Why satellites stay in orbit: the balance between gravity and velocity

We’re all used to seeing the Moon, our natural satellite, shining brightly in the night sky. But have you ever spotted an artificial one gliding silently overhead? These man-made objects, orbiting Earth, appear as steady, moving points of light — they don’t twinkle like stars or flash like airplanes. You can often see them shortly after sunset or just before sunrise, when the sky is dark but the satellite is still high enough to catch sunlight, reflecting it down to your eyes. The brightest among them is the International Space Station (ISS), which can even outshine Venus.

Fig. 1. The ISS orbiting Earth. Image generated with assistance from ChatGPT (OpenAI)

Long before the satellite era, people wondered why the Moon remains in the sky while everything on Earth falls to the ground. Sir Isaac Newton answered this question in Principia Mathematica (1687), explaining that celestial bodies maintain stable orbits thanks to a delicate balance between gravity and motion. In essence, the Moon is constantly “falling” toward Earth — but its sideways velocity keeps shifting it away just enough to trace a curved path around our planet rather than crashing into it.

Motion Without Gravity. The best way to understand why this balance works is to separate the two effects — motion and gravity — and then combine them. Let’s start by imagining a simplified world where gravity doesn’t exist (Fig. 2, left). Picture the Earth replaced by a massless dot C in space. Now send a satellite flying at the speed V past it. With no gravitational pull to alter its course, the satellite will continue moving forever in a straight line, following Newton’s first law of motion: an object in motion stays in motion unless acted upon by an external force. It won’t bend toward the dot or slow down. Instead, it will simply pass by and continue moving away at the constant speed, increasing its distance from the dot with each passing second.

Fig. 2. The formation of an orbit as a balance between straight-line motion (left) and gravitational pull (center). When both act together (right), the path curves continuously around Earth, resulting in a stable orbit.

Gravity without motion. Now let’s position the Earth in place of the dot, erase the satellite’s velocity, and let it go (Fig. 2, center). Without any horizontal motion, the satellite will be pulled toward the Earth, accelerating along the line connecting their centers. This is pure free fall. The satellite doesn’t orbit. It simply plunges straight down along the gravitational vector, decreasing its distance from Earth with each passing second.

Motion plus gravity. We combine these two effects, keeping gravity active and giving the satellite some sideways velocity (Fig. 2, right). As the satellite's velocity tries to move it along a straight line away from Earth, gravity constantly pulls it toward the Earth, bending its straight path into a curve. If the satellite's speed is too fast, gravity won't curve it enough, and the satellite will spiral away from Earth into space. If the speed is too slow, gravity will curve the path too much, making the satellite spiral inward and burn up in the atmosphere. When the speed is just right, gravity will curve the satellite's trajectory into a stable loop, allowing the satellite to maintain a stable orbit.

Higher Orbits, Lower Speeds. As altitude increases, gravity weakens, reducing the inward pull. To stay balanced, the orbital velocity must decrease accordingly. This is why satellites in higher orbits move more slowly than those in lower orbits. For example, geostationary satellites, which synchronize their rotation with Earth's 24-hour period, must orbit at a high altitude of 35,786 km, requiring a speed of 3.1 km/s. In contrast, the low-orbit satellites, such as the ISS, orbiting Earth at approximately 400 kilometers (≈ 250 miles), must travel much faster, at about 7.7 km/s, to avoid crashing into Earth. This delicate balance — stronger gravity necessitates higher speed, while weaker gravity allows for lower speed — governs all orbital motion, from satellites to the Moon.

No air, no friction, no fuel. Satellites orbit far above Earth's atmosphere, where the air is so thin that it exerts virtually no drag. Without air resistance, there’s no loss of energy and no slowdown, allowing the satellite's motion to continue indefinitely. This is why the Moon doesn't require engines to sustain its graceful dance around our planet, illuminating our nights with its majestic presence. The Moon's perfectly balanced speed will continue to counteract the Earth's gravity until the Sun burns its fuel and the solar system finishes its natural cycle. In low-orbit ISS, where a trace of atmosphere still lingers, drag slowly robs satellites of energy, which is why the ISS requires occasional engine boosts to maintain its altitude. However, once you are high enough, beyond the reach of air molecules, you can truly fall forever without actually falling.

This is the essence of orbital motion: a continuous compromise between the straight line of the sideways velocity and the gravity curving it inward. Newton imagined firing a cannon from a tall mountain; with enough speed, the cannonball would fall toward Earth but never reach it, tracing a circle around the globe. That same idea powers every spacecraft in orbit today, from the ISS to communication satellites to the Moon itself. Next time you spot a silent light moving above the horizon, remember: what you are seeing is an object falling toward Earth… but always missing.