# Liberation points in celestial mechanics and astrodynamics pdf

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- Periodic Attitudes of Libration Point Spacecrafts in the Earth-Moon System
- Methods in Astrodynamics and Celestial Mechanics, Volume 17
- Lagrange point
- We apologize for the inconvenience...

## Periodic Attitudes of Libration Point Spacecrafts in the Earth-Moon System

Normally, the two objects exert an unbalanced gravitational force at a point, altering the orbit of whatever is at that point. At the Lagrange points, the gravitational forces of the two large bodies and the centrifugal force balance each other. This can make Lagrange points an excellent location for satellites, as few orbit corrections are needed to maintain the desired orbit. Small objects placed in orbit at Lagrange points are in equilibrium in at least two directions relative to the center of mass of the large bodies.

There are five such points, labeled L 1 to L 5 , all in the orbital plane of the two large bodies, for each given combination of two orbital bodies.

For instance, there are five Lagrangian points L 1 to L 5 for the Sun—Earth system, and in a similar way there are five different Lagrangian points for the Earth—Moon system. L 1 , L 2 , and L 3 are on the line through the centers of the two large bodies, while L 4 and L 5 each act as the third vertex of an equilateral triangle formed with the centers of the two large bodies.

L 4 and L 5 are stable, which implies that objects can orbit around them in a rotating coordinate system tied to the two large bodies. The L 4 and L 5 points are stable gravity wells and have a tendency to pull objects into them. Several planets have trojan asteroids near their L 4 and L 5 points with respect to the Sun. Jupiter has more than a million of these trojans. Artificial satellites have been placed at L 1 and L 2 with respect to the Sun and Earth , and with respect to the Earth and the Moon.

In , Lagrange published an "Essay on the three-body problem ". In the first chapter he considered the general three-body problem. From that, in the second chapter, he demonstrated two special constant-pattern solutions , the collinear and the equilateral, for any three masses, with circular orbits.

The L 1 point lies on the line defined by the two large masses M 1 and M 2 , and between them. It is the point where the gravitational attraction of M 2 partially cancels that of M 1. An object that orbits the Sun more closely than Earth would normally have a shorter orbital period than Earth, but that ignores the effect of Earth's own gravitational pull. If the object is directly between Earth and the Sun, then Earth's gravity counteracts some of the Sun's pull on the object, and therefore increases the orbital period of the object.

The closer to Earth the object is, the greater this effect is. At the L 1 point, the orbital period of the object becomes exactly equal to Earth's orbital period. L 1 is about 1. The L 2 point lies on the line through the two large masses, beyond the smaller of the two. Here, the gravitational forces of the two large masses balance the centrifugal effect on a body at L 2. On the opposite side of Earth from the Sun, the orbital period of an object would normally be greater than that of Earth.

The extra pull of Earth's gravity decreases the orbital period of the object, and at the L 2 point that orbital period becomes equal to Earth's. The L 3 point lies on the line defined by the two large masses, beyond the larger of the two.

Within the Sun—Earth system, the L 3 point exists on the opposite side of the Sun, a little outside Earth's orbit and slightly closer to the center of the Sun than Earth is. This placement occurs because the Sun is also affected by Earth's gravity and so orbits around the two bodies' barycenter , which is well inside the body of the Sun. An object at Earth's distance from the Sun would have an orbital period of one year if only the Sun's gravity is considered.

But an object on the opposite side of the Sun from Earth and directly in line with both "feels" Earth's gravity adding slightly to the Sun's and therefore must orbit a little farther from the barycenter of Earth and Sun in order to have the same 1-year period. The L 4 and L 5 points lie at the third corners of the two equilateral triangles in the plane of orbit whose common base is the line between the centers of the two masses, such that the point lies behind L 5 or ahead L 4 of the smaller mass with regard to its orbit around the larger mass.

When a body at these points is perturbed, it moves away from the point, but the factor opposite of that which is increased or decreased by the perturbation either gravity or angular momentum-induced speed will also increase or decrease, bending the object's path into a stable, kidney bean -shaped orbit around the point as seen in the corotating frame of reference.

