# Basic facts about astrodynamics

Orbital mechanics, also known as astrodynamics, is the application of ballistics and celestial mechanics to the practical problems associated with the movement of rockets or another spacecraft.

The motion of objects is usually calculated from Newton’s laws of motion and the laws of universal gravity. In parallel, orbital mechanics is a fundamental science for planning and controlling space missions.

There are many different tracks that are used for different purposes, some of them are telecommunications, science, technology demonstrations, remote sensing. In fact, what we need to know about these in the first round is that the orbits are not circular but elliptical. An ellipse is a closed plane curve surrounded by two focal points, and for each point on the curve the sum of the two distances of the focal points is constant.

What else you need to know is that the elongation of the ellipse is measured by the eccentricity of ‘e’, so from e=0 if is a circle, and the two foci are at the same point, to e=1. In short, eccentricity is the distance between the center and the focus. In fact, the sum of the eccentricity of the orbit maybe more than 1, but then orbit would not be close, but would be a hyperbola.

The central body of the system is always in one of the foci of the ellipse, so the Sun in the Solar System and the Earth in the satellite orbit. The point in the trajectory where the distance between the bodies is called the smallest Periapsis, in contrast, the apoapse is when the distance between the bodies is maximal.

If we speak of the earth, we call them perigeum and apogeum. The mean of the periapsis and apoapsis result in the value of the half-major axis, which is half the distance from the largest axis of the ellipse. In addition to distances, angles outside distances must also be considered when deciding the path.

The Equatorial Plane is the first such construction to include, as its name implies, the Equator of the Earth. The second plane is the plane orbit that has the orbit. The angle between the two planes is the angle of inclination, which varies between 0 and 180 degrees.

So, if the angle is less than 90 degrees, the course call prograde or direct, or if the angle of inclination is greater than 90 degrees, the course is called indirect or retrograde. And if the angle of inclination is 90 degrees, then the orbit passes through the poles, in which case it is called the polar orbit.

The next angle is the right ascent of the take-off note (RAAN), which is the equivalent of the spatial ground length. It is measured at the equinox from the Sun, the place in the celestial sphere where the Sun crosses the celestial equator from south to north during the spring equinox, when the equator has the same number of light hours as dark. RAAN is continuously measured in a completely circle from the coincidence of Earth and the Sun in space, and from that equator, the measurement increases to the east.

Information complied by: Dezső Sándor.

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