Review article

Concepts for future missions to search for technosignatures

A variety of new developments in the last decade, such as the blossoming of exoplanet discovery and characterization science, as well as forthcoming improvements in observing capabilities from space and the ground, have opened new and exciting possibilities in the search for life in the Universe. The search for biosignatures, observational evidence of extraterrestrial life, has become one of the main science drivers for many new space missions and large telescopes. In parallel, these advances have motivated a renewed interest in the search for technosignatures (hereafter TS), defined as observational evidence for the existence of industry or technology in the Universe [for a more precise definition see 1].

In 2018, NASA began to consider including TS research as part of its research portfolio. In preparation for this, the agency organized a meeting (“NASA technosignatures workshop”,¹ held in September 2018 at the Lunar and Planetary Institute, Houston, USA) to learn about the current state of the art in the field. The workshop participants produced a report with an overall view of current possibilities in non-radio TS search [2]. In August 2020, a second workshop was sponsored by NASA (“TechnoClimes 2020”, Blue Marble Space Institute of Science²) with the goal of producing a research agenda for TS science. This paper presents some of the conclusions of that workshop. Our aim is to establish an agenda of projects or space missions that would realize new avenues to search for TS.

TS research has deep synergies with other areas of astronomy. It is often the case that TS science can be done in “commensal” mode, in which it takes advantage of data that are being acquired for other purposes (this is the same mode that had earlier received the less kind denomination of “parasitic”, e.g. [3]). For instance, missions to observe exoplanets via transit photometry are also useful to search for transit TS at almost no extra cost. It is important to exploit such opportunities to the fullest, and another report from the TechnoClimes 2020 workshop explores such possibilities [4]. Here we take a different approach. We present a set of projects and mission concepts that would explore new opportunities in this field, independent of any current limitations on funding or resources. The purpose of this approach is to provide an overview of the capabilities of current and near-term technology to conduct a systematic search for TS and present some ideas for conducting TS-specific searches. In the end, all of the concepts discussed in this paper would provide ancillary benefits in the form of extremely valuable data for other fields of astronomy, astrophysics and planetary sciences.

Even though we do not take them into consideration here, it is obvious that real-world limitations (mainly funding) are likely to play a major role in any prioritization of these concepts and, in fact, some exploratory work has been conducted in this direction [5]. Those factors would need to be carefully balanced against the ancillary benefits for other areas of science and, more importantly, some inevitably uncertain guesses about the chances of success. For the latter, it would be important to consider the relevant TS in the context of the “nine axes of merit” [6], as well as any other useful parameterization.

There are many different types of TS that might be produced by extraterrestrial technology and leave some observable evidence. Some may be intentional, such as radio or laser signals, transit beacons, or other intentional attempts at interstellar communication. Others are the byproduct of some industrial or engineering activity. Broadly speaking, for us to detect such signals at interstellar distances with our current sensitivities, such signals would need to be stronger than those produced by current human civilization, particularly the unintentional ones. It seems reasonable to assume that, as observing capabilities improve, TS searches would be increasingly sensitive to more moderate levels of technological activity. For instance, scientists in the 1960s were thinking about detecting star-system scale megastructures [7], whereas modern technology could conceivably detect planetary-scale engineering [8]. We may view this as a “technology balance” between the observer and the target technological capabilities. We are currently at the cusp of being able to detect our own TS at typical interstellar distances [9], [10]. Only those species that have constructed or developed technologies much larger or more luminous than any of our own can be detected with our current astronomical infrastructure. As this infrastructure becomes more sensitive, we will be able to detect technosignatures closer in scale to our own. It seems unlikely that civilizations with a relatively low level of technological development would enter into contact with each other, since that would require either very high sensitivities or highly visible engineering [11], [12]. This technology balance is the result of an observational bias by which “less advanced” civilizations lack the sensitivity needed to detect other civilizations unless they have built very large or luminous structures. In addition to this, a recent paper [13] presents an independent calculation that leads to the conclusion that our first contact is likely to be with a more advanced civilization. The argument for this “contact inequality”, as they call it, is based on statistical considerations about the lifespan of technological civilizations and does not take into account the observational bias. Therefore, these two independent arguments go in the same direction, reinforcing the notion that this is the most likely scenario for a first contact.

Perhaps an even more provocative idea is to consider a type of TS in the form of artifacts, for instance interstellar probes that might have been sent into the solar system a long time ago (potentially up to billions of years in the past), perhaps during a close encounter of our Sun with other stars. Such artifacts might have been captured by solar system bodies into stable orbits or they might even have crashed on planets, asteroids or moons [14], [15], [16]. Bodies with old surfaces such as those of the Moon or Mars might still exhibit evidence for such collisions. Systematic searches, which have not been conducted up to now, would at the very least provide upper limits on the existence of solar system artifacts [17], [18] and could do so at relatively low cost.

Even negative results from volume-limited surveys for certain TS would be valuable because their absence may establish quantitative upper bounds on certain types of technologies or developmental stages of civilizations in the solar neighborhood. Such results may have multiple explanations including, but not limited to, the relative rarity of intelligence in the universe and may have profound implications for humanity’s future [19].

Nevertheless, such systematic observations would provide enormous ancillary benefits on solar system research and advance our knowledge about the objects being scrutinized. In this paper we consider a number of different mission concepts covering various types of TS. In addition to the TS research, we discuss other science goals that may be pursued with each one of these mission concepts.