For the next two months, Earth will have a cosmic guest. A so-called "mini moon" will join our actual moon in circling the planet until Nov. 25.
What exactly is the mini moon? It's actually a small asteroid -- only 33 meters across, the size of a bus and 300,000 times smaller than Earth's -- that has temporarily entered Earth's gravitational pull, says Jacqueline McCleary, an assistant professor of physics at Northeastern University.
"Perturbations -- little kicks from Jupiter's gravity, the sun's gravity, Earth's gravity -- can sometimes knock an object a little bit closer so that it falls in towards the Earth," McCleary explains. "For an object to be orbiting or be a satellite, it would have to be traveling at a speed lower than the escape velocity."
"So, technically, it will be captured by the Earth's gravitational potential," she adds. "But instead of doing a complete orbit like a regular moon, it's more going to be deflected and then come back in and then kicked back out again and return to more or less its previous orbit around the sun."
This mini moon, asteroid 2024 PT5, came from the Arjuna asteroid belt, a group of small asteroids that were discovered only in the last 30 years, and it is set to return to Earth's orbit in 2055. McCleary says these moonlets are actually quite common.
"About every 10 to 20 years an object probably from the Arjuna asteroid belt falls into the gravitational influence of the Earth-moon system and has its orbit altered enough that technically it can be considered a satellite of the Earth as opposed to just moving in a heliocentric orbit," McCleary says.
Unfortunately, for those hoping to catch a glimpse of the mini moon, it's too small and dark to be seen with the naked eye. Unless you have access to a powerful telescope, Earth's little cosmic guest will be out of sight for most people.
However, skygazers will be able to catch a close glimpse of another asteroid before too long, though it might be a little too close for comfort. In 2029, Apophis, a much larger asteroid, will closely approach Earth and serve as a reminder that not every asteroid that comes Earth's way is as small or harmless as the mini moon.
Named after the ancient Egyptian serpent demon and "Lord of Chaos" Apep, the asteroid measures 1,100 feet or 355 meters. In comparison, the asteroid that exploded over a Siberian forest in 1908, the largest impact event in recorded history, was smaller than 100 meters in diameter. The asteroid that wiped out the dinosaurs was about 10 kilometers across.