There are a few hundred billion stars in our Milky Way galaxy and more than a few percent of them host a habitable Earth-mass planet. Most of the stars move on circular orbits around the Galactic center because they formed out of a cold gaseous disk, similarly to the planets around the Sun.
Our Galactic disk is fifty million times larger than the extent of our planetary system, spanning a diameter of fifty thousand light years from the opposite side to the location of the Sun. This means that any light signal we may receive from an alien civilization on the other side, was emitted around the time when the first waves of Homo-sapiens migrated to Europe and replaced the indigenous Neanderthals. The human species was indistinguishable from nature back then. By now, we have evolved to a technological civilization capable of creating spacecraft, the internet and GPT-4. But our own TV shows over the past seventy years have reached merely two hundred sun-like stars by now. Similarly, our own interstellar probes will take ten thousand years before they leave the Oort cloud of the solar system into actual interstellar territory. We are late to the Galactic party.
Most of the initially-habitable Earth-like planets became not-habitable billions of years ago, because of the cosmic star formation history and stellar evolution. By now, their sun-like stars evolved and boiled off all liquid water on their surface as the Sun will do to Earth in a billion years. Because we are late bloomers, this offset in time reduces the chance for detecting a light signal from a twin civilization which shares our century-old technological phase. An alternative way to find interstellar partners is to trace long-lived technological products that remain gravitationally bound to the Milky Way, so that we could find them even if the senders died long ago. In other words, even if we missed the party, we could still find balloons drifting to our backyard from our neighborhood. Although the balloons might seem like trash, they carry an important message about the past activities of our neighbors.
Our own “party balloons” were sent out of the solar system as chemical rockets. The tyranny of the rocket equation implies that such spacecraft can only reach a tenth of the escape speed from the Milky Way disk. This is a blessing in our search for a twin civilization that preceded us, since similar “party balloons” from other civilizations are all still confined by gravity to the disk of the Milky Way, billions of years after being launched.
At a speed of several tens of kilometers per second, chemical rockets could spread through the Galactic disk in less than a billion years, allowing those launched near older stars to reach the inner Solar system by now. As I showed in a recent paper with my student, Carson Ezell, the spatial distribution of chemical rockets above the midplane of the Galactic disk resembles that of old stars which possess a similar velocity dispersion.
A source of fun and bragging rights for Galactic civilizations could involve the activity of launching thin membranes to interstellar space. In a recent paper, I showed that for micron-thick films, the repulsive force from starlight counteracts the attractive gravity of the Milky Way disk. As a result, such membranes would float, carried by radiation pressure above the disk, just like kites are carried by the wind above Earth. In another recent paper, I showed that membranes that were designed as tiles of a Dyson sphere in order to harvest energy in the immediate vicinity of a star, would be naturally pushed out to interstellar space when the star brightens to become a red giant.
An abundant population of interstellar membranes could explain the anomalous non-gravitational acceleration of the first interstellar object, `Oumuamua, in terms of radiation pressure from the Sun. Indeed, the anomalous acceleration declined smoothly inversely with distance squared, as expected for a thin object. A similar push by sunlight was detected for the thin stainless-steel enclosure of NASA’s rocket booster 2020 SO, which was discovered three years after `Oumuamua by the same Pan-STARRS observatory.
Smaller pieces of technological space trash are likely to be far more abundant than large pieces, either because they are easier to launch or because they resulted from the breakup of larger objects. The breakup scenario explains the large abundance of shell fragments on the beach compared to the population of unbroken sea shells from which these fragments originated. The two interstellar meteors that predated `Oumuamua, IM1 from January 2014 and IM2 from March 2017, were about a hundred times smaller than `Oumuamua but a million times more abundant.
Functional devices are likely to be rarer than space trash based on our experience with plastics in the Earth’s oceans. Given that the detection rate of meter-scale interstellar meteors is once per decade, we should expect new arrivals of functional devices to be rare unless they target the Earth. Moreover, given the billion-year journey time for interstellar objects that are gravitationally bound to the Milky Way disk, such probes should have defined Earth because of its biosphere as a desired destination long before we distinguished ourselves from nature.
An interstellar encounter provides us with an opportunity to catch up on the latest advances in science and technology in our cosmic neighborhood. This is the rationale behind the Galileo Project which operates a new astronomical observatory at Harvard University, analyzes new satellite data and will soon launch an expedition to the Pacific Ocean in an attempt to retrieve relics from the first interstellar meteor, IM1.
In daily life, the experience of recovering trash in the backyard of our home is frustrating. This is because we are already aware of our neighbors. But given our ignorance about Galactic neighbors, any piece of technological trash from interstellar space found by the Galileo Project will be treasured. The pieces will be labeled at the Harvard College Observatory and loaned to museums and research teams around the world. No recycling will be allowed whatsoever.
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. His new book, titled “Interstellar”, is scheduled for publication in August 2023.