Neidon as seen from orbit.
Planet of Kerbol
Orbital Characteristics
Semi-major axis 409 355 191 706 m [Note 1]
Apoapsis 414 577 213 080 m [Note 1]
Periapsis 404 133 170 332 m [Note 1]
Orbital eccentricity 0.013
Orbital inclination 1.27 °
Argument of periapsis 0 °
Longitude of the ascending node 259 °
Mean anomaly 2.272 rad (at 0s UT)
Sidereal orbital period 1 519 864 480 s
70 364 d 0 h 34 m 40 s
Synodic orbital period 9259616.3 s
Orbital Velocity 1 671 to 1 714 m/s
Physical Characteristics
Equatorial radius 2 145 000 m
Equatorial Circumference 13 477 432 m
Surface area 5.7818185×1013 m2
Mass 2.1228217×1023 kg
Standard gravitational parameter 1.4167882×1013 m3/s2
Density 5 135.0303 kg/m3
Surface gravity 3.08 m/s2 (0.314 g)
Escape velocity 3 634.58 m/s
Sidereal rotation period 40 250.000 s
1 d 5 h 10 m 50 s
Solar day 40 251.066 s
1 d 5 h 10 m 51.1 s
Sidereal rotational velocity 334.84 m/s
Synchronous orbit 6 201.27 km
Sphere of influence 4.4157238×109 m [Note 1]
Atmospheric Characteristics
Atmosphere present ✓ Yes
Atmospheric Pressure 607.950 kPa
6 atm
Scale height  m
Atmospheric height 260 000 m
6.0×10-6 atm
Temperaturemin -222.19 °C 50.96 K
Temperaturemax -183.15 °C 90 K
Oxygen present × No
Scientific multiplier
Surface N/A
Splashed N/A
Lower atmosphere 15
Upper atmosphere 12
Near space 10
Outer space 9
Recovery 9

  1. 1.0 1.1 1.2 1.3 The distances are given from the bodies center not surface (unlike ingame)

Neidon is an ice giant and the ninth planet of the Kerbol system. It is the third planet added in the Outer Planets Mod and is the Neptune analog for Kerbal Space Program. The planet is an attractive target to visit because of the deep blue and purple colors, however its distance makes it a difficult place to visit. Neidon also has two moons: Thatmo, and Nissee.. It also has a procedural system of asteroids if Custom Asteroids is installed. As with the other gas giants, it is not possible to land on Neidon - instead, the craft is simply destroyed.

Neidon's great distance from Kerbol reduces solar panel output to under 0.11% of that at Kerbin, so bringing extra solar panels or an alternative power source (e.g. RTGs) for missions to Neidon and its moons is recommended.

In-Game Description

Due to its distance from Kerbin, it wasn't until fairly recently that Kerbalkind discovered the Kerbol system's most distant planet. At first scientists thought Neidon might be a purple, rocky planet like Eve, but they quickly realized they were in fact looking at an ice giant similar to Urlum.

Kerbal Astronomical Society


Neidon has a very cold, dense atmosphere that extends to an altitude of 260,000 meters and has an average molecular mass of 2.8 g/mol. This indicates a composition of mostly hydrogen (80%) and helium (19%), and a small amount of methane may contribute to the color. The upper atmosphere also has traces of hydrocarbons, carbon monoxide, and carbon dioxide.

Like all other atmospheres in the game, Neidon's atmosphere fades exponentially as altitude increases. The scale height varies with altitude. The pressure-altitude profile is globally constant and independent of temperature. The following table gives the atmospheric pressure at various altitudes above the datum level.

Altitude (m) Pressure (Pa) Pressure (atm)
0 607 950 6.000
10 000 446 822 4.410
20 000 312 958 3.089
30 000 208 184 2.055
40 000 133 036 1.313
50 000 83 115 0.820
60 000 51 560 0.509
70 000 31 904 0.315
80 000 19 819 0.196
90 000 12 157 0.120
100 000 7 406 0.073
120 000 2 714 0.027
140 000 1 024 0.010
160 000 416.7 0.004
180 000 185.0 0.002
200 000 86.22 0.001
230 000 28.45 0.000
260 000 0 0.000

Judging by density and planet models, it's interesting to note that the atmosphere in game is only the outer layer. At sea-level the atmosphere of a realistic planet would start to gradually become liquid until it reached the icy mantle. This is also true for Urlum.

Temperatures and Atmospheric Layers

There appears to be several layers in Neidon's atmosphere - a Troposphere, where temperatures decrease with altitude, and a Stratosphere, where temperatures increase with altitude. The boundary between the troposphere and stratosphere is called the Tropopause, and occurs at an altitude of 25 km. There is also a Thermosphere which begins around 100 km and extends out to 160 km.

Atmospheric flight

Aerobraking into Neidon's orbit from a high-speed interplanetary intercept is possible but it is very difficult to achieve due to rapid and often destructive aerodynamic heating. A successful aerocapture requires a properly designed spacecraft, with a large ablator mass and a low ballistic coefficient, and precise periapsis targeting. The required periapsis altitude depends on the spacecraft's drag characteristics and its approach velocity. The margins for error are extremely small.

You could also use parachutes in Neidon's atmosphere to slow your spacecraft down.


From the average density of the planet, it's possible to estimate models for its composition. One model suggests a solid core - likely iron (radius of 128.7 km) - surrounded by a mantle made of ices (water, ammonia, and methane with a radius of 645.3 km). The outer atmospheric layer begins to liquefy and harden to pressures that destroy any spacecraft. From the outer layer to the ice mantle could be 1,372.8 km.

The deep purple and blue color could be due to methane in the atmosphere, which absorbs red light.


There is an extremely tilted magnetic field that indicates a magnetosphere that may be formed by convective fluids in the icy mantle or lower atmospheric regions, similar to Urlum.

Possible Weather

With certain graphical modifications installed, Neidon has rotating clouds. By observing the rotation of these clouds, wind speeds can be estimated to vary between 325 to 600 m/s, the fastest wind speeds in the game.

Natural Satellites

Neidon only has two natural satellites, each with a very inclined orbit:

  • Thatmo, an icy moon, has a very thin atmosphere and a retrograde orbit.
  • Nissee, a rocky moon, has a highly inclined and eccentric orbit.

Synchronous Orbits around any of the Neidonean moons are impossible, as they all lie outside the Sphere of Influence of the moons, as is common with tidally locked bodies.


Neidon has a total of eight (atmosphere and space-only) biomes.

Biome list

Neidon Biome Map

Neidon biome map

  • Polar Storms
  • Polar Bands
  • Polar Zones
  • Temperate Bands
  • Temperate Zones
  • Tropical Zones
  • Equatorial Band
  • Equatorial Zones

Reference frames

Warp Minimum Altitude
5× 260 000 m (above the atmosphere)
10× 260 000 m (above the atmosphere)
50× 260 000 m (above the atmosphere)
100× 260 000 m (above the atmosphere)
1 000× 321 750 m
10 000× 643 500 m
100 000× 1 287 000 m