A Constantly Shifting Path
When most people picture a satellite in orbit, they often imagine a small object following a perfect, unchanging path around a perfectly spherical Earth. This image, reinforced by textbook diagrams, suggests a quiet, predictable existence, a simple balance between speed and gravity. The reality is far more complex and dynamic. An orbit isn’t a fixed railway track in the sky. It’s a constantly shifting path, subject to a surprising number of invisible forces that tug, push, and drag at any object placed in space.
From the moment a satellite is deployed, it begins a long, complex dance with the laws of physics. Its trajectory is influenced not just by the Earth’s main gravitational pull, but by the planet’s own imperfections, the distant reach of the Moon and Sun, the subtle friction of the planet’s outermost atmosphere, and even the pressure of sunlight itself. For satellite operators, managing an orbit is a continuous, active process of measurement and correction, fighting against a chorus of forces that all work to alter a satellite’s path. This article explores the many factors, both natural and human-made, that can affect the orbit of a satellite.
The Dominant Force: Gravity’s Complexities
Gravity is, without question, the main force governing a satellite’s motion. It’s the “string” that holds the satellite in orbit, constantly pulling it toward the center of the Earth as the satellite’s own velocity tries to fling it out into space. If gravity were simple – if the Earth were a perfect, uniform sphere and no other bodies existed in the universe – orbits would be perfectly predictable ellipses. But the real gravitational environment is far from simple.
Earth’s Lumpy Gravitational Field
The first major complication is the Earth itself. Our planet is not a perfect sphere. Its rotation causes it to bulge at the equator, making it slightly flattened, a shape known as an oblate spheroid. This equatorial bulge, a belt of extra mass, exerts a powerful and uneven gravitational pull on satellites. For a satellite in an inclined orbit (one that doesn’t circle directly over the equator), this bulge pulls on it more strongly as it passes the equator, causing the entire orbit to slowly twist or wobble over time.
This effect, known as orbital precession, isn’t just a nuisance; it’s a feature that satellite engineers actively exploit. A Sun-synchronous orbit is a prime example. This is a special type of polar orbit designed so that the equatorial bulge’s tug rotates the satellite’s orbital plane at the exact same rate that the Earth orbits the Sun – about one degree per day. The result is that the satellite crosses the equator at the same local solar time every single day. For Earth observation satellites, this is extremely valuable. It means they can take pictures of a specific city, for example, at 10:30 AM every time they pass, with the Sun’s shadows always in the same place, making it much easier to detect changes on the ground.
Beyond the equatorial bulge, the Earth’s mass isn’t distributed evenly. The planet is “lumpy.” It has massive mountains, deep ocean trenches, and variations in the density of the rock and magma deep within its mantle. Each of these features creates a small, local “bump” or “dip” in the planet’s gravitational field, known as a gravity anomaly. When a satellite flies over a dense mountain range, the extra mass pulls it just a tiny bit, altering its altitude and speed.
These variations are minuscule on a single pass, but their effects are cumulative. Over weeks and months, these tiny tugs can cause a satellite’s orbit to drift, particularly for those needing very precise positioning. In fact, these anomalies are so significant that NASA and other agencies have flown dedicated missions to map them. The Gravity Recovery and Climate Experiment (GRACE) mission, for example, used two satellites following each other in the same orbit. By measuring the tiny changes in the distance between them (down to the width of a human hair), they could map the Earth’s gravitational geoid in unprecedented detail. This mapping is essential not just for oceanography and geology but for accurately predicting the long-term behavior of all other satellites.
The Pull of the Moon and Sun
The Earth and its satellites don’t exist in a vacuum. They are part of a larger system, and the gravity of other celestial bodies also plays a role. This is often called third-body perturbation. For satellites orbiting the Earth, the two most significant “third bodies” are the Moon and the Sun.
The Moon, our closest neighbor, has a powerful gravitational influence. While its pull is far weaker than Earth’s, it’s strong enough to cause significant long-term changes, especially for satellites in high orbits. The most dramatic effect is seen in Geostationary Orbit (GEO). A GEO satellite orbits 35,786 kilometers (about 22,236 miles) above the equator, traveling at the same speed as the Earth’s rotation. This allows it to “hover” over a single point on the ground, making it perfect for communications and weather broadcasting.
