Keeping an Ear to the Center of the Galaxy, Southern SETI Prepares for Great Leap Forward
Posted by Amir Alexander
2007/02/26 11:00 CST
North and South
As you read these lines, SETI researchers are hard at work looking for a signal from an alien civilization in our galaxy. For nearly half a century they have tried targeted searches of specific promising stars, and surveys of the entire sky; they have listened at the magical “water hole,” as well as at other promising frequencies; some have searched for radio transmissions, others for brief flashes of light; most have used the most advanced observational and computing technologies, constantly pushing forwards the bound of the possible. And yet, it can be reasonably argued that nearly all of the Searches for Extraterrestrial Intelligence also have this in common: they have been looking in the wrong place.
The vast majority of SETI searches to date have been carried out from locations in Earth’s northern hemisphere. This is only natural, given that most of our planet’s landmass and population centers, as well as the majority of the wealthiest and most technologically advanced societies reside north of the equator. But for SETI this geographical accident is unfortunate. The central region of our galaxy, the Milky Way, can hardly be seen from anywhere in the northern hemisphere, and even where it is visible, it barely rises above the horizon and is constantly overshadowed by radio interference from Earth. In contrast, the center of the galaxy is visible high in the sky from almost everywhere in the southern hemisphere, and can be clearly observed with little interference
What it all comes down to is that SETI researchers, using northern radio telescopes, have mostly been looking for alien civilizations in the periphery of the galaxy, where stars and planetary systems are relatively sparse. The center of the Milky Way, where the star density reaches an astounding 100 million stars per square arc minute, 100 times greater than the density in outlying regions, has been left relatively unexplored. The center is where most of the stars int he galaxy reside, but if intelligent beings are hailing us from that region even now, chances are most SETI searches would never hear their call.
There is, however, one ongoing SETI search that has been keeping an ear out for the call from the center of our galaxy for more than two decades. Known, appropriately, as “Southern SETI,” it makes use of the two 30-meter dishes at the Instituto Argentino de Radioastronomia (IAR), 35 kilometers south of Buenos Aires in Argentina. Located in the southern part of the continent of South America, Southern SETI has a continuous view of densest star-fields in our galaxy. And, since 1990, it has been sponsored and supported by The Planetary Society.
Two Decades of Listening
Southern SETI’s beginnings date back to 1986 when a group of physics students at the University of Buenos Aires convinced the IAR to begin a targeted SETI search of 90 Sun-like nearby stars. One of these students was Guillermo A. Lemarchand, who has been the moving spirits behind Southern SETI throughout the years, and is still the project’s director. The search used a relatively old spectrometer with low resolution that had never been designed for SETI, but it was a start.
1990 marked a turning point for the project when, thanks to Planetary Society support, Southern SETI reached an agreement with SETI pioneer Paul Horowitz to make use of his META (Million channel Extraterrestrial Assay) system at the IAR facilities. At the time, Horowitz was already operating META on the 24-meter (80-foot) dish at the Oak Ridge Observatory in Harvard, Massachusetts. Two Argentine engineers spent a year with Horowitz learning how to build and install the same system on one of the 30 meter dishes at IAR. The new Southern SETI version of Horowitz’s design was nicknamed META II. It was dedicated on October 12, 1990, and has been operating continuously ever since.
Over the years Southern SETI has tried a series of different search strategies. First and foremost, the project has completed 6 full surveys of the southern skies, several of them in the traditional magic frequencies of 1.42 and 1.667 GHz, which represent the lower and upper boundaries of the famed “water hole,“ and additional surveys at 1.72 (another "water hole" frequency) and 3.3 GHz. In addition, Southern SETI also conducted a targeted search of 71 nearby stars and worked in close coordination with Paul Horowitz’s Harvard searches. In 1997, again with Planetary Society funding, The META II system was upgraded, allowing Lemarchand and his colleagues to use both 30 meter telescopes at the IAR facility and search on a broader range of frequency.
It has been a full decade now since that last upgrade. Electronic systems that were “cutting edge” at the time are now considered yesterday’s technology. Circuit boards that were brand new in 1997 have aged, some have been revamped and some are no longer usable. Furthermore, Lemarchand’s team has come up with a new and innovative search strategy that requires new capabilities that were not available on the old META II. It is time for a new upgrade, and again, The Planetary Society stands ready to assist the only SETI project that is constantly listening for a signal from the center of our galaxy.
