Imaging Observations of Asteroids with Hubble Space Telescope

Icarus 137, 260–268 (1999)

Article ID icar.1999.6047, available online at http://www.idealibrary.com on

Alex Storrs

Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, Maryland 21218

E-mail: storrs@stsci.edu

Ben Weiss

Amherst College, Amherst, Massachusetts

Ben Zellner

Georgia Southern University, Statesboro, Georgia

Win Burleson

Rice University, Houston, Texas

Rukmini Sichitiu

Dartmouth College, Hanover, New Hampshire

Eddie Wells and Charles Kowal

Computer Sciences Corporation, STScI, Baltimore, Maryland

and

David Tholen

Institute for Astronomy, University of Hawaii, Honolulu, Hawaii

Received April 23, 1998; revised September 8, 1998

We present the results of two Hubble Space Telescope (HST)

observing programs, consisting of 11 imaging observations of 10
asteroids. The primary focus of the projects was to search for faint
companions (satellites) of these asteroids. No binary systems were
detected. More specifically, no companions were found at more
than 0.1 arcsec separation of brightness down to 4 magnitudes
fainter than the primary asteroid. No companions down to 6 mag-
nitudes fainter than the primary asteroid were found at more than
0.5 arcsec separation. It is unlikely that companions more than 6
magnitudes fainter than a well-exposed primary would be detected.
These nondetections place very stringent limits on the existence of
companion bodies for these asteroids. A secondary goal was to re-
solve the illuminated portion of the asteroids. Nine of the 10 as-
teroids were marginally resolved, with three of these—9 Metis, 18
Melpomene, and 19 Fortuna—showing significant extension and
brightness variations up to a factor of two across the illuminated
portion of the restored image. The diameters of the resolved aster-
oids are generally in good agreement with those in the TRIAD II
file (E. F. Tedesco, 1989, In

Asteroids II

(Binzel, Gehrels, and

Matthews, Eds.)). Diameters for 19 Fortuna and 624 Hektor (which
are not in the TRIAD file) have been measured: 225 and 370

195 km, respectively.

1999 Academic Press

Key Words:

asteroids; satellites.

INTRODUCTION

Asteroids are generally believed to be the remnants of plan-

etesimals that never accumulated into a planet. As a result, they
provide some vital information about the protosolar nebula and
the formation of planets. The main belt and Trojan asteroids
appear to be in approximately their original orbits, thereby pre-
serving information on the conditions in the protosolar nebula
at the time and distance of their formation. In this respect, they
are almost unique among the small bodies of the Solar System.

Imaging studies of asteroids at very high resolution can pro-

vide data on some important questions: Do asteroids today have
satellites? How (if at all) did the parent bodies break up? If a
satellite is present, what is the mass and density of the system?
The presence or absence of satellites around asteroids would
place constraints on the dynamical history of the original as-
teroidal bodies (e.g., Van Flandern et al. 1979, Weidenschilling
et al. 1989).

By as early as 1979 there were already a number of far-

reaching predictions in the literature (Hartmann 1979). In partic-
ular, various authors have suggested companions for the objects
as listed in Table I. Such companions should be observable in
high resolution images of asteroids.

260

0019-1035/99 $30.00
Copyright c

1999 by Academic Press

All rights of reproduction in any form reserved.

HST IMAGES OF ASTEROIDS

261

TABLE I

Suggested Companions

Primary asteroid

Satellite

Separation

Asteroid

diameter (km)

(km)

9 Metis

153

1100

18 Melpomene

135

750

2.2

146 Lucina

150

1600

7.0

216 Kleopatra

103

114

221

0.2

532 Herculina

220

1000

3.2

624 Hektor

150

300

Contact binary?

624 Hektor

110

111

241

0.1

Wang et al. (1981).

Dunham (1979) (10 km topography).

Arlot et al. (1985).

Cellino et al. (1985).

Binzel and Van Flandern (1979).

Hartmann and Cruikshank (1978) (partially consolidated low-velocity col-

lision product).

