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NEAR Encounter with Asteroid 253 Mathilde: Overview
Icarus 140, 3–16 (1999) Article ID icar.1999.6120, available online at http://www.idealibrary.com on
NEAR Encounter with Asteroid 253 Mathilde: Overview J. Veverka, P. Thomas, A. Harch, B. Clark, J. F. Bell III, B. Carcich, and J. Joseph Department of Astronomy and CRSR, Cornell University, Space Sciences Building, Ithaca, New York 14853 E-mail: [email protected]
S. Murchie and N. Izenberg Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, Maryland 20723-6099
C. Chapman and W. Merline Southwest Research Institute, 1050 Walnut Street, Suite 426, Boulder, Colorado 80302
M. Malin Malin Space Science Systems, Inc., P.O. Box 910148, San Diego, California 92191-0148
L. McFadden University of Maryland, Department of Astronomy, College Park, Maryland 20742
and M. Robinson Northwestern University, Department of Geological Sciences, 309 Locy Hall, Evanston, Illinois 60208 Received May 14, 1998; revised January 28, 1999
is remarkably low: 1.3 ± 0.3 g/cm3 , a value consistent with a rubble pile structure for the interior. Assuming that Mathilde’s rock type is similar to that found in CM meteorites, the porosity of the interior must be some 50%. Shock and seismic disturbances associated with major impacts are expected to be transmitted very poorly by Mathilde’s underdense interior, a fact which may explain the remarkable degree to which surface morphology and topography have been preserved in spite of later major collisional events. Except for the lower geometric albedo (0.047 ± 0.005), the photometric properties of Mathilde are closely similar to those of Phobos. The surface is extremely homogeneous in terms of both color or albedo: specifically, no color or albedo variations associated with craters have been identified. °c 1999 Academic Press Key Words: asteroids; asteroids, Eros; surfaces, asteroids.
On June 27, 1997, the NEAR spacecraft carried out the firstever encounter with a C-type asteroid, flying by 253 Mathilde at a distance of 1212 km. We summarize findings derived from 330 images obtained by NEAR’s MSI camera which cover about 60% of the surface of the asteroid. The highest resolution achieved was about 160 m/pixel. Mathilde is a low-reflectance object (geometric albedo = 0.047) with principal diameters of 66 × 48 × 44 km. The mean radius of 26.4 ± 1.3 km is somewhat smaller than the value of 30 km suggested by previous telescopic data. Mathilde’s surface morphology is dominated by large craters, at least four of which have diameters comparable to the radius of Mathilde. The two largest, Ishikari and Karoo, have diameters of 29.3 and 33.4 km, respectively. No evidence of layering is exposed in the crater walls, but suggestions of downslope movement are present. The surface density of craters in the diameter range from 0.5 to 5 km is close to equilibrium saturation, a situation in which as many craters are being destroyed as are being produced. Observed depth-to-diameter ratios for craters in this size range are close to those observed on the lunar surface. A disruption lifetime of about 4 billion years has been estimated for Mathilde. Based on the mass determination obtained from Doppler tracking (D. K. Yeomans et al., 1997, Science 278, 2106–2109) and the volume derived from MSI images, the average density of Mathilde
1. INTRODUCTION
NEAR, the first mission of NASA’s new Discovery program, was launched on February 17, 1996, on a 3-year trajectory to the near-Earth asteroid 433 Eros. The spacecraft carries a complement of six science instruments: imager, near-infrared spectrometer, gamma-ray spectrometer, X-ray spectrometer, magnetometer, and laser rangefinder in addition to a radio science 3 0019-1035/99 $30.00 c 1999 by Academic Press Copyright ° All rights of reproduction in any form reserved.
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investigation which analyzes the tracking signals from the spacecraft (Farquhar et al. 1995). NEAR will arrive and begin its orbital mission at Eros in early 1999. On its way to Eros the NEAR spacecraft passed within 1212 km of asteroid 253 Mathilde on 27 June 1997. In this first spacecraft encounter with a C-type asteroid (the dark, probably carbon-rich type predominant in the outer part of the belt), NEAR took over 500 images (Veverka et al. 1997a) and determined Mathilde’s mass to an accuracy of about 5% (Yeomans et al. 1997). The purpose of this paper is twofold: (1) to provide an overview of the major results obtained during the encounter, results which are detailed in the six contributions that make up this Special Issue of Icarus, and (2) to compare the Mathilde findings with those provided earlier by Galileo for S-type asteroids 951 Gaspra and 243 Ida (Veverka et al. 1994a, Belton et al. 1996). For convenience, Mathilde’s fundamental properties are summarized in Table I. The Mathilde encounter took place 1.99 AU from the Sun, shortly after the NEAR spacecraft reached the greatest heliocentric distance from the Sun on its 3-year journey to Eros. Because the solar panels on NEAR are sized to provide adequate power at the closer heliocentric distances of the Eros orbital phase, there was not enough power to operate all instruments during the flyby. To conserve power and to avoid the risk of the spacecraft aborting the flyby sequence if a power shortage developed, only one of the six instruments on NEAR, the Multispectral Imager (MSI), was turned on for the Mathilde encounter. This imager, an f /3.4 refractor, provided with a 244 × 537 pixel CCD, has a 2.26◦ × 2.95◦ field of view (Veverka et al. 1997b, Hawkins
TABLE I Mathilde Parameters Groundbased Orbit Type Geometric albedo Mean radius Lightcurve Period Amplitude Probable axes (diameters)
a = 2.6 AU e = 0.23 i = 6.9◦ C 0.038 60 km 17.4 days 0.45 mag 70 × 50 × 50 km NEAR
Nominal axes (diameters) Mean radius Volumea Massb Mean density Geometric albedo a b
Thomas et al., this issue. Yeomans et al. 1997.
66 × 48 × 46 km 26.5 ± 1.3 km 3 78,000 −11,000 +12,000 km 1.033 ± 0.044 × 1020 g 1.3 ± 0.2g/cm3 0.047 ± 0.005
et al. 1997). Developed for close-up studies of Eros from orbital distances (expected spatial resolution at Eros is 3–4 m from a 30-km orbit), the instrument was not designed for imaging asteroids during distant fast flybys. Nevertheless, resolutions of some 160 m per pixel were achieved during the 1212-km encounter. The inflight performance of the MSI camera is reviewed by Murchie et al. (1999). With the maximum exposure of 1 s, the camera can detect objects as faint as +9.5 mag. Absolute calibration, in particular the conversion of MSI measured fluxes to V -mag, can be accomplished to ±5–10%. 2. THE FLYBY
Carrying out the science objectives at Mathilde involved a number of new challenges, in terms of both navigation and spacecraft operation (Yeomans et al. 1997, Harch et al. 1995, Harch and Heyler 1998). Because the spacecraft has fixed solar panels and no scan platform, the encounter had to be carried out in an orientation which permitted enough sunlight to fall on the solar panels to provide the necessary power while slewing the spacecraft attitude to allow the field of view of the MSI camera to cover the region of the sky in which Mathilde was predicted to be. To meet the power requirements the solar incidence angle on the solar panels had to be maintained at 50◦ or less, a factor which limited the total observing time around closest approach to approximately 25 min. As the approach phase angle was very large (140◦ ), it was decided to concentrate most of the imaging at and just after closest approach, when the illumination geometry was much more favorable. The four goals of the imaging investigation at Mathilde were: (1) to obtain the highest resolution image possible to study surface morphology; (2) to provide a hemispheric view of the asteroid at a resolution of about 500 m per pixel to determine the asteroid’s size, shape, and volume; (3) to obtain hemispheric coverage in color to look for evidence of compositional heterogeneity; and (4) to search the vicinity of Mathilde for possible satellites. The spacecraft approached Mathilde looking close to the direction of the Sun (approach phase angle ∼140◦ ). The actual imaging sequence (Fig. 1) began some 5 min before closest approach, when views of a crescent-illuminated Mathilde were obtained at resolutions of about 500 m. The highest resolution data (160-m resolution) were obtained at closest approach (12l2 km), when the phase angle was close to 90◦ . The imaging sequence continued for another 20 min as the spacecraft receded from the asteroid and viewed Mathilde under good illumination at a phase angle of about 40◦ . During this time multicolor global coverage was obtained at 500-m resolution using the seven color filters on MSI which cover the spectral range from 400 to 1100 nm. The imaging sequence concluded with about 200 images devoted to a satellite search. The MSI camera is sensitive enough that objects as small as 40 m across could have been detected even if they were made of material as dark as Mathilde. On approach, six special sequences of images were obtained beginning at 42 h before closest approach to perform optical
MATHILDE OVERVIEW
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FIG. 1. Schematic summary of the imaging sequence at Mathilde.
navigation by detecting Mathilde against the star background. Since NEAR approached Mathilde looking close to the direction of the Sun, the asteroid was detected first as a faint dot almost lost in the Sun’s glare, just 36 h before closest approach. For
comparison, under much more favorable viewing conditions, Galileo detected Gaspra 53 days before encounter and Ida 33 days before! By the sixth and last sequence at 11 h before encounter (Fig. 2), Mathilde had brightened to about +7.0 (±0.3)
FIG. 2. Optical navigation detection of Mathilde on approach. Ten MSI frames taken during OpNav Sequence 6 some 11 h before closest approach have been co-added and the strong background gradient due to solar glare (phase angle ∼140◦ ) has been removed.
