Celestial radio signals reaching Earth are so faint that all the energy collected in the forty-year history of radio astronomy is about equal to that released when a few snowflakes fall on the ground. *** Mars is planned, along with looks at the asteroid belt, the four major satellites of Jupiter and the rings of Saturn. lt is also possible to look "under the surface" with radar, showing the terrain which lies below a layer of dust. Venus, Earth's sister planet with the eternal cloud cover, has been a source of mystery and the target of sometimes wild conjecture for many years. ln 1964, radar signals from Arecibo were used to accurately determine Venus' period of rotation and confirmed the theory that Venus, alone of all the planets, spins on its axis in a clockwise, not counter-clockwise, direction. It was later also determined that Venus exhibits the phenomenon of "Earth lock." Each time Venus swings by Earth, it turns the same face toward Earth. ln 1968, Arecibo produced its first radar map of Venus. Mercury, the closest planet to the Sun, completes an orbit every eighty-eight days. Until 1965 it was thought that Mercury always kept the same face turned toward the Sun. Radar studies by astronomers at Arecibo showed that Mercury did, after all, rotate slowly on its axis. turning alternate faces to the Sun at each close approach. Beginning in 1970, the Arecibo radar has been used to map portions of Mercury, showing the planet's surface to be rougher than that of Venus but not quite as rough as the Moon. The universe outside of our little solar system abounds with mysteries and discoveries which stretch the mind's ability to comprehend. ln 1967 a British survey of twinkling radio sources turned up radio pulses of startling regularity coming from a certain direction in the sky. The Cambridge, England team which made the discovery did not release the news of the discoverey immediately because of debate over whether the pulses might be signals from intelligent beings elsewhere in the Galaxy. After several weeks this possibility was largely discounted and the theory advanced that the pulses were coming from a rapidly spinning neutron star. Conclusive evidence against the possibility of pulsar signals being intelligent interstellar communication came from the Arecibo discovery that the pulse interval was increasing by some 36 billionths of a second a day. This was confirmation of the theory that pulsars are neutron stars, since such a rotating star should be gradually slowing down. Neutron stars, theoretically predicted in 1933, are the [image] ANATOMY of A PULSAR PULSE remnants of giant stars which have collapsed to a radius of about 10 km while retaining approximately the mass of our sun. This results in a density of some 1014 g/m3, the density of an atomic nucleus. (For an exercise in the mind's inability to comprehend large numbers, try to imagine one cubic inch of matter with a weight in excess of ten billion tons! Of course such a density can only exist under the terrific pressures found within a collapsed star.) In some way which is not yet completely understood, the intense magnetic field of the rotating neutron star generates beams of coherent radio waves and light which appear as pulses to an observer on Earth. Perhaps the most famous pulsar is the one in the Crab Nebula, which was observed and recorded by the Chinese as an explosion in the year 1054 A.D. A nebula is a still glowing cloud of interstellar gas and dust, the remnant of a supernova, or stellar explosion. The Crab Nebula pulsar also emits light pulses and has optically identified as the exploded supernova. The Arecibo Observatory continues to do research on pulsars and is also participating in the search for black holes. Without a doubt one of the oddest celestial bodies, a black hole is the super-dense remnant of a giant star which has collapsed in such a way that it almost no longer exists. Past a certain limit the gravitational field of a black hole does not permit any interaction with the rest of the universe. A beam of light, or anything else, will "fall into" a black hole and never come out. Black holes are unescapably predicted by the general theory of relativity, but their existence has not yet been observationally verified. Of course one must use indirect methods to "observe" a black hole, such as noting the apparent influence of a large mass on a stellar system when no such mass is observable with an optical or radio telescope. Interestingly enough, something like a black hole was predicted in 1795 by Pierre-Simon Laplace. *** Of all the things which may exist outside the bounds of our planet Earth, surely the most wondrous of these is life itself. *** In addition to pulsars and black holes, there are a multitude of other interesting objects Out There - several other types of radio stars, several types of radio galaxies and, the most distant known objects in the Universe, the quasars, or quasi-stellar radio sources. Receding from us at more than half the speed of light, quasars are whole galaxies in which a very small part (only light-weeks in diameter) releases tremendous amounts of energy equivalent to the total annihilation of millions of stars. Ouasars emit enormous quantities of radio energy which, traveling at the speed of light, have taken as long as ten billion years to reach Earth. Although the 1,000 foot dish of the Arecibo radio telescope is physically the largest on Earth and there are now several radio astronomical observatories using multiple antennas with their signals combined by computer in such a way as to synthesize antennas of more than a kilometer in diameter, there are very distant or very small objects (such as quasars) which can not be adequately measured with a single radio telescope. To make such measurements a technique known as long-baseline interferometry is used. This involves combining the signals from two or more radio telescopes, often on opposite sides of the Earth, and using computers to process the signals to yield data not obtainable with a single radio telescope. Pioneer work in this area was done in 1966 by a team which included a Cornell professor and made use of the Arecibo Observatory.