Within a billion years, NASA’s Voyager 1 probe will reach the opposite side of the Milky Way disk from the Sun. By the time it arrives, the Sun will have boiled all of Earth’s oceans, making it habitable. As a result, NASA may not be around to celebrate this remarkable milestone in the journey of one of its most legendary spacecraft.
Last month I asked my brilliant Harvard undergraduate, Shokhruz Kakharov, where the Voyager 1 spacecraft will be in a billion years. Using a detailed model for the mass distribution of the Milky Way galaxy, Shokhruz was able to plot Voyager’s future orbit relative to the Sun over billions of years. The results will be presented in a forthcoming peer-reviewed paper.
All of this may sound academic rather than grounded «on Earth» as the adults in the room often pretend they are. But the reason for my question was on Earth. Actually, I wondered about this question because most stars were formed billions of years before the sun. Therefore, if a Voyager-like rocket was used on exoplanets more than a billion years ago, then corresponding space probes may have reached the Solar System from anywhere within the Milky Way disk by now. We can observe these interstellar objects with our telescopes as they pass close to Earth.
In particular, pairing a ground-based telescope with the Webb Space Telescope, a million miles away, will allow us to precisely localize the trajectory of objects and detect any non-gravitational acceleration they exhibit. It would also be extremely sensitive to detecting residual gases either as a result of cometary evaporation of natural ice or engine exhaust. But even without surrounding gas, the Webb telescope can measure the surface temperature and size of objects based on the infrared flux they emit. This would allow us to determine their reflection of sunlight within the Earth-Sun distance as long as they are much larger than Voyager.
However, on the scale of Voyager’s size, there is not enough reflected sunlight for our telescopes to detect these objects unless they get close to Earth. Even better – when they collided with Earth, they would turn out to be interstellar meteors of unusual material strength and composition. Our next expedition to the site of the interstellar meteorite, IM1, which collided with Earth on January 8, 2014 and displayed unusual material strength and composition, aims to find large pieces of the object and infer its origin.
Shokhruz and I calculated the galactic orbits of all 5 probes that NASA has launched into interstellar space so far, namely: Voyager 1, Voyager 2, Pioneer 10, Pioneer 11 and New Horizons. We also calculated the past trajectories of two interstellar meteors, IM1 and IM2, as well as the interstellar object `Oumuamu and the interstellar comet Borisov.
The fundamental question of whether any of the interstellar objects discovered near Earth are man-made will be better answered as more are discovered. The most promising path to increase 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 could find an interstellar object every few months. With my postdoc, Richard Cloete, we are developing the software needed to analyze Rubin’s data. By tracking the orbits of interstellar objects and observing them with other telescopes, we hope to identify their likely origins and the nature of the environment that spawned them.
For the same reasons that humans may not be on Earth when Voyager reaches the far side of the Milky Way, senders of interstellar probes may not be on their exoplanet due to the evolution of their parent star when we receive these packages in our near-Earth mailbox. Even if these technological objects ceased to function long ago, their existence would imply that there were once other intelligent inhabitants of the Milky Way. Their trash is our treasure. Learning about their state of mind based on what they left behind is like studying ancient civilizations on Earth that no longer exist based on the relics we find in archaeological sites.
In a recent public appearance, I was asked what I envisioned for the future of humanity. I explained that people arrogantly believe that they are important actors on the cosmic stage. But the truth is that even on Earth’s provincial stage, life survived massive catastrophes long before humans came on 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 human-caused ecological disasters. Another way to put it is that microbes are more resilient than humans. In a billion years, human existence could be a secondary footnote in the cosmic book. To gain a more balanced perspective, we must seek out 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 understand what happened on the cosmic stage and find satisfaction in the fact that our own Voyager will arrive on the other side of the Milky Way from the Sun in a billion years. Isn’t this a breathtaking achievement?
Yes, we are short-lived meter-sized beings with great physical limitations, but we are so ambitious and fearless that we can send our message in a bottle to the other side of the Milky Way, 50 thousand light years away, within a billion years.
Avi Loeb is the head of Project Galileo, founding director of the Harvard University Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and former chair of the Department of Astronomy at Harvard University (2011-2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chairman of the Committee on Physics and Astronomy of the National Academies. He is the author of the bestselling book “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and co-author of the textbook “Life in Space”, both published in 2021. His new book, “Interstellar”, is published in August 2023.