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TABLE II Mathilde OPNAVs
TABLE III Mathilde Encounter
OPNAV Time (h) Range (million km) Predicted MAG Actual MAG Delta 2 3 4 5 6
−37 −30 −24 −18 −11
1.32 1.07 0.88 0.66 0.40
7.7–8.7 7.4–8.4 6.9–7.9 6.3–7.3 5.3–6.3
∼9.6 9.5 9.0 8.2 7.0
+0.9 +1.1 +1.1 +0.9 +0.7
Note. Approach phase ∼140◦ .
visual magnitude, about a magnitude fainter than our preencounter estimate of +5.3 to +6.3 (Table II). Based on a rapid analysis of these data, the pointing was updated to ensure that the camera would be looking at Mathilde at the time of the flyby. The last update was sent to the spacecraft only 5 h before the encounter. As already noted, unlike Galileo NEAR does not have a scan platform and the whole spacecraft must be slewed to keep the camera pointed at the asteroid. The flyby was executed flawlessly. The spacecraft passed the asteroid at 1212 km, very close to the planned 1200-km miss distance (Table III). All images were obtained as planned: the final optical navigation solution was so accurate that Mathilde was captured within the central frame of the closest-approach
Date of NEAR flyby Time of closest approach Flyby distance Flyby speed Approach phase angle Departure phase angle Duration of imaging sequence Total No. of frames taken Total No. of frames of Mathilde
27 June 1997 12h 55m 54.5s UT 1212 km 9.93 km/s 140◦ 40◦ 25 min 534 330
mosaic designed to capture the highest possible resolution image of the asteroid (Fig. 3). Although the sequence executed precisely as planned, only about 330 of the 534 MSI frames exposed contain the asteroid (Table IV). Some were intentionally pointed at the region around Mathilde to search for satellites. Others were used as insurance to cover the uncertainty region of Mathilde’s position at the time of closest approach, the linear extent of which was about 10 times the camera field of view. A representative sequence of MSI images is shown in Fig. 4. Although MSI has an autoexposure capability (Hawkins et al. 1997), this feature was not used during the Mathilde flyby. All
FIG. 3. Mathilde’s actual position relative to the sequence of MSI frames taken to capture the highest resolution view of the asteroid. Shown is the 2σ uncertainty ellipsoid in Mathilde’s position. Dimensions 460 by 160 km.
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MATHILDE OVERVIEW
FIG. 4. Sample of consecutive MSI frames. See Table IV.
exposures were preplanned using model calculations, assuming a Phobos-like photometric function (Simonelli et al. 1998), with the geometric albedo lowered to 0.035 (by changing the value of the single scattering albedo) and assuming a triaxial model for Mathilde with a mean diameter of 61 km. Generally, three sets of exposures were obtained, with the middle exposure chosen to yield a maximum DN value of 1000 in the nominal albedo case. (The maximum possible DN value is 210 or 4096.) The other two exposures of each set were chosen to bracket the middle exposure by a factor of 2. This strategy proved successful, yielding properly exposed images during all phases of the encounter. The fact that on approach Mathilde turned out to be fainter than expected in the optical navigation images (see Table II) can be attributed to a slight overestimation of Mathilde’s size (see below) and to a characteristic of Mathilde’s surface: the significant effect of large shadows at extreme phase angles produced by the huge crater concavities that define Mathilde’s shape. A major goal of the Mathilde encounter was a search for satellites, motivated in part by speculations that a large satellite might help explain the asteroid’s unusually slow rotation period. On approach our ability to search for satellites was limited by several factors, but principally by the large approach phase angle. A search of the approach images which cover the whole so-called Hill sphere estimated to extend some 100 radii around Mathilde, revealed no satellite larger than 10 km in radius, assuming the same albedo as Mathilde. Approximately 200 images were obtained to search for satellites after closest approach under a much more favorable phase angle of about 39◦ (Fig. 5). Our coverage
was limited to about 20 radii around Mathilde: no satellite down to a limiting size of 40 m was detected. The duration of the post closest approach satellite search was limited by spacecraft constraints. Given that the spacecraft had to be maintained in an attitude which provided sufficient illumination to the solar panels to power the spacecraft and the MSI camera without recourse to the battery, the entire data-taking sequence during the encounter was limited to about 25 min. 3. SHAPE, SIZE, AND MEAN DENSITY
On the basis of control point and stereo measurements, as well as from constraints provided by observed limb profiles and terminator locations, Thomas et al. (1999) derive a model which reproduces accurately the observed shape of Mathilde. The fidelity of the model can be judged from Fig. 6, which includes two synthetic images of Mathilde derived from the shape model interpolated to fill a gap in our data coverage between the high phase view (α = 126◦ ) at left and the closest approach view (α = 90◦ ) at right. The mean radius of Mathilde derived by Thomas et al., 26.41 ±1.3 km, is somewhat smaller than the value of 30 km suggested by averaging several telescopic estimates.1 Some 60% of Mathilde’s surface was observed during the flyby. The NEAR data, obtained over an interval of 25 min, do not place a useful 1 The difference is consistent with the fact that we find a slightly higher value for the geometric albedo (see Table I).
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TABLE IV Mathilde Flyby Picture List
TABLE IV—Continued
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TABLE IV—Continued
TABLE IV—Continued
Note. MET, mission event time, a unique picture identifier; lat, latitude (◦ ); lon, west longitude (◦ ); range, range in km; phase, solar phase angle (◦ ); exp, exposure time (s); filter, MSI filter; hires, resolution (pixel width) km/pxl; lores, resolution (pixel length) km/pxl. Filter designations are as follows (Veverka et al. 1997b) (filter No., spectral coverage (nm)): 0, 700 ± 100; 1, 550 ± 15; 2, 450 ± 25; 3, 760 ± 10; 4, 950 ± 20; 5, 900 ± 20; 6, 1000 ± 25; 7, 1050 ± 40. MSI has rectangular pixels (27 × 16 µm), corresponding to 162 × 96 µrad/pixel.
constraint on the spin period of Mathilde, which according to Mottola et al. (1995) is 417 h (with a lightcurve amplitude of 0.45 mag). No changes in shadow positions were noted over a period of some 10 min when the highest resolution views are available, from which Thomas et al. set a lower limit of 1.8 days on Mathilde’s spin period. Neither NEAR nor telescopic observations place useful constraints on Mathilde’s pole position. The nominal shape model has dimensions of 66 × 48 × 44 km, for a volume of 77200 km3 . Thomas et al. estimate that the volume is uncertain by about ±14%, a result which yields a mean density of 1.34 ± 0.2 g/cm3 , using the mass measurement of 1.033(±0.044) × 1020 g by Yeomans et al. (1997). Thomas et al. stress that the observational constraints make it extremely unlikely that the true volume of Mathilde is smaller than their minimum estimate, implying that the density of Mathilde must be less than 1.5 g/cm3 . 4. SURFACE FEATURES AND PROPOSED NAMES
Thomas et al. produced a sketch map (Fig. 7), which includes provisional designations (yet to be approved by the IAU) for the principal features, predominantly large craters. The locations and names are summarized in Table V. As already noted, the location of Mathilde’s spin axis remains undetermined; therefore, the coordinate system shown is arbitrary and uses the J2000 reference frame as basis (Thomas et al. 1999). Given Mathilde’s very low albedo and possible carbonaceous composition, the proposed names are those of coal-mining regions and coal mines
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FIG. 5. Representative sequence of images from NEAR’s satellite search.
on Earth. A series of Mathilde views, with the names of prominent craters superimposed, is provided in Fig. 8. 5. THE GEOLOGY OF MATHILDE
Mathilde’s surface morphology is dominated by large craters, at least four of which have diameters comparable to the radius of Mathilde. The largest craters are Ishikari and Karoo, with diameters of 29.3 and 33.4 km, respectively (Fig. 8).
The geology of Mathilde is discussed in detail by Thomas et al. (1999), while Chapman et al. (1999) focus on the cratering record preserved on the asteroid’s surface. Craters down to diameters of some 500 m can be detected reliably in the NEAR images: nothing unusual has been noted about their morphology. Thomas et al. find depth-to-diameter ratios for intermediatesized craters (diameters about 1 to 5 km) to range from 0.12 to 0.25, compared to the well-determined value of 0.2 for fresh craters on the Moon (Pike 1977) and to values slightly below
MATHILDE OVERVIEW
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FIG. 6. Demonstration of the fidelity of the shape and photometric models derived from the Mathilde data. At the two ends are two real views of Mathilde (phase ∼138◦ at left; phase ∼90◦ at right). In between are two “interpolated” synthesized views showing intermediate aspects of Mathilde not imaged by NEAR. Such modeling provides a useful visualization of the relative positions of major surface features. Prominent craters are identified by letters: A, Jixi; B, Damodar; C, Karoo.
