TUCSON (Arizona) — Earth gets a new moon most months, but last month, we got two.

About 4am on Feb 15 at the Mount Lemmon Observatory, 9,000 feet above Tucson, Arizona, two astronomers from the Catalina Sky Survey, Dr Kacper Wierzchos and Dr Theodore Pruyne, watched as their computer screens registered a dot moving against a static background of stars.

“It didn’t seem to be any different than the other near-Earth asteroids that we discover,” Dr Wierzchos said, “except that it was found to be orbiting Earth instead of the sun”.

If the discovery holds up, the object, named 2020 CD3 for now, would be the second minimoon ever found.

The solar system is full of primordial crumbs, most of which circle the sun in the asteroid belt between Mars and Jupiter. Sometimes, Jupiter’s gravitational influence sends those space rocks careening toward the inner solar system, where some could threaten Earth.

While they orbit near us, they don’t orbit us. That’s what makes 2020 CD3 so rare. Around 18 months to a year ago, the Earth-moon system’s gravity captured the tiny rock in an orbital dance.

Ephemeral Earth companions may be very common, according to Dr Michele Bannister, an astronomer at the University of Canterbury in Christchurch, New Zealand.

“They are orbiting roughly the same space that we are, and some will get into the right spot where it can nudge into a ballet with us,” she said. “And then it’s like any dance: You do a couple spins together and go your separate ways. There’s something beautifully transient about it.”

Astronomers at the Minor Planet Centre, an international body that tracks asteroid discoveries, announced the find on Feb 25. With only a few nights of data, it’s too early to say exactly what 2020 CD3 is made of. But many astronomers are convinced it is not a leftover from a rocket launch or other human activity.

It might be the size of a small car. “It would probably fit in a bedroom, even in San Francisco or New York,” said Dr Alessondra Springmann, an astronomer at the University of Arizona.

More observations will help astronomers determine when it arrived. But it is expected to leave Earth’s orbit in about two weeks, said Dr Paul Chodas, who directs Nasa’s Centre for Near Earth Object Studies.

“We’re catching this little guy on its way out,” he said.

Earth shares its neighbourhood with a coterie of objects. “Quasi-moons” are asteroids that orbit the sun but are close enough to Earth to seem like tiny moons moving backward.

“Horseshoe” asteroids circle the sun, but Earth’s gravity shoos them away from our planet and forces them into odd U-shaped orbits. Two clouds of charged dust particles, known as the Kordylewski clouds, are parked in a gravitational nexus between Earth and the moon.

And Earth has one known Trojan asteroid, a rock that stays with a planet, leading or trailing its annual march. But none of these are true satellites like the moon, or now 2020 CD3.

The previous moonlet orbited Earth in 2006 and 2007 before rejoining its fellow asteroids. Some observers initially thought that object, designated 2006 RH120, was a piece of a rocket booster from the Apollo 12 mission, but astronomers eventually determined it was a rock. It’s expected to return in August 2028.

Some amateur observers said 2020 CD3 might also be space junk. But a piece of rocket would move differently through space, Dr Chodas said.

Astronomers are scrambling to swing as much glass as they can toward the object to determine its nature, but Dr Chodas said 2020 CD3 was growing dimmer and would most likely be too faint to see by June.

Whatever happens to 2020 CD3, it will not be the last space rock to join the moon around Earth.

When the forthcoming Vera Rubin Observatory begins taking pictures of the entire sky, astronomers might be able to find a new minimoon every few months, according to an analysis by Dr Grigori Fedorets, an astronomer at Queen’s University Belfast.

At any given time, the Earth probably hosts a minimoon 2 feet across, and every decade or so it captures a moonlet as large as 2020 CD3, Fedorets said.

It will take that telescope and other proposed space missions to spot them all, said Dr Amy Mainzer, an astronomer at the University of Arizona who leads a team designing a new satellite called the Near Earth Object Surveillance Mission.

“We can only see what we have the technology to see,” she said. “And necessarily, that’s not everything all of the time.” THE NEW YORK TIMES