In astronomy, and in particular in astrodynamics, the **osculating orbit** of an object in space at a given moment in time is the gravitational Kepler orbit (i.e. an elliptic or other conic one) that it would have around its central body if perturbations were absent.^{ [1] } That is, it is the orbit that coincides with the current orbital state vectors (position and velocity).

The word * osculate * is Latin for "kiss". In mathematics, two curves osculate when they just touch, without (necessarily) crossing, at a point, where both have the same position and slope, i.e. the two curves "kiss".

An osculating orbit and the object's position upon it can be fully described by the six standard Kepler orbital elements (osculating elements), which are easy to calculate as long as one knows the object's position and velocity relative to the central body. The osculating elements would remain constant in the absence of perturbations. Real astronomical orbits experience perturbations that cause the osculating elements to evolve, sometimes very quickly. In cases where general celestial mechanical analyses of the motion have been carried out (as they have been for the major planets, the Moon, and other planetary satellites), the orbit can be described by a set of mean elements with secular and periodic terms. In the case of minor planets, a system of proper orbital elements has been devised to enable representation of the most important aspects of their orbits.

Perturbations that cause an object's osculating orbit to change can arise from:

- A non-spherical component to the central body (when the central body can be modeled neither with a point mass nor with a spherically symmetrical mass distribution, e.g. when it is an oblate spheroid).
- A third body or multiple other bodies whose gravity perturbs the object's orbit, for example the effect of the Moon's gravity on objects orbiting Earth.
- A relativistic correction.
- A non-gravitational force acting on the body, for example force arising from:
- Thrust from a rocket engine
- Releasing, leaking, venting or ablation of a material
- Collisions with other objects
- Atmospheric drag
- Radiation pressure
- Solar wind pressure
- Switch to a non-inertial reference frame (e.g. when a satellite's orbit is described in a reference frame associated with the precessing equator of the planet).

An object's orbital parameters will be different if they are expressed with respect to a non-inertial reference frame (for example, a frame co-precessing with the primary's equator), than if it is expressed with respect to a (non-rotating) inertial reference frame.

Put in more general terms, a perturbed trajectory can be analysed as if assembled of points, each of which is contributed by a curve out of a sequence of curves. Variables parameterising the curves within this family can be called * orbital elements *. Typically (though not necessarily), these curves are chosen as Keplerian conics, all of which share one focus. In most situations, it is convenient to set each of these curves tangent to the trajectory at the point of intersection. Curves that obey this condition (and also the further condition that they have the same curvature at the point of tangency as would be produced by the object's gravity towards the central body in the absence of perturbing forces) are called osculating, while the variables parameterising these curves are called osculating elements. In some situations, description of orbital motion can be simplified and approximated by choosing orbital elements that are not osculating. Also, in some situations, the standard (Lagrange-type or Delaunay-type) equations furnish orbital elements that turn out to be non-osculating.^{ [2] }

In celestial mechanics, the **Lagrange points** are points of equilibrium for small-mass objects under the influence of two massive orbiting bodies. Mathematically, this involves the solution of the restricted three-body problem in which two bodies are far more massive than the third.

In celestial mechanics, an **orbit** is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a planet, moon, asteroid, or Lagrange point. Normally, orbit refers to a regularly repeating trajectory, although it may also refer to a non-repeating trajectory. To a close approximation, planets and satellites follow elliptic orbits, with the center of mass being orbited at a focal point of the ellipse, as described by Kepler's laws of planetary motion.

**Precession** is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In other words, if the axis of rotation of a body is itself rotating about a second axis, that body is said to be precessing about the second axis. A motion in which the second Euler angle changes is called *nutation*. In physics, there are two types of precession: torque-free and torque-induced.

**Orbital elements** are the parameters required to uniquely identify a specific orbit. In celestial mechanics these elements are considered in two-body systems using a Kepler orbit. There are many different ways to mathematically describe the same orbit, but certain schemes, each consisting of a set of six parameters, are commonly used in astronomy and orbital mechanics.

In mathematics and applied mathematics, **perturbation theory** comprises methods for finding an approximate solution to a problem, by starting from the exact solution of a related, simpler problem. A critical feature of the technique is a middle step that breaks the problem into "solvable" and "perturbative" parts. In perturbation theory, the solution is expressed as a power series in a small parameter . The first term is the known solution to the solvable problem. Successive terms in the series at higher powers of usually become smaller. An approximate 'perturbation solution' is obtained by truncating the series, usually by keeping only the first two terms, the solution to the known problem and the 'first order' perturbation correction.

**Celestial mechanics** is the branch of astronomy that deals with the motions of objects in outer space. Historically, celestial mechanics applies principles of physics to astronomical objects, such as stars and planets, to produce ephemeris data.

The **orbital period** is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting planets, exoplanets orbiting other stars, or binary stars.

**Orbital mechanics** or **astrodynamics** is the application of ballistics and celestial mechanics to the practical problems concerning the motion of rockets and other spacecraft. The motion of these objects is usually calculated from Newton's laws of motion and law of universal gravitation. Orbital mechanics is a core discipline within space-mission design and control.

The **Laplace plane** or **Laplacian plane** of a planetary satellite, named after its discoverer Pierre-Simon Laplace (1749–1827), is a mean or reference plane about whose axis the instantaneous orbital plane of that satellite precesses.

