Hey all! I have been exploring some DP0.3 data and ran into something that I thought I would ask about. I created a dataset queried from SSObject and SSSource containing ~4.4 million objects, then filtered it down to only the rows containing objects with corresponding MPC identifiers (resulting in a dataset of about ~1.2 M objects). I was making some plots of heliocentric distance versus heliocentric velocities, and saw an outlier on my plots which upon inspection turned out to be Pluto. The odd thing was that Pluto’s heliocentric distance turned out to be >130 AU, which didn’t make sense to me. So I used topcat’s VO feature to obtain a dataset pulled directly from MPC (MPCORB, specifically) and matched objects from the DP0.3 dataset I was using to the MPCORB entries (about 434k objects matched IDs).
I then plotted e.g. the eccentricities provided for each object in the DP0.3 dataset with the eccentricities provided in the objects queried directly from MPCORB, and found there were many objects for which the eccentricities didn’t match. See below (x axis is the DP0.3 eccentricities, y axis is the MPC eccentricities for the same objects).
I repeated this exercise for inclinations and semi-major axes, and found similar results:
Do you have any thoughts on why there are discrepancies between the values provided in DP0.3 and MPC? I am wondering if this has anything to do with the assumptions of the DP0.3 simulation; e.g. for example assuming that some objects have only been observed a few times and thus their orbital elements might be “poorly constrained” from the perspective of the simulation, and thus wouldn’t match the accepted values? Not sure if that makes sense or explains what I am seeing here, but just thought I’d ask – thank you so much for any thoughts you might have on this!
Hi @ewhite42, thanks for you question. I think the discrepancy is likely due to the differences in the MPCORB catalog in DP0.3 compared to the MPCORB catalog that is available now from the Minor Planet Center. According to our [documentation[(The DP0.3 Simulation — Rubin Observatory DP0.3), " The DP0.3 simulation contains all objects in the Minor Planet Center Orbit (MPCORB) Database as of May 1 2023, except for the ~400 objects that have no absolute magnitudes. Out of these objects, Rubin detects 97% (1.2 million) of them in the simulated 10-year survey." The differences in the the MPCORB catalog from more than 2 years ago and today is likely to lead to the differences in orbital elements between the two catalogs, especially in eccentricity and inclination, as additional observations for many objects likely better constrained the orbits for many objects in the catalog over the time between the 2023 MPCORB snapshot and that available today.
I’m going to mark this as the solution to your question, but please let us know if that’s not the case. Thank you.
Hi @sgreenstreet – thank you so much for your reply, this is very helpful! I do have one follow up question – and if you would recommend I post this as a separate question I am happy to do so – but I am just wondering if you have any thoughts on why Pluto might have shown a heliocentric distance of >130 AU. I’m just curious why it would have a value so much larger than the aphelion distance in case there might be any other objects in the sample that show issues with the heliocentric distance, or if it is just a fluke of some kind. Many thanks for any thoughts you might have on this and again for the very helpful feedback!
Hi @sgreenstreet , sorry about the delayed reply! To identify the object as Pluto, I retrieved its fullDesignation (which was 1930 BM) from my DP0.3 table and searched it in the MPC database (IAU Minor Planet Center)
I wonder if this is because of Hybrid Cat. “Hybrid Solar System Catalogue Creator (Hybridcat) was used. Hybridcat removes the closest-matching synthetic object to each real object, creating a population with all of MPCORB and most of S3M that closely match S3M’s orbital distributions” @phbern might have thoughts on this.
Yes, it’s possible this is a case of a bug that happened during the matching between the synthetic population we injected and MPCORB. I don’t remember in which space we did the matching (poke @Gerenjie who might know), but if it was done in, say, Keplerian/cometary elements or on-sky position, instead of state vector, this is plausible
(it is also the only way Pluto could show up in there! The “real” Pluto is too bright, so we’d have removed it due to saturation)
Thank you so much @mschwamb and @phbern! (And apologies for my late reply!) This is very helpful to know, I really appreciate you taking the time to explain this
