A small jump away from the gravitational pull of a rocky object reaches a height above the surface that scales inversely with the global radius of the object.
The average radius of Earth is 6371 kilometers. An upward jump by twenty centimeters from the surface of Earth requires an initial speed of 2 meters per second, easily supplied by the leg muscles of a human. The same jump would carry an astronaut to a height of 250 meters above the surface of an object like the Chicxulub impactor, 5-kilometer in radius, that killed all non-avian dinosaurs on Earth 66 million years ago. That jump would allow the astronaut to escape the gravitational binding of any solid object with a radius smaller than 1.5 kilometers, an order of magnitude above the size of the anomalous interstellar object `Oumuamua. An astronaut riding sub-kilometer objects can lift off using leg muscles with no need for rocket propulsion. Starship is not needed on a kilometer-scale rock.
There are about ten million asteroids with a radius of order a kilometer in the main asteroid belt. Large cities accommodate tens of thousands of people per square kilometers. If each kilometer-scale asteroid hosted a percent of that population density, then all of humanity could have been accommodated in the asteroid belt. This corresponds to each person living on a property of 10⁵ square feet, much larger than most people own on Earth. The population of kilometer-radius asteroids totals ten millionths of the Earth’s mass but their cumulative surface area is similar to the land area on Earth because smaller objects have a larger surface to volume ratio.
In the upcoming billions of years, when the Sun will brighten up based on its predictable evolution, humans might choose to reside on asteroids in the main belt, which is farther from the Sun — like carousel seats in an amusement park at the optimal distances from the central heat source.
To make the ride enjoyable, each asteroid could be equipped with seat belts, Sun shades to protect the passengers from the solar wind and ultraviolet light, and water fountains which convert ice reservoirs on the asteroid surface into drinking water. The ride could be made more enjoyable in pressurized underground cabins with air composition, temperature and density similar to Earth’s atmosphere. For suitable pay, the habitat could offer luxurious spaces with food, kitchen and bathroom facilities. Construction crews of robots with artificial intelligence will prepare the spaces before they are ready for human use.
Near-Earth objects can be equipped with telescopes that allow beautiful views of Earth during the brief periods of passages near humanity’s traditional home.
Aside from this futuristic vision for real estate, one can imagine that advanced extraterrestrials might have already processed near-Earth objects for their service. Just like the Chinese spy balloon which monitored North America in early 2023, an extraterrestrial civilization might wish to monitor Earth from floating surveillance posts in the Solar system. Instead of using familiar drones over a state like New-Jersey which offers dull landscapes, they might have installed more sophisticated surveillance equipment on near-Earth objects (NEOs) which astronomers would mistake for natural rocks. Equipping some of these posts with propulsion and minute course correction capabilities would cause astronomers to classify these objects as “dark comets” based on their non-gravitational acceleration without a cometary tail.
Last month, astronomers reported the discovery of a population of near-Earth objects which appear as “dark comets,” displaying non-gravitational acceleration and no signs of cometary evaporation. They divide into two distinct classes: a larger population of outer objects on eccentric orbits around the Sun, and a smaller population of inner objects on near-circular orbits. Could any of these “dark comets” be artificial in origin? Future data from the Webb telescope or the Rubin Observatory could reveal their nature.
The newly discovered near-Earth “dark comets” range in radii between tens of meters to 5 kilometers, allowing them to serve as motherships with easy release of small probes owing to their weak gravity. The release of small probes can be timed at the proper point along their trajectories so as to deliver these probes to Earth. If the released gadgets are equipped with a propulsion system, they can avoid creating a fireball as they pass through the Earth’s atmosphere. Otherwise, they would be catalogued as common meteors and any technological debris that survives their impact would be interpreted as human-made space trash by most meteorite hunters.
As robots start to explore objects in the asteroid belt, they might discover some as archaeological sites, in case they were used by extraterrestrial civilizations for billions of years before humans emerged on Earth. Any surveillance system which monitored Earth during the first 99.9 percent of its history, found no traces of humans.
One could argue that even if extraterrestrials monitored Earth now, they would have been disappointed by the level of intelligence they find. However, if we ever choose to invest more funds in the exploration of “dark comets” or anomalous interstellar objects like `Oumuamua than in military budgets on the surface of the rock we were born on, we might uncover a digital screen which says: “welcome to the club of intelligent civilizations.”
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 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. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.
