Lagrange point

In celestial mechanics, the Lagrange points /ləˈɡrɑːndʒ/ (also Lagrangian points, L-points, or libration points) are orbital points near two large co-orbiting bodies. At the Lagrange points the gravitational forces of the two large bodies cancel out in such a way that a small object placed in orbit there is in equilibrium in at least two directions relative to the center of mass of the large bodies.

Smaller objects (green) at the Lagrange points are in equilibrium. At any other point, the gravitational forces are non-equilibria.

Lagrange points in the Sun–Earth system (not to scale). A small object at L4 or L5 will hold its relative position. A small object at L1, L2, or L3 will hold its relative position until deflected slightly radially, after which it will diverge from its original position.

An example of a spacecraft at Sun–Earth L2
  WMAP ·   Earth

There are five such points, labeled L₁ to L₅, 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 L₁ to L₅ for the Sun–Earth system, and in a similar way there are five different Lagrangian points for the Earth–Moon system. L₁, L₂, and L₃ are on the line through the centers of the two large bodies, while L₄ and L₅ each act as the third vertex of an equilateral triangle formed with the centers of the two large bodies. L₄ and L₅ are stable, which implies that objects can orbit around them in a rotating coordinate system tied to the two large bodies.

Several planets have trojan asteroids near their L₄ and L₅ points with respect to the Sun. Jupiter has more than a million of these trojans. Artificial satellites have been placed at L₁ and L₂ with respect to the Sun and Earth, and with respect to the Earth and the Moon.[1] The Lagrangian points have been proposed for uses in space exploration.

History

The three collinear Lagrange points (L₁, L₂, L₃) were discovered by Leonhard Euler a few years before Joseph-Louis Lagrange discovered the remaining two.[2][3]

In 1772, 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.[4]

Lagrange points

The five Lagrange points are labeled and defined as follows:

L₁ point

The L₁ point lies on the line defined by the two large masses M₁ and M₂, and between them. It is the point where the gravitational attraction of M₂ partially cancels that of M₁. 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, or 0.01 au, 1/100th the distance to the Sun.[5]

L₂ point

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. 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 or 0.01 au from Earth.

L₃ point

The L₃ point lies on the line defined by the two large masses, beyond the larger of the two. Within the Sun–Earth system, the L₃ point exists on the opposite side of the Sun, a little outside Earth's orbit and slightly closer from the center of 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 Sun in order to have the same 1-year period. It is at the L₃ point that the combined pull of Earth and Sun causes the object to orbit with the same period as Earth, in effect orbiting an Earth+Sun mass with the Earth-Sun barycenter at one focus of its orbit.

L₄ and L₅ points

Gravitational accelerations at L₄

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.

Point Stability

The triangular points (L₄ and L₅) are stable equilibria, provided that the ratio of M₁/M₂ is greater than 24.96.[note 1][6] 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).

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 Lagrange points

Due to the natural stability of L₄ and L₅, 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 L₄ and L₅ points, which were taken from mythological characters appearing in Homer's Iliad, an epic poem set during the Trojan War. Asteroids at the L₄ point, ahead of Jupiter, are named after Greek characters in the Iliad and referred to as the "Greek camp". Those at the L₅ 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:

• The Sun–Earth L₄ and L₅ points contain interplanetary dust and at least one asteroid, 2010 TK7.[7][8]
• The Earth–Moon L₄ and L₅ points contain concentrations of interplanetary dust, known as Kordylewski clouds.[9][10] Stability at these specific points is greatly complicated by solar gravitational influence.[11]
• The Sun–Neptune L₄ and L₅ points contain several dozen known objects, the Neptune trojans.[12]
• Mars has four accepted Mars trojans: 5261 Eureka, 1999 UJ7, 1998 VF31, and 2007 NS2.
• Saturn's moon Tethys has two smaller moons in its L₄ and L₅ points, Telesto and Calypso. Another Saturn 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.
• One version of the giant impact hypothesis postulates that an object named Theia formed at the Sun–Earth L₄ or L₅ point and crashed into Earth after its orbit destabilized, forming the Moon.
• In binary stars, the Roche lobe has its apex located at L₁; if one of the stars expands past its Roche lobe, then it will lose matter to its companion star, known as Roche lobe overflow.

Objects which are on horseshoe orbits are sometimes erroneously described as trojans, but do not occupy Lagrange points. Known objects on horseshoe orbits include 3753 Cruithne with Earth, and Saturn's moons Epimetheus and Janus.

