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Photo of Jim Cordes Jim Cordes
Big Dish

Jim Cordes answers questions about how the Arecibo Observatory tunes into the universe searching for pulsars, far-away galaxies, asteroids -- and signals from distant space.

q What's more valuable when you're looking for pulsars and other objects in the universe -- an optical telescope or a radio telescope? Why?

A For most pulsars radio telescopes are the only apparatus we can use to study them because they radiate detectable emission only in the radio band of the electromagnetic spectrum. A few pulsars show optical emission that can be observed using optical telescopes (and infrared, X-ray and gamma-ray telescopes). For other kinds of objects in the Universe, such as stars, comets, galaxies, and quasars, other bands besides radio are the bands of choice. An easy way to think of it is that if our eyes were sensitive to radio waves rather than light, the universe would look very different. Or if we saw in X-rays rather than light, it would also look different. This is why we really need telescopes that can span the entire electromagnetic spectrum -- from radio to infrared to optical to gamma-rays -- in order to survey and understand the Universe.

q What year was the big dish made and completed? Was it funded by the United States government?

A The 1000-foot (305-meter) dish at Arecibo was completed in 1963 and its construction was funded by the US Air Force.

q Why was the observatory constructed in this particular place -- is it an especially good location for looking into space?

A The Observatory was located in Puerto Rico for several reasons. First, being a huge reflector, a natural valley of suitable size was needed and it was known that there were sink-holes in the karst region of Puerto Rico that might be appropriate. Secondly, the original primary purpose of the antenna was to study the atmosphere, particularly the ionosphere near the tropics, so an island location near the equator but still with easy access from the US was ideal. Finally, it was also realized that the antenna could be used as a radar to study other planets, especially Venus. By locating the antenna near the equator, the greatest amount of viewing time of planets could be achieved. One thing to remember is that with the antenna we can study objects that are within 20 degrees from the vertical direction. The planets are roughly in a plane (the ecliptic) that is at most about 23.5 degrees from the equator. At Arecibo, the latitude is 18 degrees so we can see planets for quite a bit of time.

q Why is the three-story high reflecting dome you and Alan went into called the Gregorian dome?

A The dome that we walked into is called the Gregorian dome for the following reason. Inside the dome are two additional reflectors that collect the radiation reflected off the main dish and bring it to a point focus. These two reflectors are called the secondary and tertiary reflectors. The way the three reflectors work together is according to 'Gregorian optics,' named after the Scottish seventeenth mathematician, James Gregory. He designed a reflecting telescope (optical!) whose optics serve as the prototype for many telescopes including Arecibo.

q If you go on vacation in Puerto Rico in the area where the observatory is, are you allowed to take a tour around the dish?

A The general public is not allowed to go on a tour around the dish. However, there is an excellent visitor center with hands on displays and a great view of the dish. One of the scenes from the movie "Contact" was filmed from the patio that overlooks the dish.

q Are there any comets or other objects on a collision course with earth that you have detected with the radio telescope?

A Scientists have detected radar echoes from comets that have approached the Earth, though not dangerously so. It is very unlikely that a comet or asteroid of large size will hit the Earth in our lifetimes. However, scientists are very interested in identifying all asteroids that will come inside the Earth's orbit. Many of them will be tracked using the Arecibo telescope.

q Does part of the movie "Contact" take place at the Arecibo Observatory? If you saw that movie, did you think it was realistic?

A Part of the movie does take place at Arecibo and quite a bit of filming was done there. Much of the content was realistic though other parts were dramatized to make the plot interesting. We don't usually use headphones to 'listen' to detected signals. Instead, we record data digitally and use computers to analyze the signals. Computer analysis is far more sensitive than our ears in detecting signals.

q How many scientists are stationed at the Arecibo dish? What do you think is the most exciting discovery of the scientists there so far?

