At the time, the triplet was 11 million kilometers away imaging it with Arecibo’s 75-m resolution was analogous to using a camera in New York to photograph a person in Los Angeles with 3-cm resolution. The Arecibo measurements yielded 75-m-resolution images of the 3-km-diameter asteroid, whose two moons are about 1.1 km and 400 m across. In 2008 Arecibo discovered a triple system, asteroid 2001 SN263. About 15% of them are binary systems, consisting of two objects whose orbits about their center of mass allow accurate determination of their masses and densities. The NEAs are of interest not only because of their potential to smash into us but also because the details of their structures hold clues to the origin and evolution of the solar system. The radar can obtain distances with a precision of 10 m and speeds accurate to 1 mm/s. To date, more than 300 NEAs, some as small as 50 m across, have been studied by the Arecibo radar with resolutions as fine as a few meters. So have a rapidly growing number of asteroids, especially the so-called near-Earth asteroids (NEAs) that come relatively close to our planet. Over the years, the terrestrial planets, some moons of Jupiter and Saturn, Saturn’s rings, and some comets have yielded their secrets to the Arecibo radar. The weakness of the detected signals necessitated the use of large antennas and sensitive receivers. By 1960 a few tens of galaxies had had their HI signatures measured, but the study of HI in other galaxies was still in its infancy. The first detection of HI outside the Milky Way occurred in 1953, when Frank Kerr and James Hindman detected it in the Magellanic Clouds. Its intensity, line profile, and measured frequency provide information about the physical state of the emitting gas and, via the Doppler effect, its velocity along the line of sight. Arecibo Observatory's radio telescope was instantly recognizable, thanks to its 1,000-foot-wide (305 meters) dish, trio of soaring towers and delicate web of cables and platform that held science. That hyperfine line is important because hydrogen is by far the most abundant element in the universe. That radiation comes from hyperfine transitions in which the spin of a ground-state electron flips-before the transition, electron and proton spins are parallel afterward, they are antiparallel. The 21-cm spectral line from the Milky Way’s interstellar neutral hydrogen (HI) was first detected in 1951 at Harvard University’s Lyman Laboratory by Edward Purcell and Harold Ewen.
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