Mathematical details

A contour plot of the effective potential due to gravity and the centrifugal force of a two-body system in a rotating frame of reference. The arrows indicate the gradients of the potential around the five Lagrange points—downhill toward them (red) or away from them (blue). Counterintuitively, the L₄ and L₅ points are the high points of the potential. At the points themselves these forces are balanced.

Visualisation of the relationship between the Lagrangian points (red) of a planet (blue) orbiting a star (yellow) anticlockwise, and the effective potential in the plane containing the orbit (grey rubber-sheet model with purple contours of equal potential).[13]
Click for animation.

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.[14]

L₁

The location of L₁ is the solution to the following equation, gravitation providing the centripetal force:

${\displaystyle {\frac {M_{1}}{(R-r)^{2}}}={\frac {M_{2}}{r^{2}}}+\left({\frac {M_{1}}{M_{1}+M_{2}}}R-r\right){\frac {M_{1}+M_{2}}{R^{3}}}}$

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:

${\displaystyle r\approx R{\sqrt[{3}]{\frac {M_{2}}{3M_{1}}}}}$

We may also write this as:

${\displaystyle {\frac {M_{2}}{r^{3}}}\approx 3{\frac {M_{1}}{R^{3}}}}$

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 L₁ or at the L₂ point is about three times that of the larger body. We may also write:

${\displaystyle \rho _{2}(d_{2}/r)^{3}\approx 3\rho _{1}(d_{1}/R)^{3}}$

where ρ₁ and ρ₂ are the average densities of the two bodies and ${\displaystyle d_{1}}$ and ${\displaystyle d_{2}}$ are their diameters. 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.

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:

${\displaystyle T_{s,M_{2}}(r)={\frac {T_{M_{2},M_{1}}(R)}{\sqrt {3}}}.}$

L₂

The location of L₂ is the solution to the following equation, gravitation providing the centripetal force:

${\displaystyle {\frac {M_{1}}{(R+r)^{2}}}+{\frac {M_{2}}{r^{2}}}=\left({\frac {M_{1}}{M_{1}+M_{2}}}R+r\right){\frac {M_{1}+M_{2}}{R^{3}}}}$

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:

${\displaystyle r\approx R{\sqrt[{3}]{\frac {M_{2}}{3M_{1}}}}}$

The same remarks about tidal influence and apparent size apply as for the L₁ point. For example, the angular radius of the sun as viewed from L₂ is arcsin(695.5×10³/151.1×10⁶) ≈ 0.264°, whereas that of the earth is arcsin(6371/1.5×10⁶ ≈ 0.242°. Looking toward the sun from L₂ one sees an annular eclipse. It is necessary for a spacecraft, like Gaia, to follow a Lissajous orbit or a halo orbit around L₂ in order for its solar panels to get full sun.

L₃

The location of L₃ is the solution to the following equation, gravitation providing the centripetal force:

${\displaystyle {\frac {M_{1}}{\left(R-r\right)^{2}}}+{\frac {M_{2}}{\left(2R-r\right)^{2}}}=\left({\frac {M_{2}}{M_{1}+M_{2}}}R+R-r\right){\frac {M_{1}+M_{2}}{R^{3}}}}$

with parameters M_1,2 and R defined as for the L₁ and L₂ cases, and r now indicates the distance of L₃ from the position of the smaller object, if it were rotated 180 degrees about the larger object, while positive r implying L3 is closer to the larger 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:[15]

${\displaystyle r\approx R{\frac {7M_{2}}{12M_{1}}}}$

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 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.

Net radial acceleration of a point orbiting along the Earth–Moon line.

Radial acceleration

The radial acceleration a of an object in orbit at a point along the line passing through both bodies is given by:

${\displaystyle a=-{\frac {GM_{1}}{r^{2}}}\operatorname {sgn}(r)+{\frac {GM_{2}}{(R-r)^{2}}}\operatorname {sgn}(R-r)+{\frac {G{\bigl (}(M_{1}+M_{2})r-M_{2}R{\bigr )}}{R^{3}}}}$

where r is the distance from the large body M₁ and sgn(x) is the sign function of x. The terms in this function represent respectively: force from M₁; force from M₂; and centrifugal force. The points L₃, L₁, L₂ occur where the acceleration is zero — see chart at right.

Stability

Although the L₁, L₂, and L₃ 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.e. bounded but not precisely repeating) orbits following Lissajous-curve trajectories. 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.