0.2 estimated for Gaspra (Carr et al. 1994) and for Ida (Sullivan et al. 1996). The fresher intermediate size craters on Mathilde appear bowl-shaped; no examples of complex craters with flat or ringed floors are observed. Various stages of crater degradation are present, consistent with the observation that Mathilde’s surface is at the saturation equilibrium level in terms of crater density (see below and Chapman et al. 1999). A few craters have definite raised rims, but no striking examples of ejecta blankets are noted. The extreme color and albedo blandness of Mathilde’s surface TABLE V Crater Names, Locations, and Diameters
Name
Latitude (◦ )
Longitude (◦ )
Diameter (km)
Image
Benham Maritsa Mulgildie Jerada Kalimantan Clackmannan Matanuska Quetta Similkameen Lorraine Aachen Oaxaca Enugu Lublin Teruel Otago Zulia Baganur Jixi Damodar Kuznetsk Ishikari Karoo
19.0 44.6 57.7 42.1 −7.7 18.9 27.3 45.6 −13.5 48.1 9.2 38.7 −15.3 55.3 −28.1 23.7 −39.5 14.6 −12.3 73.0 45.9 −66.2 33.5
247.2 151.5 176.1 177.3 123.7 260.8 217.3 165.5 104.7 142.5 60.9 186.3 151.6 156.8 142.7 164.5 30.9 191.6 256.5 263.8 88.9 186.9 98.4
2.2 2.4 2.5 2.5 2.7 2.8 2.9 3.2 3.4 4.1 4.8 5.2 5.9 6.5 7.6 7.9 12.3 16.4 19.9 28.7 28.5 29.3 33.4
42826360 42826370 42826360 42826360 42826488 42826360 42826360 42826360 42826622 42826370 42826360 42826360 42826488 42826360 42826488 42826488 42826686 42826488 42826112 42826112 42827050 42827050 42826622
discussed by Clark et al. (1999) makes it very difficult to detect possible ejecta blankets. No blocks of ejecta have been identified; if any exist, they cannot be larger than 200 to 300 m across. No evidence of layering is seen in the walls of the very large craters imaged at high resolution, but at least two or three examples of downslope movement have been identified (Veverka et al. 1997a, Thomas et al. 1999). No strong evidence of a pervasive global fabric such as the grooves and facets on Gaspra (Veverka et al. 1994b), is observed. There are, however, three suggestions of at least local structure or fabric: the polygonal outlines of some craters, a sinuous marking with the characteristics of an exposed layer, and a few scarps or ridges. One of these scarps, of 20 km extent, and an associated relief of some 200 km, is the largest structural feature on Mathilde noted by Thomas et al. While the absence of grooves and facets may not be surprising on a body having the presumed structure of a low-density rubble pile, the existence of some evidence of fabric, of which the polygonality of some craters is most widespread, is interesting. Both Thomas et al. and Chapman et al. point out the remarkable degree to which surface morphology and topography have been preserved on Mathilde in spite of later major collisional events. Large impact events can destroy distal topography through seismic shaking (including spallation) or by blanketing it with ejecta. There are no obvious clues on Mathilde to suggest that either process has been effective. Clearly, Mathilde’s underdense interior can be expected to transmit shock and seismic energy poorly (Thomas et al. 1999, Chapman et al. 1999, Davis 1999). The apparent absence of evidence that surface morphology has been modified by ejecta is more surprising. For example, on Ida, a body with weaker gravity than Mathilde, there is strong evidence of impact ejecta on the surface (Geissler et al. 1996), as there is even on the surface of Mars’ tiny satellite Deimos (Thomas et al. 1996a). An analysis of the cratering record preserved on Mathilde’s surface is presented by Chapman et al. (1999), who find that for craters in the range from 0.5 to 5 km the surface is close
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FIG. 7. Sketch map of Mathilde from Thomas et al. (this issue), showing provisional names for major surface features.
to equilibrium saturation, a condition in which as many craters are being destroyed as are being produced. This situation is like that observed on asteroid 243 Ida (Chapman et al. 1996a), but is unlike that on 951 Gaspra (Chapman et al. 1996b), where a relatively younger surface was observed. To derive a lower limit (because the surface is saturated, only a lower limit can be derived) on the age of Mathilde’s surface, one must know the size distribution of impacting objects, the flux of this population, and the mechanical properties of Mathilde’s surface. Chapman et al. suggest that the latter are so uncertain that no precise lower bound on Mathilde’s age can be derived. A somewhat more optimistic approach is adopted by Davis (1999). In a previous study Farinella and Davis (1991) estimated the lifetime against collisional disruption for the S-type asteroid 951 Gaspra of some 200 myr, while Greenberg et al. (1994) derived a value a factor of 5 greater. Analogous calculations are carried out for Mathilde by Davis (1999), making reasonable assumptions about the size distribution and flux of the impacting population (believed to be essentially the same as those which affect Gaspra and Ida), but making allowance for the likely weaker mechanical strength of C-type objects relative to S-type asteroids. It is concluded that a C-type asteroid the size of Mathilde has a nominal disruption lifetime of about 4 billion years and that the time interval needed to form the four or five giant craters is noticeably less, probably 400 to 2000 million years. In other words, it is not surprising that bodies large enough to form the huge craters have hit Mathilde, while at the same time Mathilde has not been demolished by a catastrophic impact. Davis suggests that Mathilde is likely to be the remnant of a larger body which lost 50% or more of its initial mass by cratering.
6. PHOTOMETRIC AND COLOR PROPERTIES
From a combination of telescopic data between 1.5◦ and 16.5◦ phase obtained by Mottola et al. (1995) and NEAR observations at phase angles ranging from 39◦ to 136◦ , Clark et al. (1999) derive the first-ever detailed photometric properties of a C-type asteroid. The resulting geometric albedo of 0.047 ± 0.005 agrees well with that estimated by telescopic techniques, once the mean radius is adjusted downward as discussed above. Albedo variations on Mathilde are very subdued. Allowing for the fact that some of the remaining brightness variations apparent in photometrically corrected data may be due to inadequately modeled small-scale topography in the Mathilde shape model, albedo variations on this asteroid are restricted to about ±10% of the mean, a smaller range than is observed on two other well studied low-albedo small bodies: Phobos (Simonelli et al. 1998) and Deimos (Thomas et al. 1996a). Albedo variations associated with crater rims and with ejecta have been noted on Phobos and to varying degrees on S asteroids Gaspra and Ida. No such variations are evident on Mathilde. This lack of albedo variation related to surface morphology is consistent with the concomitant lack of variations in color (Veverka et al. 1997a). Mathilde’s homogeneity in albedo and color is interesting in that the only somewhat brighter surface of Phobos does show measurable variations, as do different size fractions of pulverized carbonaceous meterorites and carbonaceous meteorite analogs measured in the laboratory (e.g., Johnson and Fanale 1973). One implication is that not only globally, but even locally in the proximity of crater rims, Mathilde’s regolith has a uniform texture/particle size distribution.
MATHILDE OVERVIEW
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FIG. 8. Four views of Mathilde with provisional names of prominent craters indicated. Locations of named features are given in Table V.
