This prediction update is supplied by Jan Manek of the Stefanik Observatory, Prague, and is based on USNO/Flagstaff and Table Mountain Observatory astrometry for the minor planet and the GSC 1.2 star position.

Summary:

• The updated path runs ~1 path width north of New Zealand at ~14:38 UT, enters Australia south of Brisbane at ~14:40.5 UT and runs northwest across central Australia.
• The updated track is about two track widths south of the OCCULT and Goffin predictions, with a time shift approx 1 min earlier than the OCCULT prediction.

Manek notes that this event has rather more uncertainities than usual:

1. The GSC gives a blue magnitude of 12.25, and the adopted visual magnitude is based on the B-V value of +0.65. So the true visual magnitude depends on the actual colour of the star.
2. The magnitude drop strongly depends on the colour of the star (see point 1). The redder the star, the larger the magnitude drop. This event may be at the limit of visual observation as the star will at best be the same brightness as the asteroid.
3. Only the GSC 1.2 position is available and this has an expected error of 0.3" (with no proper motion + a 20 years old position).
4. The diameter is only a guess too. Values of 129 km and 232 km are quoted in the literature.

THE EVENT AT ONE GLIMPSE:

• Date and approx. UT time of event: May, 24, 1999 @ 14:41 UT
• Magnitude of target star: 11.6 (?)
• Magnitude drop [mag]: 0.5 (?)
• Estimated maximum duration [s]: 25.1
• Path description: Path runs ~1 path width north of New Zealand at ~14:38 UT, enters Australia south of Brisbane at ~14:40.5 UT and runs to NW over central Australia. The uncertainities are rather large. Consult the map for details.

• Approximate width [km]: 240
• Uncertainty [path widths]: 2.5
• Uncertainty in time [s]: 60
• Map: See below
• Remarks: Uncertainities are given on the basis of the nominal errors in the observations.
• Circles along the path are OCCULT calculated exact centerline points.

• Name: GSC 6146 00849
• Constellation: Virgo
• J2000 position [h,m,s; o,',"]: 14h 11m 32.720s; -19o 50' 53.95"
• Position source: GSC 1.2
• Estimated accuracy ["]: 0.3
• V mag [mag]: 11.6 (using assumed B-V)
• B-V [mag]: +0.65 (estimate)
• Remarks:
  1. Average GSC magnitude is 12.25 in approximately the B photometric region. Colour estimate is only rough.
  2. Target star is 2.3' north of 10.3 mag star PPM 228422. This is in the poor SW part of Virgo.
  3. Be sure to leave enough time to locate the correct star!

• Number, name: (88) Thisbe
• Approx. diameter [km]: 232
• Orbit source: calculation Manek
• Source of used astrometry: USNO/Flagstaff (Ron Stone); TMO (Bill Owen)
• Number of used observations: 47 USNO; 13 TMO
• Number of rejected observations: 3 USNO (1999 05 06.2908, 1999 02 19.5144, 1999 02 11.5326)
• Time covered by the observations: 1998 02 26 - 1999 05 18
• Rms residuals ["] (RA,DE): 0.08", 0.06"
• Estimated positional accuracy at epoch of event ["]: 0.10"

• UT date and time of least geocentric approach: 14:47:33 UT
• Approx. V mag of minor planet at event [mag]: 11.0
• Geocentric parallax of minor planet ["]: 5.128
• Magnitude drop [mag]: 0.5
• Estimated maximum duration [s]: 25.1
• Apparent motion of minor planet ["/h]: 26.71
• Angular distance to moon, phase of moon [deg,%]: 39, 72%

IMPORTANT NOTE!

Astrometric updates such as these should not be taken as definitive, but rather only as an indication of where the true track may lie relative to the original predicted track. Observers must bear in mind that later astrometry, in which the target star is measured in the same field as the asteroid, may still reveal substantial changes to the predicted track and time of the event. For this reason it is most important that observers far from the predicted track still monitor the event.