International Occultation Timing Association
Link to Steve Preston's prediction
Cloud cover predictions, updated 2/9 5PM CST
http://www.asteroidoccultation.com/2020_02/0210_191_64120.htm
Satellite search area (between the gray lines)
On 2020 Feb 10 UT, the 100.9 km diameter asteroid (191) Kolga will occult a 4.3 mag star in the constellation Serpens for observers along a path across SE USA.
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Large asteroid Kolga will occult 4.3-mag. nu Serpentis Mon. am, Feb. 10, Okla. - S. Car.; satellite chance over most e. USA

Within a few seconds of 9:39 UT (4:39am EST or 3:39am CST) Sunday morning, Feb. 10, (191) Kolga, about 100 km across, will occult nu Serpentis for about 3 seconds in a path crossing the southeastern USA. This is the brightest star to be occulted by a sizable asteroid in the USA this year. The star is also known as HIP 84880 = SAO 160479, spectral type A2V, at J2000 RA 17h 20m 49.7s, Dec -12 deg. 50’ 49”, about 4 deg. northeast of 2nd-mag. eta Ophiuchi (Sabik).
The link to Steve Preston’s page includes finder charts of different scales to locate the star. The shape model for Kolga, shown as projected as it should appear at the time of the occultation shows that the asteroid is not highly elongated. The small-scale map of the path is a view of eastern North America as seen from Kolga, very foreshortened since Kolga and nu Serpentis are rising in the east.

Satellite search: A wide-area map shows the path (green central line and blue northern and southern limits), and two dark gray lines that enclose a much wider area (most of the eastern USA), 20 times the path width, where occultations of the star by a satellite of Kolga is possible; observers throughout this area with clear skies are encouraged to observe, for the low chance of recording a definite (high signal-to-noise) short occultation by a satellite. The path starts at longitude 103 deg. W. in Colorado; the star altitude will be only 1 deg. there, so the event will almost surely be unobservable.

The next two maps zoom in on the path, with the first one showing the path from west of Enid, Oklahoma (event altitude 5 deg. there, where if it’s clear the star should start to be visible, rising in the east, azimuth 110 deg.) to Memphis and northeastern Mississippi (altitude 13 deg. there). The second map extends to Atlanta, and Charleston (SC) and the Atlantic coast (altitude 20 deg. there). Besides the green central line and blue limits, the maps also show red lines, which are the limits in case of a very possible shift of the path to the north or south by 15 km, a “1-sigma” shift. Theoretically, there’s a 16% chance of an occultation at those red lines, and a 50% chance at the limits, but the true size of Kolga is uncertain by about 10 km, which can increase or decrease those odds; the chances rise rapidly going towards the center, where the chance is almost certain, 99.9%.

Weather: Unfortunately, much less certain is the chance of skies clear enough to observe the event; the current forecasts of that are in the last maps, showing cloud cover probability from three different forecast models. The European ECMWF and American GFS maps are quite pessimistic over nearly all of the path, while the NWS map is only a little better. With these relatively pessimistic forecasts, I have cancelled our plane reservations to fly to Charlotte, NC, to try to observe the event. But those living in or near the path can wait until closer to the time of the event, to see how the actual pattern of clouds forms, to make their decisions; I hope some of them have breaks in the clouds large enough to obtain some event timings.

Kolga orbit improved: The path prediction for this occultation, now rank 99, should be very good; the orbit has been updated by JPL’s Horizons team using techniques similar to those (using Gaia observations of the asteroid and two previous occultations with Gaia DR2 data for the occulted stars) that they used to predict four occultations by the small asteroid Phaethon in October, to better than 1-km accuracy. Kolga’s irregular shape and larger size will result in larger errors than that, but still should be quite good. The shape model, DAMIT 4072, projected for the occultation, is shown at the top of Messner’s page for the event. The best-observed past Kolga occultation, 2 years ago, ruled out the other shape model for Kolga, DAMIT 4073, but although the fit to #4072 was better, it was not good, and the distribution of the observations were not good enough to scale the shape model. I think all we can tell from the DAMIT 4072 model is that the outline will probably be roughly spherical, but the size is not-known well. An interactive map of the occultation path is on Derek Breit’s Web site, at http://www.poyntsource.com/New/Google/20200210_64120.HTM . The gray lines there can be moved by specifying offset values (distances from the center in km; positive values are south) at the top of the map. Please inform others in your area of this occultation, if you live in or near the path. If you can observe it yourself, please let me or Occult Watcher (IOTA’s free asteroidal occultation prediction and observation-coordination software available from http://www.hristopavlov.net/OccultWatcher/publish.htm  know, so that mobile observers can avoid duplicating your chord.


David, dunham@starpower.net
DAMIT 4072 model