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.
Understanding Astronautics: Orbital Decay and Reentry
The confusing part is that a satellite can lose speed yet later move faster. A small drag force first slows it at its current height. This makes the path no longer match a stable circular orbit.
The satellite begins to fall inward, often into an oval orbit with a lower closest point. Gravity then accelerates it as it approaches that lower point. A lower circular orbit requires a greater orbital speed, even though the satellite has lost total mechanical energy.
This distinction between speed at one point and total orbital energy is important. It prevents the common mistake of thinking that slowing an orbiting object must make it rise.
The rate of decay depends greatly on a spacecraft's ballistic coefficient. This describes how much mass is concentrated behind a given exposed area. A compact, heavy satellite is less affected by drag than a light satellite with wide solar panels.
Its shape and orientation matter too. If it turns so that a broad face meets the oncoming gas, it experiences more drag. Engineers can use this effect deliberately.
Some satellites change their attitude to reduce drag when they need to stay in orbit. Others use drag sails, large thin surfaces deployed near the end of a mission, to make reentry happen sooner.
The upper atmosphere changes from day to day. Energy from the Sun heats it and makes it expand upward. During strong solar activity, including solar storms, the air at a satellite's altitude can become much denser than expected.
This can increase decay enough to alter tracking predictions. Operators must regularly measure or estimate atmospheric conditions and update orbit forecasts. They may fire small rocket engines to raise an orbit before it becomes too low.
This is called an orbit maintenance maneuver. The International Space Station needs such boosts because its large size gives drag many surfaces to act on.
Reentry heating is not simply caused by rubbing against air in the everyday sense. At very high speed, air in front of the vehicle is compressed violently. The compressed gas becomes extremely hot, and heat flows into the spacecraft surface.
A heat shield protects some returning capsules by absorbing heat and slowly wearing away. Most inactive satellites do not have such protection. They break apart when heating, pressure, and spinning loads exceed the strength of their materials.
Many fragments vaporize high above the ground. Denser parts, such as tanks or engine components, have a better chance of surviving. Students should pay attention to altitude, area, mass, orientation, and solar activity because orbital decay is the result of all these factors acting together.
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.