Wednesday 6 June 2012

Understanding the MPC's discovery credit rules

In last month's Planetary notes in the April 2012 The Astronomer magazine (TA Vol 48 No 576 p327 [2012]) sub-editor Mark Kidger commented that I had discovered 3 Main Belt asteroids during March but that's not quite true(!) so it gives me an opportunity to try and explain the Minor Planet Center's (MPC) new rules relating to assigning minor planet discovery credit, published in an editorial notice in MPEC 2010-U20 on 2010 Oct. 19th.

In my astrometry report submitted to Mark I had indicated that minor planets 2012 FU1, 2012 FW1 and 2012 FX1 were Great Shefford "designations", rather than discoveries. These three objects, along with two others had been picked up on the night of March 17/18th, on images exposed for three NEO Confirmation Page objects. As a standard practice I check for any other moving objects in the field of view and if any are found, use the "Known Object Overlay" tool of Astrometrica to identify them, then measure positions and send in astrometry to the MPC. However, none of the five objects could be matched with known objects and a subsequent check against the MPC's Minor Planet Checker also did not come up with any exact matches.

Positions for all five objects were measured and, as they had not been identified with known objects I gave them my own observer-assigned temporary designations of GSC3HA - GSC3HE and immediately sent the astrometry in to the MPC. Four minutes later an automated message was received back identifying GSC3HB with main-belt minor planet 2006 BQ34, but nothing was received back for the other four objects.

The next night was clear and along with other NEO work, follow-up images were taken for the remaining four objects and positions measured. For each object the positions from the previous night were then combined with the newly measured positions and new observer-assigned temporary designations given to each object. The astrometry for the two nights for each object were then sent off to the MPC. So for instance, from the first night, three positions were sent in for object GSC3HA, on the second night another three positions were obtained for the same object and all 6 positions were sent to the MPC with new observer-assigned temporary designation of GSC3JA.

By sending in observations from just a single night the MPC's automated linking routines might have a hard task to match those positions with other observations of potentially the same object that might have been made many days or even weeks earlier. Submitting two nights at the same time, with the same ID used for both nights gives the matching routines a head start to locate other relatively recent observations. As soon as the matching routines put more than one night of observations together and don't match with any known object then they will automatically allocate a new MPC provisional designation and this is what happened for the four remaining objects, receiving provisional designations 2012 FT1, 2012 FU1, 2012 FW1 and 2012 FX1.

This would have been the first step towards achieving discovery credit if following the MPC rules prior to  2010 Oct. 19th - if further observations allowed identification to an earlier apparition for the object where a good orbit had been determined then that earlier apparition would take precedence, but otherwise, the observations that had allowed the provisional designation to be assigned would eventually lead (when the orbit was determined well enough to be numbered) to that observer being given discovery credit and the right to name the minor planet.

However, with the new rules there is nothing special about the assigning of a provisional designation such as 2012 FT1. Every single observation needs to be associated with a designation and if it appears that an object cannot immediately be identified with a known designation then a new one is automatically assigned by the MPC. In this way it is quite possible that the same object gets more than one provisional designation during a single opposition and very likely, if it has been observed at earlier oppositions it will have received one or more provisional designations from those earlier apparitions in the process too.

As the orbit gets more and more refined (as new observations are made, or older observations are identified from previous oppositions), eventually the orbital uncertainties are determined to be so small that the object is ready for permanent numbering by the MPC. It is only at this point that discovery credit is assigned. Central to this assignment is a new MPC procedure, introduced with the new rules on 19th Oct. 2010 that stores the date & time that the MPC received the astrometry from the observer, along with the more familiar details of Observatory code, date & time of observation, RA and Dec etc. These dates & times of receipt are called timetags by the MPC. Observations sent in before the new rules came into play have timetags constructed that are "related to the date of observation". Unfortunately, there is currently no means for an observer to see the timetags associated with observations that have been sent in to the MPC.

