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Running Interference
Artist concept of the fully assembled Large Binocular TelescopeUsing a single telescope to observe a planet the size of Earth around a distant star would be pretty impractical.

It would require a mirror as wide as a football field, and it would have to be deployed in space. But technologists are rapidly developing more feasible methods of studying such extremely faint objects. One of these methods, called optical interferometry, is a key technology in NASA's search for new worlds.

Image to left: The Large Binocular Telescope Interferometer (LBTI) will study the formation of solar systems and will be capable of directly detecting giant planets outside our solar system.
Credit: NASA's PlanetQuest

Optical interferometry combines the light of multiple telescopes to perform the work of a single, much larger telescope. This is possible because of the interaction of light waves, also called interference. Their interaction can be used to cancel out the blinding glare of bright stars or to measure distances and angles precisely.

To really understand how optical interferometry works, we have to understand waves.

The wavelength of a wave is the distance from one crest (hill) or trough (valley) to the next. When a crest or trough passes a certain point, the time it takes for the next crest or trough to reach that point is the period of the wave. If the period of a wave is three Examples of a wave's amplitudeseconds, then the frequency of the wave is one-third per second.

From these two numbers, we can calculate the wave's speed, or velocity. Velocity is calculated by dividing the wavelength by the period of the wave (just as we say a car goes at 30 miles per hour). A wave's amplitude is half the difference in height between a crest and the trough of a wave. Big waves have large amplitudes, and small waves have small amplitudes.

Image to right: A wave's amplitude is half the difference in height between a crest and the trough of a wave.
Credit: NASA's PlanetQuest

Different wavelength or frequency results in the different colors we see. Red light has a longer wavelength than blue light; green light is somewhere in between. At the same time, blue light has a higher frequency than red light. Whether a color is dim or bright, its wavelength remains unchanged.

Example of constructive interferenceIf two rays of light have the same wavelength, they can interact in a surprising way. When they overlap with each other, the two amplitudes of the waves add up to twice the amplitude and we get a much brighter light. This is called constructive interference.

Image to left: Constructive interference occurs when identical light waves overlap. Credit: NASA's PlanetQuest

However, if we shift one light ray by just half a wavelength, all the crests of one wave coincide with the troughs of the other wave, and the two rays cancel each other out and disappear. This phenomenon is known as destructive interference. The shift from constructive interference to destructive interference is called the phase shift, or phase difference.

Example of destructive interferenceImage to left: Destructive interference occurs when light waves cancel each other out. Credit: NASA's PlanetQuest

NASA already has one interferometer at work: the Keck Interferometer, located at the W.M. Keck Observatory on Mauna Kea, Hawaii. Part of NASA's Origins Program, the Keck Interferometer aids in the overall effort to find distant planets.

The future of interferometry is bright, with new instruments and missions on the horizon. The Large Binocular Telescope Interferometer under construction in Arizona will be capable of directly detecting giant planets outside our Solar System. In 2009, NASA will launch the Space Interferometry Mission (SIM) into an Earth-trailing solar orbit. SIM will search for new planets and seek answers in astronomy and astrophysics.

For more information, test your knowledge with the Virtual Interferometer or visit NASA's PlanetQuest.
Anna Heiney, KSC Staff Writer
NASA's PlanetQuest and John F. Kennedy Space Center