Low Earth orbit is not perfectly empty, so satellites moving there still collide with a very thin upper atmosphere. Each collision with air molecules creates drag, which removes a small amount of orbital energy. Over many orbits, this energy loss lowers the satellite into denser air, causing even stronger drag.
This feedback process is called orbital decay, and it explains why many low satellites eventually fall back to Earth.
A satellite in a circular orbit moves fast enough that gravity bends its path around Earth instead of pulling it straight down. Drag acts opposite the satellite's velocity, reducing its speed and mechanical energy. As the orbit shrinks, the satellite moves into denser atmospheric layers where heating rises sharply and structural breakup can occur.
During reentry, most small spacecraft burn up, while larger or denser pieces may survive to lower altitudes.
Key Facts
- Orbital speed near low Earth orbit is about v = 7.8 km/s.
- Drag force can be estimated by Fd = 1/2 rho v^2 Cd A.
- Orbital energy for a circular orbit is E = -GMm/(2r).
- Lower orbital radius usually means higher circular orbital speed, v = sqrt(GM/r), but drag still removes total energy.
- Atmospheric density rho increases rapidly as altitude decreases, so drag becomes much stronger during decay.
- Reentry heating scales strongly with speed because kinetic energy is KE = 1/2 mv^2.
Vocabulary
- Orbital decay
- Orbital decay is the gradual lowering of an orbit as drag or other forces remove mechanical energy from an object.
- Atmospheric drag
- Atmospheric drag is a resistive force caused by collisions with gas molecules that acts opposite an object's motion through the atmosphere.
- Low Earth orbit
- Low Earth orbit is the region of orbit close to Earth, commonly from about 160 km to 2000 km above the surface.
- Reentry
- Reentry is the process in which an object from space enters denser atmosphere and experiences strong drag, heating, and deceleration.
- Ablation
- Ablation is the removal of surface material by heating, melting, vaporization, or erosion during high-speed atmospheric entry.
Common Mistakes to Avoid
- Thinking there is no air in low Earth orbit. This is wrong because the upper atmosphere is thin but still present, and even tiny drag forces matter over thousands of orbits.
- Assuming drag makes a satellite immediately fall straight down. This is wrong because the satellite remains in orbit while its path slowly shrinks and becomes lower over time.
- Saying a lower orbit always means the satellite is slowing down in the simple circular-orbit sense. This is incomplete because drag removes energy, while the circular speed required at a smaller radius is actually larger.
- Ignoring cross-sectional area when comparing decay rates. This is wrong because larger area and lower mass per area increase drag acceleration and make orbital decay faster.
Practice Questions
- 1 A satellite in low Earth orbit moves at 7.8 km/s. What is its kinetic energy per kilogram using KE/m = 1/2 v^2? Give your answer in J/kg.
- 2 Use Fd = 1/2 rho v^2 Cd A for a satellite with rho = 1.0 x 10^-11 kg/m^3, v = 7800 m/s, Cd = 2.2, and A = 4.0 m^2. What drag force acts on the satellite?
- 3 Two satellites have the same mass and altitude, but one has solar panels spread out while the other is compact. Explain which one will decay faster and why.