“How long do satellites stay in the air?”
The Four-Year-Old’s construction site
As part of her Mars Rover Curiosity Liftoff Celebration, The Four-Year-Old built a satellite launch pad out of Legos, with only a few targeted assists from her parents. While hard at work on the cargo truck, The Four-Year-Old paused long enough to ask:
“Mommyo, how long do satellites stay in the air?”
Mother: “Oh, I don’t know. Thirty or forty years I guess.”
The Four-Year-Old: “My satellite friend only stays up there for one day.”
Mother: “Really?”
The Four-Year-Old: “Yes. It’s really cold up there.”
As this dialogue makes painfully clear, neither of us knows all that much about what life would be like for satellites in Low Earth Orbits, which the Federal Communications Commission (FCC) affectionately refers to as LEOs and defines as orbits that are about 300-1250 miles above the Earth’s surface.
1) How long a LEO satellite stays in the sky depends (in part) on its altitude.
In vastly oversimplified terms (you know, the kind I specialize in), the higher the orbit, the longer an LEO satellite can remain in place. LEO satellites may have anywhere from 10 to 100 years before their orbit degrades enough to send them crashing back toward Earth.
As a side note, satellites placed in Highly Elliptical Orbits (HEOs) or geostationary orbits can remain in space for hundreds, thousands or even millions of years.
And yes, I know that the stability of the orbit actually depends on the balance between the gravitational force exerted by the Earth and the velocity or speed at which the satellite is traveling, but the location of the satellite does affect the strength of the Earth’s gravitational pull. Work with me here. I’m just an English major explaining things to a preschooler.
2) Satellites aren’t always cold.
Space itself may be cold (a mere 2.7 degrees Kelvin, which translates to -484.81 degrees Fahrenheit or -270.45 degrees Celsius), but that doesn’t mean a LEO satellite will be. The satellite’s surface temperature varies depending on its exposure to direct sunlight, sunlight reflected off the Earth’s surface, and infrared energy from the Earth.
To get an idea of the actual temperatures experienced by a LEO satellite, I skimmed the relevant bits of Millan Fernando Diaz-Aguado’s 2005 Masters thesis. In it, Diaz-Aguado, then a student at the University of Texas, performs a series of thermal cycling and thermal soaking tests on Formation Autonomy Spacecraft with Thrust, Relative Navigation, Attitude, and Crosslink Program (FASTRAC) at NASA’s Johnson Space Center in Texas.
Before performing his tests, Diaz-Aguado first calculated the likely temperature ranges experienced in space by LEO satellites. He determined that in order to keep their internal bits working, engineers would have to design LEO satellites to withstand expected exterior temperatures ranging from -75 degrees Celsius (-103 degrees Fahrenheit) to 51 degrees Celsius (123.8 degrees Fahrenheit).
In other words, The Four-Year-Old’s satellite friend is just as likely to suffer from heat exhaustion as frostbite.
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