Amplifiers in the Sky
The new search strategy is based on a remarkable natural phenomenon known as “natural masers” (“maser” is the microwave version of a “laser,” and stands for “microwave amplification by stimulated emission of radiation”). In themselves, these masers are nothing but extremely diffuse clouds of gas that exist in interplanetary space. So diffuse, in fact, that their molecular density is about equal to the best vacuum obtainable in laboratory conditions on Earth. As can be expected, these clouds tend to interfere with radio transmissions that pass through them. Remarkably, however, when it comes to certain specific frequencies, these clouds actually amplify rather than absorb the signals instead of absorbing them. This can be a crucial consideration for a SETI search. Sending a radio signal across the vast distances of interstellar space is enormously difficult and requires a major investment of power and energy. Along the way, a signal will easily dissipate and be absorbed by interspersed clouds of interstellar particles, and by the time it reaches a listening civilization, it will likely be overshadowed by stronger, naturally occurring radio signals from space, as well as by transmissions from the receiving civilization itself. All of this makes it extremely likely that a transmission across interstellar space will never be received.
All this changes, however, if masers are used as conduits between the transmitting and the receiving civilization. A signal sent through a maser would be amplified from tens of millions to hundreds of billions of times. A transmission that would be all but lost if traveling directly across interstellar space, might now arrive at its destination crisp and clear. Is it not reasonable to suppose that an advanced alien civilization would take advantage of these enormously powerful “amplifiers in the sky” to get their message across the vast distances of space? Guillermo Lemarchand thinks that it is.
The masers that most interest Lemarchand are clouds of OH, the hydroxyl radical which, when conjoined with another hydrogen atom, makes up water. These masers have the unique property of amplifying signals in the OH emission range –- 1.66 GHz. This frequency is significant, because it marks the upper boundary of the “water hole” – that magical range of frequency between 1.42 GHz and 1.66 GHz where most SETI searches are conducted.
According to Lemarchand OH masers are not rare. There are thousands of such masers in the night sky and many more that have not yet been mapped. Each of these, he suggested, might be a conduit for an alien civilization. Maybe, just maybe, there is a transmission there, sent to us from a faraway alien civilization.
By pointing a radio telescope at masers, Lemarchand suggests, we can eliminate much of the inherent uncertainty that accompanies a SETI search. The questions of “where should we search,” and “what frequency should we monitor” have no simple answers in SETI, and are in each case determined by the theories and beliefs of the investigator. But if we focus on masers, then these questions are settled for us: we will search in the directions in the sky where masers are located, and we will listen at the frequencies that they amplify; in the case of OH masers, that frequency is 1.66 GHz.
A Great Leap Forward
Unfortunately, explains Lemarchand, the aging system put in place for Southern SETI over a decade and a half ago is not well suited for an effective search of OH masers. Currently, META II can monitor 8.4 million separate channels simultaneously, each with a bandwidth of only 0.05 Hertz. Looking for signals at such a narrow bandwidth has the advantage that it screens out all naturally occurring radio signals. This is because natural signals coming from space, such as natural cosmic masers, have a much wider bandwidth, of tens to hundreds of Hertz. But the downside of using such fine frequency resolution is that the combined range of frequencies META II is monitoring at any given moment - the so called “instantaneous bandwidth” - is also relatively narrow. 8.4 million channels of 0.05 Hertz each give a total instantaneous bandwidth of only 0.42 MHz. This is not enough, says Lemarchand. Even assuming that we are listening at the correct frequency, 1.66 GHz, the Doppler effect caused by the relative motion of the alien planet and our Earth will cause the transmission frequency to drift. This means that in order to receive the signal on Earth, we will have to listen to a slightly higher or slightly lower frequency than the original transmission. But if our entire instantaneous bandwidth is only 0.42 MHz, centered on 1.66 GHz, then the entire signal will likely fall outside the bandwidth we are monitoring, and we will never hear the distant call from the depths of space.
