One objection to SETI is that it is not falsifiable — there is no point at which a lack of signals can prove that extraterrestrial civilizations do not exist. But there are some aspects of SETI that can be falsifiable. Consider a class of objects near enough for us to investigate not only with listening efforts but with probes, one small enough to be thoroughly covered, and one most people know almost nothing about. Could these offer a listening post for ‘Bracewell probes,’ a way of watching the development of our culture and reporting home to ETI? And if so, could we combine SETI with METI to advance both disciplines without compromising our own security?

If the idea of nearby probes seems far-fetched today, it was even more so when Ronald Bracewell advanced his ‘sentinel hypothesis.’ Bracewell took the question of SETI and stood it on its ear. That was no mean feat in 1960, for SETI was just being born in that year through the efforts of Frank Drake at the Green Bank instrument in West Virginia. While Drake was, reasonably enough, asking whether we might pick up signs of an extraterrestrial civilization around another star, Bracewell had begun to wonder whether there might be a different way to study an alien culture. A long-lived probe could be planted in any system under investigation.

Image: Stanford’s Ronald Bracewell, who in 1960 advanced the idea of long-lived probes investigating other planetary systems. Credit: Stanford University.

Add Von Neumann-style self repair and such an object might stay on sentry duty for millennia, for aeons, all the while returning useful data about the changes occurring on an interesting habitable planet. And if a civilization arose on that planet and reached the level of electromagnetic communications, then the probe could be programmed to make contact, at whatever threshold its builders chose.

Jim Benford has been thinking about Bracewell probes and their possibilities of late because they offer advantages over traditional forms of SETI. For one thing — and this is a huge advantage — a contact once made with a local probe could initiate dialog in more or less real time, without interstellar lightspeed delays, although of course we would be querying an intelligence that was itself subject to those delays if it communicated with its home world.

Image: Plasma physicist Jim Benford (Microwave Sciences).

The question is interesting enough that is has inspired some top-notch science fiction, in particular David Brin’s novel Existence, where the idea is extended to not one but a series of different probes at work in Earth orbit and in the asteroids. But it would be Michael Papagiannis who in 1978 wrote seriously about the asteroid belt as a possible venue for such ‘lurkers,’ (to use Benford’s term). Benford is not sold on the asteroid belt as a target.

For we also have an all but undiscussed body of targets that can be called co-orbital objects with Earth. These small objects approach the Earth closely and on an annual basis, for they have the same orbital period as Earth. We might study them for signs of artificiality through spectroscopy in the visible or near-infrared as well as pinging them with radar or other signals.

What strange orbits these objects occupy, with some in so-called ‘horseshoe’ orbits — these can actually become quasi-satellites for a time before returning to earlier orbital parameters. Have a look at a horseshoe orbit.

Image: A horseshoe orbit. No wonder these objects took so long to find. Credit: James Benford.

And from another view:

Image: Plan showing possible orbits along gravitational contours. In this image, the Earth (and the whole image with it) is rotating counterclockwise around the Sun. Credit: Wikimedia Commons.

As Benford explains, think of a quasi-satellite as an object in a 1:1 orbital resonance with a planet, so that the object stays close to the planet over many orbital periods. Outside the Hill sphere (that region where an astronomical body dominates the attraction of satellites), quasi-satellites cannot be considered true satellites. Instead, while their period around the Sun is the same as the planet, they seem to travel in an oblong retrograde loop around it.

Beyond horseshoe orbits we also find ‘tadpole’ and ‘quasi-satellite’ orbits as shown in the figure below. Here we find stable orbits for centuries and possibly longer, much longer. Co-orbitals include Cruithne (3753), a 5-kilometer object with closest approach to Earth of 0.080 AU — interestingly, this one experienced a close encounter with Mars in historical timescales, around the time of Periclean Athens. Another is Earth Trojan 2010 TK7, which oscillates around the Sun-Earth Lagrangian point L4, and 2016 HO3, which Benford describes as “currently the smallest, closest, and most stable (known) quasi-satellite of Earth,” with a minimum distance of 0.0348 AU. A number of other quasi-satellites are known.

Image: Three types of co-orbital orbits. Credit: James Benford.

How might we investigate these objects with the tools of SETI, and why? Benford calls for a multi-year program of observations in radio and optical wavelengths as well as planetary radars, with the main burden of the work falling upon the Lick Observatory and other platforms involved in the Breakthrough Listen project. Here we’re looking for size, shape, rotation periods and spectra. At the same time, he urges SETI observations of this range of objects.

Planetary radar also comes into play, and with an interesting consequence. From the paper:

These objects have not been pinged or imaged by any planetary radar as yet. Recent developments in planetary radars have shown they can detect the presence and trajectories of spacecraft in lunar orbit, even though their size is a few meters. Whether these radars are sensitive or powerful enough to get a return signal from any of the presently known co-orbital objects requires analysis. In any case, they can ‘ping’ the objects, meaning that a signal reaches there but the return signal may be too weak to detect at Earth.

Here the Bracewell idea comes into full view:

If there is an ET probe there, it might sense that it had been noticed by us.

What an interesting campaign Benford has in mind. It includes simultaneous use of planetary radar on the target and SETI observations. Readers at this point may be recalling that Benford is on the record with strenuous objections to METI, the idea of Messaging to Extraterrestrial Civilizations, given the limits on what we know about what is around us in the cosmos, and the need for international agreement on how to proceed, as opposed to sending signals to the stars in random bursts of activity and with wildly varying content.

Yet here we are talking about an activity that, in the unlikely event there is a probe in our own Solar System, could conceivably activate it and cause it to respond. The Bracewell probe is front and center here, recalling Duncan Lunan’s 1974 proposition that a Bracewell probe could be the cause of long-delayed echoes of radio transmissions heard in the 1920s. Benford notes that the phenomena Lunan identified have subsequently been explained as unusual propagation patterns in Earth’s magnetosphere. But it’s interesting to see Benford’s response to the idea of METI in this new context:

This would be ‘Active SETI’, which could solicit a response from a hypothetical probe. This does not incur the objections to sending interstellar messages, messaging to ETI (METI), because any such alien lurkers would already know we are here. Of course, this is at very short range compared to the interstellar ambitions of METI enthusiasts. We presume that Lurkers already know that we have radar, but might not respond to a single radar painting such as we have done to many asteroids. If we want to send a message, as Paul Davies suggested for the LaGrange points in 2010, how would a signal be designed to elicit such a response?

An interesting question indeed, and as the author points out, actually working on a near-term use of METI at a nearby target could benefit research into message creation and drive the field forward. The problem is an easy one to state: What kind of message would one send to a lurking probe that would ‘awaken’ it to the possibility of communicating with us?

As to falsification and SETI:

In my view SETI has suffered from being seen as somewhat nonscientific. That’s because it doesn’t offer itself as a study with falsifiable propositions, which is the very definition of science, as Popper said.

I advance a falsifiable proposition: “There exist in near-Earth space extraterrestrial probes which are observing Earth and it may be possible for us to find and contact them.

This proposition can be disproved. We can observe them, ping them with radar, transmit messages to them, send robotic probes to them and visit them with human spacecraft missions.

What a lively concept. We blend SETI’s listening to the stars with astronomical imaging and spectroscopy, while simultaneously turning METI into what Benford calls a ‘local experiment.’ And as we do this, our efforts at studying co-orbital objects advance the cause of astronomical science, which is engaged in the great process of mapping the entire Solar System.

The paper is Benford, “Looking for Lurkers: Objects Co-orbital with Earth as SETI Observables,” submitted to the Astrophysical Journal (preprint).