Chasing `Oumuamua

Avi Loeb
5 min readMay 8, 2023

The first object discovered from outside the Solar system, `Oumuamua, had an extreme shape — most likely flat, and was pushed away from the Sun without showing cometary evaporation. Its anomalies relative to solar system asteroids or comets remain unresolved as a result of the limited data collected on it when it passed near Earth in October and November of 2017.

As a result, a team of astronomers wishes to chase `Oumuamua and study it up close within the next century. A new paper outlined the required orbital parameters for a related `Project Lyra’ mission that could reach `Oumuamua in the year 2086 or 2175.

Finding `Oumuamua as an unexpected near-Earth object resembles noticing potential dating partners in a coffee shop, but realizing your interest in knowing more about them after they left the room. Chasing them down the street is an immediate impulse. However, a more measured response to the circumstances would be to systematically search for other dating partners with similar qualities. Surely, in a large enough population of candidates, a similar one will come your way.

The latter approach is indeed suggested by the `dating app’ LSST of the Rubin Observatory in Chile, that will soon start to survey the Southern sky with its 3.2-billion-pixel camera every four days. Together with my students and postdocs within the Galileo Project, I am designing the software that will allow us to find anomalous interstellar objects like `Oumuamua in the LSST data pipeline.

On the other hand, the challenge of chasing `Oumuamua in the Solar system is daunting. This interstellar object is currently at a distance of 5.4 billion kilometers from Earth, 36 times the Earth-Sun separation. As a result, it is 60 million times fainter than it was at closest approach to Earth. Its 37.8 magnitude is orders of magnitude below the sensitivity of even the deepest images from the Webb space telescope.

`Oumuamua was estimated to have a size of order a hundred meters, implying that a meter-size telescope onboard a chasing spacecraft would be able to resolve it with 10 pixels on a side at the diffraction limit, only if the spacecraft comes within a distance of closest approach to `Oumuamua of order the diameter of Earth. This represents a trajectory precision at the level of a millionth of `Oumuamua’s distance from Earth. Since the required precision is much more demanding than the uncertainties in `Oumuamua’s trajectory, the chasing spacecraft will have to detect the light reflected or emitted by `Oumuamua as it gets close to it and navigate accordingly. This would require having a heavy telescope onboard, making the mission very expensive.

But there are additional technical challenges. Given that the spacecraft will need to exceed `Oumuamua’s speed of 26.5 kilometers per second, the encounter will be short. The encounter time will be of order the duration of traversing twice the diameter of Earth at a speed of tens of kilometers per second, about ten minutes.

Dedicating a century to reach `Oumuamua and then spending only 10 minutes in its vicinity is analogous to chasing dating partners down the street for an hour, only to see them for a thousandth of a second. Is that really worth the effort? Since most of us will not be alive to witness the encounters in 2086 or 2175, I do not think so.

A much better approach to learn about interstellar objects is to treat the 2017 discovery as a wake-up call and find additional `Oumuamua-like interstellar objects in the coming years. The parallax from monitoring any of them with the Webb space telescope and a telescope on Earth would reveal any non-gravitational acceleration to exquisite precision. Also, detecting the infrared emission from the objects would allow to infer their surface area, given that their surface temperature is dictated by their measured distance from the Sun. The simultaneous spectroscopic detection of reflected sunlight and infrared emission would allow us to infer the objects’ reflectance (albedo) and surface composition, which remain unknown for `Oumuamua.

If any of the future `Oumuamua-like objects are detected far in advance of their closest approach to Earth, one could envision launching a spacecraft that meet them along their path. The parameters of such a mission were envisioned in a recent paper by the Galileo Project team. The required velocity boost for such an encounter exceeds that of the Comet Interceptor envisioned by ESA by an order of magnitude and mandates a new mission design.

As dating coaches advise, it is far better to think about the opportunities offered by future dating partners than to obsess with an uncertain past date. Statistically speaking, the future offers many more dating opportunities than the past.

The only requirement for discovering new scientific knowledge is to keep a beginner’s mind, as advocated by the concept of shoshin in Zen Buddhism. In the context of interstellar objects, this requires that we will not insist that all of them are natural in origin. After all, the natural explanations to `Oumuamua involved objects of a type that we had never seen before, like a `dust bunny’, a hydrogen iceberg, a nitrogen iceberg or a water-hydrogen iceberg.

The uncertainty should make us all keep an open mind to the possibility that once in a while we might notice an object manufactured by an extraterrestrial technological civilization. This might represent an exotic form of space trash, like Elon Musk’s Tesla Roadster, which was launched into space as a dummy payload in 2018. Ironically, it is the space polluter himself, Elon Musk, who damps enthusiasm about the search for space trash from extraterrestrial analogs of himself.

If an artificial object has thin walls, it would exhibit a push by reflecting sunlight — as did the rocket booster 2020 SO in the data stream of the same telescope that discovered `Oumuamua. In searching for interstellar partners, we better start with a mirror image of ourselves. What could be more familiar and common than that?


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".