In theory, said Lemarchand, this problem can be circumvented if the aliens correct for the motions of their planet and ours in their transmissions. But can we simply assume that they do so? Hardly. Expecting the unknown aliens to know about us in advance, not to mention accommodate themselves for our receiving convenience, is a tall order. If we want to find that elusive SETI signal, it is much safer to assume that we must make our own corrections, and that means listening in as broad a band as possible so as not to miss anything.
To make sure that Southern SETI does just that, Lemarchand is proposing to replace the aging META II system with a brand new electronic back-end, based on SERENDIP V. Designed and built by Dan Werthimer’s U.C. Berkeley team of SETI@home fame, SERENDIP V is the latest generation SETI receiver installed at Arecibo. It is the most advanced system available today not just for conducting SETI searches, but for operating radio telescopes in general. Different versions of SERENDIP V may soon be utilized by an all-sky survey based at the Jet Propulsion Laboratory and by the Allen Telescope Array currently under construction in the Cascade Mountains in California.
In its Southern SETI configuration, according to Lemarchand, SERENDIP V will monitor between 130 million and 260 million channels simultaneously. Furthermore, instead of the current system’s ultrafine resolution of 0.05 Hertz per channel, SERENDIP V will be configured so that each channel is a full 1 Hertz wide. This resolution, explains Lemarchand, is perfectly sufficient to distinguish an artificial signal from a natural one. It also gives SERENDIP V a healthy instantaneous bandwidth of 130 to 260 MHz.
When compared with META II’s 8.4 million channels, SERENDIP V represents a 15 to 30 times increase in the number of channels monitored simultaneously; and when compared with the current system’s 0.42 MHz instantaneous bandwidth, the new setup represents a 325 to 650 times increase. This last is particularly important, for it expands the band that Southern SETI monitors at any given time enormously. It vastly reduces the possibility that an alien signal will be overlooked simply because it drifted off the main frequency.
But this is not all. A SETI search focused on masers in the sky posits additional technological challenges. Masers fluctuate constantly in the degree to which they can amplify radio signals from afar. Their signal might be very faint at times, only to increase dramatically at others. Monitoring a maser requires hours of patient continuous observing, and the adding together of the faint signals that have been received over time in a process called “integration.”
The busy radio telescopes of the northern hemisphere rarely have time to focus on a single target for hours at a time, not to mention repeating the process night after night while aiming at different locations. The IAR dishes, in contrast, can observe targets continuously for 12 hours every day, and whereas the META II system was not designed to undertake long integrations, SERENDIP V is perfectly capable of carrying out the task. All this will makes the new and revamped Southern SETI an ideal instrument for monitoring masers.
Finally, the new back-end has one other major advantage: it is portable. Unlike META II, which is permanently integrated into the electronic controls of the 30-meter dishes at IAR, the new SERENDIP V back-end can be packed up and moved to other locations. As a result Southern SETI can now travel, attaching itself to different radio telescopes and making use of their unique attributes to complement the ongoing search at IAR.
This is already happening. In recent months Lemarchand has teamed up with his Brazilian colleagues G. Giménez de Castro and E. S. Santini to propose bringing Southern SETI to the 13.7 meter dish at the Radio Observatorio do Itapetinga in Brazil. This makes sense, because the Itapetinga radio telescope can observe in the millimeter wavelengths that are not accessible to the Argentine dishes and can therefore observe masers that amplify frequencies other than the ones observed at IAR. But just as importantly, the Argentine - Brazilian cooperation shows how the search for alien civilizations light-years away can bring neighboring civilizations here at home closer together.
With SERENDIP V, Southern SETI will listen to 15-30 times as many channels over a bandwidth increased 325-650 times. It will be able to integrate signals from masers, and it is portable, enabling it to hook up with other dishes with relative ease, thus enormously expanding its search capacities. By any conceivable measure, a transition from META II to SERENDIP V represents a giant leap forward for Southern SETI.
As we have in the past, today, once again, We Make it Happen! With funding from The Planetary Society, Lemarchand is now moving forward to implement his vision of the new Southern SETI. And if sometime in the coming years we were to hear that a true signal from an alien civilization has been found, it may well be that it was detected by Southern SETI -- our constant ear aimed at the center of the galaxy.