Mathilde’s light scattering properties (or “photometric function”) show that Mathilde’s surface is not only much darker (lower single scattering albedo), but also more backscattering than the surfaces of S-type asteroids. Except for a significantly lower single scattering albedo, the photometric properties of
Mathilde are very similar to those of Phobos. Macroscopic surface roughness, as measured by Hapke’s parameter 2, does not seem to differ significantly among the three asteroids (Mathilde, Gaspra, and Ida) studied by spacecraft to date. In addition to the mean geometric albedo of 0.047, Clark et al. derive a
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phase integral of q = 0.28 and a Bond albedo of 0.013 for Mathilde. Since the NIS spectrometer on NEAR could not be utilized during the Mathilde flyby (due to insufficient spacecraft power, as discussed above), very little new information about Mathilde’s spectral properties can be added to the knowledge derived from telescopic data. The one exception is the new insight that at least on this C-asteroid, large scale and local color variations of any significance are absent. Binzel et al. (1996) found that Mathilde’s spectrum is essentially gray, with a shallow absorption shortward of 500 nm and a suggestion of a dip near 900 nm. Based on these characteristics, it was concluded that Mathilde’s spectrum does not match well those of CM or CV chondrites. Gorlovka, a shockdarkened ordinary chondrite, and Y-826162, an anomalous CI chondrite, were proposed as better matches. Observations between 1.2 and 3.3 µm by Rivkin et al. (1997) show that unlike the anomalous CI chondrite, Mathilde does not display a 3.0-µm water of hydration absorption in its spectrum. Strong arguments have also been advanced (e.g., Chapman 1996) to the effect that C-type asteroids are unlikely to be related to shocked ordinary chondrites. While an unaltered sample of the hydrated CM meteorite Murchison fails to match Mathilde’s spectral properties, as already noted by Binzel et al. (1996), a heated sample provides an excellent match (Hiroi et al. 1993): heating drives off the water of hydration, removes the 3.0-µm absorption, and flattens the slope of the near-infrared continuum. As a useful working hypothesis, we will assume in our discussion that Mathilde material is similar to dehydrated CM material. 7. DISCUSSION
The determination of Mathilde’s mean density is based on the first direct measurement of an asteroid mass from perturbations on a passing spacecraft. The mass of 243 Ida was estimated by Belton et al. (1995) based on constraints on the orbit of Ida’s satellite Dactyl. Neither Ida nor Gaspra produced measurable perturbations on the path of Galileo. The strong constraints on Mathilde’s volume (Thomas et al. 1999) and the precise mass determination (Yeomans et al. 1997) yield a surprisingly low mean density of 1.3 ± 0.2 g/cm3 , a value which argues for significant void space inside of Mathilde (Veverka et al. 1997a). As stressed by Thomas et al., it is difficult to develop a shape model for Mathilde which is consistent with observations and for which the asteroid’s mean density would exceed 1.5 g/cm3 . The idea that collisionally evolved asteroids could have porous, underdense structures is not new (Barks 1960). Unfortunately, to estimate the fraction of void space within Mathilde necessitates an assumption about the composition of Mathilde rock. As discussed above, we adopt the working hypothesis that this rock is similar to carbonaceous chondrite material, most likely CM-type rock. Table VI summarizes the fraction of void space required
TABLE VI How Porous Is Mathilde? Rock type Rock Density (g/cm3 ) Mathilde porosity (%) for mean density of 1.34 ± 0.2 g/cm3
CI
CM
CO/CV
2.25 40 ± 6
2.75 51 ± 8
3.45 61 ± 9
to explain Mathilde’s mean density assuming that the asteroid consists of carbonaceous-chondrite-like rock. The values range from 30 to 70%, with a most likely value of about 50%. An unlikely possibility is that subsurface ice contributes to the low density. Given the 0.9 g/cm3 density of water ice, the presence of water ice cannot be the major explanation for Mathilde’s low density. As summarized in Table VII, such an explanation requires that some 67 to 83% of Mathilde’s volume to be ice, a very unlikely proposition in our view, given Mathilde’s geologic evolution and current understanding of C-type asteroids. For example, it is difficult to understand why, if C-type asteroids are made up largely of water ice, no evidence of outgassing or comet-like activity has ever been reported in association with a C-type asteroid. It seems equally unlikely that significant quantities of ice would be preserved inside an object as heavily affected by severe collisions as Mathilde has been (Chapman et al. 1999, Davis 1999), especially if this object has a low-albedo surface and orbits at 2.6 AU from the Sun. It is certainly true that rather small amounts of pulverized carbonaceous material can lower the albedo of water ice significantly and perhaps even mask completely the characteristic near-infrared absorption bands (e.g., Clark 1981). However, in the case of Mathilde, several facts suggest that water ice is not an important constituent: (a) no spectral evidence of water ice or of hydrated materials has been observed, and (b) no exposures of higher albedo materials have been seen. Jupiter’s satellite Callisto provides an instructive example of how difficult it is to mask all surface evidence of water ice, even in the presence of appreciable amounts of a dark, nonicy component. Finally, we stress that no morphologic evidence of a surface rich in subsurface ice occurs in the NEAR images. Features that might be attributed to “sapping” or “solifluction” do not occur on Mathilde’s steeper slopes, nor are there any regions of collapsed terrain within craters. All of these considerations argue against water ice being a significant constituent of either Mathilde’s surface or interior. TABLE VII Is Ice the Answer?
Rock density Ice density Ice fraction (%) needed (1.34 g/cm3 )
CI
CM
CO/CV
2.25 0.90 67
2.75 0.90 76
3.45 0.90 83
MATHILDE OVERVIEW
Chapman et al. stress that very large craters, five of which have diameters between 20 and 33 km, occur with uniquely high spatial density on Mathilde. The formation of such large craters is not unexpected, assuming (as is likely) a cratering population similar to that which has impacted the Moon. It is the preservation of so many of these large craters which is noteworthy. Remarkably, the last of the major cratering events did not destroy or even noticeably modify some of the earlier ones. This apparent ineffectiveness of major impacts to modify preexisting topography except in the immediate vicinity of the event is probably explained by Mathilde’s underdense interior and likely weak mechanical strength: it might be relatively easy to produce a crater cavity, but difficult to transmit the energy effectively to ejecta or to distant parts of the asteroid. Structurally, the three asteroids visited by spacecraft to date seem to differ significantly from one another. Gaspra has a strongly faceted shape and other features suggestive of a monolithic body with a global fabric and perhaps considerable mechanical strength (Veverka et al. 1994a). Ida has a very elongated, almost bifurcated shape, possibly a somewhat underdense interior, and at least a hint of different mechanical properties at the two ends (Belton et al. 1996, Thomas et al. 1996b). Mathilde is a monolithic, very underdense body, with a generally spheroidal shape modified by large crater cavities. Clearly, not all asteroids are alike, and their continued detailed investigation by spacecraft is essential to obtaining a firm understanding of their nature and evolution. ACKNOWLEDGMENTS This contribution is dedicated to the memory of Dr. Jurgen Rahe, who played a key role in making the NEAR mission and the exploration of Mathilde a reality. We gratefully acknowledge the invaluable assistance of many colleagues on the NEAR flight team, including T. Coughlin, R. Farquhar, G. Heyler, M. Holdridge, A. Santo, D. Dunham, and B.G. Williams. We thank M. Belton and R. Binzel for helpful reviews. This research was supported by the NEAR project and by NASA Grant NAG 5-3626.
REFERENCES Barks, C. 1960. Island in the Sky. Uncle Scrooge 29, 1–18. Belton, M. J. S., C. R. Chapman, K. P. Klaasen, A. P. Harch, P. C. Thomas, J. Veverka, A. S. McEwen, and R. T. Pappalardo 1996. Galileo’s encounter with 243 Ida: An overview of the imaging experiment. Icarus 120, 1–19. Belton, M. J. S., C. R. Chapman, P. C. Thomas, M. E. Davies, R. Greenberg, K. Klaasen, D. Byrnes, L. D’Amario, S. Synnott, T. V. Johnson, A. McEwen, W. J. Merline, D. R. Davis, J-M. Petit, A. Storrs, J. Veverka, and B. Zellner 1995. Bulk density of asteroid 243 Ida from the orbit of its satellite Dactyl. Nature 374, 785–788. Binzel, R., H. Burbine, and J. Bus 1996. Groundbased reconnaissance of asteroid 253 Mathilde: Visible wavelength spectrum and meteorite comparison. Icarus 119, 447–449. Carr, M. H., R. L. Kirk, A. McEwen, J. Veverka, P. Thomas, J. W. Head, and S. Murchie 1994. The geology of Gaspra. Icarus 107, 67–71. Chapman, C. R. 1996. S-type asteroids, ordinary chondrites, and space weathering: The evidence from Galileo’s fly-bys of Gaspra and Ida. Meteoritics Planet. Sci. 31, 699–725.
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Chapman, C. R., E. V. Ryan, W. J. Merline, G. Neukum, R. Wagner, P. C. Thomas, J. Veverka, and R. A. Sullivan 1996a. Cratering on Ida. Icarus 120, 77–86. Chapman, C. R., J. Veverka, M. J. S. Belton, G. Neukum, and D. Morrison 1996b. Cratering on Gaspra. Icarus 120, 231–245. Chapman, C. R., W. J. Merline, and P. Thomas 1999. Cratering in Mathilde. Icarus 140, 28–33. Clark, Beth E., J. Veverka, P. Helfenstein, P. C. Thomas, J. F. Bell III, A. Harch, M. S. Robinson, S. L. Murchie, L. A. McFadden, and C. R. Chapman 1999. NEAR photometry of Asteroid 253 Mathilde. Icarus 140, 53–65. Clark, R. N. 1981. The spectral reflectance of water-mineral mixtures at low temperatures. J. Geophys. Res. 86, 3074–3086. Davis, D. 1999. The collisional history of Asteroid 253 Mathilde. Icarus 140, 49–52. Farinella, A. P., and D. R. Davis 1991. The collisional lifetime of 951 Gaspra. Proc. Lunar Planet. Sci. Conf. 22nd, 363-364. Farquhar, R. W., D. W. Dunham, and J. V. McAdams 1995. NEAR mission overview and trajectory design. J. Astron. Sci. 43, 353–371. Geissler, P., J.-M. Pettit, D. Durda, R. Greenberg, W. F. Bottke, M. C. Nolan, and J. Moore 1996. Erosion and ejecta reaccretion on 243 Ida and its moon. Icarus 120, 140–157. Greenberg, R., M. Noland, W. Bottke, R. Kolvoord, and J. Veverka 1994. Collisional history of Gaspra. Icarus 107, 84–97. Harch, A., and G. A. Heyler 1998. Design and execution of the asteroid Mathilde flyby. J. Astron. Sci., in press. Harch, A., D. Dunham, R. Farquhar, J. Veverka, B. Williams, and D. Yeomans 1995. Encounter strategy for a flyby of the main-belt asteroid 253 Mathilde. J. Astron. Sci. 43, 399–415. Hawkins, S. E., E. Darlington, S. Murchie, K. Peacock, T. Harris, C. Hersman, M. Elko, D. Prendergast, B. Ballard, R. Gold, J. Veverka, and M. Robinson 1997. Multispectral imager on the Near-Earth Rendezvous Mission. Space Sci. Rev. 82, 31–100. Hiroi, T., C. M. Pieters, M. E. Zolensky, and M. E. Lipschutz 1993. Evidence of thermal metamorphism on the C,G,B, and F asteroids. Science 261, 1016–1018. Johnson, T. V., and F. P. Fanale 1973. Optical properties of carbonaceous chondrites and their relationship to asteroids. J. Geophys. Res. 78, 8507–8518. Mottola, S., W. D. Sears, A. Erikson, A. W. Harris, J. W. Young, G. Hahn, M. Dahlgren, B. E. A. Mueller, B. Owen, R. Gil-Hutton, J. Licandro, M. A. Barucci, C. Angeli, G. Neukum, C. I. Lagerkvist, and J. F. Lahulla 1995. The slow rotation of 253 Mathilde. Planet. Space Sci. 43, 1609–1613. Murchie, S., M. Robinson, S. E. Hawkins III, A. Harch, P. Helfenstein, P. Thomas, K. Peacock, W. Owen, G. Heyler, P. Murphy, E. H. Darlington, A. Keeney, R. Gold, B. Clark, N. Izenberg, J. F. Bell III, W. Merline, and J. Veverka 1999. Inflight calibration of the NEAR multispectral imager. Icarus 140, 66–91. Pike, R. J. 1977. Size-dependence in the shape of fresh impact craters on the Moon. In Impact and Explosion Cratering (D. J. Roddy, R. O. Pepin, and R. B. Merrill, Eds.), pp. 489–509. Pergamon, New York. Rivkin, A. S., B. E. Clark, D. T. Britt, and L. A. Lebofsky 1997. Infrared spectrophotometry of the NEAR flyby target 253 Mathilde. Icarus 127, 255–257. Simonelli, D., M. Wisz, A. Switala, D. Adinolfi, J. Veverka, P. C. Thomas, and P. Helfenstein 1998. Photometric properties of Phobos surface materials from Viking images. Icarus 131, 52–77. Sullivan, R., R. Greeley, R. Pappalardo, E. Asphaug, J. M. Moore, D. Morrison, M. J. S. Belton, M. Carr, C. R. Chapman, P. Geissler, R. Greenberg, J. Granahan, J. W. Head III, R. Kirk, A. McEwen, P. Lee, P. C. Thomas, and J. Veverka 1996. Geology of 243 Ida. Icarus 120, 119–139. Thomas, P. C., D. Adinolfi, P. Helfenstein, D. Simonelli, and J. Veverka 1996a. The surface of Deimos: Contribution of materials and processes to its unique appearance. Icarus 123, 536–556.
