In August 2024, astronomers in South Africa discovered 2024 PT5, a rocky body moving close to Earth at record low speeds. Spectroscopic analysis showed it to be similar to lunar rocks, allowing it to be classified as the second known lunar fragment in Earth orbit.
Image source: Dennys Hess / Unsplash
The object, which was between 8 and 12 meters in diameter, was moving at a relative speed of just 4.5 miles per hour (about 7.2 km/h), making it one of the slowest near-Earth objects ever observed. Only nine asteroids are known to have achieved such a slow approach speed to Earth, which piqued the interest of researchers from the Mission Accessible Near-Earth Object Survey (MANOS), which is searching for and characterizing easily accessible near-Earth bodies.
An image taken in November 2024 by the Two-Meter Twin Telescope shows asteroid 2024 PT5, a potential lunar fragment temporarily crossing Earth’s orbit.
Image source: Two-Meter Twin Telescope / Light Bridges / Instituto de Astrofísica de Canarias
A few days after the discovery, the MANOS team led by Teddy Kareta and Nick Moskovitz pointed the Lowell Discovery telescope at 2024 PT5. Visible and near-infrared observations showed that the object was not a typical asteroid. Its spectral characteristics were similar to rocks previously returned to Earth by the Apollo missions and the Soviet Luna 24 station.
An analysis of 2024 PT5’s composition led researchers to hypothesize that it is a fragment ejected from the lunar surface by an ancient collision. In Caretta’s analogy, Earth was moving along its own “lane” on the cosmic highway, while 2024 PT5 was moving along the inner one. In 2024, the fragment changed orbit and ended up in Earth’s path, approaching it at an almost identical speed. By the end of September, it began to move away. Scientists assume that in 2055, the objects will again end up in similar sections of orbit.
Both fragments are in orbits close to Earth’s, but their origins, physical parameters, and dynamics are different. This diversity suggests that there may be a previously unrecorded population of lunar fragments near Earth. According to Kareta, some of the already classified unusual asteroids may have a lunar origin. The discovery of 2024 PT5 increases the likelihood of such bodies hidden in near-Earth space.
The orbit of 2024 PT5 in a geocentric rotating coordinate system aligned with the ecliptic plane. The left image shows the approaches of 2002–03 and 2055, and the transition to a horseshoe orbit in 2024–25. The center and right images detail the lunar approach in August 2024 and the departure in January 2025. Color corresponds to time: from green to red.
Image source: Kareta T., Fuentes-Muñoz O., Moskovitz N., Farnocchia D., Sharkey B. N. L. / The Astrophysical Journal Letters
The orbits of near-Earth objects are calculated based on the observed parameters of their motion. If some of these bodies were misclassified, and their supposed sources were incorrectly identified, this could mean that other characteristics of their orbits were distorted. According to Kareta, such an error is “almost certainly excluded”, but, as he emphasizes, “it needs to be proven”, since the accuracy of these calculations depends on understanding the possible risks of collisions with Earth in the long term.