For Sun–Earth-L₁ missions, it is preferable for the spacecraft to be in a large-amplitude (100,000–200,000 km or 62,000–124,000 mi) Lissajous orbit around L₁ than to stay at L₁, because the line between Sun and Earth has increased solar interference on Earth–spacecraft communications. Similarly, a large-amplitude Lissajous orbit around L₂ keeps a probe out of Earth's shadow and therefore ensures continuous illumination of its solar panels.

The L₄ and L₅ points are stable provided that the mass of the primary body (e.g. the Earth) is at least 25[note 1] times the mass of the secondary body (e.g. the Moon).[16][17] The Earth is over 81 times the mass of the Moon (the Moon is 1.23% of the mass of the Earth[18]). Although the L₄ and L₅ 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.[17][19]

Solar System values

Sun-planet Lagrange points to scale (click for clearer points)

This table lists sample values of L₁, L₂, and L₃ within the Solar System. 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 L₃ showing a negative location. The percentage columns show how the distances compare to the semimajor axis. E.g. for the Moon, L₁ is located 326400 km from Earth's center, which is 84.9% of the Earth–Moon distance or 15.1% in front of the Moon; L₂ is located 448900 km from Earth's center, which is 116.8% of the Earth–Moon distance or 16.8% beyond the Moon; and L₃ is located −381700 km from Earth's center, which is 99.3% of the Earth–Moon distance or 0.7084% in front of the Moon's 'negative' position.

Lagrangian points in Solar System

Body pair	Semimajor axis, SMA (×10^9 m)	L1 (×10^9 m)	1 − L1/SMA (%)	L2 (×10^9 m)	L2/SMA − 1 (%)	L3 (×10^9 m)	1 + L3/SMA (%)
Earth–Moon	0.3844	0.32639	15.09	0.4489	16.78	−0.38168	0.7084
Sun–Mercury	57.909	57.689	0.3806	58.13	0.3815	−57.909	0.000009683
Sun–Venus	108.21	107.2	0.9315	109.22	0.9373	−108.21	0.0001428
Sun–Earth	149.6	148.11	0.997	151.1	1.004	−149.6	0.0001752
Sun–Mars	227.94	226.86	0.4748	229.03	0.4763	−227.94	0.00001882
Sun–Jupiter	778.34	726.45	6.667	832.65	6.978	−777.91	0.05563
Sun–Saturn	1426.7	1362.5	4.496	1492.8	4.635	−1426.4	0.01667
Sun–Uranus	2870.7	2801.1	2.421	2941.3	2.461	−2870.6	0.002546
Sun–Neptune	4498.4	4383.4	2.557	4615.4	2.602	−4498.3	0.003004

Spaceflight applications

Sun–Earth

The satellite ACE in an orbit around Sun–Earth L₁.

Sun–Earth L₁ 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 1978 International Sun Earth Explorer 3 (ISEE-3) mission used as an interplanetary early warning storm monitor for solar disturbances.[20] Since June 2015, DSCOVR has orbited the L₁ point. 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 L₁ up to an hour before Earth. Solar and heliospheric missions currently located around L₁ include the Solar and Heliospheric Observatory, Wind, and the Advanced Composition Explorer. Planned missions include the Interstellar Mapping and Acceleration Probe (IMAP).

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,[21] so solar radiation is not completely blocked at L₂. Spacecraft generally orbit around L₂, avoiding partial eclipses of the Sun to maintain a constant temperature. From locations near L₂, 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. The James Webb Space Telescope is due to be positioned at L₂.

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[22] 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.[23]

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 has been used for a communications satellite covering the Moon's far side, for example, Queqiao, launched in 2018,[24] and would be "an ideal location" for a propellant depot as part of the proposed depot-based space transportation architecture.[25]

Sun–Venus

Scientists at the B612 Foundation were[26] planning to use Venus's L₃ point to position their planned Sentinel telescope, which aimed to look back towards Earth's orbit and compile a catalogue of near-Earth asteroids.[27]

Sun–Mars

In 2017, the idea of positioning a magnetic dipole shield at the Sun–Mars L₁ point for use as an artificial magnetosphere for Mars was discussed at a NASA conference.[28] The idea is that this would protect the planet's atmosphere from the Sun's radiation and solar winds.

