From far out in space, Earth looks like a blue ball. Since water covers three-fourths of the Earth’s surface, blue is the color we see most. The continents（陆地）look brown, like small islands floating in the huge, blue sea. White clouds cover around the Earth like a light blanket（毯子）. The Earth is shaped like a sphere, or a ball. It is 25, 000 miles around! It would take more than a year to walk around the whole planet. A spaceship can fly around the widest part of the sphere in only 90 minutes.

Even though spaceships have traveled to the Moon, people cannot visit the Moon without special suits. The Moon has no air or water. Plants and animals can’t live there either. Astronauts first landed on the Moon in 1969. After that, there were six more trips to the Moon. They brought back Moon rocks, which scientists are still studying. There are holes, or craters（火山口）, all over the Moon’ s surface. Scientists believe that meteorites（陨石）smashed into the Moon millions of years ago and formed the craters.

The Sun is the closest star to Earth. A star is a hot ball of burning gas. The Sun looks very big because it is so close. But the Sun is just a medium-sized star. Billions of far-away stars are much bigger than our Sun. The burning gases from the Sun are so hot that they warm the Earth from 93 million miles away! Even though the Sun is always shining, the night here on Earth is dark. That's because the Earth rotates, or turns around, every 24 hours. During the day, the Earth faces the Sun. Then we see light. During the night, the Earth turns away from the Sun. Then it faces the darkness of space.

Each day we learn more about the Earth, the Moon and the Sun.

Many people know that rubbish is a big problem on the planet Earth. What many people don’t know is that junk（垃圾）has become a problem in outer space too.

According to BBC News, there are more than 22, 000 pieces of space junk floating around the earth. And these are just the things that we can see from the surface of the earth by telescopes （望远镜）. There are also millions of smaller pieces of junk that we can’t see.

Objects, like bits of old space rockets（火箭）or satellites（卫星）, move around the planet at very high speed so fast that even a very small piece can break important satellites or become dangerous to astronauts. If the tiniest piece of junk crashed into a spaceship, it could damage （损害）the space tool.

To make things worse, when two objects in space crash, they break into many smaller pieces. For example, when a U.S. satellite hit an old Russian rocket in 2009, it broke into more than 2,000 pieces, increasing the amount of space junk.

To reduce additional （额外的）space junk, countries have agreed that all new space tools can only stay in space for 25 years at most. Each tool must be built to fall safely into the earth’s atmosphere （大气层）after that time. In the upper parts of the atmosphere, it will burn up.

Many scientists also suggest different ways to clean up space junk. In England, scientists are testing a special net that can be sent into space junk. The net catches the junk and then pulls it into the earth's atmosphere to burn up. The Germans are building robots that can collect pieces of space junk and bring them back to Earth to be safely dealt with.

“The problem is becoming more challenging because we’re sending more objects into space to help people use their mobile phones and computers,” says Marco Castronuovo, an Italian space researcher.

Venus, Jupiter, Mercury and Saturn are four major planets in the solar system. But do you know from where their English names originated? If not, just look ahead.