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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 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 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. Before 0.23, the craft won't stop at an altitude of 0, 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


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 96,378 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 2.8 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 planets Uranus and Neptune. 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 1 072 480 10.585
20 000 817 125 8.064
30 000 658 008 6.494
40 000 520 935 5.141
50 000 405 932 4.006
60 000 311 385 3.073
70 000 234 855 2.318
80 000 173 901 1.716
90 000 126 082 1.244
100 000 88 960 0.878
110 000 60 095 0.593
120 000 37 045 0.366
130 000 19 870 0.196
140 000 12 750 0.126
150 000 10 000 0.099
160 000 7 221 0.071
170 000 4 524 0.045
180 000 2 216 0.022
190 000 605 0.006
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

The thickness of Jool's atmosphere makes it well suited for aerobraking 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. For an intercept originating from Kerbin, a periapsis altitude of about 190,000 m should, under most conditions, result in an aerocapture.

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, he will not be destroyed, making one-way sacrificial landings possible. However, as seen near the end of this video, the kerbonaut will start to glitch out and shake uncontrollably. This only happens on 4× time warp. The game will later glitch and the Hell Kraken will strike. Unless you restart the game, Kerbin's atmosphere will be missing but it is unknown if this still happens.

This glitch, while most common on Jool, is not unique to Jool, and can even be experienced on Kerbin in extreme situations; for example, going on EVA from a high speed spaceplane as it takes off can occasionally cause this, or it can even, although much more rarely, happen in less extreme situations as simple as sliding slowly on the Mun. If you do experience this outside of Jool, it is very difficult to replicate, which is why recordings of this are so rare.

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

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, 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 a Laplace resonance, with orbital periods of 1:2:4 respectively. 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 a Laplace resonance of 1:2:4.
  • 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.
  • The barometric, gravitational and temperature measurements taken as close to 0 meters altitude are roughly 14.9785 atm, 0.785 m/s2, 989.45 °C, respectively.
  • 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.


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


  • Added a new, more distant moon named Pol.
  • Initial Release