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.
Asteroid 2020 SO may get captured by Earth from Oct 2020 - May 2021. Current nominal trajectory shows shows capture through L2, and escape through L1. Highly-chaotic path, so be prepared for lots of revisions as new observations come in. @renerpho @nrco0e https://t.co/h4JaG2rHEd pic.twitter.com/RfUaeLtEWq— Tony Dunn (@tony873004) September 20, 2020
This would be extremely noteworthy in and of itself - but there's a twist. The Earth-like orbit and low velocity of 2020 SO suggest that it's not actually an asteroid; its characteristics, according to experts, are more consistent with something human-made.
2020 SO has been classified as an Apollo asteroid in the JPL Small-Body Database - a class of asteroids whose paths cross Earth's orbit. This class of asteroid often has near-Earth encounters. But there are a few clues that 2020 SO is not like the others.
The object is on an orbit that's just a smidge over a year, and on a very low inclination with respect to Earth's orbit; that is, it's not tilted, but on the same orbital path. Its eccentricity - the deviation of the shape of its orbit from a perfect circle - is just a little higher than Earth's. And its velocity is much, much lower than the velocity of an Apollo asteroid.
"The velocity seems to be a big one," space archaeologist Alice Gorman of Flinders University in Australia told ScienceAlert. "What I'm seeing is that it's just moving too slowly, which reflects its initial velocity. That's essentially a big giveaway."
Objects that have come from the Moon have a lower velocity than asteroids, too; but, Gorman noted, 2020 SO is even slower than Moon rocks.
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.
Asteroid #2020SO is suspected of being the Surveyor 2 centaur rocket booster, launched on 20 September 1966. The Earth-like orbit and low relative velocity suggest a possible man-made object.— Kevin Heider (@kpheider) September 21, 2020
Reusable rockets are only a recent invention, because retrieval is incredibly technically difficult. The widely used solution for decades was to launch multi-stage rockets designed to fall apart. The booster stage falls back to Earth for reuse; the rest of the rocket, transporting the payload, is discarded in space once its job is done.
These discarded stages constitute a lot of space junk. And, according to Gorman, they're surprisingly easy to lose.
"There are so many factors in the space environment, like gravitational factors and other things that affect movement, that it can sometimes be quite unpredictable," she said.
"You have to keep tracking these things, or you can just sort of lose sight of them really easily. And if they do something a little bit unpredictable, and you look the wrong way, then you don't know where it's gone. It is quite astonishing, the number of things that have gone missing."
The estimated size of 2020 SO matches the properties of a 1960s-era Centaur stage. According to NASA's CNEOS database, the object is between 6.4 and 14 metres (21 and 46 feet) long; a Centaur stage measures 12.68 metres (41.6 feet).
Asteroids are detected in the sky as moving bright objects - a dot in the darkness. From that, we can infer velocity and orbit, and make an estimate of size, but it's impossible to determine shape or composition without more detailed observations.
2020 SO is due to make two close swoops of Earth. On 1 December 2020, it will pass by at a distance of around 50,000 kilometres (31,000 miles). On around 2 February 2021, it will fly by at 220,000 kilometres.
Neither is close enough to enter Earth's atmosphere - the object poses absolutely no danger. But those distances, particularly at slow speeds, may be enough to study it more closely and ascertain what 2020 SO is.
We might be able to make out a rough shape. Spectroscopy could help determine if the object is painted. And how much light it reflects could even provide information to help plan long-term space missions. If 2020 SO is that 1966 Centaur stage, it's been out in space for 54 years - a human-made spacecraft enduring the void for all those decades.
"It would be interesting to do some reflectance spectroscopy, which would show how rough the surfaces are, how much it's been pitted and decayed from being bombarded by dust and micro meteorites," Gorman said.
"It's human material that's been out in a different part of space. So, it would be interesting to compare that to the results you get from stuff in low Earth orbit, which is much, much denser in material."
And, of course, whether or not it is a rocket stage, 2020 SO's properties can help us identify other near-Earth objects in the future. If it's human-made, that means the next time we see an object with similar properties, we have a little bit more information supporting an anthropogenic origin.
If it's an asteroid, that means we have a really weird rock on our hands demonstrating that asteroids can move - really unexpectedly - like rocket stages.
So, whatever 2020 SO is, we'll have a lot to learn from our incoming mystery guest.