The First Interstellar Meteor
On March 31, 2019, I attended a radio interview about a meteor over the Bering Sea which was spotted off Kamchatka’s peninsula on December 18, 2018 after producing a blast with ten times the energy of the atomic bomb over Hiroshima. In preparation for the interview I searched the literature online and came across a catalog of all meteors over the past three decades, ordered by the strength of the fireball they produced. These objects were discovered by a classified set of sensors owned by the US government (USG), which determined the three-dimensional components of their velocity and location at the time of impact.
Impressed by the most powerful fireballs, I asked my undergraduate student, Amir Siraj, who has been working with me on `Oumuamua — the interstellar object discovered on October 19, 2017, to calculate the past trajectory of the fastest meteors in this CNEOS catalog, starting from their detected position and velocity at impact and taking account of the gravity of the Earth, the Sun and all distant planets within the Solar System.
The trajectory of the fastest object ended up being bound to the Sun since it involved a head-on collision with the moving Earth. The second fastest was definitely unbound to the Sun. It moved so fast that its past trajectory was inferred to be unbound to the Sun. We calculated that its excess speed should have been a whopping ~40 kilometers per second outside the Solar System. A few days later, we posted our paper on the arXiv.
We initially calibrated our statistical confidence by estimating error bars based on documented differences between related measurements. Critics were quick to dismiss our conclusions on Twitter, Facebook and e-mail, by repeatedly referring to another paper which asserted that the “USG sensors data are generally unreliable for orbit calculations.” But in recent days — three years after our preprint was posted, I was informed that I will receive an official USG confirmation that this meteor was indeed of interstellar origin, making it the first interstellar object to have been discovered, as it predated the discovery of `Oumuamua by 3.75 years.
In retrospect, meteoric fireballs offer a unique opportunity for learning about interstellar objects. The traditional search method for interstellar objects uses the Sun as a lamppost and finds objects from the light they reflect. This is how `Oumuamua was detected by the Pan STARRS telescope. The limitation of this search method is that it is only sensitive to large objects, in the case of Pan STARRS — larger than a hundred meters, since smaller objects are too faint to be detected. Naturally, one would expect smaller objects to be more abundant. So much so, that some of these might hit the Earth at a noticeable rate despite its small cross-sectional area. And so, the Earth’s atmosphere ends up serving as a detector for interstellar objects of roughly the size of a person.
There is an added benefit to this detection method: since meteors burn up in the atmosphere, spectroscopy of the gases they emit reveal their composition even if they leave no relic behind. And if they do leave a relic, we can put our hands on it and study its nature.
This particular interstellar meteor was spotted just north of Manus Island, off the coast of Papua New Guinea on January 8, 2014 at 17:05:34 UTC. It produced a blast of merely a percent of the Hiroshima bomb, implying a meter-size object with a mass of about 500 kg. Given the inferred impact rate of roughly once per decade, we concluded that there should be roughly a million such objects inside the orbit of the Earth around the Sun. These meter-size objects carry as much mass per volume of space as `Oumuamua-like objects that are a hundred times bigger, assuming that both populations possess random velocity distributions in interstellar space.
To make the inferred population of meter-size interstellar meteors, each star needs to eject about 10 to the power 22 objects, totaling about an Earth mass worth of material. This requirement is at tension with the expected mass in planetesimals.
In the future, astronomers may establish an alert system that triggers follow-up spectroscopic observations to an impact by a meteor of possible interstellar origin. Alert systems already exist for gravitational wave sources, gamma-ray bursts, or fast radio bursts at the edge of the Universe. Even though interstellar meteors reflect our immediate cosmic neighborhood, they offer fascinating new information about nurseries which may be very different from the Solar System.
The detection of this meteor offers new prospects for “interstellar panspermia”, namely the transfer of life between planets that reside in the habitable zones of different stars. This might be possible as long as the meteor is big enough so that it does not burn up completely in the Earth’s atmosphere. In order to seed life, the required impact rate is merely once per billion years, allowing for a much bigger object that could survive entry to the target planet.
Some interstellar objects may be artificial in origin, representing technological equipment from alien civilizations just like the spacecraft we launched away from the Solar system. They would appear like a plastic bottle swept ashore on the background of natural rocks. We do not know the composition or nature of the 2014 interstellar meteor. But we do know that NASA never launched an `Oumuamua-scale spacecraft, the size of a football field. However, it did launch many spacecraft on the scale of the 2014 interstellar meteor.
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 chairs the advisory board for the Breakthrough Starshot project, and 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.