Jool

Jool is a gas giant and the sixth planet of the Kerbolar star system. It is the most distant of the six planets and the hardest 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.

Atmosphere

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: $p_{j}=15\ e^{-altitude/9000}$ 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 feasible.

Natural Satellites

The ocean moon Laythe, about to transit Jool. Jool has four 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, it's 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. It is the third and largest body orbiting Jool, and 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 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. Gallery Jool in 0.17. Jool and its moons in 0.17. Trivia Jool has roughly the same equatorial radius as the planets Earth and Venus. The sunsets and sunrises on Jool are purple. Changes 0.18 Added a new farther out moon named Pol. 0.17 Initial Release