Esteemed physicist, Freeman Dyson of Princeton's Advanced Institute, has gone so far as to wager a bet that any contact is most likely to arise from some object other than a planet or moon.

For Scot Stride of NASA's Jet Propulsion Laboratory, one possibility is detecting evidence of a probe. His fascination with robotic exploration is partly based on first-hand experience, he wrote, since "many of the scientists and engineers at this NASA center don't see our robotic probes as just machines, but as extensions of our senses, intellect and being.

Indeed, Matt Golombeck used to humorously call the Mars Pathfinder Sojourner rover a "mini-geologist" version of himself. My views are similar. This has indirectly resulted in a personal interest in how advanced extraterrestrial intelligence (ETI) might carry out galactic exploration and the construction of interstellar robotic probes."

Indeed, the search for life is often summarized more as a search for evidence of the technology produced by life.

To underscore the differences, Director of SETI Research, Jill Tarter at the SETI Institute's wrote: " 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. " Tarter was the inspiration for the main character of Carl Sagan's novel Contact.

Stride also has taken up the question of where to seek technological evidence for life. Whether space probes might be available from other civilizations--and whether we are even technically capable of investigating this possibility--was the topic of a recent paper co-authored by Stride and Bruce Cornet in the 'Contact in Context' series, entitled "Solar System SETI Using Radio Telescope Arrays".

Astrobiology Magazine had the opportunity to talk with Scot Stride of NASA's Jet Propulsion Laboratory about the survey opportunities within our solar system.

Astrobiology Magazine (AM): Could you give some background on the history of looking for signals in our neighborhood, or Solar System SETI?

Scot Stride (SS): Solar System SETI (S3ETI) is a strategy that hypothesizes ETI, in some material or physical form, may be present in our solar system.

The idea that an ET civilization could be close enough to physically journey to Earth has its roots in ancient history with the Babylonian and Sumerian writings. Since then science has surpassed myth and superstition, and our knowledge of the universe, space travel and the prospects of discovering extraterrestrial civilizations has vastly improved.

In more recent times the search for other intelligences in the solar system can be traced to Lowell and his belief that canals on Mars were built by its industrious inhabitants. Other people of that era considered signaling to possible ET in the solar system. In these cases it was believed the ET were native to our solar system and living on some planet like Mars or Venus.

Given our present knowledge of solar system habitats, we are quite certain there are no intelligent ET now living on any of the non-terrestrial planets or moons in our solar system.

The environments are extreme in one respect or another which limits the complexity of life as we know it. Simple microbial life may exist in some remote crevice of the solar system but that is for the astrobiologists to discover.

Any ET intelligence that may be present in the solar system is expected to have originated from somewhere out in deep interstellar space.

We have enjoyed remarkable success exploring interplanetary space and planetary environments with robotic spacecraft and rovers. Likewise, a highly advanced ET civilization, if one exists, may be exploring the nearby cosmos with artificially intelligent robotic probes - a gradual program of exploration covering long timescales and interstellar distances. Just such a highly advanced robotic probe may now, by chance, be exploring our solar system.

It was hypothesized in the early 1960's that objects could be parked, suspended or trapped in either the L4 or L5 Earth-Sun Lagrange points [locations in space where gravitational forces and the orbital motion of a body balance each other].

Between 1961 and 1982 at least eight groups of researchers made observations of these regions using optical telescopes and low frequency pulsed radar. Two of these groups attempted to search specifically for robotic probe artifacts, functioning or not, in these regions. Nothing was found, but those efforts were a defining moment in the scientific search for ETI in the solar system. Other targets in the solar system, like the moon and Mars, are also considered candidates to search for ET artifacts.

"So far government-funded investigations of the planets have not included a search for signatures of ETI. However, there are ways to search indirectly for ET technology in the solar system using radio telescopes, like the Allen Telescope Array now being constructed at Hat Creek, California.

Using radio telescope arrays to find evidence ETI in the solar system is the thrust of contemporary Solar System SETI efforts.

AM: You propose to use the Allen Telescope Array as one method to search for microwave frequencies. The Allen Telescope Array will consist of 350 individual 20-foot antennas linked to form the equivalent of a single large antenna. When fully operational in 2005, the Allen Telescope Array will have more collecting area than the newly completed Green Bank Telescope in West Virginia, and better resolution than the venerable Arecibo dish in Puerto Rico. Based on your research, can you describe what a top priority might be when that work is begun around 2005?

SS: A search for Anomalous Microwave Phenomena (AMP) or Unidentified Radio Signals (URS) within the solar system can be modeled after traditional SETI searches, that is, "All-Sky" surveys or "Targeted" examinations. AMP or URS can radiate the Earth from practically any direction, but the probability is higher for regions within the plane of the solar system. In that respect there is a preference to search within � 17 degrees of the ecliptic plane, which encompasses all the planet-moon systems in the solar system.

