Lagrangian point

History

The three collinear Lagrange points (L₁, L₂, L₃) were discovered by Leonhard Euler a few years before Lagrange discovered the remaining two.

In 1772, Joseph-Louis 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.

Lagrange points

The five Lagrangian points are labeled and defined as follows:

The L₁ point lies on the line defined by the two large masses M₁ and M₂, and between them. It is the most intuitively understood of the Lagrangian points: the one where the gravitational attraction of M₂ partially cancels M₁'s gravitational attraction.

Explanation 

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 L₁ point, the orbital period of the object becomes exactly equal to Earth's orbital period. L₁ is about 1.5 million kilometers from Earth.

The L₂ 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 L₂.

Explanation 

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 L₂ point that orbital period becomes equal to Earth's. Like L₁, L₂ is about 1.5 million kilometers from Earth.

The L₃ point lies on the line defined by the two large masses, beyond the larger of the two.

Explanation 

L₃ in the Sun–Earth system exists on the opposite side of the Sun, a little outside Earth's orbit but slightly closer to the Sun than Earth is. (This apparent contradiction is because the Sun is also affected by Earth's gravity, and so orbits around the two bodies' barycenter, which is, however, well inside the body of the Sun.) At the L₃ point, the combined pull of Earth and Sun again causes the object to orbit with the same period as Earth.

The L₄ and L₅ 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 (L₅) or ahead (L₄) of the smaller mass with regard to its orbit around the larger mass.

The triangular points (L₄ and L₅) are stable equilibria, provided that the ratio of M₁/M₂ is greater than 24.96. This is the case for the Sun–Earth system, the Sun–Jupiter system, and, by a smaller margin, the Earth–Moon system. 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).

In contrast to L₄ and L₅, where stable equilibrium exists, the points L₁, L₂, and L₃ are positions of unstable equilibrium. Any object orbiting at L₁, L₂, or L₃ 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.

Natural objects at Lagrangian points

It is common to find objects at or orbiting the L₄ and L₅ points of natural orbital systems. These are commonly called "trojans"; in the 20th century, asteroids discovered orbiting at the Sun–Jupiter L₄ and L₅ points were named after characters from Homer's Iliad. Asteroids at the L₄ point, which leads Jupiter, are referred to as the "Greek camp", whereas those at the L₅ point are referred to as the "Trojan camp". See also Trojan (astronomy).

Other examples of natural objects orbiting at Lagrange points:

The Sun–Earth L₄ and L₅ points contain interplanetary dust and at least one asteroid, 2010 TK7, detected in October 2010 by Wide-field Infrared Survey Explorer (WISE) and announced during July 2011.

The Earth–Moon L₄ and L₅ points may contain interplanetary dust in what are called Kordylewski clouds; however, the Hiten spacecraft's Munich Dust Counter (MDC) detected no increase in dust during its passes through these points. Stability at these specific points is greatly complicated by solar gravitational influence.

Recent observations suggest that the Sun–Neptune L₄ and L₅ points, known as the Neptune trojans, may be very thickly populated, containing large bodies an order of magnitude more numerous than the Jupiter trojans.

Several asteroids also orbit near the Sun-Jupiter L₃ point, called the Hilda family.

The Saturnian moon Tethys has two smaller moons in its L₄ and L₅ points, Telesto and Calypso. The Saturnian moon Dione also has two Lagrangian co-orbitals, Helene at its L₄ point and Polydeuces at L₅. The moons wander azimuthally about the Lagrangian points, with Polydeuces describing the largest deviations, moving up to 32° away from the Saturn–Dione L₅ point. Tethys and Dione are hundreds of times more massive than their "escorts" (see the moons' articles for exact diameter figures; masses are not known in several cases), and Saturn is far more massive still, which makes the overall system stable.

One version of the giant impact hypothesis suggests that an object named Theia formed at the Sun–Earth L₄ or L₅ points and crashed into Earth after its orbit destabilized, forming the Moon.

Mars has four known co-orbital asteroids (5261 Eureka, 1999 UJ7, 1998 VF31 and 2007 NS2), all at its Lagrangian points.

