From Kerbal Space Program Wiki
Jump to: navigation, search
Jool as seen from orbit.
Planet of Kerbol
Orbital Characteristics
Semi-major axis 68 773 560 320 m [Note 1]
Apoapsis 72 212 238 387 m [Note 1]
Periapsis 65 334 882 253 m [Note 1]
Orbital eccentricity 0.05
Orbital inclination 1.304 °
Argument of periapsis 0 °
Longitude of the ascending node 52 °
Mean anomaly 0.1 rad (at 0s UT)
Sidereal orbital period 104 661 432 s
4 845 d 2 h 37 m 12.1 s
Synodic orbital period 10 090 901.7 s
Orbital velocity 3 927 - 4 341 m/s
Physical Characteristics
Equatorial radius 6 000 000 m
Equatorial circumference 37 699 112 m
Surface area 4.5238934×1014 m2
Mass 4.2332127×1024 kg
Standard gravitational parameter 2.8252800×1014 m3/s2
Density 4 678.7273 kg/m3
Surface gravity 7.85 m/s2 (0.8 g)
Escape velocity 9 704.43 m/s
Sidereal rotation period 36 000.000 s
1 d 4 h 0 m 0 s
Solar day 36 012.387 s
1 d 4 h 0 m 12.4 s
Sidereal rotational velocity 1 047.2 m/s
Synchronous orbit 15 010.46 km
Sphere of influence 2.4559852×109 m [Note 1]
Atmospheric Characteristics
Atmosphere present Yes
Atmospheric pressure 1 519.88 kPa
15 atm
Atmospheric height 200 000 m
1.5×10-5 atm
Temperaturemin -153.14 °C 120.01 K
Temperaturemax -48.08 °C 225.07 K
Oxygen present No
Scientific multiplier
Surface N/A
Splashed N/A
Lower atmosphere 12
Upper atmosphere 9
Near space 7
Outer space 6
Recovery 6

  1. 1.0 1.1 1.2 1.3 The distances are given from the body's center, not from the surface (unlike ingame)

Jool is a gas giant and the sixth planet orbiting Kerbol. It is the Jupiter analog for Kerbal Space Program. Aside from Kerbol itself, Jool has the largest diameter and greatest mass of all celestial bodies in the Kerbol System. Its extremely high gravity makes orbital maneuvers unpleasantly expensive. While its distance from Kerbin makes it difficult to reach, it is one of the most appealing targets for missions due to its large and complex system of five moons: Laythe, Vall, Tylo, Bop, and Pol. In KSP1, it was possible to land and plant flags on Jool[1] before v0.23. In versions following 0.23, the craft won't stop at an altitude of -100 m, instead it will continue descending until it gets to -250 m. At this point, anything that hits this altitude at any speed will be completely destroyed. Jool has animated eddies and vortices in high quality.

In-game description

KSP 1:

Jool is particularly known for being a rather large, predominantly green planet. Kerbalkind has longed to visit it since it was first spotted in the sky. Philosophers reason that the swirling green planet must be a really nice place to visit, on account of its wholesome coloration.

If you look at Jool through a telescope, it is fuzzy.''

Kerbal Astronomical Society

KSP 2:

By far the flashiest planet in the Kerbollar system, Jool is a gas giant with massive gravitational pull and five moons! There is no land to land on, and the atmospheric pressure destroys anything Kerbals throw into its center; however, the upper layers of clouds are ripe for study and watercolor paintings.


Temperature and pressure of Jool's atmosphere as a function of altitude.

Jool has an extremely dense, cold atmosphere with a mass of approximately 8.8×1019 kilograms (about one ten-millionth that of Jupiter), a datum level pressure of 5066.25 kilopascals (50 atmospheres), and a depth of 200,000 meters. Compared to the atmosphere of Kerbin, Jool's atmosphere has 1875 times the mass, 50 times the surface pressure, and nearly 3 times the depth. At an altitude of 91,554 m on Jool, the atmospheric pressure is the same as at sea level on Kerbin (1 atm).

The average molecular weight of Jool air is 2.2 g/mol, and its adiabatic index is 1.43. Although the composition of Jool's atmosphere is unknown, these values suggest that it consists mostly of hydrogen and helium. The molar mass of Jool's atmosphere is comparable to the real life planet Jupiter. Because of the low molar weight, Jool air at the datum level is only about 2 times as dense as Kerbin air at sea level.