Most main belt asteroids (4 Vesta being an obvious exception)

appear to have either undifferentiated surfaces or to be mono-
lithic “chips off the old block” of a differentiated parent body.
This observation implies that the parent bodies of the asteroids
were undifferentiated or disrupted by fairly gentle collisions that
shatter them along the interfaces between differentiated regions.
On the other hand, the observation of variegated asteroidal sur-
faces would imply a more violent collisional history. This in-
terpretation assumes that the asteroid does not become coated
with an optically thick layer of material of uniform composition,
such as dust or small, well mixed rock chips.

Many searches have been made for satellites of asteroids,

but the only unambiguous detection has been that of Dactyl,
the small satellite of 243 Ida, by the Galileo spacecraft (Belton
et al. 1995). Theoretical considerations (e.g., Prokof’eva and
Tarashchuk 1995) indicate that slower rotating asteroids should
be more likely to keep companions than faster rotators. With a

TABLE II

Asteroid Observations

Asteroid

Observed (1993)

Phase (degrees)

Program

Filter(s)

Exp. time

Dataset name

9 Metis

17 Aug. 01:29

2.321

1.485

4521

F555W

2.0

w1k10r03

18 Melpomene

18 Jan. 09:20

2.220

1.255

4764

F439W

2.0

w1930104

19 Fortuna

10 Sept. 12:44

2.301

1.534

4521

F555W

5.0

w1k10i03

109 Felicitas

20 Nov. 07:01

1.938

1.131

4521

F555W

4.0

w1k10w03

146 Lucina

6 Oct. 09:34

2.889

2.062

4764

F439W

4.0

w1930204

216 Kleopatra

2 July 15:33

3.243

2.384

4764

F439W, F555W

7.0

w1930303

434 Hungaria

27 Nov. 14:12

1.955

1.243

4521

F555W

w1k10y03

532 Herculina

30 Sept. 08:42

3.216

2.387

4764

F439W

1.8

w1930404

624 Hektor

13 June 13:54

5.239

4.333

4764

F439, F555W

100 s

w1930504

674 Rachele

10 Sept. 16:33

2.715

1.962

4521

F555W

4.0

w1k10z03

674 Rachele

22 Nov. 02:30

2.578

1.712

4521

F555W

4.0

w1k11003

period of 4.65 h, however, 243 Ida does not fit into this class.
The nondetection of satellites presented here indicates a problem
with the conventional understanding of satellite formation and
orbital evolution.

More recent analyses of the question of disruption of aster-

oids during collisions (e.g., Durda and Dermott 1997, Ryan and
Melosh 1998) show that more small bodies are produced in cata-
strophic collisions than was previously thought, and with smaller
velocities. Rather than breaking into a few large components,
large (greater than 10 km diameter) rocky bodies are likely to
shatter into a cloud of smaller particles, many with velocities
too small to escape the gravitational potential of the original
body. They will reaccrete into a “rubble pile,” forming an aster-
oid with a homogeneous surface (and relaxed shape) despite the
(potentially) differentiated origin of the material.

OBSERVATIONS

Two HST programs have surveyed 10 asteroids, in an attempt

to image the illuminated portions of those bodies and to de-
tect any possible companions. The first was program 4764, with
principal investigator (PI) Ben Weiss, and the second program
4521, with PI Ben Zellner. Both programs used the uncorrected
Wide-Field/Planetary Camera (WFPC1), and imaged the aster-
oids with the most sensitive CCD detector, PC6. The pixels are
0.043 arcs across. The B filter (F439W, centered at 4362 ˚

FWHM 468 ˚

A) was used for all objects in program 4764, and

the V filter (F555W, centered at 5430 ˚

A, FWHM 1600 ˚

A) was

used for 216 Kleopatra, 624 Hektor, and all of program 4521.
The observations are summarized in Table II.

Program 4764 targeted five asteroids that were suspected—

from either direct or occultation observations—of being binary
systems. Information on predicted companions is summarized in
Table I. Two pairs of images were made to allow for cosmic ray
removal of two-filter observations. This tactic was later modified
to use only one filter, with three images with longer exposure
times and one with a shorter time.

262

STORRS ET AL.