Earth's companion object 3753 Cruithne is in a relationship with Earth that is somewhat trojan-like, but that is different from a true trojan. Cruithne occupies one of two regular solar orbits, one of them slightly smaller and faster than Earth's, and the other slightly larger and slower. It periodically alternates between these two orbits due to close encounters with Earth. When it is in the smaller, faster orbit and approaches Earth, it gains orbital energy from Earth and moves up into the larger, slower orbit. It then falls farther and farther behind Earth, and eventually Earth approaches it from the other direction. Then Cruithne gives up orbital energy to Earth, and drops back into the smaller orbit, thus beginning the cycle anew. The cycle has no noticeable impact on the length of the year, because Earth's mass is over 20 billion (7010200000000000000♠2×10¹⁰) times more than that of 3753 Cruithne.

Epimetheus and Janus, satellites of Saturn, have a similar relationship, though they are of similar masses and so actually exchange orbits with each other periodically. (Janus is roughly 4 times more massive but still light enough for its orbit to be altered.) Another similar configuration is known as orbital resonance, in which orbiting bodies tend to have periods of a simple integer ratio, due to their interaction.

In a binary star system, the Roche lobe has its apex located at L₁; if a star overflows its Roche lobe, then it will lose matter to its companion star.

Mathematical details

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 with the minor body's centrifugal force are in balance at the Lagrangian points, allowing the smaller third body to be relatively stationary with respect to the first two.

L1

The location of L₁ is the solution to the following equation, balancing gravitation and the centrifugal force:

M 1 ( R − r ) 2 = M 2 r 2 + M 1 R 2 − r ( M 1 + M 2 ) R 3

where r is the distance of the L₁ point from the smaller object, R is the distance between the two main objects, and M₁ and M₂ are the masses of the large and small object, respectively. (The quantity in parentheses on the right is the distance of L₁ from the center of mass.) Solving this for r involves solving a quintic function, but if the mass of the smaller object (M₂) is much smaller than the mass of the larger object (M₁) then L₁ and L₂ are at approximately equal distances r from the smaller object, equal to the radius of the Hill sphere, given by:

r ≈ R M 2 3 M 1 3

This distance can be described as being such that the orbital period, corresponding to a circular orbit with this distance as radius around M₂ in the absence of M₁, is that of M₂ around M₁, divided by √3 ≈ 1.73:

T s , M 2 ( r ) = T M 2 , M 1 ( R ) 3 .

L2

The location of L₂ is the solution to the following equation, balancing gravitation and inertia:

M 1 ( R + r ) 2 + M 2 r 2 = M 1 R 2 + r ( M 1 + M 2 ) R 3

with parameters defined as for the L₁ case. Again, if the mass of the smaller object (M₂) is much smaller than the mass of the larger object (M₁) then L₂ is at approximately the radius of the Hill sphere, given by:

r ≈ R M 2 3 M 1 3

L3

The location of L₃ is the solution to the following equation, balancing gravitation and the centrifugal force:

M 1 ( R − r ) 2 + M 2 ( 2 R − r ) 2 = ( M 2 M 1 + M 2 R + R − r ) M 1 + M 2 R 3

with parameters defined as for the L₁ and L₂ cases except that r now indicates how much closer L₃ is to the more massive object than the smaller object. If the mass of the smaller object (M₂) is much smaller than the mass of the larger object (M₁) then:

r ≈ R 7 M 2 12 M 1

L4 and L5

The reason these points are in balance is that, at L₄ and L₅, 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 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.

Stability

Although the L₁, L₂, and L₃ points are nominally unstable, it turns out that it is possible to find (unstable) periodic orbits around these points, at least in the restricted three-body problem. These periodic orbits, referred to as "halo" orbits, do not exist in a full n-body dynamical system such as the Solar System. However, quasi-periodic (i.e. bounded but not precisely repeating) orbits following Lissajous-curve trajectories do exist in the n-body system. These quasi-periodic Lissajous orbits are what most of Lagrangian-point missions to date have used. Although they are not perfectly stable, a relatively modest effort at station keeping can allow a spacecraft to stay in a desired Lissajous orbit for an extended period of time. It also turns out that, at least in the case of Sun–Earth-L₁ missions, it is actually preferable to place the spacecraft in a large-amplitude (100,000–200,000 km or 62,000–124,000 mi) Lissajous orbit, instead of having it sit at the Lagrangian point, because this keeps the spacecraft off the direct line between Sun and Earth, thereby reducing the impact of solar interference on Earth–spacecraft communications. Similarly, a large-amplitude Lissajous orbit around L₂ can keep a probe out of Earth's shadow and therefore ensures a better illumination of its solar panels.