A There are about 15 scientists stationed at the Observatory. And there are several hundred scientists from all over the world who use the telescope. I think the most exciting discoveries from Arecibo involve pulsars. In 1974 Joe Taylor and Russ Hulse discovered a pulsar in orbit with another neutron star. The orbital period is only 8 hours. They went on to show that the two stars are spiraling in. The way they do so confirms Einstein's theory of General Relativity which predicts that two such stars should emit gravitational waves and thus lose energy. The other exciting discovery was by Alex Wolszczan, who found that there are several planets orbiting a pulsar. These were the first planets discovered outside our solar system. Their existence makes many people think that planets must be easy to form and in lots of ways. If they can exist around a neutron star -- a hostile object formed in a violent supernova explosion -- then they probably form around all kinds of stars. This is good news for the idea that planets hospitable for life are common.

q Would it be possible on day to put a dish like Arecibo on the moon so that you don't get interference from cell phones and stuff?

A Yes, it is possible to build a dish even larger than Arecibo on the Moon, most likely put into a large crater that provides natural excavation. It is very expensive to put something on the Moon so I doubt if it will happen very soon. But once a permanent base is established there in the next century, it will be much easier to construct a radio telescope. At the rate that interference is increasing, we will need such a site!

q My science class watched the special about Science in Paradise. I was wondering exactly how you viewed the stars and other objects in the universe. I never caught on to that in the show. Do you see them on computers? Thank you. Andrea

A Most of the time when we view astronomical objects, we stare at computer screens to look at the output of a digital camera or the results of a computer analysis of data. Sometimes we see these results in 'real time' when we're at the telescope. But other times, we may not see the results until weeks or months after we make the observations. I should say, however, that I (and other professional astronomers) enjoy to look with just our eyes when we are in a dark place and can see the Milky Way.

q Hurricane Georges passed over us in the Virgin Islands and Puerto Rico. Buck Island was spared, more or less, but Georges packed quite a punch, particularly to P.R. according to the reports. How did you weather the storm and what if any procedures do you take in advance of the storm to protect the aerial portion of the radio telescope?

A Hurricane Georges made a nearly direct hit on the mountain region in Puerto Rico where the Arecibo telescope sits. Luckily, no major damage occurred on the telescope. But quite a few trees were felled at the Observatory and many employees of the Observatory suffered through loss of power for several weeks. Some even lost the roofs of their houses. We have specific procedures to protect the Observatory from wind damage. These include wood barriers on windows of the several buildings. More important, we take down some of the line feed antennas that are attached to the carriage house on the platform and the feed arm (which rotates around the ring suspended from the platform) is locked into position. The structure is designed to withstand 125 mile per hour winds. The measured winds went up to about 110 miles per hour during hurricane Georges.

q Could an amateur scientist build a small version of a radio telescope? If so, do you have any suggestions about how to get started and where to find the components?

A It certainly is possible to build a small radio telescope and many people have. It requires quite a bit of knowledge of electronics. There is an organization called The Society of Amateur Radio Astronomers with a web site you can contact for more information:

You can search for other web sites using Yahoo, for example. Depending on your age, you might want to consider a summer research internship at one of the national radio observatories (NAIC = National Astronomy and Ionosphere Center, which runs Arecibo; NRAO = National Radio Astronomy Observatory.) NRAO also has a program for teachers that is discussed on their web site. Web sites are at

q Why did you have to pick the lock to get into the observatory with Alan Alda and the Frontiers crew?

A Well, we didn't really have to pick the lock: the film crew was late in getting to the Observatory that day and I didn't want to wait for the Observatory staff to bring a key for the gate. The reason the gate is locked is because it leads to the catwalk going to the platform; for security and safety reasons, we restrict access to that area. I didn't really expect the filming of the lock picking was going to be shown in the program!

q Which is the better telescope -- the radio telescope at Arecibo, or the Hubble telescope? Now, just a few days ago, the Hubble telescope detected the farthest and possibly oldest galaxies ever known to man. I know that the two telescopes are for different purposes, but which do you think is better?