Program 4521 was more generally targeted, imaging bright,

slowly rotating asteroids that reached opposition during April
through December 1993. The primary intent was to search for
companions, with a secondary emphasis on resolving the pri-
mary asteroid. This was a “snapshot” program for which ex-
posures were scheduled when regular observations could not be
fitted in the spacecraft calendar. Exposure times for this program
were bracketed around the expected ideal, so the usual cosmic
ray elimination routine could not be used with this data. Results
from this program are based on the single best exposure, from
which cosmic rays have been removed “by hand.”

The snapshot observations were made under gyro control in-

stead of the usual guidestar control. The pointing drift under gyro
control is about 1 m per second. This means that the longest
exposures (40 s, of 434 Hungaria) might have experienced a
drift of up to one pixel during the time the shutter was open.
As discussed below, 434 Hungaria appears point-like, there is
no evidence of drift. Of the six observations made under gyro
control, three match the TRIAD diameters exactly, two do not
appear in the TRIAD file, and one (9 Metis) appears 13% larger
than the TRIAD value. 9 Metis appears quite elongated and the
difference is probably due to the particular aspect of the asteroid
that was observed.

The HST images were subjected to the usual processing (cor-

rection of analog-to-digital conversion errors, bias subtraction,
preflash correction, and flat-fielding) using the IRAF and
STSDAS software, following the procedures in the HST Data
Handbook. The time variation of the flat field was removed by
multiplying each image by a “deltaflat.” Finally, the unpaired im-
ages (from program 4521) were subjectively cleaned of cosmic
ray strikes.

IMAGE RESTORATION

The aberrated point-spread function (psf ) of the uncorrected

HST+WFPC optics spreads a large amount of the light from a
source over the surrounding area in the image. This scattered
light can make it difficult to detect faint companion sources.
Since any companion objects would appear extended (due to
the size of the psf if not of the companion), we used iterative
image restoration techniques to remove the aberrated light and
to look for correlated groups of pixels.

We relied primarily on the maximum entropy (Gull and Daniell

1978, Wu 1994) method implemented in the STSDAS package,
although the Lucy algorithm (Richardson 1972, and Lucy 1974)
was used to corroborate the early results. These programs are
iterative; that is, they compare an estimate of the unaberrated
image convolved with the psf to the data and update the esti-
mate based on the difference between the two. The programs
require as input the degraded image, the psf, and a model for
the noise. We used a theoretical psf calculated by the Tiny Tim
software package (Krist 1993), although some early tests with
observed psfs gave similar results. The Tiny Tim psf is preferred

because it is noiseless and because it can be calculated at en-
hanced spatial sampling (discussed below). The WFPC1 has a
“gain” of 8 electrons per data number, and a read noise of 13
electrons (WFPC Instrument Handbook V.2.1, p. 60). The max-
imum entropy program was run to convergence (often 50–60
iterations for point-like objects) or until the maximum number
of iterations (200) was reached. The Lucy program was run for
only 30 iterations. Further execution of either program did not
improve the resulting image.

We ran several test procedures to determine how well these

restoration techniques worked in the particular case of asteroids.
While the restoration techniques are optimized for the usual as-
tronomical cases of fields of point sources with or without back-
ground contributions with low spatial variation, images of aster-
oids have abrupt changes in brightness over small spatial scales,
often with extended areas at the high signal level. In particular,
we want to verify that image restoration will detect faint point
sources near a well-exposed (possibly extended) source and that
the size, shape, and brightness variations in such restorations
can be trusted.

Figure 1 shows the results of such tests. The first two panels

are a well-exposed (but not saturated) stellar image, before and
after restoration. Note that 52% of the flux is contained in the
four peak pixels of the restored image—an area of one quarter of
an original Planetary Camera pixel. The third and fourth panels
are the same PSF star to which has been added scaled down
(by a factor of 10) and offset stellar images, before and after
restoration. Note how the secondary images are barely visible
in the raw image, but can be clearly seen (for most separations)
in the restored image.

The last two panels of Fig. 1 show a model of extended sources

of various sizes, and the result of convolving this model with
a psf, adding noise, and restoring the degraded image. We find
that a convolved image has a FWHM 15 to 25% broader than the
model image and that after restoration the image had a FWHM
30% smaller than the original model at the small (1–5 pixel)
sizes relevant here.

