Our
Universe:
Scientists estimate
that the edge of the universe may be 10 billion light-years away. (A light-year
is the distance light travels in a year-9.4 trillion km). But as vast as
it is, the universe consists of just two simple things: matter, anything
that contains atoms, and the energy that makes matter move. Some
10 percent of our universe’s matter is clustered into 100 billion galaxies,
very large groups of stars-some with orbiting planets. Scientist
say the other 90 percent of our universe’s matter is dark matter, invisible
mass that scientists have not yet been able to detect.
The
Milky Way: 100,000 light years across.
A lot closer
to home – and better understood- is our Milky Way galaxy. All the stars
you see in the night sky, plus more- about 200 billion in all – belong
to the Milky Way. Clouds of gas and dust make up the rest of the
galaxy. Like all galaxies, our Milky Way is always in motion, rotating
its spiral arms.
Our
Solar System: 25 million km across
Zooming in, you’ll
find our star, the Sun, and the heavenly bodies that orbit around it: the
nine planets (Mercury, Venus, Earth, Mars, and Jupiter, Saturn, Uranus,
Neptune, and Pluto); their 63 moons; and countless asteroids, meteoroids,
and comets. Scientists say this solar system formed from clouds of gas
and dust that surrounded the Sun after it formed 4.6 billion years ago.
NASA has sent 20 robotic spacecraft to explore every planet in the solar
system, except Pluto. As far as we know, Earth is the only planet capable
of sustaining life.
Our
Planet: Equatorial diameter 12,762 km
Now we’ve zeroed
in on the most familiar planet in the universe. What makes Earth such a
home sweet home for life? First, Earth is the perfect distance from the
sun-149.6 million km-so it’s not too hot or too cold. Earth also has an
atmosphere, a blanket of gases including nitrogen, oxygen, and carbon dioxide.
This atmosphere gives us the oxygen we need to live, and helps shield us
from the Sun’s radiation. And Earth is the only planet that has liquid
water, vital for life.
Western
Hemisphere: United States, 6,400 km from New York to California
As we get closer
to Earth we can see that water makes up much of our planet’s surface-70
percent. Billions of years ago, all Earth’s landmasses were joined as one.
Over time, this land mass broke apart to form seven large continents: North
America and South America, Africa, Asia, Europe, Australia, and Antarctica.
The continents are still moving today, up to 10 cm every year. This continuous
shifting of landmasses, called continental drift, is caused by the movements
of molten rock below Earth’s crust. The same forces cause volcanic eruptions,
earthquakes, and slower changes, such as the formation of mountains, lakes,
and valleys.
Washington,
DC
Washington, DC
is one of the largest cities in North America. Scientists keep a
watchful eye on our planet’s big cities and other areas with artificial
satellites – spacecraft launched into Earth’s orbit. Satellites use different
remote sensors to photograph and monitor weather, air and sea traffic,
pollution, and more. Landsat 4 is one such satellite.