Constellation-X and The Laser Interferometer Space Antenna (LISA) will use the complementary techniques of X-ray spectroscopy and gravitational waves to study black holes. They will probe space, time, and matter in the extreme environment near black holes and track their evolution with cosmic time. These two facilities will be a major resource for a broad astronomy and physics community. The National Academy of Sciences' decadal survey Astronomy and Astrophysics in the New Millennium developed community consensus on the most important science questions and funding priorities. It recommended both LISA and Constellation-X as high priorities for this decade.
Constellation-X will extend our capability for high resolution X-ray spectroscopy by 25 to 100 times. Its key goals are to determine the fate of gas falling into a black hole by tracking spectral features close to the event horizon, and to trace the evolution of black holes with cosmic time by obtaining detailed spectra of faint quasars at high redshift. The mission is optimized for these challenges but also provides the ability to observe other objects with unprecedented sensitivity. Constellation-X will also observe the first clusters of galaxies and be able to search for spectral features from the surfaces of neutron stars, which could finally determine the properties of matter at nuclear density.
LISA's gravitational waves offer an entirely new way to sense action in the Universe. Through them we will hear for the first time the mergers of giant black holes and the death spirals of stars they capture and swallow. Using these, we will map the knotted structure of space and time around a black hole and determine if the astonishing predictions of Einstein's theory are correct: the freezing of time and dragging of space around a black hole. LISA will also make the first complete map of merging binary stars in our Galaxy, future supernovae which could affect life on Earth. It will set important limits on background radiation from the early Universe and from catastrophic events, such as phase transitions in the vacuum or changes in the dimensionality of the Universe.
Both Constellation-X and LISA are critical to the goals of the Beyond Einstein program. One of these great observatories will begin development at the beginning of the program, and the other will begin development approximately three years later. NASA will determine the order based on science priority, technological readiness, and programmatic considerations.