Extraterrestrial equipment could arrive in two forms: defunct ‘space trash’, similar to the way our own spacecraft will appear in a billion years, or functional equipment, such as an autonomous craft equipped by Artificial Intelligence (AI). The latter would be a natural choice for crossing the tens of thousands of light years that span the scale of our own Milky-Way galaxy and could exist even if the senders are not alive to transmit any detectable signals at this time. Hence, space archaeology for extraterrestrial equipment is a new observational frontier, not represented in the past history of the Search for Extraterrestrial Intelligence (SETI), which focused on electromagnetic signals and not physical objects. The Galileo Project is engaged in this scientific frontier.
As an astronomer, I became interested in this subject after the observational discovery of interstellar objects. The first three interstellar objects were discovered only over the past decade (2014–2019); they include:
1. The first interstellar meteor, CNEOS 2014–01–08, detected on January 8, 2014 by US Government sensors near Papua New Guinea. It was half a meter in size and exhibited material strength tougher than iron. It was an outlier both in terms of its speed outside the Solar system (representing the fastest five percent in the velocity distribution of all stars in the vicinity of the sun) and its material strength (representing less than five percent of all space rocks). The Galileo Project plans an expedition to retrieve the fragments of this meteor from the ocean floor in an attempt to determine the composition and structure of this unusual object and study whether it was natural or artificial in origin.
2. The anomalous interstellar object, ‘Oumuamua (1I/2017 U1), discovered by the Pan STARRS telescope in Hawaii on October 19, 2017, which was pushed away from the Sun by an excess force that declined inversely with distance squared but showed no evidence for commentary gases indicative of the rocket effect. Another object, 2020 SO, exhibiting an excess push with no commentary tail, was discovered by the same telescope in September 2020. It was later identified as a rocket booster launched by NASA in 1966, being pushed by reflecting sunlight from its thin walls. The Galileo Project aims to design a space mission that will rendezvous with the next ‘Oumuamua and get high quality data that would allow it to decipher its nature. The Project will also develop software that will identify targets of interest out of the data pipeline from the Legacy Survey of Space and Time (LSST) of the Vera C. Rubin Observatory.
3. The comet, 2I/Borisov, was discovered on August 29, 2019 by the amateur astronomer, Gannadiy V. Borisov. This object resembled other comets found within the Solar system and was definitely natural in origin.
It is intriguing that two out of the first three interstellar objects appear to be outliers relative to familiar Solar system asteroids or comets.
Extraterrestrial space archaeology is engaged with the search for relics of other technological civilizations. As argued by John von Neumann, the number of such objects could be extremely large if they are self-replicating, a concept enabled by 3D printing and AI technologies. Physical artifacts might also carry messages, as envisioned by Ronald Bracewell.
Searching for objects in space resembles a survey for plastic bottles in the ocean as they keep accumulating over time. The senders may not be alive when we find the relics. These circumstances are different from those encountered by the famous Drake equation, which quantifies the likelihood of detecting radio signals from extraterrestrials. That case resembles a phone conversation in which the counterpart must be active when we listen. Not so in extraterrestrial archaeology. What would be the substitute to Drake’s equation for extraterrestrial archaeology in space? If our instruments survey a volume V, the number of objects we find in each snapshot would be,
N = n × V ,
where n is the number of relics per unit volume. Suppose on the other hand that we have a fishing net of area A, like the atmosphere of the Earth when fishing meteors. In that case, the rate of new objects crossing the survey area per unit time is:
R = n × v × A ,
where v is the characteristic one-dimensional velocity of the relics along the direction perpendicular to that area.
For life-seeking probes with maneuvering capabilities, the number density n may be higher in the vicinity of habitable planets. Correspondingly, in the outskirts of planetary systems such probes are more likely to possess plunging orbits aimed radially towards the host star. In that case, the abundance of interstellar objects could be overestimated considerably by assuming an isotropic velocity distribution for detections near Earth.