The points L 1 , L 2 , and L 3 are positions of unstable equilibrium. Any object orbiting at L 1 , L 2 , or L 3 will tend to fall out of orbit; it is therefore rare to find natural objects there, and spacecraft inhabiting these areas must employ station keeping in order to maintain their position.

Due to the natural stability of L 4 and L 5 , it is common for natural objects to be found orbiting in those Lagrange points of planetary systems.

Objects that inhabit those points are generically referred to as ' trojans ' or 'trojan asteroids'. The name derives from the names that were given to asteroids discovered orbiting at the Sun— Jupiter L 4 and L 5 points, which were taken from mythological characters appearing in Homer 's Iliad , an epic poem set during the Trojan War. Asteroids at the L 4 point, ahead of Jupiter, are named after Greek characters in the Iliad and referred to as the " Greek camp ".

Those at the L 5 point are named after Trojan characters and referred to as the " Trojan camp ". Both camps are considered to be types of trojan bodies. As the Sun and Jupiter are the two most massive objects in the Solar System, there are more Sun-Jupiter trojans than for any other pair of bodies.

However, smaller numbers of objects are known at the Langrage points of other orbital systems:. Objects which are on horseshoe orbits are sometimes erroneously described as trojans, but do not occupy Lagrange points. Known objects on horseshoe orbits include Cruithne with Earth, and Saturn's moons Epimetheus and Janus. Lagrangian points are the constant-pattern solutions of the restricted three-body problem.

For example, given two massive bodies in orbits around their common barycenter , there are five positions in space where a third body, of comparatively negligible mass , could be placed so as to maintain its position relative to the two massive bodies. As seen in a rotating reference frame that matches the angular velocity of the two co-orbiting bodies, the gravitational fields of two massive bodies combined providing the centripetal force at the Lagrangian points, allowing the smaller third body to be relatively stationary with respect to the first two.

The location of L 1 is the solution to the following equation, gravitation providing the centripetal force:. The quantity in parentheses on the right is the distance of L 1 from the center of mass.

Solving this for r involves solving a quintic function , but if the mass of the smaller object M 2 is much smaller than the mass of the larger object M 1 then L 1 and L 2 are at approximately equal distances r from the smaller object, equal to the radius of the Hill sphere , given by:.

Since the tidal effect of a body is proportional to its mass divided by the distance cubed, this means that the tidal effect of the smaller body at the L 1 or at the L 2 point is about three times that of the larger body.

We may also write:. The ratio of diameter to distance gives the angle subtended by the body, showing that viewed from these two Lagrange points, the apparent sizes of the two bodies will be similar, especially if the density of the smaller one is about thrice that of the larger, as in the case of the earth and the sun.

The location of L 2 is the solution to the following equation, gravitation providing the centripetal force:. Again, if the mass of the smaller object M 2 is much smaller than the mass of the larger object M 1 then L 2 is at approximately the radius of the Hill sphere , given by:. The same remarks about tidal influence and apparent size apply as for the L 1 point. For example, the angular radius of the sun as viewed from L 2 is arcsin Looking toward the sun from L 2 one sees an annular eclipse.

It is necessary for a spacecraft, like Gaia , to follow a Lissajous orbit or a halo orbit around L 2 in order for its solar panels to get full sun. The location of L 3 is the solution to the following equation, gravitation providing the centripetal force:.

If the mass of the smaller object M 2 is much smaller than the mass of the larger object M 1 then: [15]. The reason these points are in balance is that, at L 4 and L 5 , the distances to the two masses are equal. Accordingly, the gravitational forces from the two massive bodies are in the same ratio as the masses of the two bodies, and so the resultant force acts through the barycenter of the system; additionally, the geometry of the triangle ensures that the resultant acceleration is to the distance from the barycenter in the same ratio as for the two massive bodies.

The barycenter being both the center of mass and center of rotation of the three-body system, this resultant force is exactly that required to keep the smaller body at the Lagrange point in orbital equilibrium with the other two larger bodies of the system. Indeed, the third body need not have negligible mass. The general triangular configuration was discovered by Lagrange in work on the three-body problem.

