Last month I asked my brilliant undergraduate student at Harvard, Shokhruz Kakharov: “Where will the Voyager spacecraft be in a billion years?” By using a detailed model for the mass distribution of the Milky Way galaxy, Shokhruz was able to plot the future orbit of Voyager relative to the Sun over billions of years. The results will be featured in a forthcoming peer-reviewed paper.
Within a billion years, Voyager will get to the opposite side of the Milky-Way disk relative to the Sun. By the time it gets there, the Sun will boil off all oceans on Earth, making it inhabitable. As a result, NASA might not be around to celebrate this remarkable milestone in Voyager’s journey.
This all sounds academic and not anchored “down to Earth,” as the adults in the room often pretend to be. But the reason for my question is down to Earth. In fact, I wondered about this question because most stars formed billions of years before the Sun. Therefore, if Voyager’s-like rocketry was used on exoplanets more than a billion years ago, then the corresponding space probes might have reached the Solar system by now from anywhere within the Milky-Way disk. We can observe these interstellar objects with our telescopes as they pass near-Earth.
In particular, pairing an Earth-based telescope with the space-based Webb telescope, a million miles away, will allow us to localize precisely the trajectory of the objects and detect any non-gravitational acceleration that they display. It would also be extremely sensitive to the detection of trailing gases either as a result of cometary evaporation of natural ices or exhaust gases out of an engine. But even with no surrounding gas, the Webb telescope can measure the surface temperature and size of the objects based on the infrared flux that they emit. This would allow us to determine their reflectance of sunlight within the Earth-Sun separation as long as they are much bigger than Voyager.
However, on the size scale of Voyager, there is not enough sunlight reflected for our telescopes to detect these objects unless they arrive near Earth. Better still — if they were to collide with Earth, they would show up as interstellar meteors of unusual material strength and composition. Our next expedition to the site of the interstellar meteor, IM1, which collided with Earth on January 8, 2014, and exhibited unusual material strength and composition, aims to find large pieces of that object and infer its origin.
Shokhruz and I calculated the Galactic orbits of all 5 probes launched so far by NASA to interstellar space, namely: Voyager 1, Voyager 2, Pioneer 10, Pioneer 11 and New Horizons. We also calculated the past trajectories of the two interstellar meteors, IM1 and IM2, as well as the interstellar object `Oumuamua and the interstellar comet Borisov.
The fundamental question of whether any of the interstellar objects detected near Earth is artificial in origin, will be better answered as more of them are discovered. The most promising path for increasing the current sample of interstellar objects is with the Rubin Observatory in Chile which within a year will survey the Southern sky every 4 days with a 3.2-billion-pixel camera that just arrived a week ago at the observatory. With its unprecedented sensitivity, the Rubin Observatory might find an interstellar object every few months. With my postdoc, Richard Cloete, we are developing the software needed to analyze the Rubin data. By tracing the orbits of interstellar objects and observing them with other telescopes, we hope to identify their likely origin and figure out the nature of the environment that gave birth to them.
For the same reasons that humans might not be around on Earth when Voyager arrives at the opposite side of the Milky-Way, the senders of any interstellar probes may not be around on their exoplanet because of the evolution of their older star when we receive these packages in our mailbox near Earth. Even if these technological objects stopped functioning a long while ago, their existence will imply that there used to be other intelligent residents of the Milky-Way. Their trash is our treasure. Learning about their state-of-mind from what they left behind is equivalent to studying ancient civilizations on Earth that do not exist anymore based on the relics we recover in archaeological sites.
In a recent public appearance, I was asked what I envision for the future of humanity. I explained that humans arrogantly believe that they are important actors on the cosmic stage. But the truth is that even on the provincial stage of Earth, life survived huge catastrophes long before humans came to the scene, including a global warming event 252 million years ago that wiped out 96% of all marine species.
This gives hope that in the grand scheme of things, life on Earth will also survive the environmental catastrophes triggered by humans. Another way to put it is that microbes are more resilient than humans. In a billion years, human existence might be a minor footnote on the cosmic playbook. To get a more balanced perspective, we must search for other actors on the cosmic stage and learn from them. And if none of them survived, we can study their history based on the artifacts they left behind.
We are not in a position to claim a major role in cosmic history. But the good news is that we can figure out what happened on the cosmic stage and find pleasure from the fact that our own Voyager will reach the other side of the Milky-Way relative to the Sun in a billion years. Isn’t this accomplishment breathtaking?
Yes, we are short-lived, meter-scale creatures with major physical limitations, but we are so ambitious and fearless that we can get our message in a bottle to the other side of the Milky-Way, 50 thousand light-years away, within a billion years.
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. His new book, titled “Interstellar”, was published in August 2023.
