On May 18, 2024, a stunning fireball of a fast meteor was spotted over Spain and Portugal. According to NASA’s CNEOS catalog of fireballs, the meteor was moving at a speed of 40.4 kilometers per second relative to Earth and unleashed an explosive energy of about a percent of the Hiroshima atomic bomb.
This fast speed relative to Earth is similar to that of the two interstellar meteors in the CNEOS catalog, which were detected on January 8, 2014 (IM1) and March 9, 2017 (IM2) with speeds relative to Earth of 44.8 and 36.5 kilometers per second, respectively.
Coincidentally, a brilliant postdoctoral fellow at the Harvard Graduate School of Design, David Jiménez Moreno, who received his PhD in Spain on Space Architecture, visited my office today and told me that I am the most followed scientist in Spain at this time. This may explain why several Spanish fans of my work emailed me today to ask whether this new meteor might have been interstellar. Given that my latest book was titled “Interstellar” and my DNA is half Spanish in origin, I felt obliged to explain what makes an object interstellar.
The key for inferring whether an object originated from outside the solar system is its speed relative to the Sun, not Earth. The escape speed from the solar system is 42.1 kilometers per second at the Earth’s distance from the Sun. However, since Earth is moving around the Sun with a velocity of about 30 kilometers per second, the Earth may collide head-on with objects that are gravitationally bound to the Sun but move opposite to Earth with an impact speed larger than 42.1 kilometers per second. This means that not all fast meteors are interstellar, for the same reason that some of the highest-impact car crashes result from cars that move on opposite lanes at relatively modest speeds.
On the other hand, IM1 arrived at Earth roughly from behind the Earth’s motion around the Sun. Even though its (heliocentric) speed relative to the Sun was 60 kilometers per second, its impact speed on Earth was slower. For IM2, the heliocentric speed was 50 kilometers per second, again larger than the Earth-based (geocentric) speed.
However, the recent meteor over Spain and Portugal collided with Earth nearly head on and as a result had a higher impact speed relative to Earth than its orbital speed around the Sun.
If IM1 had a different direction of motion and collided with Earth exactly head-on, its speed relative to Earth could have been as high as 90 kilometers per second. This would have made it much brighter, increasing its fireball energy by a factor of 4.
All of this makes sense because we are not at the center of the Universe. The Earth moves around the Sun and the Sun moves through interstellar space around the center of the Milky-Way galaxy. The objects impacting the Earth originate from two distinct populations: those which are gravitationally bound to the Sun and the interstellar ones. The recent expedition of the Galileo Project to IM1’s fireball site is consistent with this notion as it recovered materials with a chemical composition that is different from familiar Solar system materials.
Based on the CNEOS catalog, the new meteor originated from a highly elliptical orbit that reached a farthest distance from the Sun (aphelion) larger by a factor of 5 from the Earth-Sun separation - likely a former member of Jupiter’s family, and a closest distance from the Sun (perihelion) that is 8 times closer than Earth’s.
The size of the new meteor was estimated as similar to the meter-scale diameter of IM1 or IM2, yielding an original object mass of order a ton. However, its material composition must have been very different from IM1 or IM2 based on the high altitude at which it disintegrated.
The European Space Agency tweeted that the new meteor was likely a piece of a comet. Its low material strength was demonstrated by the fact that it exploded at an altitude of 74.3 kilometers, where the air density is 10,000 lower than at sea level. In contrast, IM1 and IM2 disintegrated at altitudes of 18.7 and 23 kilometers, respectively, where the air density is about a tenth of the sea level value. Given the similar speeds of all three objects relative to Earth, IM1 and IM2 were subjected to a ram pressure or stress which is a thousand times bigger than the new meteor. IM1 is the record holder in material strength among all meteors in the CNEOS catalog, making it an outlier even relative to the toughest iron meteorites.
Let us put all of this in a familiar context. The mass of Elon Musk’s Tesla Roadster car, launched into space as a dummy payload by SpaceX in 2018, is also about a ton, similar to the masses of IM1, IM2 and the new meteor. This Tesla car is now on an elliptic orbit around the Sun. Detailed numerical calculations of its future dynamics suggest that within 15 million years it has a 20% probability of colliding with Earth. If such a collision were to occur nearly head-on, its fireball would resemble those of IM1 or IM2 more closely than the new meteor because the car’s engine is made of aluminum, titanium and boron steel.
This naturally raises the question of whether IM1 or IM2 were launched by an interstellar space entrepreneur that preceded Elon Musk by billions of years, the age difference between most stars and the Sun. In that case, there was plenty of time for this space trash to reach Earth from the entire Milky-Way galaxy. The only way to find out the origin is by seeking the core of IM1 or IM2 in the wreckage they left behind on the ocean floor. This is indeed the goal of the next Galileo Project expeditions. We might find an unusually tough rock or a Tesla-like car engine.
In the latter case, Elon might want to compare notes with the aliens. This may explain why he is paying attention: a day after I posted an essay about Dyson spheres this week, Elon tweeted about them.
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.