The radial acceleration a of an object in orbit at a point along the line passing through both bodies is given by:. The terms in this function represent respectively: force from M 1 ; force from M 2 ; and centrifugal force. The points L 3 , L 1 , L 2 occur where the acceleration is zero — see chart at right.

Although the L 1 , L 2 , and L 3 points are nominally unstable, there are quasi-stable periodic orbits called halo orbits around these points in a three-body system. A full n -body dynamical system such as the Solar System does not contain these periodic orbits, but does contain quasi-periodic i.

These quasi-periodic Lissajous orbits are what most of Lagrangian-point space missions have used until now. Although they are not perfectly stable, a modest effort of station keeping keeps a spacecraft in a desired Lissajous orbit for a long time. Similarly, a large-amplitude Lissajous orbit around L 2 keeps a probe out of Earth's shadow and therefore ensures continuous illumination of its solar panels.

The L 4 and L 5 points are stable provided that the mass of the primary body e. Although the L 4 and L 5 points are found at the top of a "hill", as in the effective potential contour plot above, they are nonetheless stable.

The reason for the stability is a second-order effect: as a body moves away from the exact Lagrange position, Coriolis acceleration which depends on the velocity of an orbiting object and cannot be modeled as a contour map [17] curves the trajectory into a path around rather than away from the point.

The kidney-shaped orbits typically shown nested around L 4 and L 5 are the projections of the orbits on a plane e. Calculations assume the two bodies orbit in a perfect circle with separation equal to the semimajor axis and no other bodies are nearby. Distances are measured from the larger body's center of mass with L 3 showing a negative location. The percentage columns show how the distances compare to the semimajor axis.

Sun—Earth L 1 is suited for making observations of the Sun—Earth system. Objects here are never shadowed by Earth or the Moon and, if observing Earth, always view the sunlit hemisphere.

The first mission of this type was the International Sun Earth Explorer 3 ISEE-3 mission used as an interplanetary early warning storm monitor for solar disturbances. Conversely it is also useful for space-based solar telescopes , because it provides an uninterrupted view of the Sun and any space weather including the solar wind and coronal mass ejections reaches L 1 up to an hour before Earth.

Solar and heliospheric missions currently located around L 1 include the Solar and Heliospheric Observatory , Wind, and the Advanced Composition Explorer. Sun—Earth L 2 is a good spot for space-based observatories.

Because an object around L 2 will maintain the same relative position with respect to the Sun and Earth, shielding and calibration are much simpler. It is, however, slightly beyond the reach of Earth's umbra , [21] so solar radiation is not completely blocked at L 2.

Spacecraft generally orbit around L 2 , avoiding partial eclipses of the Sun to maintain a constant temperature. From locations near L 2 , the Sun, Earth and Moon are relatively close together in the sky; this means that a large sunshade with the telescope on the dark-side can allow the telescope to cool passively to around 50 K — this is especially helpful for infrared astronomy and observations of the cosmic microwave background.

Sun—Earth L 3 was a popular place to put a " Counter-Earth " in pulp science fiction and comic books. Once space-based observation became possible via satellites [22] and probes, it was shown to hold no such object. The Sun—Earth L 3 is unstable and could not contain a natural object, large or small, for very long. This is because the gravitational forces of the other planets are stronger than that of Earth Venus , for example, comes within 0.

A spacecraft orbiting near Sun—Earth L 3 would be able to closely monitor the evolution of active sunspot regions before they rotate into a geoeffective position, so that a 7-day early warning could be issued by the NOAA Space Weather Prediction Center.

Moreover, a satellite near Sun—Earth L 3 would provide very important observations not only for Earth forecasts, but also for deep space support Mars predictions and for manned mission to near-Earth asteroids. In , spacecraft transfer trajectories to Sun—Earth L 3 were studied and several designs were considered.

