Scientists Announce Discovery of Earth’s ‘Second Moon

NASA scientists have predicted that Earth would capture a “second moon” on Sunday, September 29. This temporary “mini-moon” is actually the small asteroid 2024 PT5, which typically orbits the sun as part of a small asteroid belt that trails Earth.

While Earth’s primary moon has remained in orbit for around 4 billion years, asteroid 2024 PT5 will only be a short-term visitor, departing before the year ends.

“According to NASA’s Jet Propulsion Laboratory Horizons system, the temporary capture starts at 15:54 EDT (1954 UTC) and ends at 11:43 EDT (1543 UTC) on November 25,” said Carlos de la Fuente Marcos, a mini-moon expert and professor at Universidad Complutense de Madrid, in an interview with Space.com on Wednesday, September 25.

2024 PT5 belongs to the Arjuna asteroid belt, which contains space rocks with orbits similar to Earth’s, located at an average distance of about 93 million miles (150 million kilometers) from the sun. These asteroids are part of the near-Earth object population, Marcos explained.

Despite the intrigue of a “second moon,” such gravitational capture events are relatively common. Some objects from the Arjuna belt can approach Earth at distances as close as 2.8 million miles (4.5 million kilometers) and with low speeds of less than 2,200 miles per hour (3,540 km/h). However, 2024 PT5 won’t complete a full orbit around Earth. Marcos likened it to “a window shopper” compared to a true satellite that would “buy something in the store.”

After its brief stay, 2024 PT5 will resume its orbit around the sun as part of the Arjuna asteroid family. This isn’t the first time Earth has captured an asteroid as a mini-moon; scientists have documented two short capture events in the past, typically lasting about a week, as well as two rarer long-capture events.

There are several key differences between 2024 PT5 and the moon. For one, the asteroid will only remain near Earth for a few weeks, and it won’t be visible to most skywatchers. While the moon is a massive 2,159 miles (3,475 km) in diameter, asteroid 2024 PT5 measures just 37 feet across, making it 308,108 times smaller than the moon.

“The object is too small and dim for typical amateur telescopes and binoculars,” Marcos said. “Professional astronomers, however, may be able to capture images of it using telescopes with a diameter of at least 30 inches along with CCD or CMOS detectors.”