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SPACE ENVIRONMENT
By Editor | July 9, 2009
The earth’s atmosphere limits the lowest altitude at which a satellite can be placed into orbit. Satellites launched from earth must pass through the atmosphere in order to attain the proper orbit. The lowest altitude for a satellite in circular orbit is about 93 miles, but without propulsion, the satellite would quickly lose speed and fall back to earth.
The earth’s atmosphere is divided into five regions: troposphere, stratosphere, mesosphere, thermosphere, and exosphere. The lines between these regions are not distinct, blending into each other so fluctuations occur in size depending on the time of the day, seasons, and solar activity. A brief description of each region is below:
1) Troposphere: Almost all weather and clouds occur here at the lowest region of the atmosphere. Starting at the surface of the earth, the troposphere extends up to the tropopause, the upper boundary of the troposphere. Above two miles, a person requires supplemental oxygen or a pressurized environment. The tropopause varies in thickness from 9 to 12 miles at the equator to about six miles in the polar areas.
2) Stratosphere: This region extends from the tropopause to the stratopause, the upper boundary, at about 30 to 33 miles altitude. Airflow in the stratosphere is mostly horizontal. This region is characterized by the near absence of water vapor and clouds.
3) Mesosphere: The mesosphere extends from the stratopause at the lower boundary to the mesopause, the upper boundary at about 50 miles altitude. The mesopause is where the minimum temperature is reached, approximately 130 degrees below zero Fahrenheit. Above 30 miles altitude there is not enough atmosphere for even a high altitude jet to operate. Above this altitude, both fuel and oxidizer must be carried for a rocket engine to provide thrust.
4) Thermosphere: The thermosphere extends from an altitude of 50 miles to between 200 and 375 miles. The temperature increases with altitude from about 30 degrees below zero Fahrenheit to the thermopause where the maximum temperature occurs at 2960 degrees Fahrenheit. An altitude at which a satellite in a circular orbit can, orbit the earth for at least one revolution without propulsion. At this altitude, it takes 89 minutes to complete one revolution of the earth.
5) Exosphere: The exosphere begins where the thermosphere ends and extends out into space. The density of atoms and molecules that make up the atmospheric gases in this region is so low that all atmospheric particles which surround the earth at an altitude of 1000 miles, could be contained in one cubic centimeter at sea level. However, even at this level, satellites orbiting in the exosphere are slowed by atmospheric drag caused by friction from collisions with individual particles.
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