Lagrangian spacecraft and missions

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).[29]

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. Currently slated for launch in late 2024, the Interstellar Mapping and Acceleration Probe will be placed near 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.[30] 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[31] and was a centerpiece in his 2006 film An Inconvenient Truth.[32]

LISA Pathfinder (LPF) was launched on 3 December 2015, and arrived at L₁ on 22 January 2016, where, among other experiments, it tested the technology needed by (e)LISA to detect gravitational waves. LISA Pathfinder used 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[33]
• 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[34]
• 3 July 2009 – 21 October 2013: Planck Space Observatory
• 25 August 2011 – April 2012: Chang'e 2,[35][36] from where it travelled to 4179 Toutatis and then into deep space
• January 2014: Gaia Space Observatory
• 2019: Spektr-RG X-Ray Observatory
• 2021: James Webb Space Telescope will use a halo orbit
• 2022: Euclid Space Telescope
• 2024: Nancy Grace Roman Space Telescope (WFIRST) will use a halo orbit
• 2031: Advanced Telescope for High-Energy Astrophysics (ATHENA) will use a halo orbit

Spacecraft at Earth–Moon L₂

• Chang'e 5-T1 experimental spacecraft DFH-3A "service module" was sent to the Earth-Moon L₂ lunar Lissajous orbit on 13 January 2015, where it used the remaining 800 kg of fuel to test maneuvers key to future lunar missions.[37]
• Queqiao entered orbit around the Earth–Moon L₂ on 14 June 2018. It serves as a relay satellite for the Chang'e 4 lunar far-side lander, which cannot communicate directly with Earth.

Past and current missions

Color key:

Unflown or planned mission

Mission en route or in progress (including mission extensions)

Mission at Lagrangian point completed successfully (or partially successfully)

Mission	Lagrangian point	Agency	Description
International Sun–Earth Explorer 3 (ISEE-3)	Sun–Earth L1	NASA	Launched in 1978, it was the first spacecraft to be put into orbit around a libration point, where it operated for four years in a halo orbit about the L1 Sun–Earth point. After the original mission ended, it was commanded to leave L1 in September 1982 in order to investigate comets and the Sun.[38] Now in a heliocentric orbit, an unsuccessful attempt to return to halo orbit was made in 2014 when it made a flyby of the Earth–Moon system.[39][40]
Advanced Composition Explorer (ACE)	Sun–Earth L1	NASA	Launched 1997. Has fuel to orbit near L1 until 2024. Operational as of 2019.[41]
Deep Space Climate Observatory (DSCOVR)	Sun–Earth L1	NASA	Launched on 11 February 2015. Planned successor of the Advanced Composition Explorer (ACE) satellite. In safe mode as of 2019, but is planned to reboot.[42]
LISA Pathfinder (LPF)	Sun–Earth L1	ESA, NASA	Launched one day behind revised schedule (planned for the 100th anniversary of the publication of Einstein's General Theory of Relativity), on 3 December 2015. Arrived at L1 on 22 January 2016.[43] LISA Pathfinder was deactivated on 30 June 2017.[44]
Solar and Heliospheric Observatory (SOHO)	Sun–Earth L1	ESA, NASA	Orbiting near L1 since 1996. Operational as of 2020.[45]
WIND	Sun–Earth L1	NASA	Arrived at L1 in 2004 with fuel for 60 years. Operational as of 2019.[46]
Wilkinson Microwave Anisotropy Probe (WMAP)	Sun–Earth L2	NASA	Arrived at L2 in 2001. Mission ended 2010,[47] then sent to solar orbit outside L2.[48]
Herschel Space Telescope	Sun–Earth L2	ESA	Arrived at L2 July 2009. Ceased operation on 29 April 2013; will be moved to a heliocentric orbit.[49][50]
Planck Space Observatory	Sun–Earth L2	ESA	Arrived at L2 July 2009. Mission ended on 23 October 2013; Planck has been moved to a heliocentric parking orbit.[51]
Chang'e 2	Sun–Earth L2	CNSA	Arrived in August 2011 after completing a lunar mission before departing en route to asteroid 4179 Toutatis in April 2012.[36]
ARTEMIS mission extension of THEMIS	Earth–Moon L1 and L2	NASA	Mission consists of two spacecraft, which were the first spacecraft to reach Earth–Moon Lagrangian points. Both moved through Earth–Moon Lagrangian points, and are now in lunar orbit.[52][53]
WIND	Sun–Earth L2	NASA	Arrived at L2 in November 2003 and departed April 2004.
Gaia Space Observatory	Sun–Earth L2	ESA	Launched 19 December 2013.[54] Operational as of 2020.[55]
Chang'e 5-T1 Service Module	Earth–Moon L2	CNSA	Launched on 23 October 2014, arrived at L2 halo orbit on 13 January 2015.[37]
Queqiao	Earth–Moon L2	CNSA	Launched on 21 May 2018, arrived at L2 halo orbit on June 14.[56]
Spektr-RG	Sun–Earth L2	IKI RAN DLR	Launched 13 July 2019. Roentgen and Gamma space observatory. En route to L2 point.

