The results of CCD searches for satellites of asteroids 146 Lucina and 3 Juno are reported. Juno is one of the largest asteroids (D = 244 km); no previous deep imaging search for satellites around it has been reported. A potential occultation detection of a small satellite orbiting 146 Lucina (D = 137 km) km was reported by Arlot et al. (1985), but has not been confirmed. Using the 2.1 m reflector at McDonald Observatory in 1990 and 1991 with a CCD camera equipped with a 2.7 arc-sec radius occulting disk, limiting magnitudes of m(sub R) = 19.5 and m(sub R) = 21.4 were achieved around these two asteroids. This corresponds to objects of 1.6 km radius at Juno's albedo and distance, and 0.6 km radius at Lucina's albedo and distance. No satellite detections were made. Unless satellites were located behind our occultation mask, these two asteroids do not have satellites larger than the radii given above

Barker, Edwin S.

Stern, S. Alan

English

NASA Technical Reports Server

Lunar and Planetary Institute, Houston, 1992
A CCD Search for Distant Satellites of Asteroids 3 Juno and 146 Lucina
S. Alan Stern p..- _'_
Space Sciences Department ....
Southwest Research Institute
San Antonio, Texas
and
577
N93-19248
Edwin S. Barker
McDonald Observatory, University of Texas
Fort Davis, Texas
Abstract
We report the results of CCD searches for satellites of asteroids 146 Lucina and 3
Juno. Juno is one of the largest asteroids (D=244 kin); no previous deep imaging search
for satellites around it has been reported. A potential occultation detection of a small
satellite orbiting 146 Lucina (D=137 km) km was reported by Arlot et al. (1985), but
has not been confirmed. Using the 2.1m reflector at McDonald Observatory in 1990 and
1991 with a CCD camera equipped with a 2.7 arc-sec radius occulting disk, we were able
to achieve limiting magnitudes of mR = 19.5 and mR = 21.4 around these two asteroids.
This corresponds to objects of 1.6 km radius at Juno's albedo and distance, and 0.6 km
radius at Lucina's albedo and distance. No satellite detections were made. Unless satellites
were located behind our occultation mask, these two asteroids do not have satellites larger
than the radii given above.
I. Introduction
The search for satellites of asteroids has been a topic of interest and discussion for
some years. As described by Weidenshilling et al. (1989), evidence for asteroid satellites
includes certain anomalously slow rotation rates, the shape of certain asteroid lightcurves,
elongated asteroid images (e.g., 9 Metis) and the statistical frequency of doublet craters
on the Earth and Moon (cf., the interesting recent work by Melosh and Stansberry 1991).
Interest in binary asteroids has been further fueled by the discovery that Near-Earth
Asteroid (NEA) 4769 Castalia (1989PB) is clearly bifurcated in radar images (Ostro et al.
1990).
In principle asteroid satellites could be created by several processes, including
rotational fission, fragmentation followed by mutual capture, or as a result of ejecta from
a low-speed collision. Theoretical results (Weidenshilling, et al. 1989; Chauvineau and
Mignard 1990a,b) demonstrate that a range of long-term stable orbits exist, particularly
for close binaries, even when solar and Jovian perturbations on the orbit are included.
The most immediate application of the discovery of an asteroid satellite would be
the direct mass and density determination of the parent asteroid. Subsequent studies of
the satellite's osculating elements would then permit information to be gleaned about the
internal structure, tidal evolution, and lifetime of the parent-satellite system (particularly
if the system has not reached tidal lock). The detection of asteroid satellites or orbiting
debris would also (i) lead to new information on collision statistics in the parent-body
orbit, and (ii) clearly be of interest to spacecraft mission designers. Yet another exciting
https://ntrs.nasa.gov/search.jsp?R=19930010059 2020-03-17T08:17:53+00:00Z
578 Asteroids, Comets, Meteors 1991
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opportunity would be the potential for observations of mutual occultation events between
the asteroid and its satellite, which could reveal important details about the individual
radii, shapes, and surface markings of the parent asteroid and the satellite.
The general absence of large satellites around the brightest asteroids (my <16.5) was
noted almost 35 years ago (Kuiper et al. 1958) However, the presence of small satellites
orbiting asteroids remains an open research topic. (e.g., van Flandern, Tedesco, and Binzel
1979; Weidenshilling, et al. 1989).