The GEO orbit is aligned with Earth’s equator. The Moon’s orbit is not; it’s inclined by about 5 degrees. This means the Moon spends most of its time “above” or “below” the equatorial plane, and its gravity constantly pulls GEO satellites out of that plane. This pull slowly and relentlessly increases the satellite’s orbital inclination. If left uncorrected, a GEO satellite’s inclination would drift by almost a degree per year, making it useless as it wanders north and south in the sky from the perspective of a ground antenna.
To combat this, GEO satellites must dedicate a large portion of their onboard fuel to a process called orbital station-keeping. Every few weeks, they must fire their thrusters in a “north-south” direction to push their orbit back in line with the equator. The fuel required for this maneuver is often the limiting factor in a GEO satellite’s lifespan; when it runs out of this “station-keeping” fuel, its useful life is over.
The Sun’s gravity also has an effect, though for most Earth-orbiting satellites, it’s less pronounced than the Moon’s. For satellites in very high orbits, like Medium Earth Orbit (MEO) (home to GPS satellites) or High Earth Orbit (HEO), the combined gravitational pulls of the Sun and Moon can significantly distort the orbit’s shape and altitude over many years, requiring complex calculations to predict and manage.
The Atmosphere: A Persistent Drag
One of the most significant forces affecting satellites in lower orbits is one that many people assume doesn’t exist in space: an atmosphere. It’s a common misconception that space begins where the air “ends.” In reality, the Earth’s atmosphere doesn’t have a hard boundary; it just gets thinner and thinner with altitude.
Low Earth Orbit (LEO), the region from about 160 to 2,000 kilometers (100 to 1,240 miles) high, is home to thousands of satellites, including the International Space Station (ISS) and vast constellations like Starlink. This entire region is still within the thermosphere, the uppermost, thinnest layer of the atmosphere. While the air here is more than a million times less dense than at sea level, it’s not a perfect vacuum. It’s filled with stray atoms and molecules, mostly oxygen and nitrogen.
For a satellite traveling at over 17,000 miles per hour (7.8 km/s), hitting these few molecules is like running into a constant, subtle “wind.” This effect is called atmospheric drag. Each collision with a gas molecule transfers a tiny bit of momentum, robbing the satellite of its orbital energy. This causes the satellite to slow down.
In orbit, slowing down has a peculiar effect: it causes the satellite to drop to a lower altitude. But at a lower altitude, the air is denser. This denser air increases the drag, which slows the satellite down even more, causing it to drop faster. This feedback loop is known as orbital decay. Left unchecked, atmospheric drag will inevitably pull every satellite in LEO back to Earth, leading to a fiery atmospheric reentry.
This is not always a bad thing. It’s a natural “self-cleaning” mechanism for LEO, helping to remove old, dead satellites. It’s also intentionally used to dispose of satellites at the end of their lives. But for active missions, it’s a major operational challenge. The ISS, which orbits at a relatively low altitude of around 400 kilometers (250 miles), can lose 50 to 100 meters of altitude per day due to drag. To counteract this, it requires regular “re-boosts.” Visiting cargo vehicles, like the Northrop Grumman Cygnus or the Russian Progress, fire their own engines while docked to push the entire 450-ton station back up to a higher orbit.
The Sun’s Influence on Drag
The problem of atmospheric drag is made even more complex by the Sun. The thermosphere is not static; it “breathes,” expanding and contracting in response to solar activity. The Sun’s output follows an 11-year solar cycle, moving between periods of low activity (solar minimum) and high activity (solar maximum).
During a solar maximum, the Sun bombards the Earth with more high-energy ultraviolet and X-ray radiation. This energy is absorbed by the thermosphere, causing it to heat up and expand dramatically. The “top” of the atmosphere puffs up, extending hundreds of kilometers higher into space. For a satellite orbiting at 500 kilometers, this means the density of the air it’s flying through can increase by a factor of 10, 100, or even 1,000 almost overnight.
