NASA Report: New Worlds, New Horizons: A Midterm Assessment, National Academies of Sciences
"NASA's WFIRST, the top-ranked large space-based mission in the 2010 survey, is designed to answer questions about dark energy, exoplanets, and general astrophysics. Since the release of the survey, the WFIRST scope and design have evolved to include a 2.4-meter telescope, larger infrared detectors, and an instrument called a coronagraph that enables directly imaging an exoplanet by blocking the light emitted by its parent star. These changes, while scientifically compelling, could result in further increased costs and further delays for the mission, the committee said. It recommended that prior to final confirmation of the changes, NASA conduct an independent review of the project to ensure it does not crowd out investment in the rest of NASA's astrophysics portfolio and, if necessary, de-scope the mission. The report also finds that the driving factor in the delay or non-pursuit of some new NASA initiatives, including WFIRST, was the schedule change and increased cost associated with the James Webb Space Telescope (JWST) - a successor to the Hubble Space Telescope that is set to launch in 2018. As a result, NASA's WFIRST mission was delayed, and the Laser Interferometer Space Antenna (LISA) - a space-based gravitational wave detector that first took shape as collaboration between NASA and the European Space Agency (ESA) - did not go forward."