Human Space Travel

Astronaut Under Water? Games
When you are in water, you float. Astronauts in space float, too. Because floating in water is like floating in space, astronauts spend many hours under water preparing for space missions. Do you know why astronauts float in space?
Technology and Early Space Travel
You have lived your entire life in the space age. Most people consider the launch of Sputnik I in 1957 by the former Soviet Union to be the beginning of the space age. Sputnik I was the first artificial satellite sent into orbit around Earth. An artificial satellite is any human-made object placed in orbit around a body in space. Today, hundreds of artificial satellites operate in orbit around Earth. Some artificial satellites are communication satellites. Some observe Earth. A few observe stars and other objects in distant space.
Escaping Gravity
How do artificial satellites and other spacecraft reach space? You know that when you jump up into the air, you land back on the ground because of Earths gravity. But if you could jump fast enough and high enough, you would launch into space! Only a rocket can travel fast enough and far enough to escape Earths gravity. A rocket is a vehicle propelled by the exhaust made from burning fuel. As its exhaust is forced out, the rocket accelerates forward, as shown in Figure 1. Most rockets that travel long distances carry two or more tanks of fuel to be able to travel far enough to escape Earths gravity.
StockTrek/Getty Images
Figure 1 The exhaust from a rocket launch propels a rocket skyward.
Robotic Space Probes
The Moon is the farthest object from Earth that humans have visited. However, scientists have sent robotic missions to every planet, as well as to some moons, asteroids, dwarf planets, and comets. A space probe is an uncrewed vehicle that travels to and obtains information about objects in space. Examples of the three main types of space probes are shown in Figure 2. Probes do not return to Earth. They are equipped with cameras and scientific instruments that transmit data back to Earth.
There are many reasons to send probes instead of people into space. It costs less and is often safer to send probes. Also, objects in space are very far away. A visit to Mars and back would take more than a year. A round trip to Saturn could take 15 years. Robotic missions are dangerous, too. Only half of the missions sent to Mars have been successful. Space probes that do arrive at their destinations experience harsh conditions and often do not survive long.
The National Aeronautics and Space Administration (NASA) is the U. S. government agency responsible for most space missions and space-flight technology. Other nations also have space programs. Astronauts from more than 30 countries have traveled to space, and several countries have sent robotic missions to the Moon and beyond.
1. Key Concept CheckHow do space probes help scientists explore space?
(l) Atlas Photo Bank / Photo Researchers, Inc., (c) AP Images, (r) NASA/JPL
Figure 2 Some space probes pass by an object. Others land on the surface.
Visual CheckWhich probe would transport a rover, a craft that moves on the surface of an object?
(verb) to send something from one person, place, or thing to another
Challenges for Humans in Space
When astronauts travel into space, they must bring their environments and life-support systems with them. Otherwise, they could not withstand the temperatures, the pressures, and the other extreme conditions that exist in space.
Solar Radiation
One threat to astronauts is harmful radiation from the Sun. You read that Earths atmosphere protects life on Earth from most of the Suns dangerous radiation. However, as astronauts travel in space, they move far beyond Earths atmosphere. They must rely on their spacecraft and spacesuits to shield them from dangerous solar radiation and solar particles.
Humans need oxygen. Outside Earths atmosphere, there is not enough oxygen for humans to survive. Air circulation systems inside spacecraft supply oxygen and keep carbon dioxide, which people breathe out, from accumulating. The air humans breathe on Earth is a mixture of nitrogen and oxygen. For short trips into space, spacecraft carry tanks of oxygen and nitrogen, which are mixed into the proper proportions onboard. For long trips, oxygen is supplied by passing an electric current through water. This separates waters hydrogen and oxygen atoms.
1. Key Concept CheckWhat factors must humans consider when traveling into space?
Temperature and Pressure Extremes
Most places in the solar system are either extremely cold or extremely hot. Pressures in space are also extreme. In most places, pressure is much lower than the pressure humans experience on Earth. Environmental control systems in spacecraft protect astronauts from temperature and pressure extremes. Outside their spacecraft, astronauts wear Extravehicular Mobility Unit (EMU) suits, as shown in Figure 3. EMU suits provide oxygen, protect astronauts from radiation and meteoroids, and enable astronauts to talk to each other.
2. Reading CheckWhat are the purposes of an EMU suit?
Adastra/Getty Images
Figure 3 An EMU suit enables an astronaut to spend up to eight hours outside a spacecraft.
You might think astronauts are weightless in space. But astronauts in orbit around Earth are subjected to almost the same gravity as they are on Earths surface. Then why is the astronaut shown in Figure 4 floating? As their spacecraft orbits Earth, the astronauts inside are continually falling toward Earth. But because their spacecraft is moving, they do not actually fall. They float. If their spacecraft suddenly stopped moving, they would plunge downward.
The space environment that astronauts experience is often called microgravity. In microgravity, objects seem to be weightless. This can be an advantage. No matter how much something weighs on Earth, it can be moved with ease in space. Microgravity also makes some tasks, such as turning a screwdriver, more difficult. If an astronaut is not careful, instead of the screw turning, he or she might turn instead.
On Earth, working against gravity helps keep your muscles, bones, and heart strong and healthy. But in microgravity, astronautsbones and muscles dont need to work as hard, and they begin to lose mass and strength. Astronauts in space must exercise each day to keep their bodies healthy.
Figure 4 Astronaut Eileen Collins floats because she is constantly falling toward Earth as her spacecraft orbits Earth.
from Greek astron, meansstar”; and Greek nautes, meanssailor
MiniLab: How hard is it to hit a target?
