ABSTRACT ======== INTRODUCTION ============ Radar observations provide a powerful tool for detailed physical characterization of near-Earth objects and detailed reconstruction of asteroid three-dimensional shapes. Over the last decade detailed three-dimensional shapes have been reported for 4769 Castalia (Hudson and Ostro 1994), 4179 Toutatis (Hudson and Ostro 1995), 1620 Geographos (Hudson et al. 1999), and 6489 Golevka (Hudson et al. 2000). Here we report detailed radar observations and a 3-D model of asteroid 1999 JM8, our first look at a large, optically dark near-Earth asteroid. The images reveal an object with an extremely slow, probably non-principal axis spin state, a highly irregular shape, and numerous craters between ~100 m and 1.5 km in diameter. 1999 JM8 was discovered by LINEAR on 1999 May 13, fortuitously more than two months prior to an encounter within 0.057 AU (22 lunar distances) of Earth on 1999 July 29. 1999 JM8 was originally discovered at Palomar in April, 1990 by E. F. Helin, but it was subsequently lost. Its rediscovery on 1999 May 13 occurred with sufficient lead time to prepare an extensive campaign of radar and optical observations. 1999 JM8 made an extended close encounter within 0.1 AU of Earth between July and October of 1999 that offered an excellent opportunity for physical observations. Pravec and Sarounova (Ondrejov), Hicks (Table Mountain), and Krugly (Kharkiv) obtained photometry that indicate very slow rotation and possibly a non-principal axis rotation state. Hicks also obtained photometric colors at TMO that suggest that JM8 is an optically dark object. Hicks, Buratti, and Hanner obrtained thermal radiometry with the 5-m Hale telescope at Palomar. Howell obtained an extensive set of vis-near IR spectroscopic measurements at McDonald Observatory during September of 1999, about 6 weeks after the radar observations concluded. Table I summarizes the optically-determined physical properties of 1999 JM8. We observed 1999 JM8 at Goldstone and Arecibo on 18 days between 1999 July 18 and August 9, dates that spanned the asteroid's closest approach to within 0.057 AU of Earth on 1999 July 29. The asteroid's close approach, large size, and extremely slow rotation provided an outstanding opportunity for radar observations. The best asteroid radar target that has ever been OBSERVATIONS ============ The first radar detection of JM8 was by CW on July 18. The initial orbital solution (15) was very good due to the 9-year optical arc. We measured a Doppler correction of +1.7 Hz at the epoch 1999 July 18 21:30:00 and updated the orbit to solution 17. Attempts to range the asteroid during the first track were unsuccessful due to problems resulting from the crash of the imaging DAS during a power dip the night before the observations. On the second track (July 20) we started with 2 CW runs, measured the Doppler correction of -0.5 Hz, and then successfully obtained a range with coarse- resolution 10 and 11 usec setups. We completed this very short track with two 1-usec medium resolution ranging runs that resolved the target into about 150 pixels. We used the radar astrometry to generate solution 19. During the July 21 track we verified the echoes with one CW run and then we devoted the rest of the track to 1 us x 0.1 Hz imaging. We updated the ephemeris to solution 21, which we used during the ensuing week. Thereafter, our strategy during the Goldstone tracks was to do one or two CW runs to verify that we had echoes and then devote the rest of the track to high resolution imaging with progressively finer resolution until after August 1 as the asteroid approached Earth. For the Aug 7 and 8 Goldstone tracks, the asteroid's distance increased by ~50% relative to the minimum on July 29 so we backed off to a coarser-resolution 0.25 us x 0.075 Hz setup that still placed thousands of pixels on the target. We observed JM8 at Arecibo on August 1-6 and August 9. We updated the orbit solution twice to incorporate additional radar and optical astrometry (solution 26). We used two imaging data acquisition systems at Arecibo: the Caltech Baseband Recorder (CBR), which was designed for observing pulsars and was graciously made available to us by the Caltech group (FIND OUT WHO!), and a new system originally built by Jean-Luc Margot for observing 6489 Golevka (the Golevka sampling box, or GSB) that was then under development. The prime reason for using each system was the ability to sample fast enough to achieve 0.1 usec (15 meter) resolution. The CBR has dual polarization capability but is more cumbersome to use for radar observations than the GSB; the GSB in August, 1999 had single polarization capability. We used both systems on each day, but the analyses reported here utilize the CBR data exclusively due to the modestly stronger SNR (a few dB) and the dual-polarization capability. The Arecibo delay-Doppler images were reduced using full-length Fast-Fourier Transforms that were zero extended once achieve the finest frequency resolution possible (given that echo SNR was not an issue). The resulting frequency resolutions vary from 0.0098 Hz on August 1 to 0.0060 Hz on August 9 and are the finest S-band frequency resolutions ever obtained for any asteroid. 