In addition to removing most of the wings of the psf, these

restoration methods can increase the spatial sampling of high
signal-to-noise data. A well-exposed WFPC1 image can often
be restored at 4 times its original spatial sampling, subdividing
each original pixel into 16 subpixels. All the asteroidal images
are well exposed, to a level of 1

4 to 3

4 full well. Thus pix-

els on the asteroid have a signal-to-noise ratio (SNR) of 90 or
better, so pixels in the restored images have a nominal SNR of
at least five. The maximum entropy method with four times
increased sampling was used for most of the results in this
paper.

Figure 2 summarizes the result of the satellite detection simu-

lations described above. The separation between the well-expo-
sed parent object and the satellite point source (

S) is shown

on the X axis, and the corresponding brightness difference at
the detection limit (

M, in magnitudes) is shown on the Y axis.

Thus satellites brighter than the detection limit will occupy the

264

STORRS ET AL.

TABLE III

Asteroid Observations Results

Diameter (km)

TRIAD

Observed

Asteroid

Raw

Deconv.

V magnitude

Diameter

9 Metis

200

235

165

9.67

9.83

174

18 Melpomene

155

150

125

8.80

9.12

148

19 Fortuna

210

225

11.03

10.89

109 Felicitas

100

11.91

11.70

91.6

146 Lucina

150

125

12.63

12.81

137

216 Kleopatra

225

270

110

12.14

12.58

140

434 Hungaria

13.86

14.27

532 Herculina

230

195

10.76

10.43

231

624 Hektor

380

240

370

195

14.75

14.70

674 Rachele

145

100

11.80

11.84

101

674 Rachele

125

100

11.38

11.32

101

Consistent with a point source.

FIG. 2.

Phase plot of detectability of secondary bodies in a well-exposed

(but not saturated) image of a primary body. The closer a secondary body is to its
primary in distance, the closer it must be in brightness to be detected. Only objects
in the lower part of the plot can be detected. The dotted line shows the results for
WFPC2—the main increase in detectability is in the intermediate region, 0.2 to
0.4 arcs from the primary image. The dot-dash line is a theoretical calculation for
an FOC image with 3000 counts in the peak of the primary image. The numbers
represent the separation in space and brightness for suggested companions (see
Table I).

brightness of the asteroid was determined by measuring the
count rate in the images before restoration, and correcting for
background and instrumental effects as described in the HST
Data Handbook (1995). Magnitudes in the B (F439W) pass-
band were transformed to the V passband, assuming a G5 stellar
spectrum.

The observed magnitudes show good agreement with the pre-

dictions of the TRIAD II file (Tedesco 1989) (see Table III). No
attempt has been made to correct for the asteroidal lightcurves.
The photometric process has an error of 5–10% (HST Data
Handbook 1995). This serves primarily as a check of the HST
photometric calibration, and of our reduction of the data.

The observed diameters (FWHM of the reconstructed images)

are comparable to those determined by other sources. In all cases
but one (434 Hungaria) the asteroids had larger FWHM than a
stellar image reconstructed in a similar manner. We have cor-
rected all the diameter measurements from restored images by a
factor of 1.3 (see Image Restoration above). This factor brings
the observed diameters into good agreement with those reported
in the TRIAD file (see Table III).

Arlot et al. (1985) find a slightly smaller size for 146 Lucina

than reported in the TRIAD file. This may account for the di-
ameter measured in the restored image of this body being 9%
lower than that given in the TRIAD file.

SEARCH FOR COMPANIONS

No companions were visible in any of the raw images. How-

ever, fainter satellites might be hidden in the Airy disc and
diffraction spikes, enhanced by the spherical aberration of the
HST primary mirror. To reduce this signal, the deepest unsatu-
rated images of each asteroid were subjected to both the max-
imum entropy and Lucy restoration programs. These programs
yielded similar results: even after restoration, no companions
were visible. This result means that either (a) no companions
actually exist, (b) companions exist but are too faint and/or too
close to the main asteroid to be resolved, or (c) the imperfections
in the instrument or restoration process prevented detection of
objects that would otherwise have been visible.

