A plan for planetary defense warning stations

The White House recently released a “National Preparedness Strategy and Action Plan for Near-Earth Object (NEO) Hazards and Planetary Defense.” The document addresses the hazard of NEO impacts by leveraging and enhancing existing national and international assets and adding important capabilities across government.

Sixty-six million years ago, a 10-kilometer rock impacted Earth and caused mass extinction of 75 percent of all plant and animal species on Earth, including all dinosaurs that were not birds. In 2005, the U.S. Congress tasked NASA to find 90 percent of all NEOs bigger than 140 meters that could hit the Earth. With its 3.2 billion-pixel camera, the Vera C. Rubin Observatory aims to identify two-thirds of these objects within a decade, complementing the work of the Pan-STARRS telescope in Hawaii, which discovered the first interstellar object, `Oumuamua, after flagging it as a NEO. This discovery, ushering the research frontier of interstellar objects, was highlighted in the White House document.

The number of NEOs discovered over the past two decades ranges between 300 and 500 per year. In 2022, NASA approved a new space telescope, NEO Surveyor, to begin mission development for launch by June 2028. From its vantage point at the L1 stable Lagrange point on the sunward side of Earth, it will be able to survey along Earth’s orbit both ahead and behind Earth and closer to the direction of the Sun than possible from the ground. By using two heat-sensitive infrared imaging channels, the 50-centimeter telescope of NEO Surveyor will be able to detect the heat emitted by dark asteroids with low reflectance, and make significantly more accurate measurements of NEO sizes directly from these observations regardless of the amount of sunlight they reflect. Altogether, NEO Surveyor will carry out a five-year survey to find NEOs and make accurate measurements of their sizes, composition, shapes, rotational states and orbits. With the benefit of this mid-infrared telescope in space, it would be possible to fulfill the 2005 congressional task within the coming decade.

We currently know about all NEOs bigger than 10-kilometers, which could cause mass extinction on Earth, 40 percent of those bigger than 140 meters, which would be deadly over metro areas and cause mass casualties, and only 0.03 percent of meteors bigger than 10 meters that once per decade create a bright fireball and a blast wave that can break windows in buildings near the impact site. Altogether, Earth’s gravity attracts more than 100 tons of small objects and dust from space daily. We are protected from this material by the Earth’s atmosphere, which burns up most of the small particles. This is  because the friction power on air scales with surface area, and the smallest particles have the largest surface-to-mass ratios.

The White House document calls for the development of technologies for NEO reconnaissance and deflection and increased international cooperation and NEO preparedness.

While reading these plans, it occurred to me that an important defense strategy since ancient war times is to establish multiple remote defense stations that warn of incoming dangers at a distance. In the context of Earth, it would make most sense to make these warning stations co-orbital so that they follow the orbit of the Earth around the sun. (In particular, Trojan objects librate around the Lagrangian Points L4 and L5 which orbit 60 degrees ahead or behind the Earth around the sun.)

Another opportunity is offered by horseshoe orbits which librate around the Lagrange point L3, opposite to the Earth around the Sun. Objects sent to these Lagrange points tend to stay put. At Lagrange points, the gravitational pull of two large masses, such as the sun and the Earth, precisely equals the centripetal force required for a small object to move with them. These points can be used by spacecraft to reduce fuel consumption needed to remain in position.

A set of wide-field survey telescopes co-orbiting the sun with the Earth could alert humanity of incoming NEOs at large distances, where a small nudge could make them miss Earth.

In exoplanetary systems around other stars, such an alert system may remain intact millions of years after a civilization went extinct as a result of a natural catastrophe or self-inflicted wounds. Any technological devices placed near Lagrange points would survive on stable orbits as technological monuments, long after the civilization is gone. It is a reduced version, much easier to engineer, than grandiose megastructures such as Dyson spheres — meant to harvest most of the energy from the host star. Since the warning signal is expected to be transmitted electromagnetically to the planet, finding radio or laser transmissions from multiple transmitters in co-orbit around a star would be a telltale techno-signature to search for.

If Mars or Earth hosted earlier technological civilizations tens of millions of years ago, we might still find their warning systems intact at Lagrange points relative to solar system planets. Existing Earth-based survey telescopes are only capable of detecting the reflection of sunlight from objects bigger than `Oumuamua — the size of a football field, at these locations. But once we venture to these locations, we might find smaller technological relics from past civilizations in the solar system or beyond.

Most importantly, new space telescopes such as NEO Surveyor, will be able to find new interstellar objects like `Oumuamua and reveal their nature, be it astrophysical or technological in origin.

The most exciting archaeological monuments are yet to be found by searching up in space rather than by digging deep into the ground.

Avi Loeb is the head of Harvard University’s Galileo Project, founding director of Harvard’s Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, as well as the former chair of Harvard’s Department of Astronomy (2011-2020). He chairs the advisory board for the Breakthrough Starshot project, and 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. His new book, titled “Interstellar,” is scheduled for publication in August 2023.