# Why did they think that S509356 was in orbit around the Earth? Where is the disconnect?

This answer suggests that the object S509356 could potentially be a candidate for being called a "mini-moon", an object which temporarily becomes gravitationally bound to the Earth and enters into Earth orbit. This happens because our massive moon can "pull" on things in heliocentric orbit enough to get them to spend some time in the Earth-Moon system.

The object is noted in Wikipedia's article Claimed moons of Earth:

On April 8, 2016, an object, named S509356, was discovered with an orbital period of 3.58 days. Although it has the typical area-to-mass ratio (m^2/kg) of satellites, it has a color typical of S-type asteroids. It has not yet been identified with any known artificial satellites, because there are no known satellites currently orbiting Earth in between 2.99 and 3.79 days.

and its (potential) temporary Earth orbit is discussed in the Reddit post Mystery object S509356 spotted in strange orbit. Discussion there links to a note in Jonathan's Space Report:

S509356

Asteroid surveys have spotted an unknown Earth satellite, provisionally designated S509356, in a 30000 x 152000 km x 48 deg orbit. Looking at my historical lists, it might be an object associated with the Vela launches, although it's surprising it's only being spotted now. The region beyond GEO is poorly tracked by satellite surveillance systems but in recent years coverage by asteroid searchers has been fairly good.

I looked in JPL's Horizons to check the orbit, but was surprised to see that the object in that database named 509356 never gets closer than 200 million km from Earth in the year 2016 I've plotted the 2016 data for Earth and the object below (dot is January 1st) and it's clearly not something that could be mistaken as having a 3 or 4 day orbit around the Earth. Those units are kilometers.

Where is the disconnect here? An asteroid that far from Earth would never be mistaken as being in orbit around the Earth because it would move orders of magnitude too slowly.

Does it have anything to do with this?

(click for full size)

I can only guess 509356 is not S509356.

JPL's Horizons doesn't appear to recognise any of the identifiers in the linked TLE for that designator

# Worst residual: 6.23 km
# MJD 57389.000000 (2016 Jan  2  0:00)
1 41929U 15019C   16002.49921083  .00000000  00000-0  00000-0 0   560
2 41929 049.9344 338.1882 6058496 022.6497 174.4363 00.27904573    06


The mean motion of 00.27904573 is an orbit of 3.5836420 days

It was later identified* as an upper stage for a Chinese launch, so the provisional identifier didn't get turned into a solar system body, which might explain why you can't find it in Horizons.

It has a ground track that looks a bit like a captains sleeve insignia.

• This is a great answer, thank you! I'd never seen projectpluto.com before today, it looks like there is quite a lot to see there. Do you know if the redaction shown in the link at the end of my question just a red herring, or might it be interesting enough to ask a new question about? – uhoh May 16 '18 at 10:41
• Oh, I see, those are "home made" TLEs generated from the ephemeris. "real" TLEs didn't start until 2017. I should have used that ground track for this question! – uhoh May 16 '18 at 10:52
• the Yuanzheng-1 stage was "lost" on that mission @uhoh, and wasn't "found" until 2017 – JCRM May 16 '18 at 13:34

Short answer is that you are confusing two different names: S509356 and 509356s.

The 509356 in the JPL Small Body Database used by HORIZONS is not the same body as the S509356 you mention. Searching the JPL database (https://ssd.jpl.nasa.gov/sbdb.cgi) gives us details on 509356, a main-belt asteroid discovered in 2006. 509356 is a permanent designation given by the IAU Minor Planet Centre (MPC) to this body as it is considered a minor planet.

S509356 is as far as I can tell currently considered a distant artificial satellite by the IAU MPC, as it can be found on the page here: https://minorplanetcenter.net/iau/artsats/artsats.html. As such, it would not be included in the JPL Small Body Database and doesn't show up in your search of HORIZONS.

• Thanks for the clear explanation. It turns out I was told about the Distant Artificial Satellites Observation Page before, I should have looked there first it seems. This is your first post here, congratulations! – uhoh May 16 '18 at 10:35