Two CCD imaging surveys for asteroid satellites have been reported in the past:
Gehrels et al. (1987) and Gradie and Flynn (1988). Together these two groups searched
the fields around 22 asteroids with detection limits of ray ,-_ 18 - 22, which correspond to
satellite diameters of a few km in most cases. No detections were made. Taken at face
value, these searches indicate that less than ,-_ 4% of all asteroids have faint companions.
In fact, owing to the concentration on relatively large asteroids and distant-satellite orbits,
the actual satellite-occurrence statistics for the asteroid population as a whole could be
quite different.
We report here the resuits of a project we undertook in preparation for a new satellite
search around ,-_ 50 asteroids. In this pilot project, we made searches around two new
objects: 3 Juno and 146 Lucina.
II. 3 Juno
The search for objects orbiting the S type3 Junowas made on 7 May 1990 UT. Juno
was near aphelion at a heliocentric distance of 3.34 AU, a geocentric distance of 2.36 AU,
and less than 72 hours from opposition.
To conduct this search, we employed the 2.1m Struve reflector at McDonald
Observatory, equipped with a focal reducing camera (FRC). The FRC employs a Tektronix
512x512 CCD with 15e- readout noise. The FRC optical train was configured with an
RG5 filter to reduce Rayleigh scattered moonlight, and a 5.4 arcsec diameter occulting disk
to block the light of Juno itself. This configuration allowed us to search the entire region
beyond _ 10 arcsec from Juno with the scattered light nearly-eliminated. In the region
between 6 and 10 arcsec from Juno, which is outside the occulting mask but inside the
region where diffraction, seeing, mad finite pixel size effects contribute to scattered light,
we found the scattered/diffracted light PSF to be angularly symmetric. This allowed us to
search this region for discrete photometric peaks which might represent a close-in satellite.
However, scattered/diffracted light statistics caused our detection limits in the 6-10 arcsec
region to be _2 magnitudes less constraining than those beyond 10 arcsec. Calibration
images of Juno itself were made in order to obtain a magnitude standard for this run.
The plate scale of the FRC was measured to be 1.01 arcsec/pixel. The clear field of the
filter was 265 arcsec across. We estimate the size of Juno's sphere of influence, according
to Szebehely's tidal stability criterion (Szebehely 1967), as:
Rstab .- aj(l _ ej) (_-l) 1/3 (1)
where aj is Juno's semi-major axis, ej is Juno's orbital eccentricity, and # is the ratio of
Juno's mass to the solar mass. Taking Juno's radius to be 122 km (Tedesco et al. 1989)
and adopting a density of 3 gcm -a, we find # = 1.14 × 10 -11 andRstab = 1.56 × 104 km.
At the 2.36 AU geocentric distance on the night observed, this corresponds to a stability
Asteroids, Comets, Meteors 1991 579
radius of 128 Juno radii or _9 arcsec on the sky. Given Juno's geocentric distance on the
night observed, the 5.4" occulting disk prevented us from observing objects within _38
Juno radii of the asteroid, or 15% of the projected stability field. Assuming a bulk density
of pjuno = 3 g cm -3, a satellite orbiting synchronously with Juno would lie near 2.4Rj=no.
We guided the image on Juno while imaging the surrounding field in a series of deep
CCD exposures. Over a period of an hour, we made eight CCD exposures of the field
surrounding Juno. Juno's sky motion during our observations was 33 arcsec/hr. This
rapid motion ensured that any suspect satellites could easily be seen to move with Juno
against the fixed background star field. Assuming Juno's density is near 3 gm cm -3, an
object with a semi-major axis at the edge of the stablity region defined by Szebehely's
criterion would have a period of 115 days. An object orbiting at the edge of our occulting
disk would have a period of 18.6 days. Therefore, any satellite of Juno would be seen to
move across the background star field very nearly in concert with, and in an essentially
fixed relation to Juno.
The seeing in the Juno field images was typically 2-2.3 arcsec FWHM during our
observations. This necessitated the use of a rather wide, 5.4" occulting disk. Eleven
images of the field surrounding Juno were obtained, with exposure times ranging from 100
to 600 seconds. Seven of these images had 300 second exposure times.
