Consider the possibility that Elon Musk is not the most accomplished space entrepreneur in the Milky Way galaxy. In that case, 10-meter (30 feet) wide Starship-analogs might be floating through interstellar space. If any of them collided with Earth over the past billions of years, would we notice?
Objects with a diameter of 10-meters impact Earth every few years. Such impacts release roughly the same amount of energy as the atomic bombs dropped on Hiroshima and Nagasaki during World War II, of order 20-kiloton of TNT — equivalent to 84 trillion joules. 71% of the Earth’s surface is covered by oceans and another 10% is covered by desert. This means that 8 out of 10 historic impacts are hidden under the veil of water or moving sand.
Last week, a team of researchers from Imperial College London and Oxford University suggested a better localization for the site of a hypothesized Chinguetti meteorite measuring 40 by 100 meters, which may have been lost in the dunes of the Mauritanian desert after being reported in 1916 by the French Officer, Captain Gaston Ripert. Multiple searches to find the impactor over the past century failed. The lack of a crater can be explained by a meteor trajectory that is nearly tangential to Earth’s surface. The authors’ hope to find a large iron meteorite through its magnetic signal may, however, be futile.
The geologist in the Galileo Project team, Jeff Wynn, shared with me his insights from his past survey with Gene Shoemaker in the Wabar meteorite impact site in Saudi Arabia, first reported by Henry St. John Philby in 1933. The extremely hot Wabar site is more difficult to access than almost any other place on Earth, including Antarctica. It features three distinct craters and an asymmetric distribution of shocked white sandstone, suggesting a shallow oblique angle for the impactor’s path, less than 22 degrees from Earth’s surface. The impactor was at least 10 meters in size, and potentially associated with reports of a huge fireball seen over Riyadh in 1863 heading towards the Wabar site. The explosion distributed a small number of metallic fragments outside the craters rims and none within them. Because of its shallow angle of approach, the impactor was slowed down by the atmosphere to a speed of 7–10 kilometers per second.
The Wabar bolide was 94% iron, 4% Nickel with a small amount of Iridium. Almost all of the impact energy was transformed into heat, producing a melt of 90% local sand and 10% meteoritic material. Molten glass rain fell at least out to 850 meters away from each crater. The sand-iron glass looked superficially like basaltic flow material, misleading Philby to initially consider the impact site as a volcano even though it was located in a moving dune field.
The impact craters had a low reading in Jeff’s magnetic survey. The largest fragment — the so-called “Camel’s Hump” recovered in 1965, had a mass of about two tons. It flaked off in the last part of the atmospheric entry and bounced upon impact. One other substantial fragment the size of a rabbit was recovered at the Wabar site. More fragments, identical in chemistry, fell off about 25 km up-range at a place called Umm al-Hadida (“mother of iron”). The Wabar case implies, according to Jeff, that not much iron should be expected at the bottom of impact craters.
Not all fireballs in terrestrial deserts were created by nature. The Trinity nuclear test in 1945, coordinated by Robert Oppenheimer — director of the Los Alamos Laboratory, released an explosive energy of about 25-kiloton of TNT. The tremendous heat from the explosion melted the desert sand, mostly silica, into a glassy radioactive material — named Trinitite. The sand was initially scooped into the fireball. Inside the fireball, the molten sand behaved like water in a cloud, whereby tiny droplets aggregated into bigger droplets that eventually became too heavy to remain suspended and fell down to the ground as a rain of molten glass. The glass collected on the hot sand to form the observed puddles of Trinitite.
In our ocean expedition to the site of the first recorded interstellar meteor, we discovered differentiated spherules of unique extrasolar composition that had an aggregated morphology, probably as a result of a similar droplet formation process out of the impactor materials. Analogous compound structures of spherules were reported last month from the site of a touchdown airburst that occurred 2.3–2.7 million years ago over Antarctica. This represents the oldest record of a meteor airburst on Earth identified to date, dating back to the period when the human species emerged on Earth.
Computer simulations that were originally constructed to describe human-made nuclear explosions can help in understanding the aftermath of meteor impacts. A supercomputer simulation by scientists from the Sandia Nuclear Laboratory revealed that the Tunguska impactor in 1908 released 3–5 Megatons of TNT in a fireball blast wave that devastated a vast forest with 80 million trees without generating a crater.
The Moon and Mars are scarred by numerous impact craters, far more than Earth. Friction with air burns small impactors and protects the Earth’s surface. In addition, the Earth’s oceans and moving desert sand serve as the Earth’s cosmetic make-up, hiding residual scars from view.
Over the past decade, US Government satellites documented hundreds of meteor fireballs in the CNEOS fireball catalog. The catalog includes at least two interstellar meteors, IM1 and IM2, with estimated diameters of 0.5 and 1 meters. Identifying the unique chemical and isotopic composition of interstellar meteors with velocities above the escape threshold from the solar system, would enable the search for similar fingerprints in historic collections of meteorites. If we search long enough, we might find relics of Starship-analogs from exo-planets. The sky’s the limit.
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.