Escaping Earths gravity is a challenge for scientists launching rockets. Landing a spacecraft on a solar system object also can be difficult.
Hutchings Photography/Digital Light Source
1. Read and complete a lab safety form.
2. Use tape to make a circle about 2 m in diameter on the floor. At the center, make a target circle about 15 cm in diameter.
3. Test launch a toy vehicle toward the target starting from the larger circle. Make several attempts. You are successful only if your vehicle has all its wheels inside the target circle. In your Science Journal, record what percentage of your test launches were successful.
4. Launch your vehicle. As a class, determine the success rate of the launches.
Analyze and Conclude
1. Model What technology does the toy in this activity represent?
2. Key ConceptWhat factors must be considered when launching a spacecraft?
Living and Working in Space
Even when protected from the extremes of space, astronauts still face many challenges when living and working in space. Life in space is dramatically different from life on Earth.
International Space Station
The International Space Station (ISS), shown in Figure 5, is a large, artificial satellite that orbits Earth. People work and live on the ISS for up to six months at a time. Constructed by astronauts from over 15 nations, the ISS has been continuously occupied since the first crew arrived in 2000.
The ISS crew conducts scientific and medical experiments. These include experiments to learn how microgravity affects peoples health and how it affects plants. People living in space for long periods might need to grow plants for food and oxygen. In the future, in addition to being an orbiting research laboratory, the ISS eventually might serve as a testing and repair station for missions to the Moon and beyond.
Living in space is not easy. For example, astronauts must place a clip on a book to hold it open to the right page. They eat packaged food, using magnetized trays and tableware. Toilets flush with air instead of water. And astronauts must be strapped down while they sleep, as shown in Figure 6. Otherwise, they would drift and bump into things.
Figure 5 The International Space Station is the ninth and largest space station to be built in space. It orbits Earth at 400 km above Earths surface.
Visual CheckWhat is the combined area of the ISSs eight pairs of solar panels?
Figure 6 European astronaut Paolo Nespoli sleeps strapped into a sleeping bag.
Transportation Systems
Space transportation systems are the rockets, the shuttles, and the other spacecraft that deliver cargo and humans to space. Figure 7 shows the progression of NASAs space transportation systems. Early rockets and spacecraft, such as those used to transport astronauts to the Moon, were used only once. The early rockets and spacecraft included three programs. The Mercury program obtained the goal of putting a human into orbit. The Gemini program goals were to test the spacecraft and crewsthresholds before beginning the Apollo program. The Apollo program achieved the goal of placing the first human on the Moon.
The Space Shuttle was NASAs first reusable transportation system. First launched in 1981, there were five shuttles designed to hold a maximum of seven crew members in each shuttle. The shuttles were designed to transport astronauts to the International Space Station to conduct experiments and to service uncrewed satellites , such as the Hubble Space Telescope.
1. Reading CheckHow has the human space flight program at NASA changed over the years?
Top: (l) NASA Johnson Space Center Collection, (cl) NASA Great Images in NASA Collection, (cr) NASA Marshall Space Flight Center Collection, (r) NASA; Bottom: (l) NASA Marshall Space Flight Center Collection, (r) NASA
Figure 7 NASA has launched many different types of spacecraft carrying humans into space. This timeline shows the variety of the spacecraft.
Future Space Exploration
Space engineers are continuously working on new technologies and ideas that could advance human space flight and exploration. The last image in Figure 7 shows what future space transportation systems may look like.
Lesson Review
Visual Summary
When wearing an EMU suit, an astronaut can spend up to eight hours outside a spacecraft.
Adastra/Getty Images
Sleeping can be a challenge for astronauts in space.
The space shuttle was NASAs first reusable transportation system.
Lesson Assessment
Use Vocabulary
1. Any object made by humans and placed in orbit around a body in space is a(n) __________.
2. Distinguish between a rocket and a space probe.
Understand Key Concepts
3. Which is NOT a type of space probe?
A. flyby
B. lander
C. orbiter
D. shuttle
4. Compare how a microgravity environment is both an advantage and a disadvantage for space travelers.
5. Explain how the International Space Station is used now and how it might be used in the future.
6. Which best describes how astronauts are affected by microgravity?
A. They are constantly falling toward Earth.
B. They are lifted by oxygen in the air.
C. They are not subjected to an atmosphere.
D. They are not subjected to gravity.
7. What is the purpose of the object shown below?

Photograph of Voyager I space probe
Atlas Photo Bank / Photo Researchers, Inc.
A. to be used as a lunar outpost
B. to be used as a research station
C. to explore planets up close
D. to transport astronauts
8. How is oxygen provided on a long space voyage?
A. by collecting molecules from space
B. by harnessing nuclear reactions
C. by passing electricity through water
D. by recycling carbon dioxide
Interpret Graphics
9. Identify The photo below shows the International Space Station. What purpose does the part labeled X serve?

Artist's rendition of the completed International Space Station with an X indicating one of the solar panels
10. Summarize Copy and fill in the graphic organizer below to summarize challenges for humans traveling in space.

Example of a graphic organizer used to list four challenges in space
Critical Thinking
11. Imagine you are an astronaut on a year-long voyage. How would your life be different from your life on Earth? What kind of accommodations would you have to make? How would it be the same?
12. Evaluate the following statement: Astronauts in orbit are weightless because they are so far from Earths surface.
13. Explain why solar radiation is a challenge for astronauts traveling in space.
14. Deduce why the International Space Station is being assembled in space and not on Earth.
Writing in Science
15. You are an astronaut on the International Space Station. Write a letter of at least six lines to your best friend describing some of the challenges you face on the ISS.


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