GLAST: T minus 4 days
Published on Jun 7, 2008 at 2:06 pm.
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Filed under astronomy, space telescopes.
About 3 months ago I wrote about the GLAST (Gamma-ray Large Area Space Telescope) and the search for a new name for the mission. The satellite was supposed to launch this weekend, but it has now been delayed until at least June 11. 
If all goes according to the plan, GLAST will lift off some time around noon on that date aboard a Delta rocket from Pad 17B at the Cape Canaveral Air Force Station.
GLAST is designed to study gamma rays. These are an extremely energetic form of light, and thus gamma rays are typically produced in very energetic events. Visible light is only a tiny portion of the electromagnetic spectrum. Light that is somewhat shorter wavelength (and somewhat higher energy) than visual light is ultraviolet light. Light that is somewhat longer wavelength (and somewhat lower energy) than visual light is infrared light. Radio waves, X-rays, and gamma rays are also forms of electromagnetic radiation. When I was in elementary school, some of the books that I read also listed something called “cosmic rays” as shorter wavelength, and higher energy, than gamma rays. The modern way of thinking, tough, is that that wavelength regime is really just extra energetic gamma rays. Once in a while, though, I get a student my age or somewhat older who remembers the old charts of the electromagnetic spectrum showing cosmic rays at one end. But, the term cosmic rays has an entirely different meaning for us today.
Among the sorts of things that GLAST will study are supernova remnants, neutron stars, and black holes (including the supermassive black holes in active galactic nuclei). High energy physicists are excited about GLAST, too, because these astrophysical bodies are able to accelerate particles to far higher speeds than can be attained with Earth-bound particle accelerators. So, GLAST promises to perhaps give some insight into high energy physics, too.
GLAST is not the first gamma ray telescope put into orbit. It is merely the latest, and possibly best. So, why do we want to go to all the expense of putting a gamma ray telescope into orbit? After all, when you first begin to learn about nuclear radiation, you are taught that there are three types of radiation: alpha rays, beta rays, and gamma rays. The gamma rays are the most penetrating of them all. It is the toughest to shield against. Alpha rays can be shielded with just heavy cloth, paper, or plastic. Beta rays can be shielded with a thin layer of lead, or a thicker layer of plastic. Gamma rays, though, require many inches, or sometimes even feet of lead to shield against. So, why bother putting a gamma ray telescope into orbit? Well, the reason is that while alpha rays and beta rays are fairly easy to shield against, so are gamma rays. Yes, they are penetrating, and it takes a lot of lead or concrete to block them. But, air also blocks them. Huh? If lead has trouble with gamma rays, how can air block them? Well, air blocks gamma rays the same way that the lead does. The electrons in the air molecules absorb the gamma rays. But, air is thin. So, it takes a whale of a lot of air to block gamma rays. What lead can block in a few inches takes dozens of feet of air to block. And what takes many inches of lead to block may take hundreds of feet of air to block. But, there are dozens of miles of air between the ground and space. So, gamma rays from space are effectively blocked from reaching the ground by the air over our heads. Even mountains have too much air above them for gamma ray telescopes to be of any use. So, you need to get the gamma ray telescopes above the air. That means putting them on satellites that are in Earth orbit. That is what GLAST is doing. And, if everything goes well in four days, we will have another gamma ray telescope in orbit to study high energy astrophysical phenomena.
-Astroprof
Images courtesy of NASA