This variability makes predicting orbital decay extremely difficult. The most famous example of this was Skylab, America’s first space station. NASA had planned for it to stay in orbit for years after its last crew left in 1974, hoping to re-boost it with the new Space Shuttle. But in 1977-78, the Sun entered an unexpectedly active cycle. The thermosphere swelled, drag on the massive station increased far beyond predictions, and its orbit began to decay rapidly. NASA scrambled for a solution but couldn’t act in time, and Skylab re-entered the atmosphere in 1979, scattering debris over the Indian Ocean and Australia.
This remains a modern challenge. Predictable drag is essential for managing satellite constellations, and sudden solar-driven changes can have major consequences.
The Push of Light: Solar Radiation Pressure
Gravity pulls, and the atmosphere drags. But there is another, more subtle force that pushes satellites: sunlight itself.
Light is made of particles called photons. While photons have no mass, they carry momentum. When a photon strikes a surface, it imparts a tiny, tiny push. This constant stream of photons from the Sun creates a gentle, continuous force known as solar radiation pressure (SRP).
On Earth, this pressure is unnoticeable, completely overwhelmed by wind and other forces. But in the vacuumof space, it’s a different story. The push is minuscule – on a one-square-meter solar panel, it’s about the weight of a single grain of salt. But unlike drag, which fades at high altitudes, SRP is constant and relentless. For a satellite in orbit for years or decades, this tiny push adds up, slowly but surely altering its path.
The effect of SRP depends heavily on the satellite’s design, specifically its area-to-mass ratio. A small, dense, cannonball-shaped satellite will be affected very little. But a satellite with large, lightweight structures – like the massive, shiny solar arrays on a GEO communications satellite or the enormous, multi-layered sunshield on the James Webb Space Telescope (JWST) – presents a large “sail” for the sunlight to push against.
The satellite’s surface properties also matter. A shiny, reflective surface (like mirrored foil) will get a stronger “push” than a black, absorptive surface, as the photons “bounce” off, transferring double their momentum. This complex interplay of forces, which change as the satellite orbits and its surfaces angle toward or away from the Sun, can make the orbit slightly more eccentric (oval-shaped) and can slowly raise or lower its perigee (lowest point) and apogee (highest point).
For high-precision missions, like GPS, operators must have very accurate models of the satellite’s shape and materials to account for SRP. If they didn’t, their orbital predictions would be wrong, and the navigation data they provide would become inaccurate.
This force can also be harnessed. A solar sail is a spacecraft designed to use SRP for propulsion. By deploying a huge, thin, highly reflective sail, a spacecraft can “sail” on sunlight, gaining velocity over time without using any propellant. Japan‘s IKAROS mission successfully demonstrated this in 2010, and The Planetary Society, a non-profit organization, has flown successful LightSail demonstration missions, proving the concept in Earth orbit.
The Charged Environment: Space Weather
The space around Earth isn’t just empty and full of sunlight. It’s also an active, electrical environment, filled with invisible magnetic fields and streams of charged particles. This dynamic, and sometimes violent, set of conditions is known as space weather.
The Sun doesn’t just emit light; it constantly blows out a stream of plasma – a high-energy gas of electrons and protons – called the solar wind. This solar wind interacts with Earth’s magnetic field, creating a protective “bubble” called the magnetosphere. Satellites in orbit are flying directly through this complex, charged environment.
Satellite Charging
As a satellite moves through this plasma, it can build up a static electric charge, similar to how a person builds up static shuffling their feet on a carpet. Different parts of the satellite can charge to different levels. The side facing the Sun might be in a different plasma environment than the side in shadow, or one material might collect negative electrons while another material gets hit by positive ions.
This can lead to a “differential charging,” where one part of the satellite builds up a large negative charge while another part remains positive. The voltage difference between these two points can grow to be thousands of volts. At some point, this stored electrical energy can discharge in a sudden arc – an electrostatic discharge, essentially a tiny lightning bolt inside the satellite.
This “space lightning” is a major hazard for satellite electronics, capable of short-circuiting boards and frying sensitive components. While this is primarily a systems-failure risk, it can directly affect the orbit if the discharge damages a key component, such as the thruster control system or the attitude control computer. A satellite that can’t control which way it’s pointing or fire its engines is a satellite that can’t correct its orbit.