Future and proposed missions

Mission	Lagrangian point	Agency	Description
"Lunar Far-Side Communication Satellites"	Earth–Moon L2	NASA	Proposed in 1968 for communications on the far side of the Moon during the Apollo program, mainly to enable an Apollo landing on the far side—neither the satellites nor the landing were ever realized.[57]
Space colonization and manufacturing	Earth–Moon L4 or L5	—	First proposed in 1974 by Gerard K. O'Neill[58] and subsequently advocated by the L5 Society.
EQUULEUS	Earth–Moon L2	University of Tokyo, JAXA	6U CubeSat, launch planned in 2019 as a secondary payload onboard SLS Artemis 1.[59]
James Webb Space Telescope (JWST)	Sun–Earth L2	NASA, ESA, CSA	As of 2020, launch is planned for 2021.[60]
Euclid	Sun–Earth L2	ESA, NASA	As of 2013, launch is planned in 2020.[61]
Aditya-L1	Sun–Earth L1	ISRO	Launch planned for 2021; it will be going to a point 1.5 million kilometers away from Earth, from where it will observe the Sun constantly and study the solar corona, the region around the Sun's surface.[62]
Demonstration and Experiment of Space Technology for INterplanetary voYage (DESTINY)	Earth–Moon L2	JAXA	Candidate for JAXA's next "Competitively-Chosen Medium-Sized Focused Mission", possible launch in the early 2020s.[63]
Exploration Gateway Platform	Earth–Moon L2[64]	NASA	Proposed in 2011.[65]
Nancy Grace Roman Space Telescope (WFIRST)	Sun–Earth L2	NASA, USDOE	As of 2013, in a "pre-formulation" phase until at least early 2016; possible launch in the early 2020s.[66]
LiteBIRD	Sun–Earth L2[67]	JAXA, NASA	As of 2015, one of two finalists for JAXA's next "Strategic Large Mission"; would be launched in 2024 if selected.[68]
Planetary Transits and Oscillations of stars (PLATO)	Sun–Earth L2	ESA	Planned for launch in 2024 for an initial six-year mission.[69]
Space Infrared Telescope for Cosmology and Astrophysics (SPICA)	Sun–Earth L2	JAXA, ESA, SRON	As of 2015, awaiting approval from both Japanese and European side, launch proposed for 2025.[70]
Advanced Telescope for High Energy Astrophysics (ATHENA)	Sun–Earth L2	ESA	Launch planned for 2028.[71]
Spektr-M	Sun–Earth L2	Roscosmos	Possible launch after 2027.[72]

See also

• Co-orbital configuration
• Euler's three-body problem
• Gegenschein
• Hill sphere
• Horseshoe orbit
• Klemperer rosette
• L₅ Society
• Lagrange point colonization
• Lagrangian mechanics
• Lissajous orbit
• List of objects at Lagrangian points
• Lunar space elevator
• Oberth effect

Notes

1. Actually 25 + √621/2 ≈ 24.9599357944

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External links

• Joseph-Louis, Comte Lagrange, from Oeuvres Tome 6, "Essai sur le Problème des Trois Corps"—Essai (PDF); source Tome 6 (Viewer)
• "Essay on the Three-Body Problem" by J-L Lagrange, translated from the above, in merlyn.demon.co.uk.
• Considerationes de motu corporum coelestium—Leonhard Euler—transcription and translation at merlyn.demon.co.uk.
• What are Lagrange points?—European Space Agency page, with good animations
• Explanation of Lagrange points—Prof. Neil J. Cornish
• A NASA explanation—also attributed to Neil J. Cornish
• Explanation of Lagrange points—Prof. John Baez
• Geometry and calculations of Lagrange points—Dr J R Stockton
• Locations of Lagrange points, with approximations—Dr. David Peter Stern
• An online calculator to compute the precise positions of the 5 Lagrange points for any 2-body system—Tony Dunn
• Astronomy cast—Ep. 76: Lagrange Points Fraser Cain and Dr. Pamela Gay
• The Five Points of Lagrange by Neil deGrasse Tyson
• Earth, a lone Trojan discovered
• See the Lagrange Points and Halo Orbits subsection under the section on Geosynchronous Transfer Orbit in NASA: Basics of Space Flight, Chapter 5