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Thomas, P. C., M. J. S. Belton, B. Carcich, C. R. Chapman, M. E. Davies, R. Sullivan, and J. Veverka 1996b. The shape of Ida. Icarus 120, 20– 32. Thomas, P. C., J. Veverka, J. F. Bell III, B. Clark, B. Carcich, J. Joseph, M. Robinson, L. McFadden, M. Malin, C. Chapman, W. Merline, and S. Murchie 1999. Mathilde: Size, shape and geology. Icarus 140, 17– 27. Veverka, J., M. Belton, K. Klaasen, and C. Chapman 1994a. Galileo’s encounter with 951 Gaspra: Overview. Icarus 107, 2–17. Veverka, J., P. Thomas, A. Harch, B. Clark, J. F. Bell III, B. Carcich, J. Joseph, C. Chapman, W. Merline, M. Robinson, M. Malin, L. A. McFadden, S. Murchie, S. E. Hawkins III, R. Farquhar, N. Izenberg, and A. Cheng 1997a. NEAR’s flyby of 253 Mathilde: Images of a C asteroid. Science 278, 2109– 2114.
Veverka, J., J. F. Bell III, P. Thomas, A. Harch, S. Murchie, S. E. Hawkins III, J. W. Warren, H. Darlington, K. Peacock, C. R. Chapman, L. A. McFadden, M. C. Malin, and M. S. Robinson 1997b. An overview of the NEAR multispectral imager-near-infrared spectrometer investigation. J. Geophys. Res. 102, 23709–23727. Veverka, J., P. Thomas, D. Simonelli, M. J. S. Belton, M. Carr, C. Chapman, M. E. Davies, R. Greeley, R. Greenberg, J. Head, K. Klaasen, T. V. Johnson, D. Morrison, and G. Neukum 1994b. Discovery of grooves on Gaspra. Icarus 107, 72–83. Yeomans, D. K., J.-P. Barriot, D. W. Dunham, R. W. Farquhar, J. D. Giorgini, C. E. Helfrich, A. S. Konopliv, J. V. McAdams, J. K. Miller, W. M. Owen Jr., D. J. Scheeres, S. P. Synnott, and B. G. Wiliams 1997. Estimating the mass of asteroid 253 Mathilde from tracking data during the NEAR flyby. Science 278, 2106–2109.
NEAR Encounter with Asteroid 253 Mathilde: Overview J. Veverka, P. Thomas, A. Harch, B. Clark, J. F. Bell III, B. Carcich, and J. Joseph Department of Astronomy and CRSR, Cornell University, Space Sciences Building, Ithaca, New York 14853 E-mail: [email protected]
S. Murchie and N. Izenberg Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, Maryland 20723-6099
C. Chapman and W. Merline Southwest Research Institute, 1050 Walnut Street, Suite 426, Boulder, Colorado 80302
M. Malin Malin Space Science Systems, Inc., P.O. Box 910148, San Diego, California 92191-0148
L. McFadden University of Maryland, Department of Astronomy, College Park, Maryland 20742
and M. Robinson Northwestern University, Department of Geological Sciences, 309 Locy Hall, Evanston, Illinois 60208 Received May 14, 1998; revised January 28, 1999
is remarkably low: 1.3 ± 0.3 g/cm3 , a value consistent with a rubble pile structure for the interior. Assuming that Mathilde’s rock type is similar to that found in CM meteorites, the porosity of the interior must be some 50%. Shock and seismic disturbances associated with major impacts are expected to be transmitted very poorly by Mathilde’s underdense interior, a fact which may explain the remarkable degree to which surface morphology and topography have been preserved in spite of later major collisional events. Except for the lower geometric albedo (0.047 ± 0.005), the photometric properties of Mathilde are closely similar to those of Phobos. The surface is extremely homogeneous in terms of both color or albedo: specifically, no color or albedo variations associated with craters have been identified. °c 1999 Academic Press Key Words: asteroids; asteroids, Eros; surfaces, asteroids.
On June 27, 1997, the NEAR spacecraft carried out the firstever encounter with a C-type asteroid, flying by 253 Mathilde at a distance of 1212 km. We summarize findings derived from 330 images obtained by NEAR’s MSI camera which cover about 60% of the surface of the asteroid. The highest resolution achieved was about 160 m/pixel. Mathilde is a low-reflectance object (geometric albedo = 0.047) with principal diameters of 66 × 48 × 44 km. The mean radius of 26.4 ± 1.3 km is somewhat smaller than the value of 30 km suggested by previous telescopic data. Mathilde’s surface morphology is dominated by large craters, at least four of which have diameters comparable to the radius of Mathilde. The two largest, Ishikari and Karoo, have diameters of 29.3 and 33.4 km, respectively. No evidence of layering is exposed in the crater walls, but suggestions of downslope movement are present. The surface density of craters in the diameter range from 0.5 to 5 km is close to equilibrium saturation, a situation in which as many craters are being destroyed as are being produced. Observed depth-to-diameter ratios for craters in this size range are close to those observed on the lunar surface. A disruption lifetime of about 4 billion years has been estimated for Mathilde. Based on the mass determination obtained from Doppler tracking (D. K. Yeomans et al., 1997, Science 278, 2106–2109) and the volume derived from MSI images, the average density of Mathilde
1. INTRODUCTION
NEAR, the first mission of NASA’s new Discovery program, was launched on February 17, 1996, on a 3-year trajectory to the near-Earth asteroid 433 Eros. The spacecraft carries a complement of six science instruments: imager, near-infrared spectrometer, gamma-ray spectrometer, X-ray spectrometer, magnetometer, and laser rangefinder in addition to a radio science 3 0019-1035/99 $30.00 c 1999 by Academic Press Copyright ° All rights of reproduction in any form reserved.
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investigation which analyzes the tracking signals from the spacecraft (Farquhar et al. 1995). NEAR will arrive and begin its orbital mission at Eros in early 1999. On its way to Eros the NEAR spacecraft passed within 1212 km of asteroid 253 Mathilde on 27 June 1997. In this first spacecraft encounter with a C-type asteroid (the dark, probably carbon-rich type predominant in the outer part of the belt), NEAR took over 500 images (Veverka et al. 1997a) and determined Mathilde’s mass to an accuracy of about 5% (Yeomans et al. 1997). The purpose of this paper is twofold: (1) to provide an overview of the major results obtained during the encounter, results which are detailed in the six contributions that make up this Special Issue of Icarus, and (2) to compare the Mathilde findings with those provided earlier by Galileo for S-type asteroids 951 Gaspra and 243 Ida (Veverka et al. 1994a, Belton et al. 1996). For convenience, Mathilde’s fundamental properties are summarized in Table I. The Mathilde encounter took place 1.99 AU from the Sun, shortly after the NEAR spacecraft reached the greatest heliocentric distance from the Sun on its 3-year journey to Eros. Because the solar panels on NEAR are sized to provide adequate power at the closer heliocentric distances of the Eros orbital phase, there was not enough power to operate all instruments during the flyby. To conserve power and to avoid the risk of the spacecraft aborting the flyby sequence if a power shortage developed, only one of the six instruments on NEAR, the Multispectral Imager (MSI), was turned on for the Mathilde encounter. This imager, an f /3.4 refractor, provided with a 244 × 537 pixel CCD, has a 2.26◦ × 2.95◦ field of view (Veverka et al. 1997b, Hawkins
TABLE I Mathilde Parameters Groundbased Orbit Type Geometric albedo Mean radius Lightcurve Period Amplitude Probable axes (diameters)
a = 2.6 AU e = 0.23 i = 6.9◦ C 0.038 60 km 17.4 days 0.45 mag 70 × 50 × 50 km NEAR
Nominal axes (diameters) Mean radius Volumea Massb Mean density Geometric albedo a b
Thomas et al., this issue. Yeomans et al. 1997.
