In order to get the source for officially released versions, go to the Files tab in Orekit forge and select one of the orekit-x.y-src.zip files. The x.y part in the name specifies the version. If this is the first time you download the library and you have not yet set up your own data set with UTC-TAI history, JPL ephemerides, IERS Earth Orientation Parameters … you may want to also download the orekit-data.zip file which is an example file suitable for a quick start (see configuration for further reading about data configuration).
It is also possible to retrieve published versions from the Git repository (see next section below), if retrieving either release-x.y branches or the master branch.
The development of the Orekit project is done using the Git source code control system. Orekit Git master repository is available online. The latest developments are in the develop branch. This is the one you want to retrieve if you need the latest feature before they are published in an official release. See guidelines for the branching workflow used in Orekit.
you can clone it anonymously with the command:
git clone https://www.orekit.org/git/orekit-main.git
if you are a committer, you can clone it using your ssh credentials with the command:
git clone ssh://git@www.orekit.org/orekit-main.git