Puerto Rico is home to the largest, most sensitive radio telescope in the world. For more than 40 years, the Arecibo Observatory has measured the motions of galaxies, mapped the surface of Venus, studied the physics of pulsars, and listened for signals from extraterrestrial life.
The 1,000-foot-wide dish, which rests in a natural sinkhole, consists of 40,000 aluminum panels (1) that form a radio-reflective surface. The panels gather radio waves from the sky and focus them onto a feed antenna (2) that amplifies the signals and sends them to a control room where data are analyzed. The antenna hangs from a 900-ton platform (3) suspended 450 feet above the dish by 18 cables connected to three concrete towers. Twenty-six electric motors (4) aim the telescope and adjust its focus.
In addition to studying distant stars and galaxies, scientists use the Arecibo telescope to detect and track objects that pass close to Earth. Observations here helped astronomers refine the predicted path of the asteroid Apophis, which will narrowly miss our planet in April 2036. The new trajectory, announced last fall, indicates that the probability of impact that year is very low, around 1 in 250,000.
Last year Arecibo benefited from the transition to digital television, which opened up a 100-megahertz-wide frequency band to astronomical observations. A new receiver provides access to that bit of radio spectrum, which astronomers hope to study intensively through mid-2011. By that time, the bandwidth will most likely again be dominated by commercial broadcasts.