1999 JM8's rotation is so slow that rotational smear was negligible during each Arecibo track, so we grouped all the Arecibo images from each day into one weighted summed image. We adopted the same approach for the Goldstone images except for August 8, when the track was long enough (more than 7 hours) that subtle rotation was evident in weighted sums of images that each spanned ~2.5 hours. Table "observations" summarizes the observations. Satellite search ---------------- ? ASTROMETRY & ORBIT REFINEMENT ============================= Refer to Jon's email about future/past encounters Follow template of the Toutatis paper: combined optical and radar observations from April 1990-present permit reliable computation of the orbit +/- 1000 years. Over the upcoming millenium, no encounters closer than the one in 1999. In the past the August 1990 encounter was nearly a factor of two closer but at that time 1999 JM8 had been lost. Impact probability over next ~1000 years: Zero. Include a table of past and future close encounters with uncertainties. The semimajor axis of 1999 JM8's orbit is presently within 0.006 AU (2.2 lunar distances) of a 2:9 mean motion resonance with Earth, which is evident in the asteroid's recent pattern of close approaches in August of 1981 (0.057 AU) and 1990 (0.034 AU) and in late July of 1999. JM8 makes a distant encounter with Earth in 2008 (0.32 AU) and then the pattern of close approaches at 9-year intervals ceases. DISK-INTEGRATED PROPERTIES ========================== X-band OC cross sections, bandwidths, SC/OC. Show a sequence of X-band CW spectra from each day. Compare the nominal SC/OC with those of other radar-detected NEAs. DELAY-DOPPLER IMAGES ==================== Rotation period Shape -asymmetry -dimensions -angularity -topography Surface features: -Concavities Four obviously large concavities seen on multiple days Smaller concavities: July 31, Aug 1, 4, 5, 6 Radar-bright rim on July 28 Implications of the concavities -lineament -Other features -SC/OC images Global view ----------- Figure IMAGES shows a chronological sequence of OC delay-Doppler images from Goldstone and Arecibo and Table DELAY-DOPPLER DISPERSIONS gives the bandwidths and visible range extents on each day. The maximum visible delay extent was 5.3 km on August 6 and the maximum bandwidth (when adjusted for the differences in wavelength between S- and X-band) was on Aug. 5. The Arecibo images generally show larger delay extents than the Goldstone images due to the greater sensitivity at Arecibo (this is clearly evident on August 1, the only day of overlap at both telescopes) and possibly due to differences in the asteroid's orientation. Rotation period --------------- The echo bandwidths, visible range dispersions, and Eq. XX provide constraints the apparent rotation period. If we assume that the trailing edge of the echoes is at the center of mass, and that the true diameer is twice the visible range extent, then we obtain upper bounds on the apparent rotation period that vary between 20.6 and 7 days with a mean of 11.5 days. The clear implication from this rough estimate is that 1999 JM8 is a very slow rotator. Additional evidence for slow rotation is available in time-resolved delay-Doppler images from each day. We summed individual looks spanning about 0.8 h from the beginning and end of each track and searched the resulting images for evidence of rotation. The only day in which rotation is clearly apparent is on August 8, when the observations span 7.4 hours (Fig. IMAGES). Careful inspection of the other Goldstone images also shows very subtle differences in the shape of the leading edges on July 31 and August 1 that are probably attributable to rotation. We found no evidence of rotation in the Arecibo images, which all span less than 1.4 hours. The general lack of discernible rotation indicates that JM8 rotates with a period of at least several days. The images on each day are distinctly different between July 20 and 31, but