Like all other atmospheres in the game, Jool's atmosphere fades exponentially as altitude increases. The scale height varies with altitude, which is a change from pre-1.0 versions of the game. 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 5 066 250 50.000
10 000 1 519 875 15.000
20 000 772 776 7.627
30 000 613 670 6.056
40 000 482 628 4.763
50 000 373 138 3.683
60 000 283 031 2.793
70 000 210 136 2.074
80 000 152 282 1.503
90 000 107 299 1.059
100 000 73 016 0.721
110 000 47 263 0.466
120 000 27 869 0.275
130 000 13 529 0.134
140 000 5 319 0.052
150 000 2 000 0.020
160 000 734.7 0.007
170 000 100.0 0.001
180 000 29.57 0.000
190 000 3.675 0.000
200 000 0 0.000

Air temperatures decrease as altitude increases up to an elevation of about 123 km, where the coldest atmospheric temperatures are found. A gradual warming begins above 123 km. At an altitude of 194 km there begins a very rapid increase in temperature, suggesting the presence of a thermosphere. As a matter of fact, the temperature of the upper atmosphere is so high that solar panels will actually overheat and explode rather than simply shearing off as they would in other atmospheres.

Air temperatures vary with latitude and time of day. At the datum level (elevation = 0) the temperature is a globally constant −73 °C. As the altitude increases, latitudinal and diurnal temperature variations are observed, becoming more pronounced with increasing altitude. At an altitude of 123.45 km, temperatures at the equator vary between a nighttime low of −129 °C and a daytime high of −122 °C. At this same altitude over the poles, the temperature varies between −185 °C and −181 °C. Since Jool has no axial tilt, there are no seasonal temperature variations.

Atmospheric flight

Since version 1.0.5, Jool's atmosphere is well suited for aerocapture from a high-speed interplanetary intercept. The periapsis altitude required for a successful aerocapture depends on the spacecraft's drag characteristics, its approach velocity, and the desired apoapsis of the resulting orbit. The most effective periapsis for aerocapture is best determined experimentally; however, for a Hohmann transfer originating from Kerbin, it is found that the median value is about 155 km. A Heat shield is required to prevent destructive overheating.

Parachutes work very effectively in Jool's dense atmosphere. However, even without parachutes, a craft can reach a relatively low speed, depending on the shape of the ship, as well as its mass.

If part pressure limits are enabled in the game difficulty settings, a spacecraft will implode when decending below the 4 MPa pressure level at an altitude of 1772 m. With pressure limits turned off, a spacecraft will descend beyond the datum elevation, since Jool has no solid surface to land on, and will explode when reaching the altitude of -250 m. The message "... collided with Cloud" will be displayed in the mission summary. The game may glitch out and corrupt the save. Prior to version 0.23, it was possible to land on a solid surface, though spacecraft were inevitably and invariably devoured by the Kraken.

If a kerbonaut is put on EVA, they will not be destroyed immediately, making one-way sacrificial "landings" possible. However this Kerbal will definitely die, and other glitches may occur, such as the Hell Kraken.

Reference frames

Time warp Minimum Altitude
5× 200 000 m (above the atmosphere)
10× 200 000 m (above the atmosphere)
50× 200 000 m (above the atmosphere)
100× 200 000 m (above the atmosphere)
1 000× 300 000 m
10 000× 600 000 m
100 000× 1 200 000 m

Natural satellites

All of Jool's moons

Jool has five natural satellites, each with an orbit well-aligned with Jool's orbital plane:

  • Laythe, an ocean moon with many sandy islands, is the only moon with an atmosphere. It is the closest to Jool and second largest of its moons. Due to its high orbital speed, it is somewhat challenging to reach. Next to Eve, its size and composition make it the most similar celestial body to Kerbin.
  • Vall, an ice moon, is the third largest and second closest of Jool's moons. Its orbital path and velocity sit almost exactly between Laythe and Tylo.
  • Tylo, a rocky moon, has gravity similar to Kerbin and terrain similar to Kerbin's Mun. It has the largest SOI of Jool's moons, making it easy to encounter, but achieving orbit and landing are exceptionally difficult due to its large gravity well and lack of atmosphere.
  • Bop, a captured asteroid, is the second smallest of Jool's moons. Due to its distant, erratic orbit and low gravity, it is also challenging to reach.
  • Pol, named after its resemblance to a grain of pollen, is Jool's smallest and most distant moon. It is yellow and green, and its terrain is rocky and uneven, with tall, spiky mountains.

Laythe, Vall, and Tylo are in what appears to be a Laplace resonance, with orbital periods of 1:2:4 respectively. Actual lineup is not the same, resulting in what would be a highly unstable resonance if the moons were not on rails. Despite the fact that the moons can easily eclipse both each other and Jool, they do not.

