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Jool in 0.17.
Jool is a gas giant and the fifth planet of the Kerbolar star system.
A probe visiting the gas giant.


  • Solar Day: ... hours, or ... hours, ... seconds.

Orbital Characteristics


Aerobraking in Jool's atmosphere, with Laythe and Vall visible over the horizon.
A successful landing on Jool, which was destroyed about two seconds later.

Although Jool has a radius ten times greater than Kerbin, its atmosphere begins approximately twice as high (138.2 km). It is also extremely dense, well suited for aerobraking from a high-speed inter-planetary intercept.

Closer to the surface, the physics of the game start to break down. Due to it's extremely thick atmosphere, terminal velocity is so low that lighter spacecraft will not even need parachutes or retrorockets. It is not impossible to land on it's solid surface, though spacecraft are inevitably and invariably devoured by the Kraken.

Natural Satellites

Jool has four natural satellites:

The ocean moon Laythe, about to eclipse Jool.
  • The ocean moon Laythe, which has an atmosphere and is both the largest and closest to Jool. Due to it's high orbital velocity, it is somewhat challenging to reach.
  • The ice moon Vall, which is the second largest and second closest. It's orbital path and velocity sit almost exactly between Layth and Tylo.
  • The rocky moon Tylo, similar to Kerbin's Mun, is the third body orbiting Jool. It is perhaps the easiest of the four to reach, with a stable orbit, large gravity well, and (relatively) slow orbital velocity.
  • The captured asteroid Bop, the smallest body orbiting Jool. Due to it's distant, erratic orbit and low gravity, it is also challenging to reach.

Laythe, Vall, and Tylo are in a Laplace resonance, with orbital periods of 1:2:4 respectively.