The Juno field images were processed using IRAF and then looped on an image
display device to search for co-moving companions. Different scaling and image processing
methods (e.g., histogram equalization, smoothing, and removal of background stars) were
used to assist in the satellite search. No co-moving object detections were made.
To determine the limiting magnitudes of the search, the DAOPHOT routine
ADDSTAR (Stetson 1987) was used to create artificial stars On representative CCD images.
A total of 1000 artificial stars were used to determine limiting magnitudes. These artificial
stars were created spanning a range of magnitudes as low as 26, using an as-measured
point-spread-function (PSF) of Juno; appropriate counting statistics were added to each
artificial PSF. These artificial images were then examined visually in the same manner as
the program images to find the number of stars detected as a function of magnitude.
To convert instrumental magnitudes to observed magnitudes, the DAOPHOT routine
PHOTOMETRY was used to measure the magnitudes of 3 Juno in our magnitude-reference
frame. The differences between these instrumental magnitudes and their true observational
magnitudes were used to scale the measurements of the artificial stars. Using these
techniques we found the Juno images allowed us to detect co-moving objects as faint as
mR = 19.5 in the region beyond 10 arcsec from Juno and mR =17.6 in the region between
6 and 10 arcsec from Juno.
Juno's V magnitude on 7 May 1990 was 10.I. Based on the limiting R magnitude
of 19.5 in our search, we conclude that no objects larger than _ 3.3V_ km were orbiting
Juno beyond 37Rjuno, at the time of our observations; where a is the albedo ratio of a
prospective satellite to Juno.
III. 146 Lucina
On UT 5 and 6 May 1991 we observed the field around the C type 146 Lucina using
the same instrument and observing protocols as the 3 Juno observations conducted in 1990.
Taking Lucina's radius to be 68.5 km (Tedesco et al. 1989) and adopting a density of 3 g
cm -3, we find # = 2.03 x 10 -12 and Rma_ -- 1.08 x 104 kin. At the 1.73 AU geocentric
580 Asteroids, Comets, Meteors 1991
distance observed, this corresponds to 100 Lucina radii or _86 arcsec on the sky.
On 5 Ma-y !991 we obt_ned seven 600 second in(egrat_ons on the field surrounding
146 Lucina using the same occulting disk as in the Juno work. On 6 May 1991 we obtained
an additional 3 exposure s of the surrounding fie!d__ two of which were 1800 sec in length;
a third, 600 second expos urewas also obtained. Giventhe 1.73 AU geocentric distance
of Lucina on the two nights observed, the occulting disk prevented us from detecting any
objects located within _3400km of the asteroid (50 Lucina_rad]i; period=28 days); this
distance corresponds to about 32% of size of the projected-st-ability field (this constraint
unfortunately does_ nqt includes the 1600 l_m projected distance at which Arlot et al. (1985)
made the photoelectric detection of a putative satellite duringa stellar occultation even;
see below). Assuming a bulk density of plucina = 3 g cm -3 asatellite at the synchronous
point would He near 4.6Rl_m_. _ _
Using the same analysis techniques as with Juno, a careful search of the images
revealed no objects travelling with Lucina. In the 600 second CCD images we obtained
a limiting magnitude of 21.4 for objects > 10 arcsec from Lucina, and 19.7 for objects
6-10 a rcsec from Lucina. The 1800 sec exposures yielded a limiting magnit-udeo_ 22.6
for objects > !0 arcsec from Lucina, and 20.5 for objects 6-10 arcsec from Lucina. (The
limiting magnitudes in this run were deeper-relative i;o the i9-90-Juno r(_n than increased
exposure time alone would imply; this is because, unlike the Juno run, moonlight was not
Lucina's V magnitude on 5 and 6 May 1991 was 12.3. Based on the limiting magnitude
of our search, we concludethat no objects Witi_ aibecio equal to Lucina's larger thar_ _(}_6
km were orbiting Lucina beyond 50 Rtucin_. Because the satellite detection claimed by
arlot et al. (1985) occured _t-_23R,_,in], our results unfortunately cannot directly rule
out (or confirm) theirs. We can, however, conclude that more distant satellites larger than
our detection limit are unlikely (admitting the possi-t_ili-ty of:a sateilite having been under
the occulting disk). We further conclude that if the Arlot et al. "satellite" is real, it is
likely in an orbit with e< 0.5 and a< 5 × 103 km.