Magnetic Fields and Lorentz Force
This charging has another, more direct effect on the orbit. The satellite, now carrying its own electrical charge, is moving at high speed through Earth’s magnetic field. Basic physics states that a moving charge in a magnetic field experiences a force. This is known as the Lorentz force.
This force is another small, non-gravitational push or pull on the satellite. It’s a very complex effect, as it depends on the satellite’s charge (which is constantly changing), its velocity, and the local strength and direction of the magnetosphere (which is also constantly changing). For most satellites, it’s a minor perturbation, but for high-altitude satellites with large surface areas, it’s another small but persistent force that must be factored into orbital models.
Coronal Mass Ejections
The most dramatic and dangerous space weather events are Coronal Mass Ejections (CMEs). These are enormous, violent eruptions from the Sun’s surface, blasting billions of tons of plasma and magnetic fields into space at millions of miles per hour. If a CME is aimed at Earth, it can have severe consequences for satellites.
When a CME strikes the magnetosphere, it triggers a geomagnetic storm. This event has two major effects on orbits.
First, it floods the magnetosphere with high-energy particles, dramatically increasing the risk of satellite charging and electrostatic discharge. This can lead to widespread electronic failures.
Second, the storm dumps a massive amount of energy into the Earth’s upper atmosphere, causing the thermosphere to heat up and expand far more intensely than it does during a normal solar maximum. This sudden, severe increase in atmospheric density rapidly increases drag on all LEO satellites.
A stark example of this occurred in February 2022. SpaceX had just launched a batch of 49 Starlink satellites into their initial, very low parking orbit. Just one day later, a geomagnetic storm hit. SpaceX reported that the resulting atmospheric drag was at least 50 percent higher than in previous launches. The satellites, which were in a low-power “safe mode” and had not yet raised their orbits, couldn’t overcome this intense drag. Despite efforts to fly them “edge-on” like a piece of paper to minimize resistance, the orbital decay was too fast. At least 40 of the 49 satellites were lost, re-entering the atmosphere and burning up just days after they were launched. This event was a powerful demonstration of how solar storms can directly and catastrophically affect satellite orbits.
Human-Made Influences
The final category of orbital influences is not from nature, but from satellite operators themselves and the environment they have created.
Orbital Maneuvers and Station-Keeping
Many orbital changes are deliberate. Satellites are not passive objects; most are equipped with thrusters to adjust their paths. This orbital station-keeping is a routine and essential part of operations.
As discussed, GEO satellites constantly fire their engines to fight the pull of the Moon and Sun and stay locked over their assigned longitude. LEO satellites, like the ISS, must fire their thrusters to re-boost their altitude and combat atmospheric drag.
Modern satellite constellations, like Starlink, OneWeb, and GPS, rely on precise formation flying. Their orbits must be maintained with high accuracy to ensure global coverage and, just as importantly, to avoid colliding with each other. These satellites often use highly efficient electric propulsion systems, such as Hall-effect thrusters, which provide a very gentle, continuous thrust to meticulously manage their orbital positions.
Finally, responsible operators use a satellite’s last bit of fuel to perform a de-orbiting burn. This maneuver slows the satellite down, causing it to drop into a steep orbital decay path that ensures it re-enters and burns up over a safe, unpopulated area (usually the South Pacific Ocean) within a planned timeframe. This is a deliberate change to the orbit intended to prevent the satellite from becoming a long-term hazard.
The Growing Menace of Space Debris
The most acute and unpredictable human-made threat to satellite orbits is space debris. Decades of space activity have left LEO and GEO cluttered with “space junk”: spent rocket upper stages, dead satellites, and millions of fragments from past explosions and collisions.
In LEO, this debris travels at speeds exceeding 17,000 mph. At such speeds, the kinetic energy is enormous. A hypervelocity impact with a paint fleck can chip a space station window. A collision with a 1-centimeter object is like being hit with a hand grenade. A collision with a 10-centimeter object is catastrophic and will completely destroy a satellite.