the August 1 image bears a striking resemblance to the lower-resolution image obtained on July 24. This suggests that a major component of the asteroid's rotation has a period that is close to 8 days and it is independent confirmation of the slow rotation that Pravec et al. obtained from lightcurves. However, decoupling the effects of rotation and sky motion generally requires modeling for the asteroid's shape and estimating the spin vector (Hudson 1993), so our constraint on the rotation period based on visual inspection of the echoes should be treated as a preliminary estimate. The bandwidth increased between July 24 and August 1 by a factor of about 1.8, which, if we ignore the NPA rotation that is suggested by the lightcurves, suggests that the instantaneous subradar latitude moved south from about 72 to 57 degrees (cf. Ostro 1987). If we adopt Pravec et al.'s nominal rotation period of 6.76 days, then the July 24 and August 1 bandwidths correspond to lower bounds on the maximum pole-on dimension of about 9.2 km, or somewhat more than twice the observed range extent. This is close to what we expect if the asteroid's center-of-mass is near the trailing edge in the images. 1999 JM8's 50 degrees of westerly motion (i.e., toward decreasing right ascension) indicates that the sense of rotation between July 24 and August 1, an interval of 7.8 days, must be retrograde in order for the apparent rotation to be longer than the rotation period from the lightcurves. A period of close to 7 days is also suggested by the similarity of the August 8 images to those obtained on July 24 and August 1. Shape and elongation -------------------- The images provide strong evidence that 1999 JM8 has an irregular and asymmetric shape. For example, the leading edge is rounded on July 24, 31, and August 1, 2, 8, and 9, but portions of it are distinctly angular on July 27, August 5, and 6. In the July 28 image two hill-like features are evident on the leading edge that extend about 400 meters toward the radar relative to their surroundings. The August 5 image shows an extended, gently undulating leading edge. A rotated view of the same feature on August 6, when the feature is oriented closer to the line of sight, shows that it is close to 5 km in its maximum dimension. Using the 2-sigma level as a threshold for detection, then there is a maximum factor of 1.5 difference in the visible delay extent among the Arecibo images and a factor of 1.7 difference in the Goldstone images. Although the shape of 1999 JM8 is apparently irregular, 1999 JM8 is not bifurcated, in contrast with Toutatis (Ostro et al. 1995, Hudson and Ostro 1995), Castalia (Ostro et al. 1990, Hudson and Ostro 1994), and Bacchus (Benner et al. 1999). We estimated the elongation of JM8 from extrema in the echo bandwidths between August 1 and 9, when we obtained the strongest images with the highest frequency resolution. The August 1 and 8 images are similar and their echo bandwidths have a ratio of 1.1, suggesting that the 42 degrees of sky motion did not substantially alter the subradar latitude between the two days and that variations in the bandwidth during the intervening days result mostly from the asteroid's shape. The ratio of the maximum (Aug 5) to the minimum (Aug 2 and 3) bandwidths is only 1.2, suggesting that 1999 JM8 is not very elongated. This is comparable to the three lowest NEA elongations reported from radar observations to date: 2100 Ra-Shalom (Shepard et al. 2000), 7822 (1991 CS) (Benner et al. 1999a), and 1998 KY26 (Ostro et al. 1999b). Ultimately a more accurate estimate of the asteroid's elongation will come from the shape reconstruction, which will explicitly remove bandwidth variations caused by sky motion on the apparent subradar latitude and the effects of non-principal axis rotation. Several of the images show a distal arc or linear extension of pixels (July 24 August 1 and 8). The sequence of daily images between July 31 and August 9 makes it clear that this feature is the "indentation" that is prominent on the approaching limb on August 5. Concavities ----------- The 1999 JM8 images contain numerous examples of relatively bright pixels surrounding rounded clusters of radar dark pixels. These are the signatures of concavities that may be impact craters, as have previously been seen in delay-Doppler images of Touatatis (Ostro et al. 1995, Hudson and Ostro 1995) and Geographos (Ostro et al. 1996, Hudson and Ostro 1999). Figure CRATERS identifies possible concavities. Our concavity identifications are somewhat subjective due to the delay-Doppler resolution; the variable to low SNRs