Kerbal Space Program is still in development, although an Alpha version is available for free from the game's website. Players are given the ability to build and test rockets, exploration vehicles, and space planes of their own design. Although there are no in-game challenges, players can set their own goals such as reach space, complete an orbit, land safely, or even just get off the ground. The latest version of Kerbal Space Program is 0.18, which was released on Nov. 30th, 2012. The new update added new features like docking, flight planning, and two new celestial bodies to the already existing 13.
The game consists of two separate phases. The first is the construction phase in which the player assembles the rocket from the available parts. These include fuel tanks, rocket engines, solid fuel boosters, parachutes, guidance systems and the command module itself. Designs can also include decouplers, which are parts that can be activated in flight to separate sections of the rocket which are not needed. For example, empty fuel tanks and boosters can be separated to reduce weight, which will help increase speed. Once a rocket is complete, the design can be saved and then taken to the second phase, the rocket flight.
The rocket's flight will begin on the launchpad. The player will have control over the rockets thrust and direction, as well as being able to active guidance systems which can help keep the rocket on course. Many rocket designs will consist of several "stages," with each stage being activated with the press of a button. Only one stage can be active at a time, so the player will need to activate each stage when they feel the time is right. A simple example would be activating the first stage on the launchpad, firing the rockets engines, once the engines are out of fuel, the player would activate the second stage to release them from the rocket. The player will also need to keep an eye on engine temperatures, as high temperatures will cause rockets to explode.
The rocket flight takes place on and above the fictional planet of Kerbin. Atmosphere and gravity effect the rocket's control and speed depending on the distance from the planet, meaning the player will need to keep this in mind if they are attempting to orbit or land safely back on the planets surface. If the rocket is manned, the Kerbalnauts can be killed by explosions or impact with heavenly bodies. If the player wants to attempt a safe landing they can attach parachutes to help slow the command modules, or even fire rockets to slow themselves down.
Players are able to dock ships in the game now, allowing the construction of space stations in orbit around Kerbin, or any other celestial body in the game. There is also a flight planning mechanic that allows players to plan future maneuvers that can help players fly to other planets, or establish orbits.
There are currently six planets in the full game: Moho, Eve, Kerbin, Duna, Dres, and Jool. Many of them have moons for a total of 15 bodies to explore.
Moho, the small rocky planet closest to the sun has an inclined and slightly eccentric orbit. It's small size and proximity to the sun may make it difficult to explore.
Eve is a larger planet covered in high mountains and oceans. It has a single moon with a highly eccentric and inclined orbit called Gilly.
Kerbin is Earth-like with large oceans, frozen poles, a dense atmosphere, and even familiar continental formations. It has two moons: Mun and Minmus.
Duna is a small Mars-like planet with ice forming at both poles and a rocky moon called Ike in orbit.
Dres is classified as a dwarf planet that orbits the Sun between Duna and Jool. Dres has no atmosphere, and is a very rocky planet.
Jool, the gas giant is the furthest and largest planet and has four bodies in orbit. There are four unique moons: Laythe, Vall, Pol, and Tylo as well as a captured asteroid called Bop to explore. Jool's immense gravity and long distance from Kerbin make it the most difficult planet to explore and return safely.
Many features are planned for future releases, including a full campaign mode. Players will be able to take on different missions, like delivering a payload, or research missions to other planets. In the campaign mode, you'll be able to train astronauts, research new parts, and will have a budget to watch when building ships. There is also a community actively creating new parts for the games. An alpha version of the game is currently available for purchase, there is also a free demo which is several iterations behind the paid version.