Urlum as seen from orbit.
Planet of Kerbol
Orbital Characteristics
Semi-major axis 254 317 012 787 m [Note 1]
Apoapsis 265 815 873 613 m [Note 1]
Periapsis 242 818 151 961 m [Note 1]
Orbital eccentricity 0.05
Orbital inclination 0.64 °
Argument of periapsis 0 °
Longitude of the ascending node 61 °
Mean anomaly 5.596 rad (at 0s UT)
Sidereal orbital period 744 247 753 s
34 455 d 5 h 29 m 12.6 s
Synodic orbital period 9318782.9 s
Orbital Velocity 2 052 to 2 246 m/s
Physical Characteristics
Equatorial radius 2 177 000 m
Equatorial Circumference 13 678 494 m
Surface area 5.9556165×1013 m2
Mass 1.7903288×1023 kg
Standard gravitational parameter 1.1948797×1013 m3/s2
Density 4 142.5608 kg/m3
Surface gravity 2.52 m/s2 (0.257 g)
Escape velocity 3 313.20 m/s
Sidereal rotation period 41 000.000 s
1 d 5 h 23 m 20 s
Solar day 41 002.259 s
1 d 5 h 23 m 22.3 s
Sidereal rotational velocity 333.62 m/s
Synchronous orbit 5 806.21 km
Sphere of influence 2.5626230×109 m[Note 1]
Atmospheric Characteristics
Atmosphere present ✓ Yes
Atmospheric Pressure 709.275 kPa
7 atm
Scale height 23 524 m
Atmospheric height 325 000 m
7.0×10-6 atm
Temperaturemin -219.18 °C 53.97 K
Temperaturemax -178.15 °C 95 K
Oxygen present × No
Scientific multiplier
Surface N/A
Lower atmosphere 12
Upper atmosphere 11.5
Near space 11
Outer space 10.5

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

Urlum is an ice giant and the eighth planet of the Kerbol system. It is the second planet added in the Outer Planets Mod and is the Uranus analog for Kerbal Space Program. The planet is an attractive target to visit because of its beautiful color, faint rings, and its very unique system of four moons: Polta, Priax, Wal, and its moon Tal. If CustomAsteroids is installed, asteroid moonlets will appear in orbit around Urlum. As with the other gas giants, it is not possible to land on Urlum - instead, the craft is simply destroyed.

Solar panel effectiveness is hampered by the Urlum's large distance from Kerbol reducing output to barely 0.26% of Kerbin's at apoapsis. Therefore, bringing an alternate power source (e.g. RTGs) is recommended

In-Game Description

It took a while for Kerbal astronomers to realize Urlum was not a far away star, but in fact another planet. Since then scientists have been weary of making claims about Urlum, for fear of being wrong yet again. The ice giant is unusual in that it rotates around its axis in a clockwise fashion, unlike the anti-clockwise rotation of the other planets.

Kerbal Astronomical Society


Urlum has a very cold, dense atmosphere that extends to an altitude of 325,000 meters and has an average molecular mass of 2.8 g/mol. This indicates a composition of mostly hydrogen (83%) and helium (15%), and a small amount of methane (2.3%) may contribute to the color. The outer cloud layer contains ammonia and water ice.

Like all other atmospheres in the game, Urlum'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)
0 709 275 7.000
10 000 560 843 5.535
20 000 431 998 4.263
30 000 322 633 3.184
40 000 233 882 2.308
50 000 165 895 1.637
60 000 115 898 1.144
70 000 80 362 0.793
80 000 55 621 0.549
90 000 38 446 0.379
100 000 26.584 0.262
125 000 10 287 0.102
150 000 3 907 0.039
175 000 1 524 0.015
200 000 634.5 0.006
225 000 287.7 0.003
250 000 137.1 0.001
275 000 66.6 0.001
300 000 32.8 0.000
325 000 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 Neidon.

Temperatures and Atmospheric Layers

There appears to be several layers in Urlum's atmosphere - a Troposphere, Stratosphere, and Thermosphere. The troposphere extends to an altitude of 26 km, the stratosphere from an altitude of around 26 km to 100 km, and the thermosphere from an altitude of around 100 km and extends to 180 km.

Atmospheric flight

Aerobraking into Urlum'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.


From the average density of the planet, it's possible to estimate models for its composition. One model suggests a solid core - likely rock (radius of 84.68 km) - surrounded by a mantle made of ices (water, ammonia, and methane with a radius of 910.31 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 1,122.01 km.

The blue-ish color could be due to methane in the atmosphere, which absorbs red light.


A strong magnetosphere can be detected, but curiously, is tilted away from the axis of rotation. This may indicate some sort of axial tilt or a weakened, migrating magnetosphere.

It may also indicate a magnetosphere caused by convective liquids in the icy mantle, similar to Neidon.

Possible Weather

With certain graphic modifications installed, Urlum has cloud layers that rotate over the planet. By observing the cloud rotation, wind speeds are estimated to vary between 80 and 250 m/s.

Natural Satellites

Urlum has four natural satellites, with one being a sub satellite of another.

  • Polta, an icy moon, has striking swirls in its terrain. It shares its orbit with Priax.
  • Priax, a heavily cratered moon, shares its orbit with Polta.
  • Wal, a rocky moon, has a unique equatorial ridge with chaotic terrain.
  • Tal, a smooth moon, is a sub-satellite that orbits Wal.

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


Urlum has a total of five (atmosphere and space-only) biomes.

Biome list

Urlum Biome Map

Urlum biome map

  • Polar Zones
  • Temperate Bands
  • Tropical Belts
  • Tropical Zones
  • Equatorial Zone

Reference Frames

Warp Minimum Altitude
5× 325 000 m (above the atmosphere)
10× 325 000 m (above the atmosphere)
50× 325 000 m (above the atmosphere)
100× 325 000 m (above the atmosphere)
1 000× 326 550 m
10 000× 653 100 m
100 000× 1 306 200 m

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