Interstellar Archaeology

Avi Loeb
10 min readAug 4, 2022


Webb telescope image of the galaxy NGC 7496

In the summer of 1950, Enrico Fermi had a lunchtime discussion at Los Alamos National Laboratory about extraterrestrial technological civilizations, during which he asked: “where is everybody?

In retrospect, this appears to be a naïve proposition for a scientist, similar to asking: “where are our neighbors?” without looking out the windows or venturing to the backyard.

Surely, we must first construct telescopes which engage in the search for interstellar objects from outside the Solar system before concluding anything.

We now know, based on Kepler satellite data, that a substantial fraction of all Sun-like stars hosts an Earth-size planet roughly at the same separation. Since most of these stars formed billions of years before the Sun, the dice of “intelligent life as we know it” was rolled tens of billions of times, in the Milky Way alone, long before we came to exist. There are trillions of galaxies in the observable volume of the Universe, all the way back to the cosmic dawn when the first galaxies formed. The exquisite level of uniformity of the cosmic microwave background informs us that there is no “cliff” out to 4,000 times the size of our cosmic horizon — implying there should be at least 64 billion times more galaxies beyond the trillions we can in principle see with our telescopes.

And the story of our cosmic roots: “Let there be light” is being rewritten by new data. The first deep image from the Webb telescope revealed some of the earliest stars from 13 billion years ago. The revelation was celebrated in a dedicated White House event, hosted by President Biden and vice-President Harris. The image shows numerous red arcs stretched around a cluster of galaxies, named SMACS 0723, located about 5 billion light years away. NASA-administrator Nelson noted: “Mr. President, we’re looking back more than 13 billion years”, an unusual statement to be heard in the household of DC politics which makes plans on a timescale of four years.

Judging by our own lack of attention to climate change, it is possible that many of the other technological civilizations in our own galaxy died several centuries after they started venturing into space. The traditional approach of the Search for Extraterrestrial Civilizations (SETI), has been searching for radio or laser signals from active transmitters. It has a narrow window of opportunity, less than one part in ten million, since most of such signals escaped the Milky Way galaxy long ago and by now are faint undetectable glows billions of light years away.

However, any chemically-propelled spacecraft sent by past civilizations into interstellar space, like the five we had sent so far (Voyager 1 & 2, Pioneer 10 & 11, and New Horizons), remained gravitationally bound to the Milky Way long after these civilizations died. Their characteristic speed of tens of kilometers per second is an order of magnitude smaller than the escape speed out of the Milky Way. These rockets would populate the Milky Way disk and move around at similar speeds to the stars in it.

This realization calls for a new research frontier of “interstellar archaeology”, in the spirit of searching our backyard of the Solar system for objects that came from the cosmic street surrounding it. The interstellar objects could potentially look different than the familiar asteroids or comets which are natural relics or Lego pieces from the construction project of the Solar system planets. The traditional field of archaeology on Earth finds relics left behind of cultures which are not around anymore. We can do the same in space.

Given this perspective, it is evident that Fermi asked his question prematurely. An astronomical search for an interstellar treasure of technological artifacts accumulated over the past ten billion in the Milky Way galaxy, is only beginning now in earnest, seven decades after Fermi’s question.

The likelihood of success in the quest for interstellar artifacts is not described by the traditional Drake equation, which applies to fleeting electromagnetic signals. Instead, the number of physical relics we discover would simply be proportional to the survey volume and the sensitivity of our detectors to small, fast-moving objects.

Astronomers routinely search for asteroids, comets or meteors, that move at a speed of tens of kilometers per second. Past surveys could have easily missed tiny spacecraft moving at a fraction of the speed of light, similar to the light sails developed by the Breakthrough Starshot Initiative, whose scientific advisory board I lead.

Our search for interstellar artifacts could unravel either functional devices or defunct space trash that aged beyond its lifetime — like New Horizons would be in a billion years, or equipment that was damaged by impact of cosmic-rays, X-rays, dust and gas particles.

Functional devices that search for life might be focusing on trajectories aimed towards the habitable regions around stars. As a result, they could be far more abundant in the vicinity of Earth than on average in interstellar space. Given that the extent of the Solar system is a hundred thousand times bigger than the Earth-Sun separation, the local density enhancement of such life-seeking devices could be some fifteen orders of magnitude.

There are two primary ways to find interstellar objects. We can look for these “keys” under the lamppost of the Sun, the bright source of light that illuminates the darkness around us. This is how Solar system asteroids or comets are routinely found. The interstellar objects would be faster than Solar system objects because they are unbound to the Sun. We can also use the Earth as a fishing net and search for objects that collide with it at high speeds. This could uncover smaller object because they produce their own light as they burn up as a result of their friction with air and appear as meteors.

First Interstellar Visitors

Remarkably, the first three interstellar visitors were discovered only over the past decade. First was the 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). Second was the unusual interstellar object, ‘Oumuamua (1I/2017 U1), discovered by the Pan STARRS tele- scope 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 cometary gases indicative of a rocket effect. Another object, 2020 SO, exhibiting an excess push with no cometary 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. Finally, the interstellar comet, 2I/Borisov, was discovered on August 29, 2019 by the amateur astronomer, Gannadiy V. Borisov. This object resembled Solar system comets 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 asteroids or comets which are bound to the Solar system.

Unexplained Objects in Our Backyard?

What we regard as “ordinary” are things we are used to seeing. Such things include birds in the sky. But digging deeper into the nature of ordinary matters suggests that they are rather extraordinary. Humans were only able to imitate birds with the Wright brothers’ first flight in 1903. Similarly, what we regard as “extraordinary claims” is often based on societal conventions.