across portions of the images, particularly near the trailing edges; the modest contrast in echo power between the dark pixels and surrounding brighter pixels of some features; and due to delay-Doppler projection effects that may explain why several features are elliptical or tear-shaped. Most of the possible concavities have been identified in images on at least two days; several of the largest have been seen on as many as six days and are clearly real. It is possible that we may have missed some features or even included some features that are not concavities, but in what follows we will assume that the features we have identified are craters and explore the implications. We estimated the visible range extents of the features from the delay-Doppler images. For features visible on multiple days, the maximum range extents are consistent for some features but are different by as much as several tens of percent for others, differences that in some cases result from changes in the projected shape of the features from one day to the next in the images, perhaps due to projection effects as the asteroid spins, and also due to the weak SNR of some features near the trailing edges of the echoes that make identifying concavities difficult. Much more robust estimates of the geographic distribution and dimensions of impact craters will become available upon completion of the 3-D shape model. We identified about 40 concavities that have visible range extents between about 0.1 and 1.6 km. Features in the ~200-400 m size range comprise about ~50% of the total. There appear to be at least 5 concavities larger than about 500 meters and 3 larger than about 1 km. The upper limit has considerable uncertainty: there's clearly at least one significant, nearly central concavity visible on July 24, Aug 1, 2, 3, 8, and 9, but the maximum dimension is difficult to estimate because most of the concavity is in shadow. The best view seems to be on August 8 when the concavity is prominent along the trailing edge at frequencies just negative of zero Hz. On that day the feature has a visible range extent of about 1.5 km, but on August 1 and 2 it appears to be close to 2 km in visible extent. The three days when the most concavities are visible are August 5, when we counted about 25, and August 4 and 6, when about 15 are visible. In contrast, if we consider only the days with the highest delay resolution (19 m/pixel at Goldstone on July 31 and August 1 and 15 m/pixel at Arecibo on August 1-6 and August 9), then the fewest number of concavities that are discernible, less than 5, is on August 9. We do not have a 3-D shape model of 1999 JM8 so we cannot reliably estimate how large these concavities are relative to the asteroid's effective diameter, but if the effective diameter of 1999 JM8 is as large as ~8.4 km, which is about twice the mean visible range extent in the Arecibo images, then the largest concavity may be close to 20% of the asteroid's diameter. This would suggest that 1999 JM8 is not a small version of 243 Mathilde, which has at least 7 craters larger than 47% of its diameter, which greatly exceeds the apparent abundance of relatively large craters on 1999 JM8 (Veverka et al. 1997, Chapman et al. 1999). The largest concavities are visible on July 24, 31, Aug 1, 2, 3, 8, and 9, days that, except for August 3, do not show many small craters. This suggests that there may be a global asymmetry in the distribution of the largest craters. The days in which we see the most large craters are days in which we do not see many Several of the days in which the four largest concavities are visible (Jul 24, 31, Aug 1-3, 8, and 9) show fewer concavities are also characterized by a general paucity of abundant smaller craters. Is the apparent difference in the distribution of concavities real, or is it an artifact of geometry, SNR, and resolution? If it's real, then the implication is that the largest craters obscured the many smaller craters that likely preceded them. Then the question is why more smaller craters aren't visible on top of the large ones? Are the large craters geologically young? It could be that there _are_ craters in the larger ones but the geometry, SNR, and resolution prevent us from seeing them. These craters appear to be on the opposite hemisphere from craters seen on July 24, 31, August 1-3 and August 8-9 so I suspect that they aren't the same. The craters on the other days were less numerous and much larger. These crude estimates of the geographic distribution seem to indicate a significant difference in the abundance of large and small craters between