Lunar orbit

In astronomy and spaceflight, a lunar orbit is the path an object takes when it circles around Earth's Moon. These orbits are usually not perfect circles. When a spacecraft is farthest from the Moon, it is called apolune, apocynthion, or aposelene. When it is closest to the Moon, it is called perilune, pericynthion, or periselene. These special names come from the names and titles of the moon goddess.

To enter a lunar orbit, spacecraft use a special move called lunar orbit insertion or LOI. This helps the spacecraft slow down just enough to be caught by the Moon's gravity.

There is also something called low lunar orbit or LLO. This is a very close orbit, less than 100 kilometers above the Moon's surface. It takes about two hours to go around the Moon once in this orbit. Low lunar orbits are very important for exploring the Moon, but they can be tricky because of the Moon's gravity. Only a few special paths, called frozen orbits, stay stable for a long time, making them useful for spacecraft that plan to stay near the Moon for many months or even years.

Perturbation effects and low orbits

Further information: Mass concentration (astronomy) § Lunar mascons

Big pieces of rock that hit the Moon long ago made areas under the surface heavier. These heavy spots can change the paths of spacecraft orbiting the Moon. They can make the orbit shift a lot in just a few days. That’s why orbits very close to the Moon, lower than 100 kilometers, can become unstable.

In 1969, during the first mission to land humans on the Moon, astronauts tried to fix this problem. They adjusted their orbit to stay safe, but it turned out to be easier than they thought. When they met up again with the main spacecraft, the orbit was a little bigger than planned but still okay.

Stable low orbits

Scientists found that certain orbits around the Moon stay the same forever because of special forces. These orbits happen at angles of 27°, 50°, 76°, and 86° from the Moon's surface. Two small satellites from the Apollo missions, PFS-1 and PFS-2, helped make this discovery. PFS-1 stayed in its orbit for over a year and a half, while PFS-2, in a less stable position, lasted only 35 days before it fell back to the Moon.

Lunar high orbits

When spacecraft orbit the Moon at heights between 500 to 20,000 km, Earth’s gravity can cause changes in their path. To understand these orbits, scientists need to consider the effects of three bodies — Earth, the Moon, and the spacecraft.

The Moon’s area of influence stretches to about 60,000 km, but Earth’s gravity can make orbits unstable at distances closer than 690 km. Special points in the Earth-Moon system, called Lagrange points, can help spacecraft stay in stable paths. These include halo orbits and distant retrograde orbits.

Some halo orbits stay above certain areas of the Moon’s surface. This helps relay satellites talk to stations on the far side of the Moon. The first to do this was the Queqiao relay satellite in 2019, placed around Earth-Moon L2 about 65,000 km from the Moon. Since 2022, CAPSTONE and Lunar Gateway have used near-rectilinear halo orbits, which also use a Lagrange point.

Orbital transfer

Main articles: Trans-lunar injection and orbit insertion

There are three main ways to get a spacecraft to orbit around the Moon from Earth. These are called direct transfer, low thrust transfer, and low-energy transfer. The time it takes can be different for each method, ranging from a few days to several months.

History of missions to lunar orbit

Further information: List of extraterrestrial orbiters § Moon

First orbiters

The Soviet Union sent the first spacecraft near the Moon, called Luna 1, on January 4, 1959. It came within 6,000 kilometres of the Moon but did not orbit it. Another spacecraft, Luna 3, launched on October 4, 1959, was the first to travel around the Moon and return to Earth, giving us the first images of the Moon's far side.

In April 1966, Luna 10 became the first spacecraft to orbit the Moon. It studied the Moon's environment until May 30, 1966. A later mission, Luna 11, launched on August 24, 1966, examined the Moon's gravity and other space conditions.

The first United States spacecraft to orbit the Moon was Lunar Orbiter 1 on August 14, 1966. Its first orbit was stretched out, but it was later adjusted to a more circular path to help map the Moon. This helped find good landing spots for future missions.

Crewed and later orbiters

During the Apollo program, the Command/Service Module stayed in orbit around the Moon while the Lunar Module landed. The combined spacecraft would first enter a stretched-out orbit and then change to a circular orbit about 60 nautical miles above the Moon. The orbit time was roughly two hours. The Lunar Module would then begin its landing by lowering its orbit to avoid tall lunar mountains. Starting with Apollo 14, the orbit adjustment was changed to save fuel for the landing.

Sun-synchronous lunar orbits

A Sun-synchronous lunar orbit is a special kind of orbit around the Moon where the orbit stays almost in the same position compared to the Sun. To keep this orbit working, it would need to move about 1° each day, just like certain orbits around Earth.

But unlike orbits around Earth, these lunar orbits don't happen naturally. This is because the Moon's gravity isn't even, so keeping this orbit would need careful control. Both Earth and the Sun's gravity would also change the orbit.