AMP or URS could be caused by a naturally occurring event, like the noise bursts observed on Jupiter when the Shoemaker-Levy comet impacted its atmosphere, or it could be artificial in origin.

There is no top priority list per say, because any planet can potentially produce AMP or have an ET robotic probe orbiting it. In 2005 each planet can be observed for several hours each day. In terms of the diversity of the targets, Saturn is a good candidate because it has several moons which vary in size and possess a range of features.

In 2005 there are three major planetary conjunction alignments, with < 2.5 degrees of angular separation, that can be observed. The conjunctions of Jupiter+Uranus, Jupiter+Neptune and Saturn+Neptune offer an opportunity to carry out between 104 and 133 days of targeted observations to search for AMP or URS.

Conjunction events are rich with targets because there are numerous bodies (i.e., moons) within the target area. In the case of conjunctions, the observation needs to include some kind of direction-finding capability. Configuring the radiotelescope array to function as a phase-comparison monopulse antenna will allow the determination of the angle-of-arrival of the signal.

In that way, some information about the origin of the AMP can be learned, possibly revealing if it was near a specific moon, a planet, or in motion.

AM: You have noted elsewhere that "It might be argued that if an ETI probe were within our solar system and transmitting a signal toward Earth, intended for us or not, that we would detect it with the current SETI effort. No one with a working knowledge of the current SETI effort would accept this allegation for any frequencies other than the 1 to 3 GHz band (particularly the 18 and 21 cm lines)". This raises the question of how to hail a probe. Are there considerations in selecting a frequency (such as the water-stretch band) that might differ between what would be a planetary transmission vs. a parked spaceprobe as you describe the classical listening channels, or frequency selections by Morrison, et al.?

SS: The hydroxyl (OH) and neutral hydrogen emission lines identified by Cocconi and Morrison were good first choices for an artificial ET hailing frequency. However, after 44 years of searching around those frequencies no confirmed signals have been found, and there is some doubt whether any frequency is "magic."

It's interesting to note that in 1974 the Arecibo message was transmitted at 2380 MHz, a frequency well above the "water hole" band. In Earth's first "Active SETI" attempt we didn't transmit at a well known and preferred frequency of either 1420 or 1665 MHz. Furthermore, 2380 MHz is the second harmonic of no particularly special frequency. The Arecibo transmitter was designed for S-band planetary radar experiments and SETI used it because it was available.

ET may make a similar decision for transmitting a beacon frequency - a decision based solely upon the economy or convenience of operating their transmitter at some given wavelength. Hence, searching much wider frequency swaths, and higher bands like 12 to 60 GHz, is a worthy decision.

The Allen Telescope Array will be optimized to search between 1 and 11 GHz, which covers a significantly wider band of electromagnetic spectra than has been previously searched.

Selecting a preferred search frequency for robotic probe emissions within the solar system is difficult because the motives and electromagnetic emissions of a probe are unknown. If a probe becomes aware of our civilization and desires to engage us in communication, it can scan our planetary emissions and choose a quiet frequency or spectral band to get our attention.

There are several protected frequencies used for radioastronomy (e.g., Carbon Sulphide at 97.981 GHz) that a probe might try hailing us on with a low power beacon.

Hypothetically, if the probe were small and could only accommodate a small, say 0.1 m aperture directional antenna, then we might expect its beacon frequencies to be high in the mm-wave region. Using a system limited to detecting energy between 1 and 11 GHz could overlook higher frequency artificial emissions if they occurred. On the other hand, if the probe were larger and emitted gamma bursts from its propulsion system, we might want to concentrate on electromagnetic byproducts of such events which may be very broadband.

AM: The Arecibo telescope's beam, as used for Project Phoenix, covers all of a 100 light-year-distant solar system out to two thousand times the Earth-Sun distance. One targeting priority for SETI using the Allen Array will be based on similar suns to our own within 90 light years. Can you expand on why current SETI strategies usually treat nearby beacons as part of the extended solar system exploration program, rather than a target that is actively sought in its own right?

SS: In the microwave SETI hypothesis the targets and search space lie outside the solar system - among the stars.

Within the confines of the traditional microwave search strategy, energy-based arguments assume that ET can't get here because it's too expensive. Another reason assumes they don't know we are here because our radio leakage has only reached about 70 light years (~4,500 star systems), and the nearest civilization is expected to be farther away.

Conservatism and self-imposed constraints have kept traditional SETI from searching within the solar system.