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

The safe zones to have stable orbits without the risk of being captured by one of the satellites' sphere of influence are as follows (as in-game altitudes, unless otherwise specified):

  • Below Laythe, between 200km and 17 460km, which are Jool's atmospheric limit and Laythe's sphere of influence's lower reach, respectively.
  • Outside of Laythe's but inside Vall's orbits, between 24 908km and 34 745km, being the upper and lower limits of Laythe's and Vall's spheres of influence, respectively.
  • Outside of Vall's but inside Tylo's orbits, between 39 559km and 51 643km.
  • Outside of Tylo's but inside Bop's orbits, between 73 357km and 91 081km. It is possible to extend the apoapsis beyond 91 081km and up to 157 496km, on the condition of aligning it with Bop's apoapsis.
  • There is no safe altitude range outside of Bop's but inside Pol's orbits, since Bop's sphere of influence's upper limit is 153 919km and Pol's lower is 142 112km. However, it is possible to make out a safe orbit by playing with eccentricity, angle of ascending node, apoapsis and periapsis, to match Pol's apoapsis and Bop's periapsis.
  • Outside of Pol's orbit, from 205 668km to 2 449 985km, being the upper limit of Pol's sphere of influence and the end of Jool's sphere of influence, respectively. It may be possible to reach lower, by matching Pol's periapsis.

An orbit where both the apoapsis and the periapsis are inside one of these safe zones is a safe orbit. With a steep enough inclination, you can also put your craft in an orbit in which the periapsis lies in one safe zone and the apoapsis in another.


Research Locations

Research locations include:

  • Northern Circle
  • Northern Tropic
  • Equator
  • Southern Tropic
  • Southern Circle



Jool biome map with legend (as of v0.2.1.0 (from Orbital Survay mod))

Visual map

File:Jool visual.png
Jool visual map (as of v0.2.1.0)

Jool visual map (Imgur)


→ See also: Images of Jool 

Kerbal Space Program


  • Despite being the Jupiter analog of KSP, Jool has roughly the same equatorial radius as the real-life planets Earth and Venus, because of the fact that the whole game had to be shrunk in size for performance.
  • The sunsets and sunrises on Jool have a purple-ish hue and are roughly the same color as Eve's atmosphere, correlating with the fact that Eve's sunset has a greenish tint of which resembles Jool.
  • Like Jool's moons, Jupiter's three nearest moons, Io, Europa, and Ganymede, have relative orbital periods of 1:2:4. Unlike Jool's, Jupiters have differing arguments that result in a true Laplace resonance.
  • Interestingly, if you use an N-Body simulation, the Jool system is very unstable and will rapidly collapse into a radically different system. Vall gets thrown out into interplanetary space, as do Pol and Bop, and Laythe and Tylo become closer together. In order to solve this problem, the moons must be broken out of resonance, or Jool's mass must be drastically increased.
  • Jool, Laythe and Tylo all have a surface gravity of 7.85 m/s² (≃ 0.8 g)
  • The green gas which composes Jool could be chlorine.
  • Despite not having a surface, the game files that contain what the Kerbals say in the different reports/observations (ScienceDefs.cfg) in KSP1 also contains some reports while landed on the surface of Jool.
Spoiler: Jool surface report/observations
  • EVA Report: You're not sure how you even landed on the surface of a gas giant. But it's probably best not to think about it for too long..
  • Materials Study: You're not sure where the container stops and the samples start anymore...
  • Atmospheric Pressure Scan: Either the pressure is really high, or the instrument just melted. It's hard to tell what happened first.
  • Seismic Scan 1: The sensor doesn't even know what to do with itself here.
  • Seismic Scan 2: The sensor has informed you that the warranty has just been voided. No refunds.
  • Gravity Scan: The instrument has been crushed by the massive gravitational forces. Science!
  • Atmosphere Analysis: The instrument has mostly compressed into an unrecognizable mass of metal.
  • Jool is the only planetary body that cannot be "destroyed" by a planet-busting glitch.
  • According to a post by former KSP developer NovaSilisko [2], he regrets making Jool intensely green and speculates that the green color might be algae.
  • In KSP1, Jool is the only completely manually colored planet (i.e. it uses a color map instead of procedural height-based coloration) in the stock solar system. This is because it has no terrain, and thus no procedural noise can color it, and it only exists as a ScaledSpace mesh. Mun and Tylo are, however, *mostly* manually colored.


  1. Kerbal Space Program: Planting a Flag on Jool” by Dahud Lefthanded in version 0.20
  2. Post in the forum by NovaSilisko


Kerbal Space Program

  • New animated shader and high resolution textures.
  • Adjusted datum pressure so crushing pressures are achieved faster.
  • New atmosphere model, beginning at 200 km instead of 138 km altitude.
  • Added a new, more distant moon named Pol.
  • Initial release

Kerbal Space Program 2

  • Initial release