RASNZ Occultation Section - Orma Occultation Update

ASTROMETRIC UPDATE:
OCCULTATION BY (2517) ORMA - 2001 JUNE 8

THIS UPDATE REPLACES THE PREVIOUS UPDATE OF 5 JUNE.

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[Prediction by Edwin Goffin]

[Prediction using OCCULT software]

The path given by Edwin Goffin crosses the Pacific Ocean to the Australian coastline around Brisbane. It then runs west acoss central Australia to the Western Australian coastline between Geraldton and Carnarvon. The path given by OCCULT lies around 0.3" to the south and a whole 5.5 minutes later.

UPDATE No. 2: 8 June 2001

This prediction update has been computed by Steve Preston of the Medina, Washington (state) in the United States based on astrometry from Ron Stone of the US Naval Observatory - Flagstaff Station and Bill Owen of the Table Mountain Observatory supplemented with historical data from the AstDys database. The Hipparcos position of the target star has been used.

Summary:

This updated path lies around 0.4" to the south and about 5.5 minutes later than that given by Goffin. The updated path lies to the north of North Cape on the North Island of New Zealand - closest approach to Auckland at 14:58:30 UT. Across the Tasman Sea, the path makes landfall around Wollongong at 15:01:40 UT. The path then passes just to the north of Canberra (15:02 UT) and then crosses Adelaide at 15:03:25 UT. Across the Great Australian Bight, the path runs from a point between Albury and Esperence to around Bunbury (15:06 UT).

Observers should note that with the most recent data received, the certainty of the calculation has actually deteriorated and the error band is now considerably larger. Observers as far from the line as Brisbane, Hobart and Auckland all stand a distinct chance of observing and occultation. Observers should also note the unusually large change in the expected times for this event relative to Goffin's prediction. Please centre your observing windows around the times indicated in this update and monitor for as long as practicable to cover any eventuality.

Additional details of this and other events are available at Steve Preston's website at http://www.oz.net/~stevepr/Asteroids/asteroid.htm

THE EVENT AT ONE GLIMPSE:

Date and approx. UT time of event: June 8, 2001 @ 15:03 UT
Magnitude of target star: 8.65
Magnitude drop [mag]: 7.7
Estimated maximum duration [s]: 4.5
Path description: See above.
Goffin's original chart reference: A01_0647

The Occultation Path:

Approximate width [km]: 49
Uncertainty [path widths]: 3.4 (1 sigma error band)
Map: See below
Remarks: Uncertainities are given on the basis of the nominal star position errors and the expected asteroid positional accuracy.
Circles along the path are OCCULT calculated exact centerline points !!!!

Data for the target star:

Name: HIP 91182 = PPM 734170 = GSC 6862 668
Constellation: Sagittarius
J2000 position [h,m,s; ^o,',"]: 18h 36m 07.492s; -24^o 26' 11.30"
Position source: Hipparcos
Estimated accuracy ["]: 0.019" (0.016", 0.011")
V mag [mag]: 8.65
B-V [mag]: +1.04
Remarks:
The target star is in central Sagittarius. It lies around 2 degrees northeast of lambda Sagittarii (magnitude = 2.8) and only 25' southeast of 25 Sagittarii (magnitude = 6.5). It is also only 32' south-southwest of the bright globular cluster Messier 22.

Data for the minor planet:

Number, name: (2517) Orma
Approx. diameter [km]: 47.5
Source of used astrometry: US Naval Observatory - Flagstaff Station (Ron Stone), Bill Owen - TMO, AstDys
Number of used observations: 160
Number of rejected observations: 7
Time covered by the observations: 1955 0 16 - 2001 06 05
Uncertainy Ellipse ["] (major axis, minor axis, PA): 0.056", 0.05", 90°.

Data for the event:

UT date and time of least geocentric approach: 2001 June 8, 15:00.7 UT
Approx. V mag of minor planet at event [mag]: 16.33
Geocentric parallax of minor planet ["]: 4.10
Magnitude drop [mag]: 7.7
Estimated maximum duration [s]: 4.5
Apparent motion of minor planet ["/h]: 24.23
Angular distance to moon, phase of moon [deg,%]: 9, 94%
Update computed by: Steve Preston, Medina, Washington, USA. (OrbFit)

Orma Update

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.

Use these links for further information:
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