Earth’s Radio-Wave Halo

Earth’s Radio-Wave Halo

I have the best job in the world. That’s a pretty bold statement, and one I’ve repeated often throughout my 30-year career as a SETI scientist, for there has never been a day when I’ve ever thought about doing anything else. What could be more thrilling than the search for a sentient, technological civilization beyond our solar neighborhood?

Profile: Jill Tarter

The inspiration for the main character of Carl Sagan’s novel Contact," Jill Tarter holds the Bernard M. Oliver Chair for SETI at the SETI Institute (SETI is the Search for Extraterrestrial Intelligence).

Tarter attended Cornell University, earning a bachelor of engineering physics degree with distinction. She earned a master’s degree and a Ph.D. in astronomy from the University of California at Berkeley. Her major field of study was theoretical high-energy astrophysics.

As a graduate student at Berkeley, she became involved in SERENDIP, a small commensal search for radio signals from extraterrestrial civilizations using the Hat Creek Observatory 85-foot telescope. After completing a Nuclear Regulatory Commission resident associateship at NASA’s Ames Research Center, Tarter joined the newly formed SETI Program Office at Ames.

In 1984 she helped found the non-profit SETI Institute in Mountain View, CA. Tarter served as the project scientist for NASA’s SETI High Resolution Microwave Survey (HRMS) until its termination by Congress in 1993. Today she heads the SETI Management Group at the SETI Institute.

It has been exciting for me to watch SETI evolve from a tiny somewhat arcane astronomical niche to a large, global, and increasingly sophisticated technological endeavor that has captured the imagination of several million citizens of Earth. I became ‘hooked’ on SETI when I realized that I lived in the first generation of human beings that could try to answer the old and fundamental question "Are We Alone?" by doing experiments, instead of relying upon beliefs.

Arecibo Telescope
Arecibo. World’s largest dish, radio telescope. Puerto Rico.

Right now as a piece of our technology races towards Mars, our closest planetary neighbor, where it will roll across the dusty red surface and perhaps learn more about the prospects for life in this solar system, it is a good time to consider how ‘SETI Today’ fits into today’s search for life-of all manner-on worlds still more distant.

"Astrobiology" is the umbrella term describing the search for life on other worlds. At the SETI Institute, we call this multidisciplinary field "Life in the Universe" (LITU) science, but the terms are interchangeable. I like to define SETI as that branch of astrobiology that uses the deliberate actions of the inhabitants in order to detect habitable worlds. I often add that we use different tools in SETI, however today’s SETI shares many similarities with other searches for life. A look at the tools we use, particularly the Allen Telescope Array, the new telescope that will be dedicated to SETI and other astronomical research, shows how neatly SETI science fits within the broad search for our place in the cosmos.

All scientists who seek signs of life in the universe are keenly aware that we have but one data point to guide us, Earth. So we look for evidence of life-as-we-know-it, a distinctive signature or biomarker that we learn to recognize from our ever-increasing understanding of life and biological processes on Earth. Microbes rule! Better experimental technology and broader investigations into possible habitats for life have helped redefine our extraterrestrial search strategies as we incorporate the lessons we learn about terrestrial biology.

Similarly, in SETI, we search for the signatures of technology-as-we-know-it, specifically, communication technology, as we assume that any signals we detect will be deliberate. This assumption is a very good example of the evolution in SETI thinking that occurs over time. In the early days of SETI we did not have to contend with the enormous pressures on the telecommunications spectrum and desires for allocations that would obscure a narrowband extraterrestrial signal. Fiber optics and spread spectrum signaling had not yet dimmed the brightness of Earth’s radio-wave halo, the television, radio and radar signals "leaking" into outer space. Based upon this evolution of Earth’s communication technology, today, we no longer believe that searching for unintentional transmissions, or "leakage," is a promising strategy. As demand for bits and pieces of the spectrum become more insistant, spread spectrum, frequency hopping, and other means of reusing the shared spectrum will make the Earth (and by analogy, any advanced civilization) become, not radio quiet, but more and more like a natural noise source.

