# Perigee – Definition & Detailed Explanation – Rocketry & Propulsion Glossary

## I. What is Perigee?

Perigee is a term used in astronomy and aerospace engineering to describe the point in an object’s orbit around a celestial body where it is closest to that body. In the case of Earth’s orbit, perigee refers to the point in a satellite or spacecraft’s orbit where it is closest to Earth. This term is commonly used in the context of space missions and rocket launches to describe the closest point of approach to a celestial body.

## II. How is Perigee Calculated?

Perigee is calculated using the principles of orbital mechanics and Kepler’s laws of planetary motion. The distance of an object’s perigee from a celestial body is determined by its eccentricity, or the shape of its orbit, as well as its semi-major axis, or the average distance from the celestial body. By analyzing these factors, scientists and engineers can accurately calculate the perigee of a satellite or spacecraft in orbit.

## III. What is the Significance of Perigee in Rocketry?

Perigee plays a crucial role in rocketry and space missions because it determines the closest distance a spacecraft will be to Earth during its orbit. This is important for mission planning, as it affects the amount of fuel required for maneuvers and the timing of critical operations such as satellite deployment or rendezvous with other spacecraft. Understanding and calculating perigee is essential for ensuring the success of space missions.

## IV. How Does Perigee Affect Orbital Maneuvers?

Perigee affects orbital maneuvers by influencing the amount of energy required to change the shape or orientation of an object’s orbit. For example, raising the perigee of a spacecraft requires a significant amount of energy, as it involves accelerating the spacecraft to a higher altitude. Similarly, lowering the perigee of a spacecraft requires energy to decelerate and bring the spacecraft closer to Earth. Understanding the effects of perigee on orbital maneuvers is essential for planning and executing complex space missions.

## V. What are Some Examples of Perigee in Space Missions?

Perigee plays a critical role in many space missions, including satellite launches, space station rendezvous, and planetary exploration missions. For example, the Hubble Space Telescope orbits Earth with a perigee of approximately 547 kilometers, allowing it to capture high-resolution images of distant galaxies and celestial objects. In contrast, the International Space Station has a perigee of around 400 kilometers, enabling it to support human habitation and scientific research in low Earth orbit. Understanding perigee is essential for the success of these and other space missions.

## VI. How is Perigee Different from Apogee?

Perigee and apogee are two terms used to describe the closest and farthest points of an object’s orbit around a celestial body, respectively. While perigee refers to the closest point of approach, apogee refers to the farthest point of approach. Understanding the differences between perigee and apogee is essential for accurately describing and analyzing the orbits of satellites, spacecraft, and other objects in space. By calculating and monitoring perigee and apogee, scientists and engineers can ensure the success of space missions and maximize the efficiency of orbital maneuvers.