Asteroid 4769 Castalia (1989 PB)

Asteroid 1989 PB was discovered on August 9, 1989, by Eleanor Helin on photographic plates taken on Palomar Observatory's 18-inch Schmidt telescope.

The slightly elongated smudge in this image is the asteroid.

Two days later, orbital calculations showed that this object was approaching the Earth and would pass within the Arecibo Observatory's field of view during August 19-22 at distances likely to provide echoes much stronger than in previous asteroid radar observations.


This figure shows the orbits of 1989 PB and some other Earth-crossing asteroids. The yellow band marks the main asteroid belt.

The asteroid's trajectory passed within 0.027 AU, or 11 lunar distances, from Earth on August 25, when the Voyager 2 spacecraft was making its closest approach to Neptune. (The Goldstone 70-meter antenna was occupied with Voyager communications during the Neptune encounter and was unavailable for radar astronomy until Aug. 30.) Arecibo observations were conducted on August 19-22 and were highly successful. All the Arecibo and Goldstone observations were reported by Ostro et al. (1990, Science 248, 1523-1528). [link to reference & abstract?); the following three figures are from that paper and copyright 1990 by the AAAS.

This diagram shows the rotation phase coverage of radar observations on Aug. 20, 21, and 22. (The asteroid's 4.07-hour rotation period was apparent from the radar data.) Analysis of the Doppler-frequency extent of the echoes produced the estimate of the convex hull, or envelope, on the asteroid's pole-on silhouette. [ P36793.gif ]

The most useful observations, on Aug. 22, yielded a 64-frame sequence ofimages that resolve the asteroid in time delay (range) and Doppler frequency (line-of-sight velocity). The equivalent linear dimensions of the radar resolution cells are 150 x ~170 meters.

The 64 images are shown below in their raw resolution on the left, [ P46883a.tif ] and smoothed and color-coded for intensity on the right [ P36165BC.tif ]

These observations provided the first evidence for a bifurcated, or double-lobed, object in the solar system. When 1989 PB's orbit was considered secure, the asteroid was numbered 4769, and named Castalia, a nymph pursued by Apollo. (Fleeing his attention, she dived into the earth, whence a spring burst forth and was given her name. The mythical spring, on Mount Parnassus at Delphi, was the site of the most important oracle to ancient Greece. Castalia was sacred to the muses and was considered a divine source of poetic inspiration.) The name also refers to a genus of aquatic plants of the water-lily family, distinguished by rounded, floating leaves and large fragrant flowers of various colors.

Delay-Doppler images involve a non-intuitive, potentially ambiguous geometric projection, but Scott Hudson (Washington State Univ.) has developed techniques for using delay-Doppler images to estimating the shape and rotation state radar targets (Hudson, 1993, Remote Sensing Rev. 8, 195). Aspects of this technique and the results of applying his inversion to Castalia (Hudson and Ostro 1994) and other asteroids are on his .

Radar-derived shape models of asteroids open the door to a wide variety of theoretical investigations that previously have been impossible or have used simplistic models (spheres or ellipsoids). Optical lightcurves become at once easier to interpret and powerful sources of physical information about the asteroid's rotation or optical scattering properties; click here for a sample of Hudson's work in Hudson's Castalia section.

Dan Scheeres (JPL) has used the Castalia model to explore the evolution and stability of close orbits about this asteroid [see Scheeres et al. (1996, Icarus 121, 67-87)]. This new area of celestial mechanics has applications to the design of spacecraft rendezvous and landing missions, to studies of retention and redistribution of impact ejecta, and to questions about the origin and lifetimes of asteroidal satellites.

Radar-based models of small asteroids also allow realistic investigations of the effects of collisions in various energy regimes on the object's surface topography, internal structure, and rotation state. Erik Asphaug (NASA Ames) has begun to use the Castalia 3-D model for such calculations.