Earth Will Capture a New Moon

Raise your hand if you have never raised your eyes to the sky and been fascinated by the sight of the Moon, our faithful companion. If you are reading our blog, probably this happened to you a few times. I have good news and bad news for you. The good one: the Earth’s gravity will temporarily capture a new moon. The bad one: it won’t be visible either with the naked eye or amateur telescopes (you will need a 30-inch telescope paired with a CCD camera). Let’s try to clarify.

2024 PT5 passes by to say hello

Among the Near Earth Objects (NEOs) there’s a group of asteroids that form the Arjuna belt. They have an orbit similar to the Earth’s, which means low orbital inclination, an orbital period of around one terrestrial year, and a low eccentricity value (their orbit is almost a circumference). It is precisely from these objects that comes asteroid 2024 PT5. It is a tiny asteroid spotted by the Asteroid Terrestrial-Impact Last Alert System (ATLAS) and that will be our companion for around two months before returning to where it came from.

A calculation of asteroid 2024 PT5's position around Earth as it temporarily is captured and becomes a mini-moon. Credit: NASA JPL small body lookup database — *A calculation of asteroid 2024 PT5’s position around Earth as it temporarily is captured and becomes a mini-moon. Credits: NASA JPL small body lookup database*

As Professor Carlos de la Fuente Marcos stated to Space.com, sometimes it can happen that some objects from the Arjuna belt approach Earth at low velocities (around 3540 km/h) and close range (around 4.5 million km). Marcos added: “You may say that if a true satellite is like a customer buying goods inside a store, objects like 2024 PT5 are window shoppers.”

What are mini-moons?

Despite being a fascinating event, these so-called “mini-moons” are not that rare. Some of them remain with us for a year or two, others just for a few days, weeks, or, at most, months. Their trips outside the belt have been attributed by astronomers mainly to the Sun’s gravitational disturbances.

This image shows the far side of the moon, illuminated by the sun, as it crosses between the DSCOVR spacecraft’s Earth Polychromatic Imaging Camera (EPIC) camera and telescope, and the Earth – one million miles away. Credits: NASA/NOAA

These events are the occasion for scientists to study in detail asteroids, searching for their origins and composition. This research is important since, despite populating massively the Solar System, they are still quite mysterious.
Moreover, they could be targets for robotic missions to bring them in a stable orbit around our planet, for further investigations, or, in the future, for mining their resources.

ATLAS: the asteroid hunter

ATLAS is a system funded by NASA that is composed of 4 telescopes, two located in Hawaii (Mauna Loa and Haleakala), one in Chile, and one in South Africa. They automatically scan the sky every night looking for moving objects, which implies not only potentially hazardous asteroids but also, as in our case, mini-moons and space junk.
Each system combines a small telescope (50 cm in opening, f/2 Wright-Schmidt system) with a powerful camera (110 Megapixel CCD), able to scan 25% of the whole sky twice and see objects up to magnitude 20. As we can read on the ATLAS website, it is like observing a flame in New York from San Francisco.

From left to right: Sutherland ATLAS station during construction in South Africa. Credit: Willie Koorts (SAAO); Chilean engineers and astronomers installing the ATLAS telescope at El Sauce Observatory. Credit: University of Hawaii; Illustration of NASA’s DART spacecraft and the Italian Space Agency’s (ASI) LICIACube prior to impact at the Didymos binary system. NASA/Johns Hopkins, APL/Steve Gribben; Illustration of the NEO Surveyor spacecraft. — From left to right: Sutherland ATLAS station during construction in South Africa. Credits: Willie Koorts (SAAO); Chilean engineers and astronomers installing the ATLAS telescope at El Sauce Observatory. Credits: University of Hawaii; Illustration of NASA’s DART spacecraft and the Italian Space Agency’s (ASI) LICIACube prior to impact at the Didymos binary system. Credits:
NASA/Johns Hopkins, APL/Steve Gribben; Illustration of the NEO Surveyor spacecraft. Credits: IPAC-Caltech

The capture is estimated to happen on September 29, making 2024 PT5 our mini-moon until November 25 when it will escape the Earth’s gravity heading back to the Arjuna belt.