01 / Gravity and Forward Motion

Gravity and Forward Motion

Gravity provides the inward acceleration that continuously bends an object's path. At the same time, inertia causes the object to continue moving forward. Together, these effects produce the curved trajectory of an orbit.

The International Space Station is a familiar example. It travels around Earth at roughly 17,500 miles per hour (28,000 kilometers per hour). Earth's gravity is constantly pulling the station downward, but its forward speed carries it far enough around the planet that the surface curves away beneath it.

This continuous free fall also explains why astronauts aboard orbiting spacecraft experience weightlessness. The spacecraft and everything inside it are falling together along the same orbital path.

Spacecraft do not normally need to fire their engines continuously to remain in orbit. Once an appropriate trajectory has been established, they can coast for long periods. Engines are used when the orbit needs to be changed or when small effects such as atmospheric drag or gravitational disturbances gradually alter the spacecraft's path.

02 / Orbital Speed

Orbital Speed

Orbital speed is the velocity associated with a particular orbit around a larger body. The required speed changes with both distance and gravitational strength.

Near a massive body, gravity is stronger and a spacecraft in a low circular orbit must move quickly. At greater distances, gravity is weaker and the circular orbital speed is lower.

This relationship can be seen in the speeds of satellites at different altitudes around Earth.

03 / Speed at Different Altitudes

Speed at Different Altitudes

In low Earth orbit, at an altitude of roughly 250 miles (400 kilometers), spacecraft travel at about 17,500 miles per hour (28,000 kilometers per hour) and complete an orbit in approximately 90 minutes. The International Space Station operates in this region.

Navigation satellites such as those in the GPS constellation orbit much farther from Earth. Their speeds are roughly 8,700 miles per hour (14,000 kilometers per hour), with orbital periods of about 12 hours.

Geostationary satellites orbit farther still, traveling at approximately 6,900 miles per hour (11,100 kilometers per hour). Their orbital period matches Earth's rotation, allowing them to remain above nearly the same location on the equator.

On a much larger scale, Earth itself travels around the Sun at an average speed of about 67,000 miles per hour (108,000 kilometers per hour), completing one revolution each year.

• Low Earth Orbit (LEO): About 17,500 mph (28,000 km/h), with a period of roughly 90 minutes
• Medium Earth Orbit (GPS): About 8,700 mph (14,000 km/h), with a period of about 12 hours
• Geostationary Orbit (GEO): About 6,900 mph (11,100 km/h), with a period of one sidereal day
• Earth Around the Sun: About 67,000 mph (108,000 km/h), with a period of one year

04 / What Happens When the Speed Changes?

What Happens When the Speed Changes?

The speed of an orbiting object helps determine the shape and size of its trajectory. A spacecraft moving too slowly for its intended orbit will not remain on the same path. Its trajectory can drop closer to the central body, potentially leading to a lower orbit or, in extreme cases, reentry or impact.

If the spacecraft increases its speed, its orbit can expand outward. A sufficiently large increase can place it into a wider elliptical orbit, while an even greater increase can eventually produce an unbound trajectory.

Near Earth's surface, escape velocity is approximately 25,000 miles per hour (40,000 kilometers per hour). Reaching escape velocity does not mean gravity suddenly disappears. Instead, it means the object's motion is sufficient to continue outward without remaining gravitationally bound to Earth.

05 / Speed in Elliptical Orbits

Speed in Elliptical Orbits

In a circular orbit, an object's speed remains nearly constant. In an elliptical orbit, however, the speed changes throughout the journey.

An orbiting object moves faster when it is closer to the body it orbits and slower when it is farther away. This behavior is described by Kepler's Second Law and is a fundamental feature of elliptical orbital motion.

Comets provide a dramatic example. A comet may move relatively slowly while traveling through the distant outer Solar System, then accelerate greatly as it approaches the Sun. After passing through the inner Solar System, it slows again as it travels outward.

06 / Changing an Orbit

Changing an Orbit

Spacecraft can deliberately change their trajectories by altering their speed and direction. Carefully timed engine burns allow mission planners to raise or lower an orbit, enter an elliptical transfer path, approach another world, or depart from a planet entirely.

Increasing speed at one point in an orbit can raise another part of the orbit, while reducing speed can lower it. These maneuvers allow spacecraft to move between different orbital altitudes without continuously using their engines.

Gravity assists provide another way to reshape a spacecraft's motion. By passing close to a moving planet, a spacecraft can exchange energy and momentum with the planet's orbital motion, changing its speed and direction while reducing the amount of fuel required.

Voyager 1 and Voyager 2 used a series of planetary gravity assists to explore the outer Solar System, while Parker Solar Probe uses repeated flybys of Venus to reshape its orbit and bring it progressively closer to the Sun.

07 / Why Orbital Motion Matters

Why Orbital Motion Matters

Understanding the relationship between gravity, speed, and distance is essential for nearly every form of spaceflight. Satellites must be placed into appropriate trajectories, spacecraft must change speed precisely during orbital maneuvers, and mission planners must predict how objects will move under gravity over long periods of time.

The same principles apply far beyond artificial satellites. The Moon orbiting Earth, Earth orbiting the Sun, comets sweeping through the Solar System, and planets circling distant stars all follow the same basic relationship between gravity and motion.

Orbital motion is not the result of gravity and forward motion canceling each other out. Gravity continually changes the direction of an object's velocity, bending what would otherwise be a straight path into an orbit. The speed of the object then helps determine whether that path becomes a circular orbit, an ellipse, or an escape trajectory.