The space-borne gravitational wave detector, LISA, was studied in great detail as a collaborative mission between NASA and ESA. Ultimately, NASA and ESA decided in 2011 not to proceed with the mission. LISA was not the highest ranked mission in the 2010 Decadal Survey and funding constraints prevented NASA from proceeding with multiple large missions. In 2013, ESA selected the Gravitational Universe as the science theme for the third large mission in the Cosmic Visions Programme (L3). ESA is currently developing a mission concept for L3 which may include substantial contributions from NASA. NASA's L3 Study Team is currently studying potential US contributions to this effort.
The Laser Interferometer Space Antenna (LISA) is a joint NASA-ESA project to develop and operate a space-based gravitational wave detector sensitive at frequencies between 0.03 mHz and 0.1 Hz. LISA detects gravitational-wave induced strains in space-time by measuring changes of the separation between fiducial masses in three spacecraft 5 million kilometers apart.
LISA will discover many extraordinary astrophysical sources: tens to hundreds of inspiraling and merging massive black hole binaries out to a redshift of z ~20; tens of stellar-mass compact objects spiraling into central massive black holes out to z ~1; more than ten thousand close, compact binaries in the Galaxy; a sky map of the background made by millions more; and possibly backgrounds of cosmological origins. The all-sky instrument will see thousands of sources in the first few months of operation. Astrophysical parameters, such as mass, spin and luminosity distance, of many sources will be measured with uncommon precision.
LISA will track more energetic inspirals for weeks to months, predicting progressively more accurate sky position and luminosity distance of the merger for electromagnetic observers. Owing to the revolutionary nature of gravitational wave detection, the numbers and types of LISA sources are somewhat uncertain. Fortunately, there are guaranteed sources, close white dwarf binaries known from electromagnetic observations, and many others that LISA should see in large numbers unless the Universe is radically different than electromagnetic observations have led us to believe. By virtue of opening a new spectrum, gravitational wave astronomy promises once-in-human-history discovery potential.
Gravitational wave observations will enable studies of: the formation and growth of massive black holes and their co-evolving host galaxies; structure formation; stellar populations and dynamics in galactic nuclei; compact stars; the structure of our Galaxy; General Relativity in extreme conditions; cosmology; and searches for new physics. Information from LISA sources will provide unique insight into extraordinary astrophysical objects. Combined with electromagnetic observations, these insights will advance the broader scientific understanding.