Sometimes during SETI surveys solar system targets have been within the antenna's main beam or a sidelobe of the beam. With a single radiotelescope having limited direction-finding capability, it's difficult to know from just the Doppler drift, or Gaussian shape of the energy detected, whether the detected signal originated inside or outside the solar system.

An example of a case where a planet-moon system transited a SETI antenna beam has been found.

During the five year Megachannel Extraterrestrial Assay, or META-I, search, 37 candidates were identified that exceeded the average 1.7x10-23 W/m2 sensitivity of the receiver (see Horowitz and Sagan, "Five years of Project META - An all-sky narrow-band radio search for extraterrestrial signals", Astrophysical Journal, Part 1, vol. 415, no. 1, pp. 218-235, 1993).

None of the signals were detected upon re-observation. Two of these candidates were detected when the antenna was pointed conspicuously close to Saturn and its moons.

During the re-observation of these two signals Saturn and its moons had shifted in right ascension and declination and were not in the antenna beam when the coordinates were checked. It is unclear whether the researchers even realized that a planet-moon system had been in the antenna beam during 2 of the 37 candidate events.

In hindsight they should have done an immediate follow-up examination of Saturn using a drift-scan mode, and looked for fast-moving emissions or signals with unusual Doppler drifts.

Analysis of the Doppler drift of a signal is primarily used to determine its relative motion (linear or rotational). Traditional SETI searches compensate the detected signals for the CMB, GBC and LSR inertial frames which don't apply to solar system targets. These compensations if added to signals originating from solar system targets may cause them to be rejected if the Doppler drift doesn't fit the expected sidereal rate.

Solar system targets can be included in microwave SETI observations, but while they're within the antenna beam the detected signals must be processed differently to try to determine if the signal was close or far away.

AM: Are there any constraints in a solar system search based on the location of the array itself? For instance, what is visible in the Northern Hemisphere is a current constraint on sky searches from Arecibo, and the question is are there any nearby objects that are not accessible from the orbital mechanics, such as the far-side of the moon as a trivial example?

SS: All the solar system bodies, with the exception of Pluto, lie very close to the ecliptic plane. They are all visible during certain times of the year from the Hat Creek (i.e., ATA) latitude. One parameter that does affect the search is the amount of time a body is observable during the year. The observing time for a planet also depends on where it is in relation to the sun. Mercury has the fewest number of observable days during a given year because it has a higher frequency of a small angular separation from the sun, or transits the sun more than the other planets.

Planets that come within a separation angle of less than 3 degrees from the sun are not deemed observable because thermal noise from the disk of the sun would dominate any weak emission that might emanate near the body under observation - the SNR would be unacceptable. The Sun-Earth-Planet separation angle is an observational constraint. Another constraint is the time period a body is visible to two antennas. Independent verification of a signal requires two or more antennas detect the same signal from the same astronomical coordinates.

Arecibo is a great resource for verifying signals that are detected by the ATA. However, the Arecibo antenna can't observe certain bodies when the ATA can and vice-versa. If a signal happens to be detected when the Arecibo can't observe the coordinates where the ATA is pointed then real-time verification will be a problem. In that case, other antennas will need to be sequestered to assist in verifying the signal.

AM: Proponents of this style of search target list are looking for technical markers of probes, proxies, machines, craft and phenomenon of suspected extraterrestrial origin which are inside heliocentric radius of the Earth's Solar System, or near the Earth. In such a nearby or solar system search, does a parked probe need a definitive purpose that may not involve such passive or unintentional announcements? In other words, does it have to be an active beacon?

SS: No, the Solar System SETI strategy attempts to detect any signals from an ET probe artifact whether they are leakage or intentional.

Note that "intentional" and "beacon" usually mean a signal that's designed to be detected by us or some other civilization. There is no reason to assume that a robotic probe would come to our solar system strictly to communicate with Earth.

SETI authorities point out that contact via microwaves or optical can occur over interstellar distances and does not necessitate launching space probes. If that's true, then intentional signals are less likely than leakage.

If intentional signals are emitted they may not be for our consumption, but rather meant to signal someone or something outside the solar system. If advanced robotic probes transmit their signals in the infrared-visible-ultraviolet [IR-VIS-UV] wavelengths using a pulsed laser then leakage from microwave pump amplifiers used to drive the laser may be detectable.

An active beacon would be a great find, but leakage is more likely.

AM: How does one include an error analysis to exclude what would be terrestrial 'leakage' of radiation in a nearby search? You include for instance the emissions from comets and meteors which presumably would be too weak when pointing to a distant object. Does this differ from traditional far-away search problems in any fundamental ways?