The MPC rules state in MPEC 2010-U20 :

"Discoverers will be defined only when an object is numbered.  At that time, the timetags on all the observations included in the solution will be examined.  The discovery observation will be that observation which is the earliest-reported observations at the opposition with the earliest-reported second-night observation.  The discovery observation will then define the discoverer."


"Objects that have multiple-opposition orbits as of now (2010 Oct. 19) will be grandfathered into the old scheme of assigning discovery credit"

There are several points to note in these statements:
a) For discovery credit to be assigned there must be at least two nights of positions reported, no assignment is possible where only a single night is available during an opposition.

b) The early reporting of astrometry matters. If an observer detects a new object on night 1 and sends in his astrometry on night 3, but in the meantime, another observer detects the same object on night 2 and sends his astrometry in immediately, then the second observer would be selected for discovery credit in preference to the first because the night 2 astrometry would have been received by the MPC first.

c) By the time an object is numbered it will quite likely have been observed on at least 3 oppositions, but the discovery credit will be assigned to the opposition that has the earliest-reported second-night observation, not necessarily the earliest opposition. So for example, if old NEAT images from 2003 are mined for moving objects and positions measured and submitted today for an object on several nearby nights back in 2003 they might end up being the only observations from the chronologically earliest observed opposition, but would have been reported last and therefore would not be considered for discovery credit.

d) An object observed at more than one opposition before the new rules came into effect will have the previous rules applied and therefore discovery credit is likely to go to observations from one of those old oppositions. An object observed at only one opposition before the new rules will have the new rules applied (but with timetags for the old observations based on the date of observation, not date submitted) so credit is also likely to go to observations made at the old opposition.

So, returning to the four objects I observed on March 17th that had received provisional designations, at the time of writing (6th June 2012):

2012 FT1: Now identified with observations on 3 nights in 2005 obtained by Spacewatch (code 691) and Moletai Astronomical Observatory in Lithuania (code 152) so discovery credit is likely to go to one of those observatories.

2012 FU1: Observations from Spacewatch (code 691) on March 16 and Mt Lemmon (code G96) on March 17 pre-date my positions from March 17th and very likely 691 and G96 submitted their observations in near-real time, so if no earlier observations are uncovered, discovery credit is likely to go to either 691 or G96, depending on which submitted first.

2012 FW1: Observations from Pan-STARRS (code F51) on Feb. 27 and Mt Lemmon (code G96) on March 13 pre-date my positions from March 18th and again, F51 and G96 are likely to have submitted their observations in near-real time, so if no earlier observations are uncovered discovery credit is likely to go to F51.

2012 FX1: Observations from Pan-STARRS (code F51) on March 16.5UT pre-date my positions from March 18.11UT and again, F51 probably submitted their observations in near-real time, so if no earlier observations are uncovered discovery credit is likely to go to F51.

Without being able to examine the timetags for 2012 FU1 and especially 2012 FX1, it can't be completely ruled out that my positions were in fact the first submitted, even though it is very, very unlikely! Additionally, as mentioned earlier, discovery credit is only assigned when the objects are finally numbered and therefore these guesses at how the credit will be assigned may all change if still unidentified observations that have been reported possibly years earlier are linked as further observations accumulate in upcoming oppositions. So I am fully expecting that all of those four objects will be assigned to other observatories by the time they each get permanent numberings.

So my advice to observers wanting to try and get discovery credit for moving objects they find in their images is:

- Measure and report unidentified objects as soon as possible, preferably on the same night as the exposures are obtained

- Try to get at least a second night of astrometry and preferably a third, just in case no other positions have been reported from anywhere else this opposition

- Assume the worst, it is very rare now that an object will be found without any earlier 2 or more night oppositions and very difficult to find an object that has not already been picked up and reported by one of the big surveys earlier in the same opposition. Difficult but not impossible.

- Don't hold your breath! Even if you have a promising candidate, it could be some years before you know whether your observations will definitely receive discovery credit or not.

[This post has minor updates from the originally published article that appeared in the May 2012 The Astronomer magazine (TA Vol 48 No 577 p19-21 [2012])]