On June 25, 2021, the Office of the Director of National Intelligence released a report on 143 UAP, and admitted that UAP data is rarely discussed openly because “Sociocultural stigmas and sensor limitations remain obstacles to collecting data on UAP… reputational risk may keep many observers silent, complicating scientific pursuit of the topic.” On Tuesday, May 17, 2022, the first open congressional hearing in half a century was held on the topic of UAP.

There are two possible interpretations of technological objects: either they were made by humans or they were made by extraterrestrial civilizations. In the first case, the government wishes to know which technologies are used by other nations. In the second case, scientists wish to know which technologies were developed by extraterrestrial civilizations.

Obviously, government officials are concerned with UAP as a threat to national security. Their job definition is twofold: to protect the safety of our military personnel and the security of the nation. From their perspective, reports by military staff members are of primary importance for the first task, and data from military training or patrol sites are linked to the second objective. They need to know what the vast majority of UAP are, and for that purpose they must attend to data of compromised quality such as the blurry videos shared during the hearing. However, the task for scientists is complementary to that. They do not need to explain most of the reports. Even if only one object is of extraterrestrial technological origin among the clutter of many others that are human-made, it would represent the most consequential discovery in human history. Its significance would resemble our first visit to the kindergarten when we realized that there is a smarter kid on the block. To figure this out, scientists must have access to the highest quality data, such as a high-resolution image of an object showing a label “Made on an Exo-Planet”, or a maneuver at a fraction of the speed of light or a set of buttons that demonstrate technical specifications of a futuristic “iPhone 100”.

Scientists are concerned with all possible geographical locations irrespective of whether they host military assets or national facilities. Extraterrestrial equipment might not adhere to national borders in much the same way that a biker navigating down the sidewalk does not care which of the possible pavement bricks is occupied by a small colony of ants.

UAP reports are most likely a mixed bag. Many objects may have mundane explanations.

But in order to figure out whether an anomalous object exists for which human-made or natural origins can be excluded, we need to retrieve new data with the best possible instruments at our disposal today.

This was the rationale in July 2021 for establishing the Galileo Project — that I lead. Funded by private donations, the Project is assembling its first telescope system on the roof of the Harvard College Observatory. The system will monitor the sky continuously in the infrared, visible and radio bands as well as audio, magnetic and muon signals. The data will be analyzed by state-of-the-art software that uses artificial intelligence and machine learning to identify objects in the sky and interpret their properties. Once the system will work as desired, the Project will make copies of it and distribute them in various geographical locations.

The Galileo Project has two additional branches of research. One involves the design of a space mission to rendezvous with unusual interstellar objects like ‘Oumuamua, in the spirit of NASA’s OSIRIS-REx mission — which landed on the asteroid Bennu, or ESA’s plan for a future Comet Interceptor — which is limited in its maneuvering speed. The Galileo Project will develop software that will identify interstellar objects that do not resemble familiar asteroids or comets from the Solar system. This software will be applied to the pipeline of data from the Legacy Survey of Space and Time (LSST) of the Vera C. Rubin Observatory data pipeline. LSST will serve as the “dating app” for dating the next `Oumuamua. Since this expensive date will cost more than a billion dollars, we will “swipe to the left” most of the time.

Finally, a third branch of the Project involves a plan for an expedition to retrieve fragments from the first interstellar meteor CNEOS 2014–01–08 from the ocean floor near Papua New Guinea. It would be particularly interesting to determine the composition and structure of this unusual object and infer whether it was natural or artificial in origin. But even if it is natural, its non-solar composition could provide independent evidence that it originated in a distant planetary system.

New Clues from Webb’s Web

From its vantage point at the second Lagrange Point (L2), located a million miles away from Earth, the Webb Telescope just started to unravel new insights about the Universe. Are there exciting prospects for using data caught in the “spider web” of the 18 hexagonal segments of Webb’s primary mirror in searching for extraterrestrial technological civilizations?

There are several ways by which the Webb Telescope can detect signs of intelligent life. First, it could study interstellar objects that arrive near Earth from outside the solar system and infer their composition and motion to check if they might be extraterrestrial spacecraft. Looking at these objects from L2 and Earth would allow us to accurately determine their trajectory in three dimensions and infer any excess propulsion beyond motion that is shaped by gravity or cometary evaporation.

Second, the Webb telescope can search for city lights on the night side of habitable planets around nearby stars. Within the Solar system, one can distinguish an artificial source of light from an object reflecting sunlight by the way it dims as it changes its distance from the lamppost of the Sun. A source that produces its own light, like a light bulb, dims inversely with distance squared whereas a distant object that reflects sunlight dims inversely with distance to the fourth power, because the amount of light bouncing off it scales inversely with distance squared.

And third, the Webb telescope could search for industrial pollution in the atmosphere of planets that transit in front of their host star. Artificial molecules, such as chlorofluorocarbons (CFCs), may survive long after the industrial civilization that produced them perished.

Final Thoughts

We invest major funds in the search for the nature of dark matter that has minimal impact on our society, but minimal funds on interstellar archaeology. As a result, the lack of “extraordinary evidence” for “everybody” in Fermi’s question is often self-inflicted ignorance. We might figure out the nature of anomalous interstellar objects long before we understand dark matter, if we would only be brave enough to collect and analyze new data, based on the scientific method.


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.



Avi Loeb

Avi Loeb is the Baird Professor of Science and Institute director at Harvard University and the bestselling author of “Extraterrestrial” and "Interstellar".