Sun–Earth

Sun–Earth L₁ is suited for making observations of the Sun–Earth system. Objects here are never shadowed by Earth or the Moon. 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. Since June 2015, DSCOVR has orbited the L₁ point.

Sun–Earth L₂ is a good spot for space-based observatories. Because an object around L₂ 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, so solar radiation is not completely blocked. From this point, the Sun, Earth and Moon are relatively closely positioned together in the sky, and hence leave a large field of view without interference – this is especially helpful for infrared astronomy.

Sun–Earth L₃ was a popular place to put a "Counter-Earth" in pulp science fiction and comic books. Once space-based observation became possible via satellites and probes, it was shown to hold no such object. The Sun–Earth L₃ 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 0.3 AU of this L₃ every 20 months).

A spacecraft orbiting near Sun–Earth L₃ 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 L₃ 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 2010, spacecraft transfer trajectories to Sun–Earth L₃ were studied and several designs were considered.

Missions to Lagrangian points generally orbit the points rather than occupy them directly.

Another interesting and useful property of the collinear Lagrangian points and their associated Lissajous orbits is that they serve as "gateways" to control the chaotic trajectories of the Interplanetary Transport Network.

Earth–Moon

Earth–Moon L₁ allows comparatively easy access to Lunar and Earth orbits with minimal change in velocity and this has as an advantage to position a half-way manned space station intended to help transport cargo and personnel to the Moon and back.

Earth–Moon L₂ would be a good location for a communications satellite covering the Moon's far side and would be "an ideal location" for a propellant depot as part of the proposed depot-based space transportation architecture.

Sun–Venus

Scientists at the B612 Foundation are planning to use Venus's L₃ point to position their planned Sentinel telescope, which aims to look back towards Earth's orbit and compile a catalogue of near-Earth asteroids.

Spacecraft at Sun–Earth L1

International Sun Earth Explorer 3 (ISEE-3) began its mission at the Sun–Earth L₁ before leaving to intercept a comet in 1982. The Sun–Earth L₁ is also the point to which the Reboot ISEE-3 mission was attempting to return the craft as the first phase of a recovery mission (as of September 25, 2014 all efforts have failed and contact was lost).

Solar and Heliospheric Observatory (SOHO) is stationed in a halo orbit at L₁, and the Advanced Composition Explorer (ACE) in a Lissajous orbit. WIND is also at L₁.

Deep Space Climate Observatory (DSCOVR), launched on 11 February 2015, began orbiting L₁ on 8 June 2015 to study the solar wind and its effects on Earth. DSCOVR is unofficially known as GORESAT, because it carries a camera always oriented to Earth and capturing full-frame photos of the planet similar to the Blue Marble. This concept was proposed by then-Vice President of the United States Al Gore in 1998 and was a centerpiece in his film An Inconvenient Truth.

LISA Pathfinder (LPF) was launched on 3 December 2015, and arrived at L₁ on 22 January 2016, where, among other experiments, it will test the technology needed by (e)LISA to detect gravitational waves. LISA Pathfinder uses an instrument consisting of two small gold alloy cubes.

Spacecraft at Sun–Earth L2

Spacecraft at the Sun–Earth L₂ point are in a Lissajous orbit until decommissioned, when they are sent into a heliocentric graveyard orbit.

1 October 2001 – October 2010—Wilkinson Microwave Anisotropy Probe

November 2003 – April 2004—WIND, then it returned to Earth orbit before going to L₁ where it still remains

July 2009 – 29 April 2013—Herschel Space Telescope

3 July 2009 – 21 October 2013—Planck Space Observatory

25 August 2011 – April 2012—Chang'e 2, from where it travelled to 4179 Toutatis and then into deep space

January 2014 – 2018—Gaia Space Observatory

2018 — James Webb Space Telescope will use a halo orbit

2020 — Euclid Space Telescope

2024 — Wide Field Infrared Survey Telescope (WFIRST) will use a halo orbit

2028 — Advanced Telescope for High Energy Astrophysics (ATHENA) will use a halo orbit