Unless someone asks you to hold your breath, you probably never think about how lucky you are to be surrounded with just the right amount of oxygen in the atmosphere around you. The Earth's atmosphere not only provides air for us to breathe, but it also helps to regulate the temperature of the planet. The sun providesthe heat the planet needs to stay warm, but it also causes the winds that cool it down again. If someoneasked you about the atmosphere, you would probablybe thinking about all the layers that surround the earth, especially the ozone layer that helps protect us from those UV rays. Did you know that all of those layers combined extend to around 300 miles up into the air? The weight of all those layers on your body is called atmospheric pressure. Do you feel tired at the end of the day from all of that weight? If you've ever taken a trip through the mountains, you may have felt it was harder to breathe when you got to the top of the peaks because oxygen also gets thinner as you travel up through the atmosphere. If you had to guess, do you think the atmospheric pressure would be higher or lower as you neared the mountain's top?

Before people started traveling in space, they wondered if the earth was the only place in the universe that had an atmosphere. Scientists now know that many of the planets in our solar system along with other objects in the universe have atmospheres. Because these atmospheresaren't filled with air like ours, but rather other combinations of gases, they would be very dangerous to a person's health. After many years of studying space, they still aren't sure that anywhere else has the right blend of gases that would allow people or animals to survive!