Planetary Defense Warning Stations

Avi Loeb
5 min readApr 8


Last week, the White House 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% of all plant and animal species on Earth, including all non-avian dinosaurs. In 2005, the US Congress tasked NASA to find 90% 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 an NEO. This discovery, ushering the research frontier of interstellar objects, was highlighted in the White House document.

Number of large NEOs discovered per year (Credit: NASA)

The number of NEOs discovered over the past two decades ranges between 300–500 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 cause mass extinction on Earth about once per 100 million years, 40% of those bigger than 140 meters which are deadly over metro areas and cause mass casualties, and only 0.03% of meteors bigger than 10 meters that about once per decade create a bright fireball and a blast wave that can break windows in buildings near the impact site. A meteor the size of a person strikes the Earth every year and releases as much energy as the Hiroshima atomic bomb. Altogether, Earth’s gravity attracts more than a hundred 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 of 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, 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 warning systems intact at relevant 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 may yet be found by searching up in space rather than by digging deep into the ground.


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.



Avi Loeb

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