Even at this early stage in our space faring age, humanity has already begun sending probes that will eventually reach other solar systems, even if that was not their original intention. Five robotic explorers - Pioneer 10 and 11, Voyager 1 and 2, and New Horizons - are all on escape velocities out of the solar system, and might someday enter another one. They will no longer be operational at that point, but they serve as a proof of concept that spacefaring civilizations do indeed build interstellar probes. Which raises the obvious question - has anyone else sent their own robotic explorers to ours? In a recent paper, published in the Proceedings of the IAU Centenary Symposium, astronomer T. Joseph W. Lazio, points out a painful truth - we still have no idea, and our technology will need to get much better if we plan to find out.

To break down where we currently stand in terms of our ability to detect technosignatures, Dr. Lazio uses a four-quadrant matrix originally developed in a W.M. Keck Institute for Space Studies report. In this framework, artifacts are structured by where they are and whether they’re still working.

Passive probes - dead or inert objects just passing through the solar system, likely on a hyperbolic trajectory Active probes - operational spacecraft using either internal or solar energy to conduct measurements, transmit data, and maneuver around. Passive Surface Artifacts - Impact remnants or leftover hardware silently sitting on the surface of a moon or planet Active Surface Artifacts - Still operational machinery on the surface of a planet or asteroids, such as a mining or automated monitoring station

In the paper, Lazio tests a single falsifiable hypothesis - One of more physical extraterrestrial technosignatures are present in the Solar System today. Can humanity, at its current technological stage, falsify this? Not even close.

Fraser discusses finding technosignatures with Dr. Jacob Haqq-Misra

To be fair, we have a decent chance of finding a dead interstellar probe if it's still floating through space. The problem is differentiating between a probe and a completely natural asteroid or comet. Every time we receive a new interstellar visitor, such as 3I/ATLAS, plenty of people, even well-respected scientists, immediately jump to the conclusion that it’s an alien craft sent here to monitor us. But what really showcases how hard this can be is the case of object 2020 SO.

Detected back in 2020, this object was moving in a strange orbit, though it was originally classified as an asteroid. Since it was so anomalous, scientists decided to take a closer look, and realized the near-infrared spectra of the object was precisely what stainless steel and polyvinyl fluoride would look like. In other words, 2020 SO wasn’t a space rock - it was in fact a Centaur rocket booster from NASA 1966 Surveyor 2 mission.

So the issue with finding free-floating “passive” artifacts isn’t so much detecting them outright, it's proving they aren’t just one of the millions of other passive rocks floating throughout the solar system. But what about artifacts on planets? After all, we’ve been able to find parachutes and even rover tracks left behind by our ground-based probes. We should be able to find an alien artifact using similar techniques right?

Fraser discusses the general search for technosignatures

Possibly - the problem is we haven’t really checked everywhere in the solar system as closely as we did to find those specific technological artifacts. And in fact, most of the solar system is covered at an average resolution that would fail to catch anything other than artifacts that absolutely dwarf anything humanity has made to date. On Saturn’s satellites, for example, our resolution is only at the scale of around 1 kilometer per pixel. Even on the Moon, where we have 0.5m per pixel resolution capabilities, only a small fraction of the lunar surface has been covered in that detail.

Even if we did have the resolution, finding it would mean that the artifact itself remained intact. Obviously if something flew into Jupiter itself it’s probably lost permanently. But even on the comparatively benign surface of Mars, micrometeorite impacts, solar radiation, and dust storms can wear down a passive surface artifact within a few million years - a blink of an eye in the history of the solar system.

Active probes, on the other hand, should at least in theory be easier to find. One key way is that they have to abide by the laws of thermodynamics - or at least we assume they do. That means they have to get rid of the waste heat generated by their activity - meaning they would look “hot” relative to what we would expect for a passive system. Large scale surveys like WISE have already found several objects that have anomalous thermal properties, but there are plenty of intricacies with modeling space rock temperatures, so there’s no way to make a definitive call permanently, and we don’t have the resources to monitor each of those anomalous objects closely enough to truly determine what they are.

Fraser discusses the search for technosignatures with Dr. Chenoa Tremblay

But WISE isn’t the only survey that will profile these small objects. The Vera C. Rubin Legacy Survey of Space and Time, SPHEREx, and the Near-Earth Object Surveyor Mission are on track to provide millions of highly detailed object profiles. Sorting through these data treasure troves could lead to highly anomalous objects that are worth a closer look.

Until we get an actual probe out to one, though, we likely won’t be able to say for certain whether it's natural or not. But the Search for Extra Terrestrial Artifacts (SETA) might finally be getting its time to shine - or observe as the case may be.

Learn More:

T. J. W. Lazio - Solar System Technosignatures

UT - The SETI Institute Releases Technosignature Report on 3I/ATLAS

UT - What Technosignatures Would Interstellar Objects Have?

UT - The Best Place to Look for Alien Megastructures Might Be Moon Dust