Didymos and his satellite Dimorphos will occult a bright (9th magnitude) star on January 21, 23:30 UT (for Europe). The star, in Gemini, will be at very high elevation in Europe (70-75 degrees). The event is relatively "slow" (0.38 s max duration).

Prediction accuracy is really excellent and observers can be confident that they will get a result. A maximum cross-track displacement of about 200 m could be present, a very reasonable uncertainty for an object that is 860 km in maximum diameter. A great opportunity to better refine its shape! 

Path views

General view of the path (from Asia to Mexico) with main occultation data (orbit JPL #200, update with occultation astrometry obtained in December 2022) : 

2023 01 21 65803 Didymos

Zoom on different areas of the path (click for full size version):

2023 01 21 65803 DidymosUSzoom2023 01 21 65803 DidymosEastZoom

2023 01 21 65803 DidymosEUzoom


Detailed path corrected for elevation, with fence lines

Please see it online here, with topography corrections, based on JPL orbit s201. With respect to the previous version there is a ~200 m cross-track shift for the event, towards North. This is NOT a major shift (about 1/4 the size of the object), and must be CONFIRMED by observing this additional event. Fence line to cover and time tags are also indicated (click on the round tag to obtain local information). There are more FENCE LINES to the South, in order to cover a possible South shift compatible with the previous orbit solution. 

REGISTER to choose your preferred fence line in this spreadsheet.

If all fence lines are taken:

- Duplicate coverage of same fence lines is useful, especially if well separated in longitude (to avoid as much as possible weather issues).  

- Intermediate positions between fence lines are also useful, in particular close to the central portion of the path, where also Dimorphos could be observed (with fast enough frame rates). 


Finding charts

Use those on OWC (top of the page, close to the star image, links to Aladdin) but NOT the path shown there (see above): https://cloud.occultwatcher.net/event/750-65803-103092-649936-T01892-1#

Position of Dimorphos

During the event, the positions of Dimorphos will be as illustrated below (for two populated areas, well separated along the path: Orlando/Florida and Toulouse/France. Courtesy S. Chesley, JPL).

occ dimorphos USA 22Janocc dimorphos FRA 21Jan

The path of the star relative to the asteroid is represented by the dashed lines in the background. As you can see on these diagrams, observers that "see" the event by Didymos could also catch the secondary event by Dimorphos which will be much shorter (max 0.07 s). The uncertainty of the position of Dimorphos along the orbit is represented by the three positions in red, but it could also be larger (nearly double). 


Observers are invited to register and review the Occultation Portal (https://occultation.tug.tubitak.gov.tr) where the high priority events among our predictions are also accessible. Use Occultation Portal to provide your results! Note that the full list of other more difficult events are still available only in the files that can be downloaded from the ACROSS web site,"predictions" page. 

We have recently updated our predictions of NEA occultations both on our website (https://lagrange.oca.eu/fr/prediction) and in  OccultWatcher (https://cloud.occultwatcher.net/events/campaign/ACROSS). For the  first set, February and March, we failed to account that a lot of  events would not be observable in any landmass, resulting in flooding  OccultWatcher with unusable occultations. Steps were taken to make  sure this doesn't happen, and the feeds for April and May were  filtered to only include events with viable observability. The  unfiltered sets of predictions can still be found on our website.

At the end of each month, we will be updating our predictions on both  sources, and we will send a reminder for anyone interested to  participate in our campaigns. For now, the visible events are those for April and May.

After a lightcurve is obtained, it is necessary to convert it into the astrometric data of the asteroid, which will then be used to refine the asteroid’s orbit. Here we compare the results obained by the open source SORA astrometry module, to the approach based on the bayesian model sampler DIAMONDS.

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