Mun
Mun, also known as the Mun and sometimes written as Mün, is a relatively large moon orbiting Kerbin. It can be thought of as an analogue to Earth’s own moon, which frequently is simply called “the Moon” (Latin: Luna).
It is gray in appearance with craters of various sizes and mountains exceeding 5029 m in height. The gravitational pull on the surface is 1.63 m/s², approximately 5/6 and 8.18 m/s² lower than that of Kerbin and about the surface gravity of Earth’s Moon (1.624 m/s²). It is possible to land on the Mun or use it for gravity assist (gravitational slingshots) to outer bodies such as Minmus or into Kerbol orbit, though such maneuvers do not save very much fuel and are more imprecise. It is, however, useful for plane changes with proper timing.
New players should note that Mun (like the majority of celestial bodies in the game) has no atmosphere. That means parachutes do not work when descending to the surface.
There are anomalous geological formations that can be found on the surface of the Mun.
Synchronous orbit around the Mun is not possible, since it would have to occur at an altitude of 2 970.58 km, beyond the Mun’s sphere of influence. However, putting your spacecraft just outside Mun’s SOI and having the same semi-major axis would make it appear stationary. A normal 25 km orbit can be achieved using around 800 m/s delta V. A semi-synchronous orbit with half the rotation period of the Mun is possible at approximately 1797.41 kilometers.
Minmus
Minmus is the smallest moon of Kerbin. From the surface of Kerbin or the Mun, it is a cyan speck one or two pixels wide which appears to slightly oscillate vertically. This is a result of viewing its rotation from afar. Up close, it is off white in appearance with what seem to be icy buttes, large hills, and frozen lakes. The highest areas are over 5.7 km in altitude. It requires somewhat more delta-v to reach Minmus orbit than Mun orbit; however, given its very small gravity, it requires much less to land on the surface and return. This makes Minmus a primary source of Science in the early game after achieving an orbit of Kerbin.
Tylo
Tylo is the largest moon of Jool and the Kerbol System, and the largest celestial body without an atmosphere. It is the analog for Ganymede. It is the third moon from Jool, is the same diameter as Kerbin, and is tidally locked. Tylo orbits Jool in roughly nine days. Synchronous orbits around Tylo are impossible, as they would lie far outside of its SOI at a radius of 14,758,067 meters.
Tylo is a relatively difficult target to visit. Its gravity is similar to Kerbin’s, but without an atmosphere, there is no way to aerobrake, and a large amount of fuel must be used to land and take off. However, due to its large gravity well, Tylo is ideal for a gravity assist to enter or leave the Jool system with minimal fuel consumption.
According to former developer NovaSilisko, Tylo was planned to be made into a high-contrasting body like Saturn’s moon Iapetus.
Laythe
Laythe is the innermost of the five natural satellites of Jool. Although mostly covered in seas, Laythe has numerous rocky, sandy islands. Laythe is tidally locked to Jool. Synchronous orbits around Laythe are not possible, as they would lie outside of its SOI, at a radius of 5,186,399 meters. A circular archipelago prominent in surface maps of the moon suggests that Laythe suffered a massive impact in its distant past.
Achieving a stable orbit around Laythe from sea level requires a delta-V of ≈2900 m/s.
Laythe has no real-world analogue. In the real-world Jupiter system, the moons Ganymede, Europa and Io form a group with an orbital resonance just like KSP’s Tylo, Vall and Laythe. But while Tylo and Vall have a certain resemblance to their real-life counterparts, Laythe is very unlike Io, which is dry, volcanic, and only has a tenuous atmosphere. Instead, the presence of a liquid ocean, plus a dense atmosphere, makes Laythe more similar to the Saturnian moon Titan. Laythe also seems very similar to what happened to Europa in 2010: Odyssey Two.
Although Laythe has liquid water on its surface, its polar regions have temperatures below zero degrees Celsius even outside its ice caps. This means Laythe’s oceans must contain another compound which reduces its freezing point, most likely ionic, such as salt. Surface samples indicate that much salt is present on Laythe’s surface. Like that of Kerbin, the air can be breathed by Kerbals (according to EVA reports – only from 1.6.0 onwards). The air however does have a strange smell about it. It is therefore possible that there are also high concentrations of salt in the air, as well as on the surface, assuming that the salt is sodium chloride. Laythe may also be a reference to the planet Damogran from the radio/tv/book/movie series The Hitchhiker’s Guide to the Galaxy, as they are almost, but not quite, entirely not unlike one another.
According to former developer NovaSilisko, Laythe was planned to have volcanic activity and high radiation levels in later versions, making it a much more hostile moon than in 1.6.0
Vall
Vall is the second of the five natural satellites of Jool. Like the other large Joolian moons, Laythe and Tylo (of which Vall is the smallest), Vall is tidally locked to its parent. Synchronous orbits around Vall are not possible, as they would lie outside of its SOI, at a radius of 3,893,254 meters.
Vall is KSP’s analog to Jupiter’s moon Europa.
Though the bodies have topical similarities, landing on Vall and returning is significantly more challenging than a landing on the Mün, due to the former’s higher gravity, more uneven terrain and distance from Kerbin.
According to former developer NovaSilisko, Vall was planned to be made more like Eeloo, with cracks in the ice and a more chaotic surface than in 0.20.2
Ike
Ike is a moon and the only natural satellite of Duna. Ike has steep, rocky slopes and no atmosphere. Ike and Duna are tidally locked due to their proximity and Ike’s size. This means that Ike is in synchronous orbit around its parent body, and that each body is only visible from approximately half of its partner’s surface. (It would be exactly half, but Ike’s slightly eccentric orbit creates regions for which Ike can be seen to set and rise. See Libration, below). It is the largest among planetary satellites relative to the size of the planet that it orbits in the Kerbol system.
An encounter with Ike is quite likely when approaching Duna, provided the player’s trajectory is near the plane of Ike’s orbit. This is because Ike’s sphere of influence is very high relative to the semi-major axis of its orbit — in fact, no other moon in the game has a larger SOI-to-semi-major-axis ratio and no other moon is as close in mass to its parent – Ike has about 6% the mass of Duna.
Stationary orbit around Ike is not possible — it would require an altitude of 1 133.90 km, outside of Ike’s sphere of influence. However, since Ike and Duna are tidally locked to each other, a spacecraft in stationary orbit around Duna will see Ike in a near stable, non-rotating position, as if it were in a stationary Ike orbit. It is not perfectly stationary as Ike has both a non-zero orbital eccentricity and a very small orbital inclination.
Although Duna is the game analog for Mars, Ike does not seem to represent Phobos or Deimos. The Duna-Ike system could be interpreted as an analog of Pluto-Charon – in the real solar system, no moon has a mass as similar to its parent body as Ike.
