10. GAMMA RAY TELESCOPES:

Gamma rays are photons with energies greater than about 70 keV. They are produced by the most extreme environments in space -- objects with the highest temperatures and the highest energies. Gamma rays are also emitted by radioactive atomic nuclei. With gamma ray telescopes, we can observe processes occurring near black holes, the collisions of cosmic rays with interstellar gas, and the debris of exploding stars.

There are several technical difficulties in building a telescope for gamma ray astronomy. The first is that gamma ray photons will pass right through a lot of matter before they interact. It takes a few centimeters thickness of lead to stop a gamma ray. Thus, gamma ray detectors must be very thick and heavy to have a reasonable chance of converting a gamma ray photon into an electronic signal. For the same reason, we don't know of any way to reflect a gamma ray. Therefore, we can't focus gamma rays with a mirror. But we have another way to determine the direction from which a gamma ray photon arrives. When the gamma ray strikes an atom, a very fast electron emerges in roughly the same direction as the original gamma ray. By measuring the path of this electron electronically, we can infer the direction of the original gamma ray. But this technique is not so accurate, and so the best gamma ray telescope so far has an angular resolution of only about 2 degrees. That is why the gamma ray images of the Multiwavelength Milky Way are more blurred than the images in the other wavelengths.

The most powerful gamma ray telescope to date is the 17-ton Compton Gamma Ray Observatory, (CGRO) which NASA launched in 1991. The CGRO mission came to an end on June 4, 2000, when NASA engineers deliberately caused the aging spacecraft to crash into the Pacific Ocean.

The next big advance in gamma ray astronomy began on October 17, 2002 with the launch of the European Space Agency's International Gamma Ray Astrophysics Laboratory (INTEGRAL) mission. INTEGRAL is especially good for measuring gamma ray emission lines from atomic nuclei, which occur at relatively low energies. It has high spectral resolution: it can discriminate among gamma rays differing in energy by 0.2%. By comparison, CGRO could observe gamma ray lines with a spectral resolution of only about 8%.

Sometime later in this decade, NASA plans to launch the Gamma Ray Large Area Space Telescope (GLAST). GLAST is similar in concept to the CGRO, but it is about 30 times more sensitive.

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Last modified November 6, 2002
Copyright by Richard McCray