A contract is a request by an agency to perform a task. Contracts were added in version 0.24 for career mode. The first contracts are predefined, but afterward an infinite supply of random contracts will be generated based on a set of templates.
Accepting a contract is optional - any contract may be declined out-of-hand without penalty - but taking contracts can lead to lucrative rewards. Contracts can reward science, funds and/or reputation if successful. Failing a contract will cause loss of funds, and possible a loss of reputation if the failure involved the loss of kerbonauts. Multiple contracts can be active at the same time, and each can have its own deadline (or none at all). It's also possible for multiple contracts to be completed by the same craft. Their difficulty ranges from trivial tasks to exceptionally difficult missions.
As of version 0.24.2 the game can generate contracts which are impossible to fulfil, specifically because they require the TT18-A Launch Stability Enhancer to be transported away from the launchpad., creating problems for challenges which forbid declining contracts.
Cancelling a contract does not count as failure and will not incur the failure penalty. Instead, when a contract is cancelled, the advance is taken back and reputation is left unchanged.
This can be useful for contracts whose deadline will come before it is possible to complete the contract, or if a more appealing contract appears that will take much longer than the contracts already accepted, such as a contract to explore Jool, which will take multiple in game years.
Rather than potentially launching multiple missions to finish up active contracts first, before launching a long-term exploration vessel (which might take multiple real life hours to days), contracts which will expire before the vessel is due back may be cancelled free of penalty. The funds given as an advance will be lost, but this is a small price to pay compared to contract failure, and results in having the same level of funds as before the contracts were accepted.
Types of contracts
As of version 0.90[outdated], there are ten types of contracts.
Part testing involves transporting a certain part to a certain situation and activating it. Different contracts may specify different conditions under which the part must be tested, for example, it may need to be tested when the craft has landed on a certain body, or is in the water on Kerbin, or is at a certain altitude and speed. If the player has not unlocked the required part, it will be available in the VAB/SPH until the contract has been completed or failed. The item will marked with **EXPERIMENTAL** during this period.
For most part tests, the stage with the part has to be activated when the exact conditions are met. However, in some cases the test has to be run by right-clicking on the part and choosing "run test".
Rescue contracts ask that an unfortunate kerbonaut be retrieved, brought down to the surface, and safely recovered. Accepting a rescue contract will spawn a new kerbonaut in a low orbit around Kerbin, who will be visible in the Tracking Station and map view. Kerbals spawned by rescue contracts can't be controlled until a craft is within 2.2 km of the object; once a craft has maneuver close enough the [ or ] keys will switch control over to him. 
Once the Kerbal has been rescued, he will be added to the crew roster in the Astronaut Complex. If the Kerbal is killed the contract is considered failed and the penalty will be assessed.
Data collection contracts require that scientific data be gathered in a specific location- for example, orbiting the Mun. This may be done by either transmitting the data with an antenna or returning the craft to Kerbin and recovering it. These contracts usually reward quite a large sum of cash, but a negligible amount of science. Stationing a satellite equipped with a repeatable experiment like a 2HOT Thermometer and an antenna in orbit is a great way to fulfill these contracts several times making them a quick and easy source of cash.
It doesn't matter if no actual science is earned; running any experiment in the designated situation is enough to fulfill the contract.
Exploration contracts require that a craft be sent to the SOI of a planet or moon, land on its surface, and transmit or return the data from there.
Flag planting contracts require a kerbonaut to plant a flag on a moon or planet. Although the flavor text of these contracts mentions that the client would like to see their flag on the surface, it doesn't matter which insignia the player picks for the mission.
Satellite delivery contracts require the player to place a powered probe core with an antenna in a specific orbit around a celestial body. The probe must be launched by a vehicle newly rolled out to the launch pad after the contract is accepted--a satellite that is already in orbit cannot be repositioned to fulfill the contract. The probe can be delivered by a crewed or uncrewed vessel but in order to meet the terms of the contract the probe must be stable for 10 seconds without the assistance of a crewed vehicle. Some of these contracts also require that the probe carry a specific scientific experiment with it, though the experiment need not be run to fulfill the terms.
Survey contracts require that a Kerbal conduct an experiment in a specific location. This report need only be taken, not returned, in order to count for the contract. Some of the locations require the kerbal to be in flight above a certain altitude and others that the craft be landed. The game will display a message when the craft enters the specified location, and the locations can be navigated to using the map view and nav ball. Several contracts combine multiple experiments in different locations into one multi-part contract.
As with data collection, it doesn't matter if the player already has completed an experiment from that specific situation already and no science needs to be earned, so long as the craft is in the correct location.
Space station construction
These contracts require the player to construct a space station with specified components in a specific orbit. The station may require a certain crew capacity, labs, and other capabilities, and it may have to be built into an asteroid.
Planetary base construction
Planetary base construction contracts require the player to build a base on a particular celestial body. They are subject to much the same specifications as the space station, except they must be landed on the surface.
These contracts require that an asteroid be captured and moved to a particular location, be it a planet, moon, orbit, or even an escape trajectory from the solar system.
Further contract types can be added using mods. In the 0.90 update, the Fine Print mod was implemented, which adds new contract types as well as more variation on the existing contracts.
- Integration of Fine Print
- Initial release
- “First Contract Playthrough” by Miguel (Maxmaps) uploaded on the official Kerbal Space Program channel released on the 11 July.
- “Version 0.24 - Overview (Kerbal Space Program)” by HOCgaming released on July 14, 2014
- “Stability Enhancer... On the Mun” forum thread by Daddy Cecil
- “Kerbal Space Program - Early Access Look At Update v0.24” by Scott Manley released on July 14, 2014
- "Fine Print" mod