29769 (1999 CE28)

Early in the morning of Tuesday, May 29, 2018, I was fortunate enough to record a 3.2 second occultation of the 12.6 magnitude star UCAC4 359-140328 in Sagittarius by the unnamed asteroid 29769, originally given the provisional designation 1999 CE28.

Not only is this the first time this asteroid has been observed to pass in front of a star, it is the smallest asteroid I have ever observed passing in front of a star.  At an estimated diameter of 14.7 miles, had I been located just 7.4 miles either side of the centerline of the shadow path, I would have missed this event altogether!  This is also the first positive event I’ve recorded for an (as yet) unnamed asteroid, and the first positive event I’ve recorded for an asteroid having more than a four-digit number (29769).

As you can see in the map above, the predicted shadow path was quite a ways northwest of my location.  Even though I used the Gaia DR2 position for UCAC4 359-140328 for the path prediction, the existing orbital elements for asteroid 29769 did not yield a correspondingly accurate position for the asteroid.

Though a single chord across an asteroid does not give us any definitive information about its overall size and shape, it does give us a very accurate astrometric position that will be used to improve the orbital elements for this asteroid.

The central moment of this occultation event was 6:00:02.414 UT on May 29, 2018, which was about 20 seconds later than predicted.  The astrometric equatorial coordinates for the star UCAC4 359-140328 referenced to the J2000 equinox (using Gaia DR2 with proper motion applied) are

UCAC4 359-140328
α = 18h 21m 01.6467
δ = -18° 20′ 46.282″


Using JPL Horizons (with the extra precision option selected), the astrometric equatorial coordinates for the asteroid 29769 (1999 CE28), again referenced to the J2000 equinox, are

29769 (1999 CE28)
α = 18h 21m 01.6388
δ = -18° 20′ 46.320″


As we can see above, the actual position of the asteroid at the time of the event was 0.0079 seconds of time east and 0.038 seconds of arc north of its predicted position.  This observation will provide a high quality astrometric data point for the asteroid that will be used to improve its orbit.  Gratifying!

As of this writing, there are 523,584 minor planets that have sufficiently well enough determined orbits to have received a number.  Of these, only 21,348 have received names (4.1%).  So, I guess you could say there is quite a backlog of numbered asteroids awaiting to receive names.  The IAU should consider naming some minor planets after the most productive asteroid occultation observers around the world.  There aren’t very many of us, and this would certainly be an encouragement to new and existing observers.