NASA is pushing back the launch date of the James Webb Space Telescope (JWST), the successor to the Hubble Space Telescope. It was going to enter Earth's orbit in October 2018, but now the launch has been scheduled for sometime between March and June 2019.
No issues have appeared with the telescope, but it's taking longer than anticipated to complete work on the spacecraft that'll bring it to orbit. NASA wants to proceed at a reasonable pace with the testing and therefore a delayed launch is necessary.
Webb's spacecraft and sunshield are larger and more complex than most spacecraft. The combination of some integration activities taking longer than initially planned, such as the installation of more than 100 sunshield membrane release devices, factoring in lessons learned from earlier testing, like longer time spans for vibration testing, has meant the integration and testing process is just taking longer. Considering the investment NASA has made, and the good performance to date, we want to proceed very systematically through these tests to be ready for a Spring 2019 launch. - Eric Smith, program director, JWST