USA Apollo 8
July 21, 1969 First humans on the Moon (Neil Armstrong, Buzz Aldrin) USA Apollo 11

The Space Race between the United States and the Soviet Union reached a peak in 1969 when the U.S. put the first humans on the Moon. However, the competition between the two countries' space programs continued for many more years.

Reaching the Moon


On May 25, 1961, shortly after the first American went into space, President John F. Kennedy presented the following challenge to the U.S. Congress:

I believe that this nation should commit itself to achieving the goal, before this decade is out, of landing a man on the Moon and returning him back safely to the Earth. No single space project in this period will be more impressive to mankind, or more important for the long-range exploration of space; and none will be so difficult or expensive to accomplish.

Eight years later, NASA's Apollo 11 mission achieved Kennedy's ambitious goal. On July 20, 1969, astronauts Neil Armstrong and Buzz Aldrin were the first humans to set foot on the moon, as shown in Figure 23.23.

Figure 23.23: Neil Armstrong took this photo of Buzz Aldrin on the Moon during the Apollo 11 mission. Armstrong and the Lunar Module can be seen in the reflection in Aldrin's helmet.

Following the Apollo 11 mission, four other American missions successfully put astronauts on the Moon. The last manned mission to the moon was Apollo 17, which landed on December 11, 1972. To date, no other country has put a person on the Moon.

In July 1975, the Soviet Union and the United States carried out a joint mission called the Apollo-Soyuz Test Project. During the mission, an American Apollo spacecraft docked with a Soviet Soyuz spacecraft, as shown in Figure 23.24. Many considered this to be the symbolic end of the Space Race.

Figure 23.24: The docking of an Apollo spacecraft with a Soyuz spacecraft in 1975 was a symbolic end to the Space Race.

Exploring Other Planets


Both the United States and the Soviet Union also sent probes to other planets during the Space Race. A space probe is a spacecraft that is sent without a crew to collect data by flying near or landing on an object in space, such as a planet, moon, asteroid, or comet. In the Venera missions, the USSR sent several probes to Venus, including some that landed on the surface. The U.S. sent probes to Mercury, Venus, and Mars in the Mariner missions, and landed two probes on Mars in the Viking missions.

In the Pioneer and Voyager missions, the U.S. also sent probes to the outer solar system, including flybys of Jupiter, Saturn, Uranus, and Neptune. The Pioneer and Voyager probes are still traveling, and are now beyond the edges of our solar system. We have lost contact with the two Pioneer probes, but expect to have contact with the two Voyager probes until at least 2020.

Lesson Summary

  • Rockets have been used for warfare and ceremonies for many centuries.
  • Newton's third law explains how a rocket works. The action force of the engine on the gases is accompanied by a reaction force of the gases on the rocket.
  • Konstantin Tsiolkovsky, Robert Goddard, and Hermann Oberthall came up with similar ideas for improving rocket design. These included using liquid fuel and using multiple stages.
  • A satellite is an object that orbits a larger object. Moons are natural satellites. Artificial satellites are made by humans.
  • Newton's law of universal gravitation explains how the force of gravity works, both on Earth and across space. Gravity hold satellites in orbit.
  • Artificial satellites are used for imaging Earth and other planets, for navigation, and for communication.
  • The launch of the Sputnik 1 satellite started a Space Race between the United States and the Soviet Union.
  • The United States' Apollo 11 mission put the first humans on the Moon.
  • The U.S. and Soviet Union also sent several probes to other planets during the Space Race.

Review Questions

  1. Use Newton's third law to explain how a rocket moves.
  2. List the three great pioneers of rocket science.
  3. What is the difference between a rocket and a satellite? How are they related?
  4. What is the name of Earth's natural satellite?
  5. Explain why a satellite in polar orbit will be able to take pictures of all parts of the Earth over time.
  6. Describe three different types of orbits.
  7. What event launched the Space Race?
  8. What goal did John F. Kennedy set for the United States in the Space Race?
  9. What are the advantages of a multi-stage rocket instead of a single-stage rocket?


geostationary orbit
A satellite placed at just the right distance above Earth to orbit at the same rate that Earth spins.
low Earth orbit
Satellites that orbit relatively close to Earth.
To travel in a circular or elliptical path around another object.
polar orbit
A path for a satellite that goes over the North and South Poles, perpendicular to Earth's spin.
A device propelled by particles flying out of it at high speed.
An object, either natural or human made, that orbits a larger object.
space probe
A spacecraft that is sent without a crew to collect data by flying near or landing on an object in space.
Space Race
A competition between the United States and the Soviet Union to have the best space technology.
The forward force produced by gases escaping from a rocket engine.

Points to Consider

  • The Space Race and the USA's desire to get to the Moon brought about many advances in science and technology.
  • Can you think of any challenges we face today that are, could be, or should be a focus of science and technology?
  • If you were in charge of NASA, what new goals would you set for space exploration?

Telescopes · Recent Space Exploration