X-Ray Spectrometer (XRS)
The XRS has a limited life of about 2.5 years before the neon and/or helium runs out. The XRS is special because, for the first time, it will provide both high spectral resolution (measuring small differences in the energies of X-ray photons) and high throughput (measuring lots of X rays) in one instrument.
X-ray Imaging Spectrometer (XIS)
For more info, visit: http://www.nasa.gov/mission_pages/astro-e2/spacecraft/index.html