hemispheres. Why would one side have more much larger craters than the other? Perhaps the side with large craters might not have as many small ones because the big craters could have "erased" the smaller ones. Curiously, an asymmetric distribution of the largest craters was also seen on Ida. Are there any concavities with radar-bright annuli surrounding them? Near the center of the July 28 image is a radar-bright annulus encompassing a group of pixels that are about 100 m in diameter, suggesting a small impact crater with a radar-bright feature surrounding it. Crater 37, which is visible on July 31 and August 1 and has a range extent of ~170 m, also has a radar-bright annulus. This feature is more obvious in the Arecibo image than in the Goldstone image. Crater 36 also appears to have a radar-bright loop that appears to overlap its rim in the August 1, 2, and 3 images. The ellipsoidal delay-Doppler distribution of bright pixels associated with this feature suggest an oblique view, so perhaps the "rim" is radar bright because of relatively abundant facets along the rim that are normal to the line of sight. Radar-bright features surrounding impact craters have not been seen previously in radar images of asteroids, but they have been found surrounding impact craters in Haystack (Thompson et al. 1981) and Arecibo (Stacy et al. 1997) radar images of the Moon and in Arecibo (Burns and Campbell 1985, Campbell et al. 1990), Goldstone (Jurgens et al. 1988), Venera (Basilevsky et al. 1987), and Magellan (Phillips et al. 1991) images of Venus. Whether or not radar bright elliptical rings surrounding concavities on 1999 JM8 are extended features, perhaps due to near-surface roughness, or due to facets oriented orthogonal to the line of sight, is not clear due to the coarse resolution and S/N. If these are roughness features, then they have implications for regolith retention and/or near-surface shaking on JM8 and the relative age of the concavities. None of the crater-like features on 1999 JM8 is complex: there are no central peaks, peak rings, or multi-ring basins, or craters that clearly overlap. Of course, complex crater structures are not expected for such a small object due to its weak gravity (Melosh 1989). We do not see any ejecta in the images. There are suggestions of crater clusters but we do not see clear evidence of overlapping concavities, thus we cannot discern the stratigraphic sequence of surface features. We do not see compelling evidence of terracing or slumping in any craters, but there are enigmatic groups of bright pixels that appear in two of the largest craters that were best seen in Arecibo images on August 1 and 2. There are hints, but not compelling evidence, that some of the concavities may have irregularly shaped rims. Ultimately we hope to understand the geographic and size distributions of craters on JM8 in order to compare its collisional history with those estimated from crater distributions on Gaspra, Ida, and Mathilde. Ultimately the shape reconstruction will provide the best constraints on the abundance, dimensions, and geographic distribution of craters on 1999 JM8. Linear features --------------- The images show at least two approximately linear features that to appear to be unrelated to impact craters. The most prominent is seen on August 4 as a narrow undulatory curve of bright pixels extending about 0.5 Hz from the positive leading edge toward the negative leading edge. This feature is also visible on August 3 and 5. A second linear feature is evident in images obtained on July 31 and August 1. In the July 31 image it occurs at positive frequencies between the leading edge and at least two craters, the larger of which it appears to intersect. It is about 0.5 Hz wide on August 1. A plausible interpretation is that the features are radar bright due orientation of facets nearly normal to the radar line of sight. The origin of the features is unknown, but they might be ridges, fault scarps, or grooves. Grooves have been seen in spacecraft images of Gaspra (Veverka et al. 1994) and Ida (Sullivan et al. 1996), and linear features were seen on Mathilde (Thomas et al. 1999), but no linear features have been seen previously on any NEAs imaged by radar. Other surface features ---------------------- Collection of radar bright pixels inside craters on August 1 and 2. The August 2 pixels are associated with the high SC/OC feature. Surface features ---------------- The first hint of surface features appears in the July 21 image in the form of two groups of dark pixels (near the trailing edge) that are surrounded by annuli of