A Temporary Earth Moonlet

[Recusant]

This story brought to mind the ongoing QI "How many moons?" item.

"Earth Seems to Have Captured an Additional Moon, And We Didn't Notice For 3 Years" | ScienceAlert

In the skies above Earth, astronomers with the Catalina Sky Survey have spotted what might be a new friend: an asteroid temporarily captured by our planet's gravity, what we call a minimoon.

It's named 2020 CD3, a small chunk of likely carbonaceous rock between 1.9 and 3.5 metres (6.2 and 11.5 feet) in diameter. And here's the kicker - the rock's trajectory indicates it's been in orbit for around three years already.

The near-Earth neighbourhood is a relatively busy place, with boatloads of asteroids zipping past. The precise numbers, however, are unclear; estimates put the number at millions, but as of February 25, the number discovered was only 22,211.

That's because asteroids are really small, we don't know where they are (so we don't know where to look), and they typically don't give off a lot of light, even when they're reflecting sunlight.

You'd think that, with so many rocks flying around, they'd get slurped up by Earth's gravity all the time. Well, they do; but most of them don't hang around long enough to become minimoons.

These briefly captured space rocks either head straight into the atmosphere, where they are burned up on entry, becoming spectacular fireballs; or they skim around for a partial orbit before their velocity carries them onwards and upwards.

It's only the smallest number of passing space rocks that are likely to become minimoons. According to a 2012 supercomputer simulation including 10 million virtual asteroids, only 18,000 got captured in Earth orbit.

So not only are they hard to spot, they are super-rare.

[Continues . . .]

[Recusant]

Another moonlet will visit the Earth, though this may be space junk rather than a natural body. 

"Earth Is About to Capture a Minimoon, But There's Something Odd About This One" | ScienceAlert

We all know and love our Moon. It's been Earth's constant companion for billions of years, a mainstay of the skies. But it's not our only companion.

Every now and again, a smaller object gets temporarily captured in our planet's orbit, hanging around for a short period of time - a few months or years - before being flung out back into space.

We call these objects minimoons, and while we have made a few tentative detections of such temporarily captured asteroids, only two have ever been confirmed - 2006 RH120, which visited in 2006 and 2007, and 2020 CD3, in Earth orbit from 2018 to 2020.

Now astronomers have spotted a new object, named 2020 SO, on an incoming trajectory that is likely to see it temporarily captured by Earth's gravity. Projections have an object arriving next month, in October 2020, and hanging around until May 2021, when it will depart for environs elsewhere.

As you can see in the simulation below, the object's trajectory suggests it will enter and leave through two of Earth's Lagrange points, gravitationally stable points created by Earth's gravitational interaction with the Sun.

[Link to tweet with a depiction of the orbit of the object.]

[Snipped description of oddities in velocity and trajectory.]

All this points to the object potentially being space junk; specifically, according to Paul Chodas of JPL, the discarded Centaur stage of a rocket that launched an experimental payload called Surveyor 2 to the Moon in September 1966.

[Continues . . .]

[Tank]

I hope they keep a detailed eye on it.