Targeted campaigns

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Apophis - 2021

99942 Apophis is a Near Earth Asteroid discovered on June 19, 2004, that temporarily reached the highest level of impact risk in the Torino scale (level 4), due to a possible impact with Earth in 2036. Later on, it was shown that the impact probability was vanishingly small, and the rating was lowered to the current level 0. However, this asteroid remains a priority target, as it has several close encounters with our planet, and is one of the small objects for which accurate astrometry can refine the measured drift in semi-major axis due to the Yarkovsky effect.

Apophis has an average diameter of 370 m, but a flattened shape (approx. 450 x 18 m) as measured by radar ranging (Brozoviç et al. 2018).

Occultation challenge

Given the several accurate astrometric measurements (including radar observations) Apophis is a high priority target for stellar occultations. A couple of interesting events are coming for observers in the US (March 6-7) and Europe (March 11), but they remain a challenging observation mostly due to:

- A maximum duration of occultations of 0.1 s or less. This requires high frame rate and enough photons to be recorded;

- An ephemeris uncertainty, for the events below, initially around 1.6 km (1 sigma), i.e. several times the nominal width of the occultation path (equivalent to the object size) - but then collapsed to 30 m (!!) after the first event.

One event was successfully observed:

March 6-7, 2021 (N America) - Occultation of 8.4 mag star NY Hydrae (HIP 45887, VizieR data base link to Gaia EDR3 5746104876938414592), spectal type G5V.

This event produced an amazing collapse of the ephemeris uncertainty, that is now around 30 m on the sky (only!!!) for the forthcoming event:

March 11, 2021 (Europe, Arabic peninsula, Pacific area) - Occultation of the 9.3 mag star UCAC4 436-049782, VizieR data base link to Gaia EDR3 5763200427324592896)

From the linked VizieR pages you can get finding charts by clicking on "start AladinLite" at the top left.

Predicted paths - March 11, 2021

The JPL orbit has been essentially confirmed by the Lucky Star prediction by J. Desmars BUT OBSERVERS SHOULD USE THIS MAP that includes (important!!!) corrections for tophography (credits: Marc Buie, SWRi; orbit: JPL). It also contains added site lines that correspond to locations where the observers should be (see the spreadsheet here below). Note the very narrow path and the compact 1- to 3- sigma width!

Coordination of the observation

In order to assign the chords to observers, please check and fill this spreadsheet if you plan to observe. As OccultWatcher does not take into accunt topography, it is not recommended for the coordination of this campaign.

Special warning: path width

Beware: the path width in the links provided above considers the SMALLEST projection of the object as predicted (unfortunately, 170 m average diameter, only!). For March 11, this situation corresponds to a nominal maximum duration of 0.035 s!

Large scale path map with timing details:

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-- PaoloTanga - 26 Feb 2021