Hunting `Oumuamua’s-Cousin with Eyes Wide Open

Starting in 2025, the Rubin Observatory in Chile will survey the southern sky every 4 days with a 3.2 gigapixel camera. Statistically, it is expected to discover an interstellar object, like `Oumuamua, every few months.

The fundamental question that my research team will address is whether technological objects lurk among the vast population of interstellar rocks that cross the orbit of the Earth around the Sun. One way to separate artificial objects from natural rocks is through their anomalous propulsion with no gaseous trail, similar to `Oumuamua’s non-gravitational acceleration.

While passing near Earth in 2017, `Oumuamua showed no evidence for cometary gas or dust coming off its surface that would provide a natural push through the rocket effect. The decline in `Oumuamua’s non-gravitational acceleration with distance from the Sun was not measured accurately enough by the Hubble Space Telescope and ground-based observatories.

The scaling was consistent with the excess push being inversely proportional to distance squared relative to the Sun, as expected from radiation pressure acting on a thin membrane. Another anomalous object detected by the same Pan-STARRS telescope in Hawaii three years later, 2020 SO, displayed a similar push by reflecting sunlight and was identified as a rocket booster with thin walls, launched by NASA in 1966.

By now, `Oumuamua is a hundred million times fainter than it was at closest approach to Earth and cannot be observed by our telescopes. It is impractical to chase blindly `Oumuamua with a spacecraft given the uncertainties in its trajectory. Alternatively, it would be prudent to use the Rubin Observatory in hunting additional objects from its family, `Oumuamua’s-cousins.

As it was tumbling every 8 hours, `Oumuamua’s brightness varied by a factor of 10, suggesting a disk-like shape at the 91% confidence level. Given the multiple anomalies of `Oumuamua, there is great interest in learning more about another object from its family. If the Rubin Observatory were to discover a cousin of `Oumuamua near Earth, what could we learn about it now?

A key advantage relative to 2017, is the opportunity to use the Webb telescope. It was my privilege to serve on the first Science Advisory committee that designed this telescope thirty years ago, as I look forward to harvesting its fruits in the coming years. The exquisite infrared sensitivity would allow the Webb telescope to detect the heat emitted by `Oumuamua’s-cousin and hence infer its surface temperature and surface area given its distance.

The area can be used to determine the reflectance of sunlight by the surface, and potentially the surface composition. As `Oumuamua’s cousin tumbles, these properties could be mapped across its various sides. But most importantly, the Webb telescope would track the trajectory of `Oumuamua’s-cousin with unprecedented precision. Let me explain.

There is a good reason for why natural selection equipped us with two eyes rather than one in the context of survival of the fittest. Imaging data from two separated eyes can be used to assess the distance of an approaching predator by observing it from slightly different directions. In 2017, the precision of measuring the distance of `Oumuamua’s was limited by the separation of the Hubble Space Telescope from ground-based telescopes.

Currently, the Webb telescope is separated from Earth by a million miles, about 3,000 times farther than Hubble’s altitude, allowing us to localize interstellar objects much better. Moreover, Webb’s aperture is about 2.7 times larger than Hubble’s mirror size, enabling detection of fainter objects at larger distances.

In collaboration with my Harvard College student, Sriram Elango, I proposed to use forthcoming imaging data from the Webb telescope in localizing the three-dimensional trajectory of interstellar objects that will be discovered in the coming years by the Rubin Observatory.

We demonstrated that with very modest observing time, it would be possible to determine to exquisite precision the radial dependence of the non-gravitational acceleration of `Oumuamua’s-cousin. This will allow us to easily distinguish the rocket effect of cometary evaporation from other sources of non-gravitational acceleration, including artificial propulsion of a functional technological artifact.

An anomalous interstellar artifact, signaling the existence of superhuman intelligence, would offer a new context to the concept “Übermensch“, proposed by the philosopher Friedrich Nietzsche.

Why is the search for interstellar artifacts worthwhile? Because knowing that we have cosmic neighbors might inspire us to venture into interstellar space. This inspiration was highlighted in an email I received this morning, which read:

“Good morning, Avi. I have been reading your book “Interstellar”. The book is now full of highlighted passages. I find myself putting down the book and reflecting on multiple paragraphs. Sometimes it hits me so hard I just have to sit down and say nothing at all to anyone for a while. I want you to know that this book is having a profound impact on me…

I am always saying, “beam me up Scotty, there’s no intelligent life here!” or “why have I been dropped on this horrible rock that makes absolutely no sense? Please take me home!” I think I say both of those things to myself because I really don’t feel that I and many others belong here, and that our real home lies elsewhere.

However, when I read the paragraph at the bottom of page 133, I found that for the first time in 57 years, the prospect of having to move to another planet was made real by what you have said there, and I am struggling with the feelings that I have about the prospect of how serious that reality is. It may come a lot sooner than we expected.

I’m having trouble putting this into words, but I just wanted you to know that what you are saying is having a profound effect on me and I’m sure thousands, and hopefully millions of people.

I think that is a big part of your intention with the book, isn’t it? For people to stop and think about the challenges facing our Interstellar future.”

Adding hope, this morning Starship’s rocket booster was successfully caught by SpaceX’s tower.