The final decision was reached today. Within a couple of months, I will be leading an expedition to collect the fragments of the first interstellar meteor. This meteor is the first near-Earth object ever detected by humans from outside the solar system. In anticipation of meeting it, I would not mind sleeping on the open deck of a boat and taking risks associated with a trip to the Pacific Ocean. Elon Musk dreams of dying on Mars. I feel content with staying on Earth, as long as I will have the opportunity to hold an interstellar fragment in my hands.
The Galileo Project expedition received more than a million dollars in funding. We have a boat. We have a dream team, including some of the most experienced and qualified professionals in ocean expeditions. We have complete design and manufacturing plans for the required sled, magnets, collection nets and mass spectrometer. And most importantly, today we received the green light to go ahead.
What is all the fuss about?
On January 8, 2014, an object from interstellar space, now labeled IM1, collided with Earth at a speed of 45 kilometers per second. As a result of its friction with air, the object disintegrated into tiny fragments about a hundred kilometers off the coast of Manus Island in Papua New Guinea. The fragmentation increased the collective surface area and hence the friction, accelerating the release of heat and generating a runaway fireball. The explosion liberated a few percent of the energy associated with the Hiroshima atomic bomb within a fifth of a second. The bright flare was detected by US Government cameras. The location was listed in the CNEOS fireball catalog of JPL/NASA to one significant digit after the decimal point in longitude and latitude.
After noticing the object in 2019, I wrote a paper with my student, Amir Siraj, that identified it as the first interstellar meteor ever discovered. The interstellar origin was confirmed in 2022 at the 99.999% confidence level in an official letter from the US Space Command under the Department of Defense to NASA. The confirmation letter was accompanied by the light curve of the fireball, which showed three distinct explosions separated by a tenth of a second. This fireball data allowed us to conclude in a follow-up paper that the meteor was tougher than all other 272 meteors in the CNEOS catalog. Intrigued by this conclusion, I established a team that designed a two-week expedition to search for the meteor fragments at a depth of 1.7 kilometers on the ocean floor. Analyzing the composition of the fragments could allow us to determine whether the object is natural or artificial in origin. The confirmation of our discovery of the first interstellar meteor was recognized by CNN as one of “2022’s extraordinary cosmic revelations and moments in space exploration.”
The published coordinates define the fireball location to within a 10-kilometer region, too large for an efficient search. Gladly, we found that the blast wave from the meteor explosion generated a high-quality signal in a seismometer located at Manus Island. The sound signal includes two broad peaks separated by about a minute, each lasting for tens of seconds. The sound speed in air is much smaller than in water or ground. The first peak begins with a sound path that goes through air from the explosion straight down to the ocean surface and then through the water and ground to the seismometer. The shortest path through air goes directly from the explosion to the seismometer and defines the beginning of the second peak in the seismometer signal. The envelope of that second peak involves the sum over paths where the spherical blast wave in air reflects off the ocean surface in circles of different radii at different times, and with an amplitude that declines inversely with distance from each reflection point. By using a simple geometry of a spherical blast wave bouncing off the ocean surface, Amir and I were able to reproduce the timing of the first peak and the shape of the second peak. Altogether, the model provides many more constraints than free parameters and measures the explosion elevation and distance tightly. We constrained the meteor path to a narrow line within the original USG localization box, narrowing down the search area by nearly two orders of magnitude.
Our “fishing expedition” may collect fragments of different sizes. The meteor size is inferred to be half a meter based on its speed and explosion energy. The huge explosion melted the object into tiny droplets. The smallest fragments were stopped quickly by their friction on air owing to their large surface area per unit mass. They fell straight down from the explosion site as hot rain, raised steam from the ocean surface, and sank down to the ocean floor. Larger fragments continued farther along the original path of the meteor. As a result, we expect to have a strip of fragments on the ocean floor, oriented along the original path of the meteor, with the smallest fragments marking the beginning of the strip straight below the initial explosion site and larger fragments farther along.
