Thursday, June 12, 2008

Looking For Gamma Rays

The GLAST telescope The Incredible Hulk
Image Credits: NASA / CBS

Yesterday, NASA launched a new telescope into orbit, The Gamma-Ray Large Area Space Telescope, or GLAST. I've been hearing about the preparation of this mission for at least a decade, so it is great to see it underway! My congratulations to the team.

But why would astronomers want to look at gamma rays? And don't we already have telescopes looking at gamma rays? Gamma rays are the most energetic form of light in the Universe. The wimpiest gamma rays are about 20,000 times more energetic than visible light, and there is no theoretical limit to how strong they can become (though there are many practical limits). This makes gamma rays very dangerous to living organisms; unsafe levels of exposure can cause all kinds of cancers and other nasty effects. (It was an overdose of gamma ray radiation that turned mild-mannered Bruce Banner into the Incredible Hulk at least one evening a week back in the late 1970s and early 1980s.)

Most gamma rays people encounter come from nuclear reactions and radioactive decay; gamma rays are the most dangerous form of radiation from nuclear waste. There are also gamma rays flying around you all the time from naturally-occurring radioactive elements. But, unless you are involved in a nuclear accident, the numbers of gamma rays on Earth are too low to cause much harm.

Many objects in space also produce gamma rays. Our atmosphere is opaque to gamma rays, so we are protected from this potentially dangerous radiation. But since gamma rays are produced by some of the most energetic and mysterious astronomical objects, like black holes, neutron stars, exploding stars, and the radioactive remnants of these exploding stars, we astronomers would like to study them. So we have to launch telescopes into space to look at gamma rays.

So, why don't we use the Hubble to look at gamma rays? Why spend lots of money on a totally different telescope? It's because gamma rays are so energetic, we can't look at them with normal mirrors. Gamma rays just pass right through the Hubble's mirror. So GLAST uses a very clever technique that relies on Einstein's most famous equation, E=mc2.

Behind that famous equation is the idea that matter (electrons, protons, atoms, rocks, hamsters, etc.) is just another form of energy, like light, heat, and motion. And it is possible to change energy from one kind into another. Our car engines convert chemical energy from gasoline into the motion energy of travel, as well as into heat energy (which is why the engine gets hot!). On a sunny day, the interior of that same car converts the light energy from the sun into heat energy. Nuclear reactors change some of the matter in the atomic fuel into light and heat energy. And the GLAST telescope changes the light energy of gamma rays into matter: two or more subatomic particles (and any leftover energy is turned into motion energy of the particles). The telescope then tracks the position and speed of these particles, which, through some complex but well-understood physics, lets us surmise the original energy and direction of the gamma ray.

But gamma rays are rare, and gamma ray telescopes aren't very efficient at converting light into matter. So, it is important to make the telescope big ("large area") so we can detect as many gamma rays as possible.

GLAST is NASA's second big gamma ray telescope, after the Compton Gamma Ray Observatory, launched by the space shuttle in 1991. NASA has another gamma-ray telescope in orbit, the Swift Telescope, but Swift just looks for the mysterious flashes of gamma ray light called gamma ray bursts. GLAST can detect gamma ray bursts, but its primary mission is to look for other, steady sources of gamma rays.

Gamma rays are produced by matter about to fall into a black hole. The matter gets sped up to high speeds by the black hole's gravity, and before it falls into the black hole's event horizon it can emit gamma rays that we can detect here on Earth. Energetic jets spewing from the regions around black holes can also act as atomic particle accelerators, which can create all kinds of subatomic particles that then collide and release gamma rays. The remnants of exploding stars, such as Cassiopeia A, also glow in gamma rays from radioactive elements created in the giant explosion that destroyed a dying star.

Don't expect many spectacular pictures from GLAST. It's just not possible to make sharp, focused pictures. While the Hubble Space Telescope can see details as fine as 0.05 arcseconds (an angle something like the size of a penny seen from 50 miles away), GLAST can only see as sharp as 1 arcminute (600 times worse than Hubble). GLAST would have trouble resolving details the size of a penny about 380 feet away, a feat that sharp-eyed people can do in excellent conditions. But it is still much better than the Compton Gamma Ray Observatory, which could only resolve the said penny about 40 feet away, something anyone with normal vision can easily do.

But what GLAST can't do in sharpness, it can make up for in its field of vision. While Hubble can only look at a sliver of sky about 1/80th the size of the full moon, GLAST can look at 20% of the entire sky at once!

It'll probably be a year or two before the first GLAST science not dealing with gamma ray bursts comes out. Until then, GLAST will be staring hard, catching elusive gamma rays from deep space. Let's just hope that the scientists are nice to their telescope and don't make it angry. You wouldn't like the telescope when it gets angry.

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