What is the point of apoapsis?
The point of apoapsis is the point in an orbit farthest from the body being orbited. It is also called apsis or apocenter. The point of apoapsis depends on the type of orbit and the host body. For example, the point of apoapsis for a celestial body orbiting the Sun is called aphelion, for a body orbiting the Earth is called apogee, and for a body orbiting a star is called apastron . The point of apoapsis is where the orbital speed is the lowest, according to Kepler’s second law. The opposite of the point of apoapsis is the point of periapsis, which is the point of closest approach to the host body.
The point of apoapsis has important implications for space exploration and astronomy. For example, a spacecraft can use a gravity assist maneuver at the point of apoapsis to increase its speed and change its trajectory. The point of apoapsis can also affect the climate and seasons of a planet, depending on its orbital eccentricity and axial tilt. For instance, Earth reaches its aphelion in early July, which coincides with summer in the northern hemisphere and winter in the southern hemisphere. The point of apoapsis can also influence the apparent size and brightness of celestial objects, such as the Moon and planets.
One way to calculate the point of apoapsis is to use the vis-viva equation, which relates the orbital speed, the orbital radius, and the standard gravitational parameter of the host body. The point of apoapsis occurs when the orbital speed is the minimum, which means that the orbital radius is the maximum. Therefore, by solving for the orbital radius when the orbital speed is equal to the square root of the standard gravitational parameter divided by twice the semi-major axis of the orbit, one can find the point of apoapsis. The semi-major axis is half of the longest diameter of the orbit.
The point of apoapsis varies depending on the shape and orientation of the orbit. Orbits with high eccentricity have a large difference between the point of apoapsis and the point of periapsis, while orbits with low eccentricity have a small difference. For example, a circular orbit has zero eccentricity and no point of apoapsis or periapsis, while a parabolic or hyperbolic orbit has an eccentricity greater than one and only a point of periapsis. The point of apoapsis can also change over time due to gravitational perturbations from other bodies or non-uniformities in the host body’s mass distribution.
The point of apoapsis affects satellite communications in several ways. For satellites in geostationary orbit, which have a constant position above the Earth’s equator, the point of apoapsis is irrelevant because their orbit is circular. However, for satellites in geosynchronous orbit, which have a constant period but not a constant position, the point of apoapsis can cause variations in their altitude and speed, which can affect their signal strength and coverage area. For satellites in low Earth orbit, which have a short period and a high speed, the point of apoapsis can influence their orbital decay and atmospheric drag, which can shorten their lifespan and require more frequent maneuvers.