## Methods in Astrodynamics and Celestial Mechanics, Volume 17

The three-body problem has a special relevance, particularly in astrophysics and astrodynamics. In general, the three-body problem is classified into two types:. Thus, the general problem has some applications in celestial mechanics such as the dynamics of triple star systems and only a very few in space dynamics and solar system dynamics , whereas the restricted problem plays an important role in studying the motion of artificial satellites. It can be used also to evaluate the motion of the planets, minor planets and comets. The restricted problem gives an accurate description not only regarding the motion of the Moon but also with respect to the motion of other natural satellites. Furthermore, the restricted problem has many applications not only in celestial mechanics but also in physics, mathematics and quantum mechanics, to name a few. Furthermore, in modern solid state physics, the restricted problem can be used to discuss the motion of an infinitesimal mass affected not only by the gravitational field but also by light pressure from one or both of the primaries, which is called the photogravitational problem.

Orbital maneuvers between the Lagrangian points and the primaries in the Earth-Sun system. Almeida Prado. Member, ABCM; prado dem. This paper is concerned with trajectories to transfer a spacecraft between the Lagrangian points of the Sun-Earth system and the primaries. The Lagrangian points have important applications in astronautics, since they are equilibrium points of the equation of motion and very good candidates to locate a satellite or a space station. The results show families of transfer orbits, parameterized by the transfer time.

Methods in Astrodynamics and Celestial Mechanics is a collection of technical papers presented at the Astrodynamics Specialist Conference held in Monterey, California, on September , , under the auspices of the American Institute of Aeronautics and Astronautics and Institute of Navigation. The conference provided a forum for tackling some of the most interesting applications of the methods of celestial mechanics to problems of space engineering. Comprised of 19 chapters, this volume first treats the promising area of motion around equilibrium configurations. Following a discussion on limiting orbits at the equilateral centers of libration, the reader is introduced to the asymptotic expansion technique and its application to trajectories. Asymptotic representations for solutions to the differential equations of satellite theory are considered. The last two sections deal with orbit determination and mission analysis and optimization in astrodynamics.

PDF | This book is designed as an introductory text and reference for The juxtaposition of celestial mechanics and astrodynamics is a Stability of Motion Near the Lagrangian Points. The “Vision for Space Exploration” would be funded by resources liberated by the end of the International Space.

## Lagrange point

Scientific Research An Academic Publisher. Three-Body Problem formulated by Newton provided route to the analysis of closed form analytical solution. This solution remains elusive even today, as one has never been found for the three-body problem.

*Description The contents of this book represent the latest and some of the most interesting applications of the methods of celestial mechanics to problems of space engineering. This area of research involves advanced dynamical and astronomical theories and the application of such theories to the selection of new trajectories, the analysis of proposals for future new space experiments, and the setting of design parameters for proposed new spacecraft of the future.*

### We apologize for the inconvenience...

Normally, the two objects exert an unbalanced gravitational force at a point, altering the orbit of whatever is at that point. At the Lagrange points, the gravitational forces of the two large bodies and the centrifugal force balance each other. This can make Lagrange points an excellent location for satellites, as few orbit corrections are needed to maintain the desired orbit. Small objects placed in orbit at Lagrange points are in equilibrium in at least two directions relative to the center of mass of the large bodies. There are five such points, labeled L 1 to L 5 , all in the orbital plane of the two large bodies, for each given combination of two orbital bodies.

Hou, L. Motions around the collinear libration points in the elliptic restricted three-body problem are studied. Literal expansions of the Lissajous orbits and the halo orbits are obtained. These expansions depend on two amplitude parameters and the orbital eccentricity of the two primaries.

Hou, L. Motions around the collinear libration points in the elliptic restricted three-body problem are studied. Literal expansions of the Lissajous orbits and the halo orbits are obtained. These expansions depend on two amplitude parameters and the orbital eccentricity of the two primaries. Numerical simulations are done to check the validity of these literal series and to compare them with the results in the circular restricted three-body problem. According to the properties of these literal expansions, three kinds of symmetric periodic orbits around the collinear libration points are discussed. First, the linear model is discussed.

Thus, more effort in the astrodynamics community has been directed toward collinear libration points in the Sun–Earth/Moon three-body problem have been the Methods of Celestial Mechanics (English Translation), Vol.