|Urlum as seen from orbit.|
|Planet of Kerbol|
|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 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|
|Equatorial radius||2 177 000 m|
|Equatorial Circumference||13 678 494 m|
|Surface area||5.9556165×1013 m2|
|Standard gravitational parameter||1.1944574×1013 m3/s2|
|Density||4 141.0965 kg/m3|
|Surface gravity||2.52 m/s2 (0.257 g)|
|Escape velocity||3 312.62 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||5 805.27 km|
|Sphere of influence||2.5622607×109 m [Note 1]|
|Atmosphere present||✓ Yes|
|Atmospheric Pressure||709.275 kPa|
|Atmospheric height||325 000 m|
|Temperaturemin||-219.18 °C 53.97 K|
|Temperaturemax||-178.15 °C 95 K|
|Oxygen present||× No|
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 Custom Asteroids 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
|“|| 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)|
|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|
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.
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.
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.
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.
|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|