X rays and gamma rays are emitted by the hottest gases and the most energetic events in the universe. Because of their penetrating power, they enable us to see into regions that are inaccessible in other wave bands, and because of their energy, they probe matter under the most extreme conditions. They also allow us to see out to large distances, observing the universe when it was much younger than it is today. X rays and gamma rays can only be observed from space, so their use for astronomy is young compared with other wavelengths. Still, dramatic discoveries of cosmological gamma-ray bursts, magnetars, baryon-rich clusters of galaxies, iron lines from accretion disks, and microquasars have led to a better understanding of these energetic environments and have taken us closer to a number of long-range scientific quests: finding the first light of the modern universe, elucidating relativistic gravity by directly imaging black holes, and understanding the origin of the elements that are critical for forming planets and life.

The technological capability is at hand to take the next steps toward these goals. Accordingly, the Panel on High-Energy Astrophysics from Space of the Astronomy and Astrophysics Survey Committee recommends a program for the coming decade that will require the building of three new telescopes:

• The Constellation-X Observatory (Con-X) is a major, high-spectral-resolution, broad-bandpass, x-ray spectroscopy mission. It is proposed as a launch of four telescopes on two rockets well away from Earth. Their combined sensitivity will improve upon that of existing and imminent x-ray missions by factors of 20 to 100, depending on wavelength.

• The top-priority, intermediate-class mission is the Gamma-ray Large Area Space Telescope (GLAST), which will use technology developed for particle physics experiments to detect high-energy gamma rays from quasars, pulsars, and gamma-ray bursts.

• The second-priority, intermediate-class mission is the Energetic X-ray Imaging Survey Telescope (EXIST), which will be attached to the International Space Station. It will monitor the whole sky at hard x-ray energies every 90 minutes.

In addition, the panel proposes a prioritized, advanced technology program that will comprise three missions: the Microarcsecond X-ray