From late September to late November this year, a “mini-moon,” dubbed 2024 PT5 by astrologers who spotted it approaching, will orbit the planet. Although this mini-moon is not visible to the naked eye (it is only 10 meters in diameter), it can be observed using a powerful telescope.
Mini-moons are asteroids that have been pulled by Earth’s gravity into orbit around the planet and remain there until they are dislodged and drift away again. The length of time these mini-moons remain in orbit depends on the speed and trajectory at which they approach Earth.
Most mini-moons that enter Earth’s orbit are difficult to see because they are too small and not bright enough to be seen against the darkness of space.
What exactly is a mini-moon?
Mini-moons are extremely rare. Asteroids are usually pulled into Earth’s orbit by the planet’s gravity once every 10 to 20 years, but a few have appeared in recent years. They can remain in the exosphere, which is about 10,000 km above the Earth’s surface.
On average, mini-moons remain in orbit around Earth for a few months to two years, with the asteroid eventually breaking free of Earth’s gravitational pull and then returning to space to resume a trajectory away from the planet.
Similar to other rocky bodies in space, mini-moons can be composed of a mixture of metallic substances, carbon, clay and silicates.
According to a 2018 study on minimoons published in the Swiss journal Frontiers in Astronomy and Space Sciences, most minimoons come toward Earth from the asteroid belt between Mars and Jupiter.
Unlike Earth’s permanent moon, mini-moons do not have a stable orbit. Instead, they follow a “horseshoe” trajectory due to asteroids being constantly pulled forward and backward by Earth’s gravity.
This orbital instability allows asteroids to gradually move away from Earth’s gravitational pull. Once the mini-moon has escaped Earth’s gravitational pull, it is released into space.
Although mini-moons are generally rare, several have been identified in Earth’s orbit since 2006.
In 2006, RH120, Earth’s first confirmed mini-moon, about 2 to 4 meters in diameter, was captured orbiting Earth for about a year. It was the only mini-moon ever photographed. The Southern African Large Telescope (SALT) was used to capture the image. It was spotted by the Catalina Sky Survey (CSS), set up by NASA using telescopes near Tucson, Arizona, in 1998 to search for “near-Earth objects.”
The mini-moon NX1 2022, with a diameter of between 5 and 15 meters, was first observed in 1981 and again in 2022.
It is scheduled to return to Earth’s orbit to resume a horseshoe orbital trajectory in 2051.
What do we know about the last mini-moon?
The asteroid currently approaching the planet is known as 2024 PT5. It was first spotted on August 7 by the NASA-funded Asteroid Earth Impact Alert System (ATLAS), located at the Haleakala Observatory on the island of Maui, Hawaii.
The system continuously scans the sky while identifying and tracking near-Earth objects that could pose a threat to Earth or provide an opportunity to gather important scientific knowledge.
“Any time an object with an orbit so similar to Earth’s is discovered, there is a chance that we are simply recovering space debris,” said Raúl de la Fuente Marcos, an astronomer at the Complutense University of Madrid and co-author of the study.
However, astronomers in the study have now confirmed that 2024 PT5 is an asteroid.
Astronomers have determined that the mini-moon will complete a full orbit around Earth from September 29 to November 25, before finally breaking free of Earth’s gravitational pull and heading into space.
Are there other types of moon?
Besides our permanent moon, which can appear in different forms depending on conditions, there are other types of “moon.”
Ghost Moons
Also known as Kordylewski clouds, ghost moons are concentrations of dust typically found in the Lagrange points of the Earth-Moon system.
These Lagrange points, sometimes called gravitational “ideal points,” are the places where the gravitational forces of the Earth and the Moon meet, allowing the phantom Moon to maintain a stable position.
These clouds can measure up to 100,000 km in diameter and were first discovered by Polish astronomer Kazimierz Kordylewski in the 1960s using a technique called polarimetry, which measures the direction in which light waves vibrate. These dust clouds were later confirmed in 2018 by the Royal Astronomical Society.
Quasi-moons
These moons share an orbit with Earth around the Sun, but do not orbit Earth themselves. Instead, a quasi-moon follows a path around the Sun that closely matches Earth’s orbit, but is not exactly the same.
In 2016, HO3, a quasi-moon, was discovered by astronomers using the Pan-STARRS 1 telescope in Hawaii. Pan-STARRS (Panoramic Survey Telescope and Rapid Response System) is a project designed to detect near-Earth objects such as asteroids or comets, which originate from regions further away than the asteroid belt between Mars and Jupiter.
2016 HO3 is between 100 and 300 meters in diameter and, according to scientists, will continue to orbit the Sun for hundreds of years. It is not known how long it has been orbiting the Sun.
Other celestial bodies, such as planets, moons, and asteroids, can also be orbited by quasimoons. Venus, Jupiter, Saturn, Neptune, and Pluto all have quasimoons that will eventually change course and leave their orbit.
Even the asteroid Ceres, currently located in the constellation Sagittarius and classified as a dwarf planet with a diameter of about 940 km, has its own quasi-moon.
The first quasi-moon ever detected, Zoozve, was discovered on November 11, 2002, by astronomer Brian A. Skiff at the Lowell Observatory in Arizona. The asteroid has a calculated diameter of about 236 meters (about 775 feet).
Fortunately, no known quasi-moon has escaped its orbital path and nearly hit Earth.
Are we able to study these asteroids?
Yes. China’s Tianwen-2 mission is a space exploration project scheduled to launch in 2025. The mission aims to collect samples from the near-moon asteroid 469219 Kamo’oalewa, which is about 40 to 100 meters long. Asteroid 469219 Kamo’oalewa was discovered on April 27, 2016, by the Pan-STARRS 1 asteroid survey telescope at the Haleakala Observatory in Hawaii.
The Tianwen-2 mission is not the only project to collect samples from an asteroid, however. The first mission to successfully collect samples from an asteroid was the Hayabusa mission, launched on May 9, 2003, by the Japan Aerospace Exploration Agency (JAXA).
The spacecraft landed on the 535-meter asteroid 25143 Itokawa on September 12, 2005, and successfully collected samples on November 19 and 25, 2005, then returned to Earth on June 13, 2010.
Several other asteroid-collecting missions have also been launched from Japan. The Hayabusa 2 mission was launched on December 3, 2014, to collect samples from the 900-meter-large asteroid 162173 Ryugu. Samples were successfully collected on February 21 and July 11, 2019. The spacecraft returned to Earth on December 6, 2020.
The OSIRIS-REx mission was launched by NASA on September 8, 2016, to collect samples from the near-Earth asteroid 101955 Bennu (492 meters). OSIRIS-REx arrived at Bennu on December 3, 2018, and collected samples on October 20, 2020. The samples returned to Earth on September 24, 2023.
NASA has announced that OSIRIS-APEX, a follow-up mission to OSIRIS-REx, will study the asteroid Apophis as it makes a close approach to Earth in 2029.