Allen Telescope Array
Allen Telescope Array (ATA)

Searches in the optical spectrum also exemplify the change and growth driving today’s search for technology-as-we-know-it. For most of my SETI career, Dr. Charles Townes, inventor of the laser and recent winner of the Frank Drake Award for Innovation in SETI and Life in the Universe Research, was a rather lonely proponent of the optical spectrum’s use in SETI. Technological development in optics and lasers needed sufficient development before SETI scientists could embrace the expanded search methodology. In particular, affordable photo diodes capable of counting arriving photons within any nanosecond, are the key to this search strategy.

Clearly, the search for life, or for evidence of technology produced by life, across interstellar distances requires a thorough understanding of what to look for. The question of where to seek life is another domain in which astrobiology and SETI are inextricable. Today’s SETI is working to expand its target list of stars each time a new planet is found, a happy reality that was virtually unthinkable a scant decade ago. Our improved knowledge of the extreme conditions in which life can thrive has forced us to reexamine our conception of habitable zones around stars, again enlarging the scope of today’s SETI search.

Perhaps nothing says ‘SETI Today’ more than the Allen Telescope Array, which we are building in collaboration with the Radio Astronomy Lab of the University of California, Berkeley. While the array is often described as a "dual use instrument," it would be more correct to say that it is a multi-tasking instrument. The simple implication that the array will conduct "SETI" and "other astronomy" is misleading, Arecibo Observatory does that today via sequential scheduling. With the ATA, several types of astronomical projects will be conducted simultaneously on this instrument of which SETI is a major type, but not so terribly distinct from other types of radio astronomy. All the projects will share the hardware and software to run both the RAL correlator and beamformers. Our SETI signal processors will be highly specialized-as will be the other signal processors that analyze the signals received by the array-and all of the research is astronomical. As you can see, there is nothing arcane about the science behind SETI.

Star field
In a universe brimming with stars, the search is on if life exists elsewhere

For me, the most compelling use of the array (after SETI, of course!) may be its exploration of the very early universe, the search for primordial dark matter concentrations, and investigations of transient events such as supernova explosions and gamma ray bursts. As a scientist who finds great power and beauty in evolution as an organizing theme that ties together so much of astrobiology, including SETI, it is very satisfying to know that the array will be probing emergent cosmic evolution and the evolution of technology, two extremes on a continuum that stretches from primal matter to minds that contemplate their origins.

As I reflect on ‘SETI Today’, I am struck by the confidence with which we step into ‘SETI Tomorrow’. We are confident in our science, confident that we use the best technology to search with the best methods, and confident that we are finding our place in an astrobiological community while it matures into a rich and lively field of study.

I think I have always seen ‘SETI Today’ as a step towards ‘SETI Tomorrow’-because in a sense it has always been, and will always be, a discipline that forces its practitioners to think broadly across disciplines and anticipate the future. This quality keeps my work exciting. Is it any wonder then that I say-with confidence-that I have the best job in the world!

What’s Next

When the Allen Telescope Array turns on in 2005, it will be capable of searching to the farthest of 17,000 nearby habitable stars, just beyond 300 parsecs. For those search distances, an electromagnetic communication, if detected, would have begun broadcasting around a millennium ago, just about 1000 AD on a terrestrial calendar (for a transmission originating from a distance of 978 light-years from Earth).

Related Web Pages

Hipparcos Catalog
Allen Telescope Array Capabilities
The Astrophysical Journal (March 2003, v. 145,pp. 181-198): "Target Selection for SETI: I. A Catalog of Nearby Habitable Stellar Systems" (PDF)
SETI Institute
How To Find An Extrasolar Planet
SIM (NASA’s Space Interferometry Mission
GAIA – The Galactic Census Project
FAME: Full-sky Astrometric Mapping Explorer
Extrasolar Giant Planet Detection with Next Generation Instruments (M. Turnbull, et al)