Ixion.20100719 Occultation July 19, 2010

(last updated 2010 06 15)



Across the globe pictured above, the three solid lines correspond to the northern limit, centerline, and southern limit of Ixion's shadow. The northern and southern limits correspond to a radius of 325 km (using an assumed albedo of 0.1). The upper and lower dashed lines indicate the effects of unknown systematic errors such as catalog zone errors. Previous KBO appulse analyses have shown the observed error to be 1.5-2 times that of the prediction's random error. The shaded area represents where the sun is more than 12 degrees below the horizon.

The scientific goals of this event are (i) to accurately determine the diameter of Ixion, (ii) to probe for a tenuous atmosphere of Ixion, and (iii) to detect possible nearby satellites of Ixion.

Observations are worthwhile and encouraged from any location at which the event is visible. Due to the possibilities of the occultation star being a multiple star system or the KBO having a satellite; either of these could move the event by more than the systematic effect uncertainties shown.


Table 1: Prediction Details
Geocentric Mid-time (yyyy month dd hh:mm:ss)

2010 July 19 01:54:11± 00:01:221 UT

Minimum Geocentric Separation 0.2693± 0.05681 arcsec
Position Angle (55636 relative to the star; measured north through east) 3.69 degrees
Geocentric Velocity 17.9 km/sec
Occultation Star UCAC2 magnitude 11.52
KBO Visual Magnitude 19.5
KBO Angular Radius 0.011 arcseconds (approximate)
1One standard deviation of random error.
2The UCAC bandpass (579-642nm) is between V and R.


Table 2: Reference Star Position
Reference star position:
(UCAC2, at epoch of event)
RA (h:m:s; J2000) Dec (d:m:s; J2000) Notes
Ixion.20100719 Catalog 16 54 25.8760± 0.0411 –24 24 22.856± 0.038
Table 3: Projected KBO Offsets from Reference Ephemeris at the Time of the Event
Body RA (arcsec) Dec (arcsec)  

+0.3671± 0.0303

+0.172± 0.017 See Notes 5 and 6

4All "offsets" are defined in the ("corrected" – "reference") or ("observed" – "calculated") sense. The offsets should be added to reference positions to get the measured positions, which we use to calculate the prediction.

4Data analyzed using UCAC2 reference network. A weighted average of the data from the two telescopes was used to calculate the RA and DEC. The errors given are 1 standard deviation.

5The reference position for Ixion is that given by JPL Horizon's ephemeris (Ixion source file: JPL#7; Earth center source file: DE405).

6Data from the SMARTS 0.9m, Lowell 42-inch, du Pont 2.5m, and Swope 1.0m telescopes over the span of 4 years were reduced with respect to stars in the UCAC2 catalog. A model was developed to fit the residuals obtained from our measured positions compared to that of the object's JPL ephemeris. All residuals obtained from the different telescopes were consistent with the model. The model includes the first-order effects of errors in the orbital elements of Ixion: (i) constant offsets in RA and Dec, (ii) linear (in time) offsets in RA and Dec, and (iii) sinusoidal terms with the Earth's orbital period. The model was propagated to obtain the predicted position and error of the KBO at the time of the occultation. The errors listed for the KBO are 1 standard deviation.


Table 4: Site Information

The sites in the table below are listed in order of their distance from the predicted centerline, starting with the most northern site.

East Longitude

Site Altitude6






SVH Observatory
-74 56 45
+40 57 41
2572 S.
Wallace Observatory
-71 29 06
+42 36 36
2477 S.
Williams College
-73 12 06
+42 42 42
2482 S.
center of Earth
8318 S.

6Altitude of each observatory is measured in kilometers above sea level.

7"Distance" refers to the closest approach distance of the "Site" to the center of Ixion's shadow in the shadow plane. The errors on all closest approach distances are ±1198 km (one standard deviation). "S." means the site is south of the center of Ixion's shadow. "N." means the site is north of the center of Ixion's shadow.

Table 5: Ixion.20100719 Occultation Predictions for Individual Sites

Ixion Immersion (UT)9
UT Mid-Time
KBO Altitude
Solar Altitude9

Ixion Emersion (UT)9

SVH Observatory
Wallace Observatory
Williams College

9The errors on all times are ±1:22 (1 minure and 22 seconds; one standard deviation). The solar altitude is given for locations where it is relevant (solar altitude greater than -18°). No entry in the immersion and emersion columns indicates that the occultation is not predicted to be visible at that site. But due to unknown effects (stellar or KBO duplicity), some of these sites may see an occultation event that would be centered on the time in the "UT Mid-Time" column.

Last updated by Carlos Zuluaga (czuluaga@mit.edu) 2010-06-15 15:00

Please direct all inquiries to Planetary Astronomy Lab (planetary-astronomy@mit.edu)