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Probing for ETI's Probes in the Solar System
by Scot Stride
sstride @

Working at JPL for many years and subscribing to its charter tends to affect a person's worldview. 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 ETI might carry out galactic exploration and the construction of interstellar robotic probes.

It turns out that a great deal of research and writing on the subject of ETI probes has been done (Freitas, et. al), most of it within the context of complementing radio SETI. Presently, the scientific community, news media and general public associate the term SETI with large radio telescopes and the search for weak signals from far away. Most people know there is a possibility of radio communication (CETI), but many don't realize the unfavorable odds of it working in practice over vast distances and multi-generations of human participants. Professional SETI scientists and engineers know full well the potential of microwave/mm-wave for both radio astronomy and for deep space telecommunications.

The SETI focus so far has been on the detection of narrow-band beacons or leakage from ETI civilizations, complemented by radio astronomy observations and mappings to better understand the origins of ET life. However, the SETI lenses that focus so clearly on the task of searching for far away signals are philosophically out of focus when it comes to searching for ETI telecommunications signals that may originate within our solar system.

Some time beginning in the early 1970's LDE's (Long-Delayed-Echoes) were a hot topic of discussion. First recorded in the 1920's by Burrows and later advocated by Lunan in the 1970's, these signals were first thought to be radio returns from ETI robotic probes residing in the solar system. It was later showed by Lawton et. al. that these echoes were likely caused by plasma and dust the Earth's upper ionosphere. LDE's are a surprising and unusual natural phenomena that is not fully understood, but they are far too ambiguous to be from ETI robotic probes.

Russian scientists have tried some limited searches for probe radio signals within the solar system. Freitas and Valdes did an optical search for probe artifacts (SETA) at the five earth-moon-sun libration points. These searches, also done within the SETI context, were primarily negative and inconclusive.

This fleeting, yet serious, research was not embraced by mainstream SETI scientists and for the most part ignored. It's chilling to think what the reaction would have been if Freitas and Valdes had detected and verified a robotic probe stationed at L5. Aside from these few studies, nothing else has been done within the SETI context to actively search for radio signals from possible ETI probes in the solar system, but there is room for hope.

Presently at least one SETI telescope is periodically observing robotic probe transmissions emanating just beyond 75 AU. These are not ETI, but from NASA's Pioneer 10 spacecraft. Pioneer 6 has been observed occasionally, as is Galileo when it's Jovian orbit is suitable. Detection of these S-band signals demonstrates that both radio and optical SETI have the capability to search the solar system for signals that could be considered ETI in origin. ETI probe radio transmissions would be clearly distinguishable from those of our own deep space robotic probes, because we know the locations, frequencies and Doppler of our spacecraft.

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). Millimeter-wave or optical signals from an ETI probe may be illuminating Earth right now, and we would never know it.

Why not? Because a wideband, all-sky survey is not actively underway. This kind of effort, which I term Solar System SETI (S3ETI), was briefly carried out at the JPL Deep Space Tracking Network during 1992-93, as a part of the NASA HRMS (High Resolution Microwave Survey) effort. At the time, the intent was not to search for ETI probe microwave transmissions within the solar system, but it certainly could have found them if they were there and between 1 and 10 GHz. Nothing was detected, but one year is not very long to find much of anything.

Project Argus, under the direction of the SETI League, is now diligently trying to recapture some of that sky coverage in the 1 to 3 GHz band. The goal of Argus is 100% sky coverage continuously; quite different from the JPL HRMS effort. This goal, if met even at 1-3 GHz, would outperform even the HRMS effort in terms of sky coverage. If this search were extended in frequency to cover 1 to 40 GHz, Argus would be the best bet for detecting strong leakage, a powerful beacon or robotic probe transmissions in the solar system.

It is doubtful that ETI probes would transmit telecommunications signals below 2 GHz, and in all likelihood the preferred frequencies are mm-wave or optical. This means that if an S3ETI all-sky survey system is built it should be done at frequencies up to 40 GHz and have a receive sensitivity to a radial distance of 50 AU. Limiting the detection range to <50 AU (Pluto's approximate farthest distance) simplifies receiver design in terms of amplifiers, noise figure and integration times. There is nothing magic about 40 GHz, it is within the microwave window, and there is a moderate selection of commercial hardware available with which to build a mm-wave SETI station. Easing the range requirements allows the use of broad beam antennas (e.g. slightly flared WR28 waveguide) and inexpensive GaAs PHEMT MMIC LNA's that don't need liquid nitrogen cooling.

At 50 AU two-way telecommunication delay with a probe would be about 14 hours, allowing expedient contact between civilizations to occur if that's one objective. Were the detection range reduced and the bandwidth increased, an effort like Argus, could possibly detect radio emissions from an ETI probe in the solar system. Many members of the SETI League will admit that the prospect of detecting a telecommunications signal from a nearby robotic probe is certainly more exciting than the detection of a faraway single-tone beacon. Food for thought!

Author's Note: The opinions expressed in this editorial are personal and in no way represent officially sanctioned attitudes, beliefs, interests or policy of NASA, JPL or Caltech.

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