A direct collision is the most violent way an orbit can be altered. It instantly changes the satellite’s velocity and spin, sending it and a cloud of thousands of new fragments tumbling into new, unpredictable, and dangerous orbits. The most infamous example was the 2009 Iridium 33 and Kosmos-2251 collision. Two large satellites – one active American communications satellite and one defunct Russian military satellite – collided over Siberia, obliterating both and creating a debris cloud of over 2,300 large, trackable fragments that will threaten other satellites in that altitude band for decades.
This event highlighted the reality of the Kessler syndrome, a scenario proposed by NASA scientist Donald J. Kessler in 1978. He theorized that if the density of debris in LEO becomes high enough, a single collision could set off a chain reaction. The debris from one collision would hit other satellites, creating more debris, which would then hit more satellites, and so on. This cascading effect could eventually make certain orbital altitudes completely unusable.
Today, the threat of collision changes orbits just as much as an actual collision. The United States Space Force (USSF) and other international bodies use a sophisticated Space Surveillance Network of ground-based radars and optical telescopes to track tens of thousands of debris objects larger than a softball. They run high-speed computers to predict “conjunctions,” or close passes, between these objects and active satellites.
When the system flags a high-probability “conjunction,” satellite operators are notified. They must then perform a Collision Avoidance Maneuver (CAM). This involves firing the satellite’s thrusters to make a small, planned change to its orbit, moving it “up,” “down,” or “sideways” to get out of the way. The ISS has had to perform these maneuvers dozens of times, sometimes with the crew sheltering in their return capsules as a precaution. These avoidance maneuvers are now a routine part of operations for hundreds of satellites. Each maneuver consumes precious fuel, shortening the satellite’s operational life, all in response to the orbital “noise” created by decades of human activity.
Subtle and Complex Effects
Beyond the main factors, there are even more subtle forces at play that high-precision missions must account for.
Albedo and Earthshine
Solar radiation pressure comes from the Sun. But light also reflects off the Earth. The planet’s albedo is its measure of reflectivity. Sunlight that hits bright surfaces like clouds or the polar ice caps is reflected back into space as “Earthshine.”
This reflected light also carries momentum and exerts its own tiny pressure on a satellite. This force is much weaker than direct sunlight and is extremely variable. A satellite passing over a dark ocean experiences less pressure than one passing over the bright white top of a storm system. This variable push is another small perturbation that must be modeled for precise orbit determination.
Relativistic Effects
Finally, on the very edges of orbital mechanics, are the effects of Albert Einstein‘s theory of General Relativity. Newton’s laws of gravity are an excellent approximation, but Einstein’s theory describes gravity as a curvature of spacetime caused by mass.
This has tangible, measurable effects on satellites. The Global Positioning System (GPS) is the classic example. The atomic clocks on GPS satellites are in a weaker gravitational field (and moving at high speed) compared to clocks on the ground. According to relativity, this means their clocks “tick” faster than ground-based clocks by about 38 microseconds per day.
This isn’t a “force” pushing the satellite, but it’s a fundamental property of its orbital state (its position in spacetime). If this relativistic effect were not calculated and corrected for, the entire GPS system would fail. The timing errors would accumulate, causing the navigational positions to be wrong by more than 10 kilometers (6 miles) every single day. This demonstrates that for the most demanding applications, even the fabric of spacetime itself is a factor in managing a satellite’s mission from orbit.
Summary
A satellite’s orbit is not the simple, perfect path many imagine. It is a dynamic and ever-changing trajectory, sculpted by a complex interplay of forces. The dominant pull of Earth’s gravity is complicated by the planet’s own lumpy, oblate shape. The distant but persistent gravitational tugs of the Moon and Sun work to pull satellites out of alignment.
In lower orbits, the whisper-thin upper atmosphere creates a constant drag, powered and puffed up by the Sun’s own variable activity, that inevitably pulls satellites down. At all altitudes, the constant, gentle push of sunlight itself, solar radiation pressure, slowly nudges satellites off course. The invisible environment of space weather can charge satellites, creating new forces and posing a direct threat during violent solar storms.
On top of all this is the human element. Operators must constantly perform maneuvers to correct for these natural perturbations, while also dodging the ever-growing cloud of space debris created by our own activities. The business of orbital mechanics is not about a static state, but about the continuous management of these many competing influences to keep our vital infrastructure in the sky safe and functional.
