From September 29 to November 25, Earth temporarily hosted a small moon, designated 2024 PT5. This 11-meter rock orbits the Sun but was briefly captured by Earth's gravity, tracing a loop around the planet before resuming its journey through space. Recent observations confirm that 2024 PT5 is an ancient fragment of the Moon, which itself originated from Earth.
Discovered in August by the South African telescope in Sutherland as part of the ATLAS project, 2024 PT5 was identified by astrophysicists Carlos and Raúl de la Fuente Marcos from the Complutense University of Madrid. Their trajectory calculations, published in September, indicated that this near-Earth object (NEO) would act as a quasi-satellite for nearly two months.
This marks the sixth temporary mini-moon identified by scientists, although some debate its classification due to its incomplete orbit. During its stay, multiple observatories studied 2024 PT5 to determine its origin.
According to Carlos de la Fuente Marcos, the mini-moon belongs to the Arjunas, a secondary asteroid belt following an orbital path similar to Earth’s. Potential origins for these objects include the main asteroid belt between Mars and Jupiter, the inner Solar System, or fragments ejected from the Moon due to impacts. The lunar origin is considered the most likely for 2024 PT5.
Three new studies, awaiting formal publication, support the lunar origin hypothesis. One study, likely to be published in A&A Letters, involved the de la Fuente Marcos brothers and scientists from the Institute of Astrophysics of the Canary Islands. Using the Gran Telescopio Canarias and other telescopes, they refined the orbital calculations of 2024 PT5.
Analysis of the visible light spectrum reflected by the mini-moon reveals a basaltic composition consistent with lunar material and a rotation period of less than one hour. This suggests 2024 PT5 formed during the last million years from an impact crater. Other studies corroborate these findings, with one set to be published in Astrophysical Journal Letters, which includes infrared spectrum data.
The Lowell Observatory began investigating 2024 PT5 in August when it was brightest. Researchers found that the only material reflecting light like PT5 consists of lunar rocks and dust. Both Carlos de la Fuente Marcos and planetary scientist Theodore Kareta emphasize that the mini-moon’s orbit, closely resembling Earth’s, is rare for main-belt asteroids, further supporting its lunar origin.
2024 PT5 is the second known lunar fragment in space, following Kamo'oalewa, identified in 2016. The presence of two such NEOs suggests there may be more awaiting discovery. Carlos de la Fuente Marcos states that this finding implies many Arjunas may be of lunar origin, enhancing understanding of our Moon's history.
Additionally, the origin of these NEOs poses implications for planetary defense, as they represent a new source of potential impact threats to Earth. Discoveries like 2024 PT5 underscore the vigilance of telescopes in the ATLAS project, which continuously monitor for potential space hazards.