This is a guest post by Stephen Kerby, a graduate student at Penn State.

Imagine you are a galaxy-spanning species, and you need to transmit information from one star to another.  You can just point your radio dish at the other star, but space is big, and your transmission is weak by the time it reaches its destination.  What if you could use the gravitational lensing of a nearby star to focus your transmission into a tight beam while monitoring local probes? What if you could use this nice yellow star right here, the locals call it the Sun? What if the locals notice your transmitting spacecraft from their planet right next to the star?

Recently, there has been renewed interest among human scientists in using the solar gravitational lens (SGL) to focus light for telescopic observations (as in the FOCAL mission) or for interstellar communication (as described in Maccone 2011). A spacecraft positioned >500 AU from the Sun could collect focused light bent by the Sun’s gravitational field, dramatically increasing the magnification of a telescope or the gain of a transmitter for a point on the exact opposite side of the Sun (the antipode). The picture below shows how the SGL could be used for transmission of an interstellar signal, and the arrangement can be reversed to focus light onto a telescope.

In the Astro 576: “The Search for Extraterrestrial Intelligence” graduate course at the PSU Dept. of Astronomy and Astrophysics, I participated in a collaboration with over a dozen colleagues to examine a parallel question; might an extraterrestrial intelligence (ETI) be using an SGL scheme to build an interstellar transmission network? If so, we might be able to detect the transmitting spacecraft if its transmissions intersect the Earth’s orbit (as proposed by Gillon 2014). Such a spacecraft would visible opposite on the sky from its interstellar target and would be most visible if it is along the ecliptic plane (the same plane as Earth’s orbit).

While the collaboration focused on conducting a prototype search at the antipode of Alpha Centauri using Breakthrough Listen at the Green Bank Telescope (paper forthcoming!), I also conducted a side project to make predictions about what sort of engineering would go into such a transmission scheme.  A paper based on that project and co-authored by Dr. Wright was recently accepted for publication in the Astronomical Journal and is now available on the arXiv (http://arxiv.org/abs/2109.08657).

Initially, my project set out to tackle a broad question; it’s physically possible to use the SGL for an interstellar transmission, but is it productive from an engineering standpoint? After all, if an ETI needs to overcome myriad challenges to get the SGL transmission system online, it might be easier just to skip the mess and be more direct.  If we can quantify the challenges facing an SGL scheme, we might be able to predict which stars might be included in an ETI transmission network and whether our Sun is a likely host.

First, we focused on the difficulty of maintaining an alignment with the target star. Normally, when transmitting using a radio dish, you need to point the dish to within a few arcminutes of the target, depending on the gain (degree of focus) of the outgoing beam.  However, the impressive gain boost of the SGL means that the interstellar transmission could be only an arcsecond across, 60x narrower and much more intense. A spacecraft trying to aim at a target star needs to stay aligned with that much precision.

We soon found that there are numerous dynamical perturbations on the spacecraft-Sun-target alignment.  First, the Sun is pulling the spacecraft inwards; if the craft drifts closer than about 500 AU to the Sun, it can’t transmit using the SGL.  Next, the Sun is being jostled around by its orbiting planets (shown in the GIF below); the spacecraft needs to expend propellant to counter these motions, coming out to 10x greater than the inwards force. A couple of linear effects like the proper motion of the target star are small corrections as well.

This has implications for local artifact SETI searches. While the Sun has several perturbations (mostly the reflex motion from Jupiter), it is a much better host for an SGL than a star with a close binary companion or a close-in giant planet. Close binary systems like Alpha Centauri and Sirius are terrible hosts for SGL spacecraft because of the reflex motions from the other stars in the systems. If we are trying to detect an SGL interstellar transmission network, we could focus on nearby stars that are unperturbed by massive planets, like Proxima, Barnard’s Star, or Ross 154.

Next, we addressed how those challenges might be overcome.  Clearly, a spacecraft could just fire its engines and counter the perturbations to maintain the alignment with the target.  Doing a quick back-of-the-envelope calculation, we found that a modern chemical, nuclear, or electric rocket engine could maintain alignment with an interstellar target for up to a few thousand years. Table 2 from the paper shows how long different propulsion systems could resist the perturbations of the sun’s gravity (~0.5 m/s/year acceleration) or including the reflex motions imparted on the Sun by the planets (~8 m/s/year).

On a human timescale, this is a long time; Voyager 2, our longest-lived active probe, is 44 years old, and there are obviously other challenges to operating autonomously for such a long period. In artifact SETI, ten thousand years is a blink of an eye.  The universe has existed for billions of years, which means that an ETI might have activated their relay spacecraft around the Sun millions of years ago. We could only detect it actively transmitting if it has survived and maintained alignment for the whole time.

So, how could an ETI extend the longevity of their spacecraft? They could reduce the total gain of the system so that they can ignore perturbations by the planets, but that blunts the benefits of an SGL arrangement. They could use advanced rocketry like fusion engines or solar or EM sails to dramatically increase their propulsive capabilities. They could use clever navigational techniques to get efficiency in exchange for simplicity or downtime. Finally, they could just let their probes die off and fall derelict, sending along a constant stream of replacements when needed.

So, we’ve used the dynamical features of the Sun and solar system to predict a few engineering challenges that must be overcome to use the SGL for transmission or science.  Then, we used those challenges to predict what to look for during an artifact/radio SETI search at the antipode of a nearby star.  As mentioned earlier, a collaboration is analyzing observations at one such antipode.  With a few proposals flying around, it looks like it will soon be an exciting time to be a gravitational lens!

If I were an eccentric trillionaire and wanted to help detect signals from an ETI, I would fund the construction of the All-Sky-All-Time-All-Wavelengths array.  Placing millions of telescopes of all kinds around the globe and across the solar system, I could survey every single spot in the sky at all wavelengths, nonstop. Certainly, if an ETI is sending signals then we should be able to detect them with a system like that. Sadly, no amount of money in the world can make this dream a reality, so we need to narrow down our SETI investigations. We can’t look for signals all the time or at all wavelengths or at every position.

A valuable avenue of SETI research is making predictions to guide observations to those with a reasonable chance of providing valuable scientific results.  In the past, notable predictions of this type include the hypothesis of “watering hole” frequencies and focused searches on stars that can observe Earth as a transiting exoplanet. Artifact SETI, the search for signs of physical ETI technology near our own solar system, starts with educated guesses about what that technology looks like.

Of course, it’s impossible to say now whether there actually is an ETI-placed spacecraft using the SGL to transmit until we’ve surveyed more antipodes. Still, our research into the challenges of operating an SGL relay is informative both for our SETI searches, and for aspirational proposals to use the SGL for our own science.