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Astronomers have discovered a small object in space that will pass close to Earth on December 1, 2020 with the small margin of only 50,000 km. Not only that, but the gravity of our planet will change its trajectory so much that it will become a temporary moon of the earth!
Here’s the real kicker – this item almost certainly is not an asteroid. Instead, astronomers think it is actually a rocket depleted from a robotic mission to the Moon launched in 1966!
The object, called 2020 SO, was discovered in September 2020 by the Pan-STARRS telescope, which surveys the sky for, in part, objects close to the Earth. It didn’t take long to see that the orbit was peculiar … as it was familiar. Its size, shape and geometry are strangely close to those of Earth’s orbit.
That would be very unusual for an asteroid, but that’s exactly what you expect from a rocket or space probe. So the astronomers plotted the orbit backwards and discovered something surprising: in September 1966 it was very close to Earth! If it were an asteroid, it means it would have passed it, but if it were actually on a space mission, that could be the launch date.
And, as it happens, there was then a spaceship was launched: Surveyor 2, a mission to land a probe on the Moon.
Aha!
Indeed, it improves. Surveyor 2 was launched on September 20, 1966, using an Atlas-Centaur rocket. The first Atlas stage performed well and the Centaur upper stage then pushed the spacecraft towards the moon. However, a mid-way correction made by Surveyor 2 went wrong, causing the spacecraft to fall which could not be recovered. It crashed into the moon days later at nearly 10,000 km / h. Ouch.
But that second stage, the Centaur enhancer, moved on. It passed the Moon and entered orbit around the Sun.
Could it be 2020 so that Centaur rocket?
It is very likely. The brightness of 2020 SO indicates that it is about 4-10 meters wide. The centaur measures approximately 3×13 meters, so it fits.
And there is also a more subtle reason to think they are the same thing. Very careful measurements of the object’s orbit show that it is heavily influenced by the pressure of sunlight. Photons from the Sun strike the object and are reflected, changing its momentum over time. This force (similar to the YORP effect) slowly changes an object’s orbit, but is more pronounced for less massive (hence, usually smaller) objects. A spent rocket thruster is a large hollow tube, so this effect should be strong … just like astronomers have discovered.
Since 2020 the orbit of SW is very similar to that of Earth, when one overtakes the other it happens relatively slowly (like two cars on a highway can move quickly, but from one car it looks like the other is passing at a slow pace). In November 2020 it passed into what is called the Earth’s Hill sphere, the volume of space around the Earth where our planet’s gravity dominates the Sun. This volume has a radius of about 1.5 million kilometers.
Normally an interplanetary object would pass right through, but 2020 SO is moving slowly enough to be captured by Earth … for a short time. It will take about four months to make a single large ring around us, then on its second pass the gravity of the Moon and Earth will give it just enough energy to escape again, once again becoming a satellite of the Sun.
When December approaches, astronomers hope to be able to obtain observations that will tell us its composition. This could confirm the case of its origin.
I will notice that this has happened before, with old space hardware passing Earth and initially mistaken for an asteroid. Plus, we also had temporary moons! Asteroid 2020 CD3 orbited the Earth for a few years before restarting in early 2020. Another asteroid, 2006RH120, orbited the Earth for a few months in 2006/7. Yet another object (only 20cm in diameter) actually burned in our atmosphere like a meteor after orbiting the Earth for a while.
These are nicknamed minimaons, although the technical term is objects captured temporarilyor TCO. It’s a quirk of orbital mechanics, but an interesting one. I wonder if, in the not-so-distant future, we can send a space probe to one, as their slow speed relative to us makes them a juicy target.
… but what a surprise it would be if, like 2020 SO, it turns out to be the depleted upper stage of an old space mission instead of an asteroid! That should be cool. Scientists might be disappointed, but I bet there would be very interesting engineering data – such as solar wind erosion or micrometeorite impacts – that would make rocket scientists drool to get their hands on. And if it were an asteroid, well, it wouldn’t be that bad.
So stay tuned. In a few weeks, we should know a lot more about this enigmatic visitor from space.
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