Figure 2 shows the positions of satellite “detections” from

the literature as asteroid numbers in the plot. Any pair of objects
(parent and satellite) with the characteristic coordinates (mag-
nitude difference

M, and projected separation

S) falling in

the top region of the plot cannot be observed in well exposed
WFPC1 images, even after restoration. Conversely, all satel-
lites with coordinates located in the bottom region at the time
of observation should be observable (except in the rare case of
the satellite being in conjunction with the parent). For example,
Table I shows a prediction of a 60-km diameter to 9 Metis, with a
1100-km separation. This corresponds to

2 and

1”

at the time of observation. This is represented as the number “9”
in Fig. 2. Such a companion should be easily seen in our data.

Because the WFPC2 phase space resembles that of WFPC1

except in the region

S between 0.2 and 0.4 arcs, we can

HST IMAGES OF ASTEROIDS

265

reasonably eliminate case (c) (above). That is, we conclude that
if any satellites of the observed asteroids exist in these regions at
the time of the observations, then those satellites are undetected
because of faintness or projected proximity to the main asteroid.

Figure 2 shows that:

(1) We should have detected multiplicity in asteroids 9, 18,

and 532 if their characteristic geometries were accurately pre-
dicted. Our nondetection of companions suggests that either
these predictions are inaccurate (the companion is either not
present, or is significantly smaller than reported or darker than
the primary) or we were unlucky and the satellite appeared
too near the primary asteroid at the time of observation to be
distinguished.

(2) Detection of multiplicity in asteroids 146, 216, and 624

would be marginal given their predicted geometries. Thus we can
neither verify nor negate these predictions. These asteroids were
imaged in the hopes that there might be more satellites farther
from the primary than had been reported, or that the reported
companions might have been observed in a close approach to
the primary on an eccentric orbit. Note that both 216 Kleopatra
and 624 Hektor appear quite elongated, however (see Fig. 3).

(3) Future attempts with WFPC2 may be able to resolve large

outer satellites for 216 and 624. The dotted line in Fig. 2 shows
the

S line for the WFPC2, determined in the same way

as for the WFPC1. The main improvement in detectability is
in the intermediate range—very close to the primary, the core
of the PSF (largely unaberrated even in the WFPC1) limits the
process, while at larger distances the low signal of the satellite
image (limited by the dynamic range of the analog to digital
converter) is the limiting factor. Note that if the image of the
primary is saturated, a fainter companion may be detected, but
image restoration becomes problematical. The dot-dash line in
Fig. 2 shows the theoretical limits of the FOC, with 3000 counts
in the primary image.

RESOLVED BODIES

All of the asteroids except 434 Hungaria had broader FWHM

in their restored images than would be expected in the restored
image of a point source. Three showed clear disks (Fig. 3):
9 Metis, 18 Melpomene, and 19 Fortuna. Care should be taken in
the interpretation of individual features in the restored images,
as discussed at the end of this section.

9 Metis appears oblong, with a long axis of 235 km and a short

axis of 165 km. This is 13% larger than the 174-km diameter
determined by Kissling et al. (1991) and is more elongated than
the 1.27 axis ratio reported by Drummond et al. (1991). The
shape appears irregular, with several brightness enhancements
along the limb.

18 Melpomene is less elongated, about 150 by 125 km, in good

agreement with the 135-km size reported by Dunham (1979).
The brightness enhancements on the sunward and antisunward
ends of the image probably reflect the effect of the asteroid’s

shape on the image restoration process, rather than any variation
in surface properties.

19 Fortuna appears nearly circular, 225 km in diameter. The

slight elongation is in good agreement with the minimum axis
ratio of 0.93, from Drummond et al. (1991). The brightness
enhancement in the sunward direction is not balanced by one on
the terminator side of the disk.

Note that bright spots at the edge of the disk in a restored

image are common, both in the data and in simulations of the re-
stored images. In the simulations (compare the last two panels of
Fig. 1), these enhancements are due to the restoration algorithm
overcompensating for the large brightness gradient at the limb of
the model. There are no features in the restored images that can-
not be explained by the restoration process overcompensating
for the sharp limb of the body.