66 × 48 × 46 km 26.5 ± 1.3 km 3 78,000 −11,000 +12,000 km 1.033 ± 0.044 × 1020 g 1.3 ± 0.2g/cm3 0.047 ± 0.005
et al. 1997). Developed for close-up studies of Eros from orbital distances (expected spatial resolution at Eros is 3–4 m from a 30-km orbit), the instrument was not designed for imaging asteroids during distant fast flybys. Nevertheless, resolutions of some 160 m per pixel were achieved during the 1212-km encounter. The inflight performance of the MSI camera is reviewed by Murchie et al. (1999). With the maximum exposure of 1 s, the camera can detect objects as faint as +9.5 mag. Absolute calibration, in particular the conversion of MSI measured fluxes to V -mag, can be accomplished to ±5–10%. 2. THE FLYBY
Carrying out the science objectives at Mathilde involved a number of new challenges, in terms of both navigation and spacecraft operation (Yeomans et al. 1997, Harch et al. 1995, Harch and Heyler 1998). Because the spacecraft has fixed solar panels and no scan platform, the encounter had to be carried out in an orientation which permitted enough sunlight to fall on the solar panels to provide the necessary power while slewing the spacecraft attitude to allow the field of view of the MSI camera to cover the region of the sky in which Mathilde was predicted to be. To meet the power requirements the solar incidence angle on the solar panels had to be maintained at 50◦ or less, a factor which limited the total observing time around closest approach to approximately 25 min. As the approach phase angle was very large (140◦ ), it was decided to concentrate most of the imaging at and just after closest approach, when the illumination geometry was much more favorable. The four goals of the imaging investigation at Mathilde were: (1) to obtain the highest resolution image possible to study surface morphology; (2) to provide a hemispheric view of the asteroid at a resolution of about 500 m per pixel to determine the asteroid’s size, shape, and volume; (3) to obtain hemispheric coverage in color to look for evidence of compositional heterogeneity; and (4) to search the vicinity of Mathilde for possible satellites. The spacecraft approached Mathilde looking close to the direction of the Sun (approach phase angle ∼140◦ ). The actual imaging sequence (Fig. 1) began some 5 min before closest approach, when views of a crescent-illuminated Mathilde were obtained at resolutions of about 500 m. The highest resolution data (160-m resolution) were obtained at closest approach (12l2 km), when the phase angle was close to 90◦ . The imaging sequence continued for another 20 min as the spacecraft receded from the asteroid and viewed Mathilde under good illumination at a phase angle of about 40◦ . During this time multicolor global coverage was obtained at 500-m resolution using the seven color filters on MSI which cover the spectral range from 400 to 1100 nm. The imaging sequence concluded with about 200 images devoted to a satellite search. The MSI camera is sensitive enough that objects as small as 40 m across could have been detected even if they were made of material as dark as Mathilde. On approach, six special sequences of images were obtained beginning at 42 h before closest approach to perform optical
MATHILDE OVERVIEW
5
FIG. 1. Schematic summary of the imaging sequence at Mathilde.
navigation by detecting Mathilde against the star background. Since NEAR approached Mathilde looking close to the direction of the Sun, the asteroid was detected first as a faint dot almost lost in the Sun’s glare, just 36 h before closest approach. For
comparison, under much more favorable viewing conditions, Galileo detected Gaspra 53 days before encounter and Ida 33 days before! By the sixth and last sequence at 11 h before encounter (Fig. 2), Mathilde had brightened to about +7.0 (±0.3)
FIG. 2. Optical navigation detection of Mathilde on approach. Ten MSI frames taken during OpNav Sequence 6 some 11 h before closest approach have been co-added and the strong background gradient due to solar glare (phase angle ∼140◦ ) has been removed.
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TABLE II Mathilde OPNAVs
TABLE III Mathilde Encounter
OPNAV Time (h) Range (million km) Predicted MAG Actual MAG Delta 2 3 4 5 6
−37 −30 −24 −18 −11
1.32 1.07 0.88 0.66 0.40
7.7–8.7 7.4–8.4 6.9–7.9 6.3–7.3 5.3–6.3
∼9.6 9.5 9.0 8.2 7.0
+0.9 +1.1 +1.1 +0.9 +0.7
Note. Approach phase ∼140◦ .
visual magnitude, about a magnitude fainter than our preencounter estimate of +5.3 to +6.3 (Table II). Based on a rapid analysis of these data, the pointing was updated to ensure that the camera would be looking at Mathilde at the time of the flyby. The last update was sent to the spacecraft only 5 h before the encounter. As already noted, unlike Galileo NEAR does not have a scan platform and the whole spacecraft must be slewed to keep the camera pointed at the asteroid. The flyby was executed flawlessly. The spacecraft passed the asteroid at 1212 km, very close to the planned 1200-km miss distance (Table III). All images were obtained as planned: the final optical navigation solution was so accurate that Mathilde was captured within the central frame of the closest-approach
Date of NEAR flyby Time of closest approach Flyby distance Flyby speed Approach phase angle Departure phase angle Duration of imaging sequence Total No. of frames taken Total No. of frames of Mathilde
27 June 1997 12h 55m 54.5s UT 1212 km 9.93 km/s 140◦ 40◦ 25 min 534 330
mosaic designed to capture the highest possible resolution image of the asteroid (Fig. 3). Although the sequence executed precisely as planned, only about 330 of the 534 MSI frames exposed contain the asteroid (Table IV). Some were intentionally pointed at the region around Mathilde to search for satellites. Others were used as insurance to cover the uncertainty region of Mathilde’s position at the time of closest approach, the linear extent of which was about 10 times the camera field of view. A representative sequence of MSI images is shown in Fig. 4. Although MSI has an autoexposure capability (Hawkins et al. 1997), this feature was not used during the Mathilde flyby. All
FIG. 3. Mathilde’s actual position relative to the sequence of MSI frames taken to capture the highest resolution view of the asteroid. Shown is the 2σ uncertainty ellipsoid in Mathilde’s position. Dimensions 460 by 160 km.
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MATHILDE OVERVIEW
FIG. 4. Sample of consecutive MSI frames. See Table IV.
exposures were preplanned using model calculations, assuming a Phobos-like photometric function (Simonelli et al. 1998), with the geometric albedo lowered to 0.035 (by changing the value of the single scattering albedo) and assuming a triaxial model for Mathilde with a mean diameter of 61 km. Generally, three sets of exposures were obtained, with the middle exposure chosen to yield a maximum DN value of 1000 in the nominal albedo case. (The maximum possible DN value is 210 or 4096.) The other two exposures of each set were chosen to bracket the middle exposure by a factor of 2. This strategy proved successful, yielding properly exposed images during all phases of the encounter. The fact that on approach Mathilde turned out to be fainter than expected in the optical navigation images (see Table II) can be attributed to a slight overestimation of Mathilde’s size (see below) and to a characteristic of Mathilde’s surface: the significant effect of large shadows at extreme phase angles produced by the huge crater concavities that define Mathilde’s shape. A major goal of the Mathilde encounter was a search for satellites, motivated in part by speculations that a large satellite might help explain the asteroid’s unusually slow rotation period. On approach our ability to search for satellites was limited by several factors, but principally by the large approach phase angle. A search of the approach images which cover the whole so-called Hill sphere estimated to extend some 100 radii around Mathilde, revealed no satellite larger than 10 km in radius, assuming the same albedo as Mathilde. Approximately 200 images were obtained to search for satellites after closest approach under a much more favorable phase angle of about 39◦ (Fig. 5). Our coverage
was limited to about 20 radii around Mathilde: no satellite down to a limiting size of 40 m was detected. The duration of the post closest approach satellite search was limited by spacecraft constraints. Given that the spacecraft had to be maintained in an attitude which provided sufficient illumination to the solar panels to power the spacecraft and the MSI camera without recourse to the battery, the entire data-taking sequence during the encounter was limited to about 25 min. 3. SHAPE, SIZE, AND MEAN DENSITY
On the basis of control point and stereo measurements, as well as from constraints provided by observed limb profiles and terminator locations, Thomas et al. (1999) derive a model which reproduces accurately the observed shape of Mathilde. The fidelity of the model can be judged from Fig. 6, which includes two synthetic images of Mathilde derived from the shape model interpolated to fill a gap in our data coverage between the high phase view (α = 126◦ ) at left and the closest approach view (α = 90◦ ) at right. The mean radius of Mathilde derived by Thomas et al., 26.41 ±1.3 km, is somewhat smaller than the value of 30 km suggested by averaging several telescopic estimates.1 Some 60% of Mathilde’s surface was observed during the flyby. The NEAR data, obtained over an interval of 25 min, do not place a useful 1 The difference is consistent with the fact that we find a slightly higher value for the geometric albedo (see Table I).