In astrodynamics and celestial dynamics, the **orbital state vectors** of an orbit are Cartesian vectors of position and velocity that together with their time (epoch) uniquely determine the trajectory of the orbiting body in space.

A **sphere of influence** (**SOI**) in astrodynamics and astronomy is the oblate-spheroid-shaped region around a celestial body where the primary gravitational influence on an orbiting object is that body. This is usually used to describe the areas in the Solar System where planets dominate the orbits of surrounding objects such as moons, despite the presence of the much more massive but distant Sun. In the patched conic approximation, used in estimating the trajectories of bodies moving between the neighbourhoods of different masses using a two body approximation, ellipses and hyperbolae, the SOI is taken as the boundary where the trajectory switches which mass field it is influenced by.

**JPL Horizons On-Line Ephemeris System** provides access to key Solar System data and flexible production of highly accurate ephemerides for Solar System objects.

In astronomy, **perturbation** is the complex motion of a massive body subjected to forces other than the gravitational attraction of a single other massive body. The other forces can include a third body, resistance, as from an atmosphere, and the off-center attraction of an oblate or otherwise misshapen body.

The **invariable plane** of a planetary system, also called **Laplace's invariable plane**, is the plane passing through its barycenter perpendicular to its angular momentum vector. In the Solar System, about 98% of this effect is contributed by the orbital angular momenta of the four jovian planets. The invariable plane is within 0.5° of the orbital plane of Jupiter, and may be regarded as the weighted average of all planetary orbital and rotational planes.

The **orbital plane** of a revolving body is the geometric plane in which its orbit lies. Three non-collinear points in space suffice to determine an orbital plane. A common example would be the positions of the centers of a massive body (host) and of an orbiting celestial body at two different times/points of its orbit.

In differential geometry, an **osculating curve** is a plane curve from a given family that has the highest possible order of contact with another curve. That is, if *F* is a family of smooth curves, *C* is a smooth curve, and *p* is a point on *C*, then an osculating curve from *F* at *p* is a curve from *F* that passes through *p* and has as many of its derivatives at *p* equal to the derivatives of *C* as possible.

The **proper orbital elements** or **proper elements** of an orbit are constants of motion of an object in space that remain practically unchanged over an astronomically long timescale. The term is usually used to describe the three quantities:

**Tisserand's parameter** is a value calculated from several orbital elements of a relatively small object and a larger "perturbing body". It is used to distinguish different kinds of orbits. The term is named after French astronomer Félix Tisserand, and applies to restricted three-body problems in which the three objects all differ greatly in mass.

In celestial mechanics, a **Kepler orbit** is the motion of one body relative to another, as an ellipse, parabola, or hyperbola, which forms a two-dimensional orbital plane in three-dimensional space. A Kepler orbit can also form a straight line. It considers only the point-like gravitational attraction of two bodies, neglecting perturbations due to gravitational interactions with other objects, atmospheric drag, solar radiation pressure, a non-spherical central body, and so on. It is thus said to be a solution of a special case of the two-body problem, known as the Kepler problem. As a theory in classical mechanics, it also does not take into account the effects of general relativity. Keplerian orbits can be parametrized into six orbital elements in various ways.

**Orbit modeling** is the process of creating mathematical models to simulate motion of a massive body as it moves in orbit around another massive body due to gravity. Other forces such as gravitational attraction from tertiary bodies, air resistance, solar pressure, or thrust from a propulsion system are typically modeled as secondary effects. Directly modeling an orbit can push the limits of machine precision due to the need to model small perturbations to very large orbits. Because of this, perturbation methods are often used to model the orbit in order to achieve better accuracy.

- ↑ Moulton, Forest R. (1970) [1902].
*Introduction to Celestial Mechanics*(2nd revised ed.). Mineola, New York: Dover. pp. 322–23. ISBN 0486646874. - ↑ For details see: Efroimsky, M. (2005). "Gauge Freedom in Orbital Mechanics".
*Annals of the New York Academy of Sciences*.**1065**(1): 346–74. arXiv: astro-ph/0603092 . Bibcode:2005NYASA1065..346E. doi:10.1196/annals.1370.016. PMID 16510420. S2CID 10820255.; Efroimsky, Michael; Goldreich, Peter (2003). "Gauge symmetry of the N-body problem in the Hamilton–Jacobi approach".*Journal of Mathematical Physics*.**44**(12): 5958–5977. arXiv: astro-ph/0305344 . Bibcode:2003JMP....44.5958E. doi:10.1063/1.1622447. S2CID 5411288.

- Diagram of a sequence of osculating orbits for the escape from Earth orbit by the ion-driven SMART-1 spacecraft: ESA Science & Technology - SMART-1 Osculating Orbit up to 25.08.04
- A sequence of osculating orbits for the approach to the Moon by the SMART-1 spacecraft: ESA Science & Technology - SMART-1 Osculating Orbit up to 09.01.05

- Videos

- Osculating orbits:
*restricted 3-Body problem*on YouTube (min. 4:26) - Osculating orbits:
*3-Body Lagrange problem*on YouTube (min. 4:00) - Osculating orbits:
*4-Body Lagrange problem*on YouTube (min. 1:05) - Osculating orbits: in:
*the Pythagorean 3-Body problem*on YouTube (min. 4:26) - Minor Planet Center:
*Asteroid Hazards, Part 3: Finding the Path*on YouTube (min. 5:38)

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