A This is a tough one because both telescopes have yielded extraordinary new results and, as you realize, they were designed for different purposes. I really cannot say which one is better for those reasons. Arecibo is the largest radio telescope on the planet in terms of collecting area. Radio interference is getting worse, rapidly, and some of us fear that Arecibo will lose some of its ability to probe new areas of astrophysics. I think this will take a while because we are also developing new methods for excising interference from the data we get. There is no question that the future of astronomy is in space and the Hubble telescope is a harbinger of this future.

The need for going into space depends on the type of electromagnetic wave you want to use. For optical and infrared light, the atmosphere distorts images and causes telescopes to be less sensitive than otherwise. High energy photons (X-rays and gamma-rays) do not penetrate the atmosphere at all, so one must put telescopes sensitive to these photons in space. For radio wavelengths, people are contemplating orbiting antennas or even large apertures built into craters on the Moon.

There are several reasons:
(1) some radio wavelengths cannot penetrate the ionosphere of the Earth;

(2) using collections of antennas together allows us to gain angular resolution; the bigger the separations of the antennas, the better the resolution and it is easier to get wide spacings in space; and

(3) with its low gravity and potential shielding from terrestrial human-made interference, the Moon is a great platform for building a very large aperture antenna, perhaps man miles in size.

q What are the plans for the future with the big dish?

A In the solar system, Arecibo will be used in its radar mode, transmitting one million watts of power so that echoes from planets, satellites, asteroids and comets can be studied. Of particular interest are asteroids on orbits that cross the Earth's orbit; these potentially could impact the Earth some day, so we want to monitor them closely. But radar also allows us to determine the composition of asteroids, information that will be useful in the future when we may mine them.

Going beyond the solar system, immediate plans for using Arecibo are to find new, exotic pulsars such as those in orbit with black holes and also pulsars that are spinning near their "break-up" limit, which is about 2000 times per second.

The recently upgraded telescope will now allow us to use frequencies where characteristic emissions from various molecules can be detected. These molecules are found in dense clouds where new stars are being formed all the time, so we can use Arecibo to probe the process of star formation.

Cosmology is an area where Arecibo will make great contributions. With its greater sensitivity and better frequency coverage, we can use it to detect more distant galaxies in a variety of ways. Galaxies are perfect 'road markers' for measuring the expansion rate of the Universe and they allow us to study how they have evolved over time. The further we can look, the further back in time we can look. So the light from the most distant galaxies is really fossil light that tells how things were in the distant past.

q I was wondering just how close we've gotten to finding intelligent life out there, and how can you could tell by the signal if someone or something in the universe were trying to contact us.

A There has been quite a bit of effort in the last ten years to search for signals from other civilizations in the Galaxy. We know enough about what is 'out there' to distinguish radio (or optical or infrared, etc.) emission from natural sources from unnatural signals. What many people expect to see is radiation concentrated in to a very narrow frequency band, much narrower than is seen from natural radiation. This concentration is an efficient use (by the ETs) of transmitter power. To establish that such a signal is from some place in the Galaxy, we would localize it using telescopes operating as an interferometer. Assuming the transmission stays on for many days, we could then establish that its distance and rule out that it is, say, one of our own transmissions reflecting off the Moon or from one of our own satellites or spacecraft like Voyager.

In spite of the great amount of effort that has gone into SETI (search for extraterrestrial intelligence) in the 1990's, we have searched only a minuscule fraction of the possibilities. So we really cannot answer yet the question of how many civilizations like ours might exist in the Galaxy. But the answer may come within our lifetimes or ... even tomorrow.

Jim Cordes is also featured in Cool Careers in Science. Check it out!


Scientific American Frontiers
Fall 1990 to Spring 2000
Sponsored by GTE Corporation,
now a part of Verizon Communications Inc.