Interstellar tourist agencies need to use a computer code that allows them to plan trips to destinations within the Milky-Way galaxy by forecasting orbits in the galaxy’s gravitational field. The calculation would provide a trip schedule, including arrival times at various destinations along the path, with or without propulsion. If such maps exist, I would love to see them because they can be deciphered to infer the distribution of mass in the Milky-Way, including invisible dark matter and black holes. These maps must be time-sensitive because the Milky-Way gained mass through coalescence with nearby galaxies, like the major merger with the so-called Gaia-Enceladus-Sausage galaxy that took place about 8–11 billion years ago.
Those who know me, would tell you that I have little affinity to science fiction. I enjoy practicing science and reading fiction, but I dislike turning science into a virtual reality that distorts the laws of physics for the benefit of artistic pleasure. The lack of a cosmic judicial system that penalizes those who violate the laws of quantum mechanics or gravity tells us that these laws cannot be broken, irrespective of our desires or wishful thinking.
So why am I considering Milky-Way tourism? Because over the past decade astronomers discovered the first interstellar objects from outside the Solar system and the follow-up question is: “Where did these objects come from?”
When `Oumuamua was discovered on October, 19 2017, many people speculated about its source star based on its direction of arrival into the solar system. This rationale, however, is futile for a simple reason. The Oort Cloud of the Solar system extends half-way to the nearest star, Proxima Centauri. Therefore, analogs of the Oort cloud around other stars are tightly packed, touching each other in interstellar space like billiard balls tacked together on a pool table. Along any line from the Solar system through the Milky Way, there are thousands of Oort clouds that happened to coincide with that direction. Limited information that includes only the direction of motion of an interstellar object, is not sufficient for inferring its origin. Objects in the outskirts of a planetary system are easiest to dislodge into interstellar space by gravitational kicks from passing stars.
The only way to remove the ambiguity about the origin of an interstellar object is by knowing the duration of its journey, enabling to cap the length of its trajectory and determine its starting point. Science fiction aficionados might suggest searching for a label stating: “Made in Exoplanet X near Star Y on Year Z after the Big Bang.” But as a scientist, I seek a more `down-to-Earth’ solution. Gladly, I came across such an insight on the deck of the ship “Silver Star” in mid-June 2023, as I was leading an expedition in the Pacific Ocean to retrieve fragments of an interstellar object that collided with Earth and appeared as a meteor in our sky.
The first two interstellar meteors, IM1 and IM2, were identified by US Government satellites on January 8, 2014 and March 9, 2017, respectively. Their measured velocities near Earth were translated to velocities of 60 and 40 kilometers per second relative to the Local Standard of Rest (LSR) of the Milky-Way outside the solar system. The LSR represents the frame that rotates around the Milky-Way center at a speed of about 233 kilometers per second at an orbital radius of 25 thousand light years.
Our ocean expedition retrieved about 50 milligrams of unique “BeLaU”-Type spherules from IM1’s fireball site in the Pacific Ocean, as described in the latest paper of our research team. In our next ocean expedition, we will aim to retrieve grams of material from one of the interstellar objects, IM1 or IM2. If IM1 or IM2 resulted from a natural process, such as the tidal disruption of a rocky planet by a dwarf star described in the recent paper I published with Morgan McLeod, then these interstellar objects were most likely ejected early in the history of their parent star. Figuring out the age of the meteoritic materials would provide a reasonable cap to their travel time.
The age of meteors can be determined from radioactive isotopes, such as Uranium-238 (half-life of 4.5 billion years, equal to the age of the solar system), Thorium-232 (half-life of 14 billion years, equal to the age of the Universe) or Samarium-147 (half-life of 106 billion years). Most stars have ages that can be clocked by these isotopes.
Knowing the object’s velocity vector upon entry of the solar system, allows us to integrate the trajectory back in time through the duration of the journey in interstellar space. This is where time-dependent maps of the mass distribution of the Milky-Way are very helpful.
On top of the ordered circular motion of the LSR around the Milky-Way center, interstellar objects exhibit significant drifts from the source neighborhood over travel times of millions to billions of years. The orbital time of the Sun around the Milky-Way center is 220 million years. The Solar system circled the Galactic center 21 times since it was born 4.6 billion years ago.
The motion of interstellar objects like IM1 or IM2 relative to the LSR results in periodic radial oscillations at the so-called epicyclic period. Near the Sun, the epicyclic oscillation period is about 160 million years. The radial excursion over this times is dictated by the radial component of the interstellar object’s velocity relative to the LSR, and is of order ten thousand light years for IM1 and IM2. In the direction perpendicular to the plane of the Milky-Way, the period of the vertical oscillations near the Sun is about 90 million years.
Time-dependent gravitational perturbations to the motion of IM1 and IM2, from galactic spiral arms, a galactic bar, or a merger with another galaxy, could trigger radial migration, either towards or away from the Galactic center. An inward radial migration at an average speed of merely 1 kilometer per second would allow interstellar objects to reach the Galactic center in 10 billion years. Such motions are expected to produce significant radial mixing of stars in the Milky-Way disk over ten billion years.
Those who wish to reach a destination on the other side of the Milky-Way galaxy in less than a billion years, must board a spacecraft with an engine that propels it to a speed of hundreds of kilometers per second in the opposite direction to the Galactic rotation, so as to cancel the centrifugal force that blocks them from crossing the Milky-Way center. Such speeds are an order of magnitude larger than those provided by the chemical rockets that propelled NASA’s five interstellar probes so far.
Our current model for the mass distribution of the Milky-Way has uncertainties that make it difficult to associate interstellar objects with particular sources over travel periods of billions of years. Hence, if we ever find other tracers that link an interstellar object to a particular astrophysical source, we would be able to constrain the mass distribution of the Milky-Way better than today. This would be the scientific equivalent to finding maps from interstellar tourist agencies.
Stay tuned for the findings of our next ocean expedition!
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.