The resolved observations were made at moderate phase

angles: 17

◦

for 9 Metis, 9

◦

for 18 Melpomene, and 20

◦

for

19 Fortuna. 18 Melpomene was observed at about half the phase
angle of the other two asteroids and has about half the contrast
between the brightest spot and the disk center in the restored
image than is seen in the other two asteroids. This observation
is suggestive that at least part of the brightness variation across
the disk may be real. If so, it is probably due to the greater
contrast between dark space and the sunlit limb of the aster-
oid compared to the contrast between the terminator limb and
space—the greater the contrast, the greater the enhancement.

The lack of images in other wavelength bands (the purpose of

the programs was primarily a satellite search) does not allow a
mineralogical interpretation like that of Binzel et al. (1997). We
note however, that similar restorations of HST images of 4 Vesta
showed similar brightness enhancements on the sunward limb
(Zellner et al. 1997). This enhancement was about 30%, and the
observations were made at a phase angle of 11

◦

. This supports

the trend of increasing brightness enhancement with increasing
phase. Note that while the bright spots reported in this paper can
usually be minimized by adding a constant to the input image
before the restoration process, the sunward brightness gradient
in the images of 4 Vesta cannot be eliminated in this manner.
The spatial resolution and disk size of the 4 Vesta dataset are
much better than that of the asteroids reported in this paper.

The small extent of the resolved images, as well as the am-

biguity introduced in the restoration process by the bright spots
along the limb, make it impossible to determine if there are any
albedo features on these asteroids and so renders a determination
of surface heterogeneity difficult. The azimuthal nonuniformity
of the brightness enhancement is due to the interaction between
the shape of the asteroid and the observation and reconstruction
process and not to any compositional variation.

Note that, while not necessarily showing a definite disk,

216 Kleopatra and 624 Hektor show axial ratios of around a fac-
tor of two. This is in keeping with the suggestions of Hartmann
and Cruikshank (1978), Arlot et al. (1985), and Cellino et al.
(1985) that these objects may be contact or near-contact bina-
ries. At the time of these observations, the two bodies (if present)

HST IMAGES OF ASTEROIDS

267

FIG. 4.

Restored images of 216 Kleopatra (left) and 624 Hektor (right). Although suggested companions or binary nature (see Table I) cannot be confirmed,

the elongated appearance does not rule out a contact binary nature for these asteroids. Note that the brightness enhancements at the ends of the reconstruction of
216 Kleopatra are consistent with artifacts induced in the image reconstruction (Conclusions). The scale bars are all 0.1 inch long and point to celestial north from
the vertical centerline of the images.

could not be unequivocally distinguished. Figure 4 shows max-
imum entropy restorations of images of these objects. Note that
although 216 Kleopatra appears to be two bodies with a bright-
ness dip between them, the ends of the asteroid’s image are en-
hanced by a factor of two over the middle. This enhancement is
easily explained by the effects of the image restoration discussed
above—these observations are unable to distinguish between a
very elongated asteroid, a contact binary, or two bodies in close
orbit, for 216 Kleopatra. 624 Hektor is not quite so elongated,
and does not show brightness enhancements at its ends—it is
probably one body.

CONCLUSIONS

No companion bodies were found in images of 10 aster-

oids (674 Rachele was observed twice). The upper limits for
these nondetections are significant compared to previous sug-
gestions of companions for all but three asteroids (146 Lucina,
216 Kleopatra, and 624 Hektor).

All asteroids except 434 Hungaria have larger FWHM than

point sources observed in a similar manner. Simulations of the

convolution and restoration processes indicate that the sizes of
restored images are about 30% smaller than the original model
sizes. After correction for this factor, the sizes and axial ratios
of the asteroids observed here agree satisfactorily with values in
the literature.

Image restoration of three best resolved asteroids show bright-

ness enhancements generally on their sunward limbs of 50 to
100% over disk center. The brightness enhancements are prob-
ably due to the image restoration process. This effect is much
larger than any possible albedo features, and no such features
(other than these limb enhancements) are seen in the restora-
tions. Thus no constraint on the surface heterogeneity can be
made from this data.

ACKNOWLEDGMENTS

ADS wishes to thank Clark Chapman and an anonymous referee for excellent

suggestions and references. Based on observations with the NASA/ESA Hubble
Space Telescope, obtained at the Space Telescope Science Institute, which is
operated by the Association of Universities for Research in Astronomy, Inc.,
under NASA Contract NAS5-26555.

268

STORRS ET AL.

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