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Lagrangian point

(Lagrange point, L-point, libration point, Lagrangian libration point)
(points around two orbiting bodies where small objects can sit)

A Lagrangian point (or Lagrange point, L-point, libration point, or Lagrangian libration point) is point in the vicinity of two objects orbiting each other at which a less-massive third object would not be drawn toward either of the two more-massive orbiting objects, i.e., the three would orbit in sync, maintaining their intervening distances. The points are used as places to locate spacecraft to maintain their position relative to the two bodies. Some of the points are naturally stable, and natural objects are sometimes found at these since an object will remain if it ever previously migrated sufficiently close.

An obvious such point is between the two more-massive bodies where their gravity balances any acceleration due to its not being centered between them. This point, known as L1, is unstable: any slight movement from the point toward either move-massive object puts the less-massive object in the influence of forces that will take it toward one of the two more-massive objects. Other points:

L4 and L5 can be stable in that any slight motion causes forces to bring them back to the L4 or L5 point respectively. For example, an object in the L5 point for the Earth and Moon, if it drifted toward the Moon, the Moon's gravity would draw harder, but that would speed its orbit around the Earth, causing its Earth orbit to expand, which in turn speeds its orbit around the Moon, causing its lunar orbit to expand, bringing it back to L5. The condition for their stability (Lagrange point stability) is a specific minimal mass ratio, the minimum being a number close to 25.

L1, L2, and L3 are unstable, but the positive feedback away from the point acts slowly, and can be counteracted by an occasional small push, a strategy that spacecraft use to park in these spots: a reasonable amount of propellant (light enough to be launched and carried there) can keep a spacecraft at the point (stationkeeping) for years or a decade.

The stable points naturally support orbits encircling or crossing the point. The unstable points also support such orbits (e.g., halo orbits) given the occasional propulsion to counteract the instability.

L4 and L5 points, which are also known as Trojan points, can collect debris since anything drifting to them will tend to stay. In the solar system, some moons and asteroids are in such Lagrange points. I imagine the name libration points is used because and an object can sit in a small orbit around any of the stable points, thus oscillating. A Trojan asteroid is one orbiting the Sun in such a position (by default, assuming an L4/L5 relation with Jupiter). The term Trojan planet refers to an extra-solar planet residing in another's L4 or L5 point (as of 2020, no candidates have been widely accepted).

All the points for the Earth, Sun, and Moon are candidates for space observatories and other satellites. Sun-Earth L1 is popular for observatories watching the Sun and Sun-Earth L2 is popular for observatories that are not. Some current and planned space platforms at Sun-Earth L1:

At L2:

Further reading:

Referenced by pages:
Advanced Composition Explorer (ACE)
asteroid belt
corotation resonance (CR)
gravitational instability (GI)
halo orbit
High Definition Space Telescope (HDST)
Herschel Space Observatory
Hill radius
International Cometary Explorer (ICE)
NEO Surveyor
New Worlds Mission
Origins Space Telescope (OST)
Roche limit
Roche lobe
Solar and Heliospheric Observatory (SOHO)
Spektr-RG (SRG)
2010 TK7
James Webb Space Telescope (JWST)
Roman Space Telescope (RST)
Wilkinson Microwave Anisotropy Probe (WMAP)