Jool is a gas giant and the sixth planet of the Kerbol star system. It is the 6th planet from Kerbol, making it one of the harder planets to reach. It is one of the most appealing targets for missions, however, because of its large and complex system of five moons: Laythe, Vall, Tylo, Bop, and Pol. Orbits well-aligned with the orbital plane, and many intersections with moon orbits can easily be captured by a couple moons with little to no ΔV before having to correct your orbit.
Aerobraking in Jool's atmosphere.
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 you a period of 94.7437 minutes, and a speed of 6,785 m/s. You typically do not need to go lower than 60 km to aerobrake.
In general, the atmospheric pressure on Jool at an altitude expressed in meters is:
The effects of landing on Jool's "surface" as a Kerbonaut
Closer to the surface, the physics of the game start to break down. Due to its 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 its solid surface, though spacecraft are 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 if you're on time warp 4. The game will later glitch and cut to a black screen with the altimeter going through numbers rapidly. Unless you restart the game, Kerbin's atmosphere will be missing.
The ocean moon Laythe
, about to transit Jool.
Jool has five natural satellites:
- The ocean moon Laythe, currently the only moon possessing an atmosphere, is the closest to Jool and second largest of its moons. Due to its high orbital velocity, it is somewhat challenging to reach. Next to Eve, its size and composition make it the most similar celestial body to Kerbin
- The ice moon Vall, which is the third largest and second closest. Its orbital path and velocity sit almost exactly between Layth and Tylo.
- The rocky moon Tylo, similar to Kerbin's Mun, although it has similar gravity to Kerbin. While having the largest SOI, making it easiest to encounter, achieving orbit and landing are exceptionally difficult due to it's large gravity well coupled with lack of atmosphere.
- The captured asteroid Bop, the smallest body orbiting Jool. Due to its distant, erratic orbit and low gravity, it is also challenging to reach.
- In 0.18, a new moon was added, Pol, named so after a Pollen grain which it resembles. A yellowish/greenish moon, rocky and uneven with tall spiky mountains. It orbits Jool beyond Bop and is around twice as large. An image can be found here.
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.
A probe deep within Jool's atmosphere which is eventually crushed by the extreme pressure.
Jool and its moons in 0.17.
A Probe that has winglets to steer. Note that as you descend deeper into the atmosphere, it gets harder to move at all, rendering, RCS useless
- 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 Io, Europa, and Ganymede, Joolean moon have a Laplace resonance 1:2:4
- Added a new farther out moon named Pol.