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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 10090901.7 s
Orbital Velocity 3 927 to 4 341 m/s
Physical Characteristics
Equatorial radius 6 000 000 m
Surface area 4.5238934×1014 m2
Mass 4.2332635×1024 kg
Standard gravitational parameter 2.8252800×1014 m3/s2
Density 4 678.7834 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
Scale height 30 000 m
Atmospheric height 200 000 m
1.5×10-5 atm
Temperaturemin -73.15 °C 200 K
Temperaturemax -73.15 °C 200 K
Oxygen present × No
Scientific multiplier
Surface N/A
Lower atmosphere 12
Upper atmosphere 9
Near space 7
Outer space 6

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

Jool is a gas giant and the sixth planet of the Kerbol star system. It is the Jupiter analog for Kerbal Space Program. Aside from Kerbol, Jool has the largest diameter and greatest mass of all celestial bodies. 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. 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.

In-game description

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


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, a datum level pressure of 1519.875 kilopascals (15 atmospheres), and a depth of 200,000 meters. Compared to the atmosphere of Kerbin, Jool's atmosphere has 1875 times the mass, 15 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 1 519 875 15.000
10 000 1 060 650 10.468
20 000 783 625 7.734
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, were 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.

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.

Jool has no solid surface to land on; descending spacecraft will explode when they reach 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

Paid (0.19.1)
Rotational/Inertial transition 1 000 000 m
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

The ocean moon Laythe, about to transit Jool.

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.


→ See also: Images of Jool 


  • Jool has roughly the same equatorial radius as the planets Earth and Venus.
  • The sunsets and sunrises on Jool are a shade of yellowish brown.
  • 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) 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.
  • Notes

    1. Kerbal Space Program: Planting a Flag on Jool” by Dahud Lefthanded in version 0.20


    • New atmosphere model, beginning at 200 km instead of 138 km altitude.
    • Added a new, more distant moon named Pol.
    • Initial Release