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TABLE IV Mathilde Flyby Picture List
TABLE IV—Continued
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MATHILDE OVERVIEW
TABLE IV—Continued
TABLE IV—Continued
Note. MET, mission event time, a unique picture identifier; lat, latitude (◦ ); lon, west longitude (◦ ); range, range in km; phase, solar phase angle (◦ ); exp, exposure time (s); filter, MSI filter; hires, resolution (pixel width) km/pxl; lores, resolution (pixel length) km/pxl. Filter designations are as follows (Veverka et al. 1997b) (filter No., spectral coverage (nm)): 0, 700 ± 100; 1, 550 ± 15; 2, 450 ± 25; 3, 760 ± 10; 4, 950 ± 20; 5, 900 ± 20; 6, 1000 ± 25; 7, 1050 ± 40. MSI has rectangular pixels (27 × 16 µm), corresponding to 162 × 96 µrad/pixel.
constraint on the spin period of Mathilde, which according to Mottola et al. (1995) is 417 h (with a lightcurve amplitude of 0.45 mag). No changes in shadow positions were noted over a period of some 10 min when the highest resolution views are available, from which Thomas et al. set a lower limit of 1.8 days on Mathilde’s spin period. Neither NEAR nor telescopic observations place useful constraints on Mathilde’s pole position. The nominal shape model has dimensions of 66 × 48 × 44 km, for a volume of 77200 km3 . Thomas et al. estimate that the volume is uncertain by about ±14%, a result which yields a mean density of 1.34 ± 0.2 g/cm3 , using the mass measurement of 1.033(±0.044) × 1020 g by Yeomans et al. (1997). Thomas et al. stress that the observational constraints make it extremely unlikely that the true volume of Mathilde is smaller than their minimum estimate, implying that the density of Mathilde must be less than 1.5 g/cm3 . 4. SURFACE FEATURES AND PROPOSED NAMES
Thomas et al. produced a sketch map (Fig. 7), which includes provisional designations (yet to be approved by the IAU) for the principal features, predominantly large craters. The locations and names are summarized in Table V. As already noted, the location of Mathilde’s spin axis remains undetermined; therefore, the coordinate system shown is arbitrary and uses the J2000 reference frame as basis (Thomas et al. 1999). Given Mathilde’s very low albedo and possible carbonaceous composition, the proposed names are those of coal-mining regions and coal mines
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FIG. 5. Representative sequence of images from NEAR’s satellite search.
on Earth. A series of Mathilde views, with the names of prominent craters superimposed, is provided in Fig. 8. 5. THE GEOLOGY OF MATHILDE
Mathilde’s surface morphology is dominated by large craters, at least four of which have diameters comparable to the radius of Mathilde. The largest craters are Ishikari and Karoo, with diameters of 29.3 and 33.4 km, respectively (Fig. 8).
The geology of Mathilde is discussed in detail by Thomas et al. (1999), while Chapman et al. (1999) focus on the cratering record preserved on the asteroid’s surface. Craters down to diameters of some 500 m can be detected reliably in the NEAR images: nothing unusual has been noted about their morphology. Thomas et al. find depth-to-diameter ratios for intermediatesized craters (diameters about 1 to 5 km) to range from 0.12 to 0.25, compared to the well-determined value of 0.2 for fresh craters on the Moon (Pike 1977) and to values slightly below
MATHILDE OVERVIEW
11
FIG. 6. Demonstration of the fidelity of the shape and photometric models derived from the Mathilde data. At the two ends are two real views of Mathilde (phase ∼138◦ at left; phase ∼90◦ at right). In between are two “interpolated” synthesized views showing intermediate aspects of Mathilde not imaged by NEAR. Such modeling provides a useful visualization of the relative positions of major surface features. Prominent craters are identified by letters: A, Jixi; B, Damodar; C, Karoo.
0.2 estimated for Gaspra (Carr et al. 1994) and for Ida (Sullivan et al. 1996). The fresher intermediate size craters on Mathilde appear bowl-shaped; no examples of complex craters with flat or ringed floors are observed. Various stages of crater degradation are present, consistent with the observation that Mathilde’s surface is at the saturation equilibrium level in terms of crater density (see below and Chapman et al. 1999). A few craters have definite raised rims, but no striking examples of ejecta blankets are noted. The extreme color and albedo blandness of Mathilde’s surface TABLE V Crater Names, Locations, and Diameters
Name
Latitude (◦ )
Longitude (◦ )
Diameter (km)
Image
Benham Maritsa Mulgildie Jerada Kalimantan Clackmannan Matanuska Quetta Similkameen Lorraine Aachen Oaxaca Enugu Lublin Teruel Otago Zulia Baganur Jixi Damodar Kuznetsk Ishikari Karoo
19.0 44.6 57.7 42.1 −7.7 18.9 27.3 45.6 −13.5 48.1 9.2 38.7 −15.3 55.3 −28.1 23.7 −39.5 14.6 −12.3 73.0 45.9 −66.2 33.5
247.2 151.5 176.1 177.3 123.7 260.8 217.3 165.5 104.7 142.5 60.9 186.3 151.6 156.8 142.7 164.5 30.9 191.6 256.5 263.8 88.9 186.9 98.4
2.2 2.4 2.5 2.5 2.7 2.8 2.9 3.2 3.4 4.1 4.8 5.2 5.9 6.5 7.6 7.9 12.3 16.4 19.9 28.7 28.5 29.3 33.4
42826360 42826370 42826360 42826360 42826488 42826360 42826360 42826360 42826622 42826370 42826360 42826360 42826488 42826360 42826488 42826488 42826686 42826488 42826112 42826112 42827050 42827050 42826622
discussed by Clark et al. (1999) makes it very difficult to detect possible ejecta blankets. No blocks of ejecta have been identified; if any exist, they cannot be larger than 200 to 300 m across. No evidence of layering is seen in the walls of the very large craters imaged at high resolution, but at least two or three examples of downslope movement have been identified (Veverka et al. 1997a, Thomas et al. 1999). No strong evidence of a pervasive global fabric such as the grooves and facets on Gaspra (Veverka et al. 1994b), is observed. There are, however, three suggestions of at least local structure or fabric: the polygonal outlines of some craters, a sinuous marking with the characteristics of an exposed layer, and a few scarps or ridges. One of these scarps, of 20 km extent, and an associated relief of some 200 km, is the largest structural feature on Mathilde noted by Thomas et al. While the absence of grooves and facets may not be surprising on a body having the presumed structure of a low-density rubble pile, the existence of some evidence of fabric, of which the polygonality of some craters is most widespread, is interesting. Both Thomas et al. and Chapman et al. point out the remarkable degree to which surface morphology and topography have been preserved on Mathilde in spite of later major collisional events. Large impact events can destroy distal topography through seismic shaking (including spallation) or by blanketing it with ejecta. There are no obvious clues on Mathilde to suggest that either process has been effective. Clearly, Mathilde’s underdense interior can be expected to transmit shock and seismic energy poorly (Thomas et al. 1999, Chapman et al. 1999, Davis 1999). The apparent absence of evidence that surface morphology has been modified by ejecta is more surprising. For example, on Ida, a body with weaker gravity than Mathilde, there is strong evidence of impact ejecta on the surface (Geissler et al. 1996), as there is even on the surface of Mars’ tiny satellite Deimos (Thomas et al. 1996a). An analysis of the cratering record preserved on Mathilde’s surface is presented by Chapman et al. (1999), who find that for craters in the range from 0.5 to 5 km the surface is close
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FIG. 7. Sketch map of Mathilde from Thomas et al. (this issue), showing provisional names for major surface features.
to equilibrium saturation, a condition in which as many craters are being destroyed as are being produced. This situation is like that observed on asteroid 243 Ida (Chapman et al. 1996a), but is unlike that on 951 Gaspra (Chapman et al. 1996b), where a relatively younger surface was observed. To derive a lower limit (because the surface is saturated, only a lower limit can be derived) on the age of Mathilde’s surface, one must know the size distribution of impacting objects, the flux of this population, and the mechanical properties of Mathilde’s surface. Chapman et al. suggest that the latter are so uncertain that no precise lower bound on Mathilde’s age can be derived. A somewhat more optimistic approach is adopted by Davis (1999). In a previous study Farinella and Davis (1991) estimated the lifetime against collisional disruption for the S-type asteroid 951 Gaspra of some 200 myr, while Greenberg et al. (1994) derived a value a factor of 5 greater. Analogous calculations are carried out for Mathilde by Davis (1999), making reasonable assumptions about the size distribution and flux of the impacting population (believed to be essentially the same as those which affect Gaspra and Ida), but making allowance for the likely weaker mechanical strength of C-type objects relative to S-type asteroids. It is concluded that a C-type asteroid the size of Mathilde has a nominal disruption lifetime of about 4 billion years and that the time interval needed to form the four or five giant craters is noticeably less, probably 400 to 2000 million years. In other words, it is not surprising that bodies large enough to form the huge craters have hit Mathilde, while at the same time Mathilde has not been demolished by a catastrophic impact. Davis suggests that Mathilde is likely to be the remnant of a larger body which lost 50% or more of its initial mass by cratering.