How many fragments should we expect of different sizes? This was the focus of a recent paper that I wrote with an intern, Amory Tillinghast-Raby, and Amir. Our forecast depends on composition. For an iron meteorite, we predict about a thousand fragments bigger than a millimeter, whereas for a stainless-steel composition we expect larger sizes, with tens of fragments bigger than a centimeter.
The unusual material strength is not a rare finding within the interstellar meteor population. Recently, I wrote another paper with Amir that identified a second interstellar meteor, IM2, which was detected near Portugal on March 9, 2017 and was also extremely tough.
Both interstellar meteors, IM1 and IM2, collided with Earth from a trajectory that was gravitationally unbound to the Sun. In other words, the objects arrived to the Solar system from interstellar space and were moving faster than the escape speed from the Sun when they were collected by the “fishing net” of the Earth’s atmosphere.
The second interstellar meteor was ten times more massive and roughly a meter in size. IM2 was moving at a speed of 40 (compared to 60 for IM1) kilometers per second relative to the Local Standard of Rest, the local frame of reference of the Milky Way that averages over the random motions of all the stars in the vicinity of the Sun.
Remarkably, both IM1 and IM2 disintegrated low in the Earth’s atmosphere despite their unusually high speeds. The ram pressure, which is the product of the air mass density and the square of the meteors’ speed when they disintegrated, provides an estimate for the yield strength of their material. The inferred yield strengths of 194 Mega-Pascals (MPa) for IM1 and 75 MPa for IM2 imply that both were tougher than iron meteorites which have a maximum yield strength of 50 MPa.
IM1 and IM2 ranked number 1 and 3 in the distribution of material strengths among all 273 meteors in the CNEOS catalog. The probability of drawing the material strength of the first and second interstellar meteors out of the familiar population of Solar system rocks is roughly the square of (3/273), or equivalently one part in 10,000. This means that the population of interstellar meteors is different from Solar system meteors at the 99.99% confidence level. This conclusion is corroborated by fitting the distribution of CNEOS meteors with a Gaussian shape in the logarithm of material strength. Both IM1 and IM2 lie on the far tail of the distribution, at 2.6 and 3.5 standard deviations away from the mean, making their combined likelihood less than a part in a million in this context.
This tantalizing conclusion about the extremely rare material strength of IM1 and IM2, implies that interstellar meteors may not be rocks from planetary systems like the Solar systems. In that case, what could be their origin?
The Earth collides with interstellar objects along its orbit around the Sun. The simplest assumption to make is that these are natural objects that arrive into the Solar system on random trajectories in the Local Standard of Rest. Based on the detection rate of IM1 and IM2 in the CNEOS catalog, roughly once per decade, one finds that up to a third of all refractory elements in the Milky-Way galaxy must be locked in meter-scale interstellar objects if IM1 and IM2 are natural in origin. This extraordinarily high abundance again seems to defy a planetary system origin.
Supernovae have been observed to produce iron-rich bullets. For example, X-ray imaging of the Vela supernova remnant revealed bow shocks from objects flying out of the explosion site, a discovery that I attempted to explain three decades ago. It is possible that IM1 and IM2 have unusually high material strength because they were produced in the ejecta of an exploding star or in collisions of two neutron stars. These explosive events produce the heaviest elements, but the ejecta must be slowed down to the speed of tens of kilometers per second, characteristic of IM1 and IM2, before making these objects.
Alternatively, it is also possible that IM1 and IM2 are tough because they are artificial in origin, resembling our own interstellar probes but launched a billion years ago from a distant technological civilization. The advantage of an artificial origin is that it reduces the inferred abundance of interstellar objects from nearly 10 to the power of 24 (a trillion trillions) per star like the Sun to a much more reasonable number.
In case we recover a sizable technological relic from the Pacific ocean, I promised the curator of the Museum of Modern Art, Paula Antonelli, that I will bring it for display in New York. This piece would represent modernity for us, even though it is a relic of ancient history for the senders. Such a technological relic could be of great interest not only to art collectors but also to entrepreneurs from Silicon Valley. For my take on the context, click here.
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. His new book, titled “Interstellar”, is scheduled for publication in August 2023.
