Unit 1: Earth Science Chapter 1: The Nature of Science     The Nature of Science One of the most famous images of Earth was taken by Apollo 17 astronauts in 1972 as they orbited the planet. In this image, featured on posters, book covers, and calendars, Earth looks like a lonely place in space. The expressions "spaceship Earth" and "pale blue dot" find their origins in this dramatic photo, shown below. Study it carefully. It will give you an idea of the scale and size of Earth in the vastness of space as you begin your study of Earth science. Alone in Space As the Apollo photo suggests, Earth really is isolated in space. If our planet was reduced to the size of a basketball with a diameter of 30 cm, the Sun would be about 3.5 km away and would have a diameter of 32.7 m. Venus would be another basketball almost 1 km from Earth. Jupiter, in contrast, would be more than 3 m in diameter and nearly 2 km away. Tiny Pluto, the smallest planet, would be a mere golf ball some 140 km from Earth. Alpha Centauri, the Sun's closest neighboring star, would be located almost 1 million km from our solar system. Earth is one of nine planets that orbit the Sun. The Sun, in turn, is one of hundreds of billions of stars in the Milky Way Galaxy, our home galaxy. And the Milky Way is one of hundreds of billions of galaxies in the universe. The distances between bodies in space are huge compared with their sizes. Thus, outer space is mostly empty. Measuring Distances in Space To measure the vast distances between planets in our solar system, scientists use astronomical units (AU). One AU equals the average distance from Earth to the Sun, or about 150 million km. The orbits of the other planets in relation to the Sun range from 0.387 AU (Mercury) to 39.44 AU (Pluto). To measure the even greater distances between stars and galaxies, scientists use a unit of measure called a light-year. One light-year equals the distance light travels in one year, or about 9.5 trillion km. Alpha Centauri is about 4 light-years away from the Sun. If you look up at the night sky and spot Alpha Centauri, the light you see left the star four years earlier. The Milky Way Galaxy is about 100,000 light-years across. The Andromeda Galaxy is approximately 2 million light-years away from our galaxy-it is the most distance object the human eye can see without a telescope. Light from the Andromeda Galaxy travels for 2 million years to reach Earth. Astronomers use powerful telescopes to detect remote objects, such as galaxies that are several billion light-years away from the Sun. When scientists study these faraway galaxies, they are literally looking back in time and seeing the galaxies as they were billions of years ago. LINK-UP: Find out more information about telescopes. Other Solar Systems Using telescopes, astronomers have recently discovered other planets outside our solar system that orbit stars just as Earth orbits the Sun. But these planets appear to be gas giants that are more similar to Jupiter than to Earth. This is largely because huge gaseous planets are much easier to detect than small, rocky planets. Astronomers hope that within a decade or so, they will have the technology to pinpoint Earth-like planets orbiting distant stars. Until then, Earth remains very much alone in space. Activity NASA space probes have traveled to distant planets in our solar system. However, no astronaut has ever gone beyond the Moon. Research some of the problems humans would face on long-distance space flights. You may want to focus your research on Mars-the most likely candidate for human exploration. Afterwards, share your results with the class, and debate the advantages and disadvantages of long-distance space flights by humans.