6. PHOTOMETRIC AND COLOR PROPERTIES
From a combination of telescopic data between 1.5◦ and 16.5◦ phase obtained by Mottola et al. (1995) and NEAR observations at phase angles ranging from 39◦ to 136◦ , Clark et al. (1999) derive the first-ever detailed photometric properties of a C-type asteroid. The resulting geometric albedo of 0.047 ± 0.005 agrees well with that estimated by telescopic techniques, once the mean radius is adjusted downward as discussed above. Albedo variations on Mathilde are very subdued. Allowing for the fact that some of the remaining brightness variations apparent in photometrically corrected data may be due to inadequately modeled small-scale topography in the Mathilde shape model, albedo variations on this asteroid are restricted to about ±10% of the mean, a smaller range than is observed on two other well studied low-albedo small bodies: Phobos (Simonelli et al. 1998) and Deimos (Thomas et al. 1996a). Albedo variations associated with crater rims and with ejecta have been noted on Phobos and to varying degrees on S asteroids Gaspra and Ida. No such variations are evident on Mathilde. This lack of albedo variation related to surface morphology is consistent with the concomitant lack of variations in color (Veverka et al. 1997a). Mathilde’s homogeneity in albedo and color is interesting in that the only somewhat brighter surface of Phobos does show measurable variations, as do different size fractions of pulverized carbonaceous meterorites and carbonaceous meteorite analogs measured in the laboratory (e.g., Johnson and Fanale 1973). One implication is that not only globally, but even locally in the proximity of crater rims, Mathilde’s regolith has a uniform texture/particle size distribution.
MATHILDE OVERVIEW
13
FIG. 8. Four views of Mathilde with provisional names of prominent craters indicated. Locations of named features are given in Table V.
Mathilde’s light scattering properties (or “photometric function”) show that Mathilde’s surface is not only much darker (lower single scattering albedo), but also more backscattering than the surfaces of S-type asteroids. Except for a significantly lower single scattering albedo, the photometric properties of
Mathilde are very similar to those of Phobos. Macroscopic surface roughness, as measured by Hapke’s parameter 2, does not seem to differ significantly among the three asteroids (Mathilde, Gaspra, and Ida) studied by spacecraft to date. In addition to the mean geometric albedo of 0.047, Clark et al. derive a
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phase integral of q = 0.28 and a Bond albedo of 0.013 for Mathilde. Since the NIS spectrometer on NEAR could not be utilized during the Mathilde flyby (due to insufficient spacecraft power, as discussed above), very little new information about Mathilde’s spectral properties can be added to the knowledge derived from telescopic data. The one exception is the new insight that at least on this C-asteroid, large scale and local color variations of any significance are absent. Binzel et al. (1996) found that Mathilde’s spectrum is essentially gray, with a shallow absorption shortward of 500 nm and a suggestion of a dip near 900 nm. Based on these characteristics, it was concluded that Mathilde’s spectrum does not match well those of CM or CV chondrites. Gorlovka, a shockdarkened ordinary chondrite, and Y-826162, an anomalous CI chondrite, were proposed as better matches. Observations between 1.2 and 3.3 µm by Rivkin et al. (1997) show that unlike the anomalous CI chondrite, Mathilde does not display a 3.0-µm water of hydration absorption in its spectrum. Strong arguments have also been advanced (e.g., Chapman 1996) to the effect that C-type asteroids are unlikely to be related to shocked ordinary chondrites. While an unaltered sample of the hydrated CM meteorite Murchison fails to match Mathilde’s spectral properties, as already noted by Binzel et al. (1996), a heated sample provides an excellent match (Hiroi et al. 1993): heating drives off the water of hydration, removes the 3.0-µm absorption, and flattens the slope of the near-infrared continuum. As a useful working hypothesis, we will assume in our discussion that Mathilde material is similar to dehydrated CM material. 7. DISCUSSION
The determination of Mathilde’s mean density is based on the first direct measurement of an asteroid mass from perturbations on a passing spacecraft. The mass of 243 Ida was estimated by Belton et al. (1995) based on constraints on the orbit of Ida’s satellite Dactyl. Neither Ida nor Gaspra produced measurable perturbations on the path of Galileo. The strong constraints on Mathilde’s volume (Thomas et al. 1999) and the precise mass determination (Yeomans et al. 1997) yield a surprisingly low mean density of 1.3 ± 0.2 g/cm3 , a value which argues for significant void space inside of Mathilde (Veverka et al. 1997a). As stressed by Thomas et al., it is difficult to develop a shape model for Mathilde which is consistent with observations and for which the asteroid’s mean density would exceed 1.5 g/cm3 . The idea that collisionally evolved asteroids could have porous, underdense structures is not new (Barks 1960). Unfortunately, to estimate the fraction of void space within Mathilde necessitates an assumption about the composition of Mathilde rock. As discussed above, we adopt the working hypothesis that this rock is similar to carbonaceous chondrite material, most likely CM-type rock. Table VI summarizes the fraction of void space required
TABLE VI How Porous Is Mathilde? Rock type Rock Density (g/cm3 ) Mathilde porosity (%) for mean density of 1.34 ± 0.2 g/cm3
CI
CM
CO/CV
2.25 40 ± 6
2.75 51 ± 8
3.45 61 ± 9
to explain Mathilde’s mean density assuming that the asteroid consists of carbonaceous-chondrite-like rock. The values range from 30 to 70%, with a most likely value of about 50%. An unlikely possibility is that subsurface ice contributes to the low density. Given the 0.9 g/cm3 density of water ice, the presence of water ice cannot be the major explanation for Mathilde’s low density. As summarized in Table VII, such an explanation requires that some 67 to 83% of Mathilde’s volume to be ice, a very unlikely proposition in our view, given Mathilde’s geologic evolution and current understanding of C-type asteroids. For example, it is difficult to understand why, if C-type asteroids are made up largely of water ice, no evidence of outgassing or comet-like activity has ever been reported in association with a C-type asteroid. It seems equally unlikely that significant quantities of ice would be preserved inside an object as heavily affected by severe collisions as Mathilde has been (Chapman et al. 1999, Davis 1999), especially if this object has a low-albedo surface and orbits at 2.6 AU from the Sun. It is certainly true that rather small amounts of pulverized carbonaceous material can lower the albedo of water ice significantly and perhaps even mask completely the characteristic near-infrared absorption bands (e.g., Clark 1981). However, in the case of Mathilde, several facts suggest that water ice is not an important constituent: (a) no spectral evidence of water ice or of hydrated materials has been observed, and (b) no exposures of higher albedo materials have been seen. Jupiter’s satellite Callisto provides an instructive example of how difficult it is to mask all surface evidence of water ice, even in the presence of appreciable amounts of a dark, nonicy component. Finally, we stress that no morphologic evidence of a surface rich in subsurface ice occurs in the NEAR images. Features that might be attributed to “sapping” or “solifluction” do not occur on Mathilde’s steeper slopes, nor are there any regions of collapsed terrain within craters. All of these considerations argue against water ice being a significant constituent of either Mathilde’s surface or interior. TABLE VII Is Ice the Answer?
Rock density Ice density Ice fraction (%) needed (1.34 g/cm3 )
CI
CM
CO/CV
2.25 0.90 67
2.75 0.90 76
3.45 0.90 83
MATHILDE OVERVIEW
Chapman et al. stress that very large craters, five of which have diameters between 20 and 33 km, occur with uniquely high spatial density on Mathilde. The formation of such large craters is not unexpected, assuming (as is likely) a cratering population similar to that which has impacted the Moon. It is the preservation of so many of these large craters which is noteworthy. Remarkably, the last of the major cratering events did not destroy or even noticeably modify some of the earlier ones. This apparent ineffectiveness of major impacts to modify preexisting topography except in the immediate vicinity of the event is probably explained by Mathilde’s underdense interior and likely weak mechanical strength: it might be relatively easy to produce a crater cavity, but difficult to transmit the energy effectively to ejecta or to distant parts of the asteroid. Structurally, the three asteroids visited by spacecraft to date seem to differ significantly from one another. Gaspra has a strongly faceted shape and other features suggestive of a monolithic body with a global fabric and perhaps considerable mechanical strength (Veverka et al. 1994a). Ida has a very elongated, almost bifurcated shape, possibly a somewhat underdense interior, and at least a hint of different mechanical properties at the two ends (Belton et al. 1996, Thomas et al. 1996b). Mathilde is a monolithic, very underdense body, with a generally spheroidal shape modified by large crater cavities. Clearly, not all asteroids are alike, and their continued detailed investigation by spacecraft is essential to obtaining a firm understanding of their nature and evolution. ACKNOWLEDGMENTS This contribution is dedicated to the memory of Dr. Jurgen Rahe, who played a key role in making the NEAR mission and the exploration of Mathilde a reality. We gratefully acknowledge the invaluable assistance of many colleagues on the NEAR flight team, including T. Coughlin, R. Farquhar, G. Heyler, M. Holdridge, A. Santo, D. Dunham, and B.G. Williams. We thank M. Belton and R. Binzel for helpful reviews. This research was supported by the NEAR project and by NASA Grant NAG 5-3626.
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