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What Is Atmospheric Drag?
Atmospheric drag, sometimes called air resistance, is the force put on an object by the atmosphere surrounding it.
In orbital space crafts, like space stations and telescopes, atmospheric drag caused by collision with gas molecules is the main reason for orbital decay, whereby the orbital trajectory of an object degrades overtime until ultimately it collides with the object it is orbiting. This is addressed through corrective orbital boosts, which reposition the object on its orbital trajectory.
Orbital decay from atmospheric drag brought down the United States’ first manned spaceship, Skylab, and a closely-monitored orbital decay was used to remove space station Mir from orbit as well.
What Affects Atmospheric Drag?
Atmospheric drag is affected by the density of the atmosphere—a more dense atmosphere will cause more atmospheric drag than a lower density one. At higher altitudes, the atmosphere is less dense.
Solar activity such as solar radiation pressure increases atmospheric density as solar energy causes the air molecules in the relatively denser lower atmosphere to rise into the less dense upper atmosphere. Thus space weather, like solar wind, has a direct impact on the atmospheric drag orbital objects experience.
How Does Atmospheric Drag Work?
The atmosphere is created by the earth’s magnetic field which emerges from the earth’s interior. This creates a gravity field that shapes the atmosphere.
In space research and exploration, atmospheric drag affects both the entry into orbit via rocket propulsion and the object’s orbital trajectory once in space.
In rocket propulsion, atmospheric drag works to keep objects from penetrating and exiting the upper atmosphere and entering orbit; this significant force must be accounted for when designing and launching space crafts to penetrate the atmosphere.
In fact, in space missions, atmospheric drag is the single greatest obstacle to getting objects like a space shuttle into orbital space as it must penetrate multiple atmospheric layers such as the mesosphere and thermosphere.
Orbital elements like spacecrafts, space stations, and satellite orbits, though in relatively thinner air density, nonetheless must account for atmospheric drag since atmospheric drag is the main cause of orbital decay, a pressing concern for all objects in circular orbit. This must be addressed regularly by orbital mechanics according to certain time scales.
What Is the Relationship Between Atmospheric Drag and Orbital Decay?
In a space environment, orbital decay slowly changes the trajectory of objects in orbit, bringing them closer to the object they orbit (most often, Earth). Left unchecked, orbital decay will ultimately pull objects back to the Earth’s atmosphere, where they will likely burn and disintegrate upon re-entry into the upper atmosphere.
Orbital decay caused by atmospheric drag is characterized by positive feedback, meaning the more it affects an object, the more that object is subject to its force. This is the case because:
- Atmospheric drag brings objects closer to the objects they are orbiting, lowering their altitude.
- The lower an object’s altitude, the denser the air surrounding it.
- This increases the drag on the object.
- As this feedback increases, the attendant heat also increases.
- The object ultimately burns up as the atmosphere surrounding it becomes too dense.
This is the case with all objects in low earth orbit, including space debris which is regularly pulled into the earth’s atmosphere.
In order to prevent unintentional satellite re-entry, orbital space crafts like satellites, telescopes, and space stations all must regularly implement corrective orbital boost or altitude boost, where the ship counteracts the drag and orbital decay with controlled propulsion away from the object it is orbiting.
What Is the Effect of Atmospheric Drag On Space Stations?
Overtime, atmospheric drag will affect space stations and other artificial satellites in low earth orbit by contributing to their orbital decay, ultimately bringing them closer and closer to the object they orbit, Earth.
Thus, without regular orbital altitude corrections in an orbital period, the altitude of a satellite will decrease; orbital corrections are a regular part of keeping space stations aloft.
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