SS: Terrestrial noise and interference affect all microwave SETI efforts. Solar System SETI efforts must leverage off the same interference analysis and filtering techniques of modern SETI efforts and those proposed for the ATA. Interfering signals in the vicinity of solar system targets could be our own interplanetary probes, like Cassini, or probes orbiting asteroids or comets.

These signals have well known carrier frequencies and sidebands so they can be eliminated when detected. Noise bursts detected while targeting the gas giants could be caused by natural impact events, like small comets or asteroids. These are notable because we need to discriminate between them and something possibly artificial. Looking for periodicity in spurious noise bursts is one way to determine artificiality which must be followed immediately by testing for manmade interference.

We must also not forget pulsars which are both periodic and energetic in the microwave region. If periodic pulses are detected while observing a solar system target, these can be checked against the periods of known pulsars. It should be fairly straightforward to determine if the object is a newly discovered pulsar because its motion would be sidereal relative to an orbiting planet.

Another analysis involves examining the polarization of the signal. Naturally occurring emissions should have random polarizations and not be coherent. Detecting a statistical weighting of more energy in one polarization (e.g., right-hand) implies a non-natural source.

Another concern is the constellation of spacecraft orbiting Earth. Some satellites will undoubtedly transit the SETI antenna beam, but these kinds of interferers affect near and far searches alike and are treated the same and rejected.

AM: One of the most interesting examples from our own solar system exploration was the Galileo flyby of Earth. The spacecraft initially could not verify that the earth itself was hospitable from a chemical spectrum, because the closeness of the Earth saturated its detection and it was calibrated for the more intense Jupiter flyby yet to come. Is this example illustrative of differences in a nearby vs. faraway search strategy?

SS: The Galileo flyby example highlights concerns about using an instrument to observe a certain target it was not calibrated for nor intended to observe.

Using the ATA to observe solar system targets when it was designed for far away searches is not an obstacle for Solar System SETI. Indeed, some of the targets in the solar system should be avoided during SETI searches while using highly sensitive phased arrays.

The most obvious target is the sun. Other than using the sun as a known "hot body" noise source for determining system noise calibrations, it serves to lessen the sensitivity of the receiving system and should be avoided.

The moon and Jupiter are also sources of hot body noise. While observing these targets some amount of degradation of the system noise temperature is expected, but it is not enough to saturate the detection system with noise.

Furthermore, using a configurable phased array allows nulls to be placed on noise sources. Non-thermal radio sources like Cassiopeia A, the Crab Nebula, M87 and Cygnus A can be nulled out, if necessary, during observations where they could transit the antenna beam during a targeted search of some region in the solar system.

Radio telescopes like the ATA are wonderful because they can be configured so that during solar system searches noise sources can be rejected. Following the construction of the ATA is the Square Kilometer Array (SKA) which could also serve the needs of traditional SETI and Solar System SETI.

AM: Are there future plans for expanding your research?

SS: Beyond that of a published paper on the strategy ["Solar System SETI Using Radio Telescope Arrays"], it's too early to predict whether Solar System SETI can expand or not.

Carrying out solar system observations with the ATA or SKA for fundamental radioastronomy research should be acceptable to everyone in the scientific community since the intent is not to search for ETI. Radioastronomy proposals in this area will no doubt be submitted and some should be accepted. Some of the success of Solar System SETI depends on the willingness of the ATA and SKA managers to accept Solar System SETI proposals.

The first hurdle is to submit technical proposals to the ATA; get them accepted and actually secure some observing time, either prime or piggyback, to search for anomalous microwave signals. Winning observing time on the ATA opens the doors of opportunity to alternate search strategies which SETI definitely needs. There will be a lot to learn about the operation of the ATA and the implementation of Solar System SETI observational experiments.

Solar System SETI research can only expand and grow if we gain practical experience in carrying out the search. If SETI doesn't carry out alternatives due to conservatism surrounding what's out there, we'll never know if ETI has discovered our solar system or not.

What's Next
Projected to come online in 2005, the development of the Allen Telescope Array is marked by many innovations crafted with the express purpose of building a world-class state-of-the-art astronomical facility at a fraction of the price of existing radio telescopes.

Although the physical structure of the Allen Telescope Array is dominated by the network of many small dishes--or "metal in the meadow"-- what truly makes it distinctive is that it will be one of the first digital radio telescopes to allow astronomers to look at completely different frequencies at the same time, and to observe completely different parts of the sky concurrently. This means that the Allen Telescope Array is not just one instrument, but in effect, many.

There are no currently planned searches for solar system SETI.

When the Allen Telescope Array turns on sometime next year, it will be capable of searching to the farthest of 17,000 nearby habitable stars, just beyond 300 parsecs (a distance of 978 light-years from Earth). 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.

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