Biggest is Best

Mar. 19, 2002
by Seth Shostak - Senior Astronomer


Daniel Altschuler, the Uruguayan-born site director of the Arecibo observatory, put his hands behind his head, and leaned back in his chair.  “Well, there are a few things that come to mind ... For example, determining the rotation rate of Mercury, or mapping the surface of Venus.”

I had asked Altschuler what he thought were the major discoveries of the massive telescope sitting in a limestone bowl a few hundred feet from his office. As the world’s largest antenna, Arecibo has been used day and night by astronomers for roughly four decades.  Altschuler gazed out the window: “Then there was the work on binary pulsars done by Joe Taylor and his student. They were looking for a slowing down caused by gravity waves. They found it and won the 1993 Nobel Prize.”

When the local population is asked what Arecibo is used for, they usually assume that SETI is the dominant activity. In reality, Project Phoenix gets only about 5% of the available observing time. The study of pulsars and galaxies are the bread and butter work of the telescope.  Indeed, Alex Wolszczan, who found the first planets outside the solar system a decade ago using this instrument, is at the observatory now, making daytime observations and uncovering yet more planets around pulsars. When I first came to Arecibo in the early 1970s, it was to investigate the behavior of galaxies.

Why pulsars and galaxies? For the same reason that Project Phoenix prefers Arecibo: sensitivity. The antenna simply outclasses the competition in terms of size, and that translates into an ability to find faint or rapidly changing signals.

A few nights ago, I was the guest on a radio program when a caller from the Southwest asked why we didn’t take our SETI experiment to the large antenna array in New Mexico. The Very Large Array, to which he referred, and which (together with Arecibo) figures in the movie “Contact”, consists of 27 antennas spread out over a desert plain. Configuring that passel of antennas to occasionally run SETI experiments would be a forbidding undertaking.  But even if you managed to do so, you’d still end up with an instrument only about one-fourth as large as Arecibo.  The antennas of the Array would get lost in the 20 acres of Arecibo’s gargantuan dish.

When it comes to hunting for weak signals – the prey of SETI researchers – a bigger snare has a better chance.

In a way it’s ironic that Arecibo has become such a valuable property. When first conceived in the late 1950s by William Gordon of Cornell University (which still runs the telescope) the idea was to build an instrument that could investigate the thin soup of electrons in the Earth’s ionosphere, roughly 40 miles up. Gordon figured this could be done using a powerful radar transmitter on a 1,000 foot diameter antenna. His original design would have had the transmitter mounted on a tower in the center of the dish.  While this would have sufficed for the ionosphere experiment, it would have precluded the use of the antenna for any astronomy observations, where the ability to aim in different directions and follow objects across the sky is essential.

The idea of studying the planets with Gordon’s souped-up radar quickly led to the present design, in which the receiver is movable so that the telescope, like a good bird dog, can point and track. But planets are overhead only near the equator, so the Cornell scientists sought a tropical location for their giant instrument. There’s a well-circulated (but apparently undocumented) story that one of these radio pioneers slid a quarter around a topographic contour map of Puerto Rico, looking for natural “bowls” in the limestone karst where the antenna could be fitted. Choosing such a bowl location minimized the amount of expensive earthworks required for construction.

Thanks to continuous upgrades and a highly trained staff, Arecibo remains not only competitive, but dominant in large areas of radio astronomy.  Every week, a dozen or so scientists are in residence here. For Project Phoenix, it truly is today’s best place to be.  And if we succeed, Daniel Altschuler will have another item for his list of Arecibo’s greatest hits.