IV. Outlook
Our detection limits set new constraints on the probability of satellites around two
asteroids not previously searched by imaging techniques. Unless by chance a satellite was
hidden behind our occulting disk during our runs, neither 3 Juno nor 146 Lucina appear to
have satellites > 1.6 and > 0_.6km in radius orbiting them at > 40 and > 50 parent-asteroid
radii, respectively. _
We point out that groundbased coronographic searches for asteroid satellites in the
main belt cannot escape the catch-22 imposed by seeing, which necessitates using an
occulting disk larger than the apparent size of the synchronous orbit where tidal forces
could be expected to bring debris after a collision. Therefore, although deep searches such
as our can make progress in the more distant regions of asteroid stability fields, either Space
Telescope or groundbased speckle techniques would be expectedto yiel_d- the best results
for searches in the prime, inner region near each p_ent asteroid. However, for NEAs, deep
coronographic searches like ours (which probe t0 much fainter magnitudes than speckle
techniques) can often probe inside the synchronous point, making such targets attractive
future search-candidates.
These findings constitu, te the first results of a planned McDonald CCD satellite-search
Asteroids, Comets, Meteors 1991 581
campaign intended to triple the total number of asteroids surveyed for satellites by direct
imaging techniques. We hope to survey several dozen asteroids (including many NEAs).
Operating during dark time and with 1-1.5" seeing, we expect to routinely reach a limiting
magnitude near mR = 23.5 at 1-2 arcsec from our search targets.
Acknowledgments
Alan Harris and Tom Gehrels provided valuable critiques of this manuscript. This research
was partially supported by a NASA operating grant to McDonald Observatory.
References
Arlot, :i., Lecacheux, :i., Richardson, C., and Thuillot, W. 1985. A possible satellite of
(146) Lucina. Icarus, 61,224.
Chauvineau, B., and F. Mignard, 1990a. Dynamics of Binary Asteroids I. Hill's Case.
Icarus, 83,360.
Chauvineau, B., and F. Mignard, 1990b. Dynamics of Binary Asteroids II. Jovian
Perturbations. Icarus, 87, 377.
Gehrels, T., Drummond, :I.D., and N.A. Levenson (1987). The absence of satellites of
asteroids. Icarus, 70, 257.
Gradie, :i. and L. Flynn (1988). A search for satellites and dust shells around asteroids:
negative results. Lunar Planet. Sci, XIX, 405. (abstract).
Kuiper, G.P., Fujita, Y., Gehrels, T., Groeneveld, I., Kent, :i., Van Biesbrock, G., and :i.
van Houten, 1958. Survey of asteroids. Astrophys. J. Supp:, 3, 799.
Melosh, H.:I., and :I.A. Stansberry, 1991. Doublet craters and the tidal disruption of binary
asteroids. Icarus, 94, 171.
Ostro, S.J. et al., 1990. Radar observations of asteroid 1989PB. Science, 248, 1523.
Szebehely, V., 1967. The Theory of Orbits: The Restricted Problem of Three Bodies,
Academic Press, New York, 668pp.
Stetson, P.B., 1987. DAOPHOT: A
Photometry. PASP, 99, 191.
Computer Program for Crowded-Field Stellar
Tedesco, E., 1989. Asteroid Colors, Magnitudes, and Diameters. In Asteroids H (R.P.
Binzel, T. Gehrels, and M.S. Matthews, eds.). U. Az. Press, Tucson. pp. 1090.
Van Flandern, T.C., Tedesco, E.F., and R.P. Binzel (1979). Satellites of Asteroids. In
Asteroids (T. Gehrels, ed.). U. Az. Press, Tucson, 443.
Weidenshilling, S.W., Paolicchi, P., and V. Zappal5_ (1989). Do asteroids have satellites?
In Asteroids H (R.P. Binzel, T. Gehrels, and M.S. Matthews, eds.). U. Az. Press, Tucson.
643.
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A CCD search for distant satellites of asteroids 3 Juno and 146 Lucina

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