The Advanced Satellite for Cosmology and Astrophysics (ASCA, formerly named ASTRO-D) was the fourth cosmic X-ray astronomy mission by JAXA, and the second for which the United States provided part of the scientific payload.

In this phase the observing program was open to astronomers based at Japanese and U.S. institutions, as well as those located in member states of the European Space Agency.

[3][4] ASCA was the first X-ray astronomy mission to combine imaging capability with a broad passband, good spectral resolution, and a large effective area.

With these properties, the primary scientific purpose of ASCA was the X-ray spectroscopy of astrophysical plasmas, especially the analysis of discrete features such as emission lines and absorption edges.

In addition, ASCA's suite of instruments provided the best opportunity at the time for identifying the sources whose combined emission makes up the cosmic X-ray background.