Didymos Occultations - ACROSS project

In the post DART impact period, Didymos's orbital uncertainties have been reduced enough that we can predict stellar occultations with better accuracy. This in turn led to the first positive observations of stellar occultations by Didymos. In the table below we present a short list of observed events from around the world along with the observers and number of observed chords. 

Attempts to record stellar occultation by Didymos occurred before the DART impact. Of course, the orbital uncertainties were much larger which added an extra degree of difficulty to the expenditions. Despite the only negative results, these expeditions can provide restrictions to the orbit of Didymos. The table below lists some of the pre-impact attempts.

NEA Occultations - ACROSS project

(385 186) 1994 AW1

This NEA with a diameter estimated 0.811 km, was observed on August 5th 2022 from Jerzmanowice, Poland. There was one chord and 2 negative observations registered which provided the astrometric position of the asteroid and uncertainties of 1-2 mas order of magnitude.

Lightcurve of the (385 186) 1994 AW1 event. Using SORA (python) library we were able to achieve precision of 0.002s in the event's timing

Single chord event by the asteroid (385 186) 1994 AW1. We assume spherical shape for the asteroid. In the picture one can see the observed chord(blue line) and its uncertainties (red lines) as well as the best fitted circle (black) and the fitted circles within 1-sigma uncertainty limit. Using SORA library we extract the astrometric position and uncertainties ~2mas. 

NEA Occultations prior to this project

(3 200) Phaethon

 This NEA, with a Diameter approximately 6 km, was campaigned for in 2019, resulting in 6 recorded events with at least one positive chord in that year, plus two other events in the years that followed. This marked a turning point on the prospects of searching for NEA events, a task not feasible only a couple of years priors, and made possible thanks to the recent Gaia Data Releases, as well as the ever-increasing list of potential NEA targets.

6 chords registered in the first event. Uncertainties of ~2 mas. (Source: Occult)

Lightcurve of (3 200) Phaethon event obtained by the OCA team. Thanks to extensive preparation, a precision of 0.002s was obtained in the event's timing!

15-chord event of (3 200) Phaethon registered on 2021/10/03. A non-spherical shape is clearly visible. (Source: Occult)

List of (3 200) Phaethon events so far (source: Occult)

(99 942) Apophis

 This NEA was considered a potentially hazardous object, even reaching Level 4 on the Torino Scale at one point. In 2021, fears of a potential impact from this object on Earth were mitigated, in no small part thanks to a campaign of occultations that resulted in 5 events with at least one positive chord in the span of two months. The low uncertainties usually associated to occultations allowed an improvement to the orbit of Apophis, clearing any chance of impact in the foreseable future.

On 2021/03/07, the first occultation by (99 942) Apophis with positive chords was confirmed. 3 chords were reported for this event. (Source: Occult)

4 chords on a single (99 942) Apophis event, the best so far. (Source: Occult)

List of (99 942) Apophis events so far (source: Occult)

There have been observations to two other NEA, (433) Eros and (1 036) Ganymed. However, due to their large sizes (> 15 km), they are left out of these examples, as these occultations do not accurately represent the type of events we are preparing for. 

Last Edited: 2022/03/02