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.
Rockets
Humans did not reach space until the second half of the 20thcentury. However, the main
technology that makes space exploration possible, therockethas 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 13thcentury. 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
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:
- An object in motion will remain in motion unless acted upon by a net force.
- Force equals mass multiplied by acceleration.
- 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 ice skater inFigure23.16. When the skater pushes
the wall, the skater’s force—the “action”—is matched by an equal force by the wall on the
skater in the opposite direction—the reaction.
Once the skater is moving, however, she has nothing to push against. Imagine now that the
skater is holding a fire extinguisher. When she pulls the trigger on the extinguisher, a fluid
or powder flies out of the extinguisher, and she 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 skater backward.
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 skater
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