Astrophysics for People in a Hurry (Astrophysics for People in a Hurry Series)

by Neil deGrasse Tyson

Earth’s atmosphere is commonly described as extending dozens of miles above Earth’s surface. Satellites in “low” Earth orbit typically travel between one hundred and four hundred miles up, completing an orbit in about ninety minutes. While you can’t breathe at those altitudes, some atmospheric molecules remain—enough to slowly drain orbital energy from unsuspecting satellites. To combat this drag, satellites in low orbit require intermittent boosts, lest they fall back to Earth and burn up in the atmosphere. An alternative way to define the edge of our atmosphere is to ask where its density of gas molecules equals the density of gas molecules in interplanetary space. Under that definition, Earth’s atmosphere extends thousands of miles. Orbiting high above this level, twenty-three thousand miles up (one-tenth of the distance to the Moon) are the communications satellites. At this special altitude, Earth’s atmosphere is not only irrelevant, but the speed of the satellite is low enough for it to require a full day to complete one revolution around Earth. With an orbit precisely matching the rotation rate of Earth, these satellites appear to hover, which make them ideal for relaying signals from one part of Earth’s surface to another.