Low Earth orbit feels like space, but it is not a perfect vacuum. A very thin atmosphere still reaches hundreds of kilometers above Earth, and satellites moving through it collide with rare gas particles at about 7 to 8 km/s. Those collisions create atmospheric drag, a tiny braking force that acts continuously.
Over days, months, or years, this drag can lower a satellite orbit enough to require reboosts or lead to reentry.
Key Facts
- Drag force can be modeled by Fd = 1/2 rho v^2 Cd A, where rho is atmospheric density, v is speed, Cd is drag coefficient, and A is cross-sectional area.
- Low Earth orbit speed is about v = 7.8 km/s near 400 km altitude.
- Orbital energy per unit mass for a circular orbit is epsilon = -GM/(2r), so losing energy makes the orbit radius shrink.
- Drag acceleration is ad = Fd/m = 1/2 rho v^2 Cd A/m.
- Ballistic coefficient is beta = m/(Cd A), and a larger beta means less deceleration from the same atmosphere.
- Solar activity heats and expands the upper atmosphere, increasing rho at low orbit altitudes and causing faster orbital decay.
Vocabulary
- Atmospheric drag
- Atmospheric drag is the resistive force caused by a spacecraft colliding with gas particles in the upper atmosphere.
- Low Earth orbit
- Low Earth orbit is a region of orbit around Earth, commonly below about 2000 km altitude, where satellites move very fast and can experience measurable drag.
- Reboost
- A reboost is a planned engine burn that raises a spacecraft orbit after drag has reduced its altitude or energy.
- Ballistic coefficient
- Ballistic coefficient is a measure of how strongly an object resists drag, equal to mass divided by the product of drag coefficient and area.
- Solar activity
- Solar activity refers to changes in the Sun, such as ultraviolet radiation and geomagnetic storms, that can heat and expand Earth's upper atmosphere.
Common Mistakes to Avoid
- Assuming low Earth orbit is a perfect vacuum. This is wrong because even trace gas particles can create important drag when a satellite is moving several kilometers per second.
- Thinking drag only reduces speed but does not affect altitude. This is wrong because drag removes orbital energy, causing the orbit to shrink and the satellite to move to lower altitudes.
- Using sea-level air density for orbital drag calculations. This is wrong because upper-atmosphere density is many orders of magnitude smaller and changes strongly with altitude and solar activity.
- Ignoring satellite orientation and area. This is wrong because drag depends on the cross-sectional area facing the flow, so a wide solar panel orientation can increase orbital decay.
Practice Questions
- 1 A 500 kg satellite at low orbit experiences a drag force of 0.020 N. What is its drag acceleration in m/s^2?
- 2 Use Fd = 1/2 rho v^2 Cd A for a satellite with rho = 4.0 x 10^-12 kg/m^3, v = 7800 m/s, Cd = 2.2, and A = 6.0 m^2. Calculate the drag force.
- 3 Two satellites have the same mass and orbit altitude, but one presents twice the cross-sectional area to its direction of motion. Explain which satellite will decay faster and why.