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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 10000 m
Atmospheric height 138 155.11 m
1.5×10-5 atm
Temperaturemin -86.13 °C 187.02 K
Temperaturemax 976.55 °C 1249.7 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. What happens with 0.23 is the craft won't stop at an altitude of 0, instead it continues descending until it get 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


Aerobraking in Jool's atmosphere.
The effects of landing on Jool's "surface" as a Kerbonaut before 0.23

Although Jool has a radius ten times greater than Kerbin, its atmosphere begins just twice as high (138.2 km). It is extremely dense, well suited for aerobraking from a high-speed interplanetary intercept. Orbiting just outside the atmosphere will give a period of 94.74 minutes, and an orbital speed of 6,785 m/s. Typically an altitude lower than 100 km is not needed for aerobraking, and it may be wise to stay in the upper atmosphere because a bug can occur causing the ship to spontaneously explode or glitch out Kerbals on EVA.

In general, the atmospheric pressure on Jool at an altitude expressed in meters is:

p_j = 15 \text{atm} \cdot e^\frac{-altitude}{10000 \text{m}}

Closer to the surface, the physics of the game start to break down. Due to its extremely thick atmosphere, terminal speed is so low that lighter spacecraft will not even need parachutes or retrorockets. It was possible to land on its solid surface though spacecraft were inevitably and invariably devoured by the Kraken before 0.23. The craft can descend below the surface, but the parts will explode once they are 250 m below the surface. The message "... collided with Cloud" will be displayed in the mission summary. The game may glitch out and corrupt the save.

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

Paid (0.19.1)
Rotational/Inertial transition 1 000 000 m
Warp Minimum Altitude
5× 138 155 m (above the atmosphere)
10× 138 155 m (above the atmosphere)
50× 138 155 m (above the atmosphere)
100× 150 000 m
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
TinyKerbol.png TinyMoho.png TinyEve.png TinyKerbin.png TinyDuna.png TinyDres.png TinyJool.png TinyEeloo.png
(The Sun)
Moho Eve Kerbin Duna Dres Jool Eeloo
Gilly Mun
Ike Laythe