Both n and v are likely to be functions of the size of the objects. NASA launched more small spacecraft than large ones. In addition, the launch of faster objects increases the specific energy requirements and therefore may be restricted to smaller objects that are more challenging to discover. Astronomical searches often target speeds of several tens of kilometers per second in the vicinity of Earth, as they are characteristic for asteroids or comets bound to the Sun. Advanced propulsion methods, such as light sails, could potentially reach the speed of light, which is four orders of magnitude larger. Fast-moving objects might have been missed in past astronomical surveys, and should be considered in LSST data. Humanity’s accomplishments thus far are modest. Over the past century, NASA launched five spacecraft that will reach interstellar space within tens of thousands of years: Voyager 1, Voyager 2, Pioneer 10, Pioneer 11 and New Horizons.
The detection threshold of surveys which rely on reflected sunlight sets the minimum size of a detectable object as a function of its distances from the observer and the Sun. Moreover, comets are more easily detectable than non- evaporating objects, because their tail of gas and dust reflects sunlight beyond the extent of their nucleus. Meteors, on the other hand, are found through the fireball they produce as they disintegrate by friction with air in the Earth’s atmosphere. This makes meteors detectable at object sizes that are orders of magnitude smaller than space objects.
For example, CNEOS 2014–01–08 was merely ~0.5 meter in size whereas a sunlight-reflecting object like ‘Oumuamua was detectable within the orbit of the Earth around the Sun because its size was ~100−200 meters. The nucleus of the comet Borisov was ~200–500 meters in size, and its evaporation made the comet detectable even farther because of its larger tail. NASA never launched spacecraft as big as ‘Oumuamua. Interstellar objects like CNEOS 2014–01-08 are a million times more abundant than ‘Oumuamua near Earth, but they were not detectable by the Pan STARRS survey which discovered ‘Oumuamua.
Radio or laser signals escape from the Milky-Way galaxy and reach cosmological scales over billions of years. However, chemical rockets are generically propelled to speeds of tens of kilometers per second, which are an order of magnitude smaller than the escape speed from the Milky Way. Coincidentally, this speed is sufficient for escape from the habitable zone of a Sun-like star when combined with the orbital speed of a parent Earth-like planet. Moreover, this speed is comparable to the velocity dispersion of stars in the disk of the Milky Way. As a result, interstellar chemical rockets remain gravitationally confined to the Milky-Way disk within roughly the same vertical scale-height as their parent stars, a thousand light years. The cumulative abundance of such objects would be set by an integral over their production history per star following the star formation history of the Milky Way.
Just like terrestrial monuments, interstellar artifacts provide evidence for past civilizations. They continue to exist in the Milky Way even if the technological era of their senders lasted for a short window of time relative to the age of the Galaxy, such that none of these senders transmits radio signals at present.
In contrast to electromagnetic signals, the abundance of artifacts which are gravitationally-bound to the Milky-Way disk would grow over cosmic time. The abundance of small objects is likely to be much larger than large objects, in part because some of them may represent fragments generated by the destruction of larger objects.
Based on the cosmic star formation history, most stars formed billions of years before the Sun, allowing sufficient time for chemical rockets to disperse through the Milky-Way disk if civilizations like ours emerged with the same time lag after the formation of other Sun-like stars. But even if one civilization had launched self-replicating probes, the abundance of artificial probes can be very high within the entire Milky-Way galaxy.
An interstellar object of future interest could be studied in great detail by the James Webb Space Telescope (JWST) as it passes nearby. Since JWST is located a million miles away from Earth at the second Lagrange Point L2, it would observe the object from a completely different direction than telescopes on Earth. This would allow us to map the three-dimensional trajectory of the object to exquisite precision and determine any forces acting on it in addition to the Sun’s gravity. Moreover, JWST would be able to detect the spectrum of infrared emission and reflected sunlight from the object, allowing JWST to potentially infer the composition of its surface.
Here’s hoping for a new era of discoveries regarding the nature of interstellar objects.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He chairs the advisory board for the Breakthrough Starshot project, and is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021.