In my recent TED talk, I noted that if we find an interstellar tennis ball in our back yard within the solar system, we would know that our cosmic neighbor plays tennis. Such tennis balls could not be made of ice. Interstellar snowballs would be eroded by energetic particles, known as cosmic rays, that fill up the Milky-Way galaxy.
In a paper I co-authored with Minh Phan and Thiem Hoang which followed on an earlier paper, we showed that icy interstellar objects which are smaller than about a hundred meters, the length of `Oumuamua or Starship, would not survive a journey of billions of years through the Milky-Way galaxy, irrespective of whether they are made of water, nitrogen, carbon dioxide, carbon monoxide, hydrogen, oxygen or methane. If any of these constituents is used as rocket fuel, the rocket’s travel through interstellar space requires a tough shield of its fuel tank that would withstand the steady bombardment by cosmic rays and interstellar gas particles. The risk of damage increases with speed, and is particularly acute close to the speed of light, as we showed in another paper.
But even a heavily shielded craft must avoid the immediate vicinity of exploding stars.
When the core of a star more massive than 8 times the mass of the Sun consumes its nuclear fuel, it collapses to make a neutron star, packing up to twice the mass of the Sun within 12 kilometers, the scale of a city. About 99% of the binding energy released during the birth of the neutron star is carried out by neutrinos, which are coupled very weakly to matter and therefore leak the explosion heat most effectively. The coupling of the remaining percent of the released energy to the envelope of the progenitor star results in a powerful explosion that enriches the interstellar environment with heavy elements, previously cooked by nuclear fusion during the lifetime of the star. We owe our existence to these violent explosions. The early Universe produced hydrogen and helium, but most of our body mass is carried by oxygen, which was dispersed by supernova explosions.
The latest core-collapse supernova in the Milky Way galaxy was observed in 1987. The detection of neutrinos from this rare event, labeled SN 1987A, inspired my first paper in astrophysics during a visit to the Institute for Advanced Study at Princeton 37 years ago. Our most up-to-date forecast predicts a few core-collapse supernovae per century in the Milky Way, suggesting that we are due for another display of cosmic fireworks in our galaxy soon. If the next supernova will be closer than 163,000 light years, the distance to SN 1987A, then we would detect many more neutrinos from it and potentially also the gravitational wave signal generated by the turbulent birth of its central compact object.
The energy released from a core collapse supernova can vaporize interstellar objects near the explosion site. During the age of the Milky-Way galaxy, about ten billion years, every location within its stellar disk was exposed to a supernova explosion at a distance of about 30 light years. An interstellar object intercepts a small fraction of the explosion energy, given by the ratio of its cross-sectional area and the surface area of a sphere with a radius equal to the supernova distance.
Taking account of the binding energy of the toughest shields that our technologies can manufacture, I calculated before my morning jog today that the protective layer of an interstellar craft must be thicker than a meter in order for it to withstand passage within a few light years from a supernova explosion. This happens to be the characteristic distance between stars in the vicinity of the Sun. When designing an interstellar journey, one must avoid passing within that distance from massive stars that could potentially explode.
Those travelers who unfortunately came too close to an exploding star had their ashes spread by the supernova ejecta through the ocean of interstellar space. I wonder whether `Oumuamua could have been a broken piece from the wreckage of an interstellar craft. If a craft was destroyed by the radioactive nuclear ejecta of an exploding star, we could call its debris an `interstellar broken arrow’.
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.