relatively bright pixels, which may correspond to concavities. The July 23 image has a diffuse group of relatively bright pixels beyond the negative leading edge, suggesting an intervening topographic low. The first obvious surface features are two tear-shaped clusters of radar-dark pixels surrounded by much brighter pixels that appear on either side of zero Hz in the July 24 image. These are the delay-Doppler signature of concavities and a plausible interpretation is that they may be impact craters. The July 24 image also reveals a concavity ~1.4 km in diameter near the trailing edge; this may be the same apparent concavity that was seen on July 23. There appear to be other surface features near the sub-radar point and along the approaching limb, but it is not clear what they are. The July 24 images span about 2 hours, so we checked images obtained at the beginning and end of the track for evidence of rotation but we did not find any. For images with resolution of 75 m x 0.075 Hz, this suggests that the the apparent rotation period is at least several days. The July 27 image shows a pronounced angular, asymmetric distribution of echo power including a pointed subradar region that extends about 400 m toward Earth relative to an adjacent, nearly flat region at more positive frequencies (near zero Hz). The negative limb appears to have two "steps" as a function of range that are also the brightest regions in this image, suggesting surfaces with more facets nearly normal to the line of sight than elsewhere on the target on this day. There are no obvious concavities in the July 27 image. The general weak echo strength may be due, in part, to a dearth of nearly normal surface facets to reflect the radar, as was seen in end-on Goldstone images of 1620 Geographos. The July 28 image is dramatically different from the July 27 image. The distribution of echo power is asymmetric with more power evident at negative frequencies than at positive frequencies. The leading edge has two prominent "hills" with sub-Earth points that are at nearly the same time delay. Between the two hills the LE is about 400 m back in range, suggesting that the "hill" at negative frequencies may be the "pointed" feature seen on July 27. There is also an arcuate feature of bright pixels just inside the positive limb that is similar to other features seen on July 24. Between July 31 and August 9 we imaged 1999 JM8 on each day at intervals that were generally close to or slightly less than 24 hours. This produced images in which the rotation of prominent features from day-to-day is obvious. JULY 31: This and the August 1 image are the highest resolution images obtained at Goldstone. The image does not obviously repeat features that were seen on previous days. The shape of the LE is somewhat angular but there are no pointed ends or twin peaks visible as there were on July 27 and 28. New surface features are visible at ranges beyond the LE. For example, at frequencies just positive of zero Hz and beginning about 20 range gates in from the LE there's a large approximately circular feature that is probabably an impact crater. Its range extent is about 700 meters and its bandwidth is about 0.9 Hz, although the range extent isn't really obvious due to the low SNR and an adjacent feature that may be more of the crater. If so, then then the range extent is close to 880 m. Adjacent to this crater at slightly larger range gates is another much smaller, nearly circular feature that may also be an impact crater that extends about 190 m in range and 0.3 Hz in Doppler frequency. At the same range gates as this crater-like feature but at immediately more positive Doppler frequencies is a small radar- bright region spanning about 0.2 Hz and about 90 meters. The resolution is not high enough to determine definitively what this feature is. A third possible crater that has range and Doppler extents of about 400 m x 0.5 Hz is present adjacent to but at larger time delays than the bright feature. There is a weak roughly circular feature near the TE along the positive limb that may be another crater; it appears to be close to 1 km in range extent and about 0.9 Hz in Doppler frequency. The TE near zero Hz is has a broad distribution of weak pixels that suggest a large concavity with a range extent of about 1.2 km. The other prominent feature in this image is a linear group of bright pixels between the positive limb/LE and the large crater that starts about 20 range gates behind the LE. The LE negative of zero Hz is almost linear--that is, it's not parabolic as would be expected from a spherical target. AUGUST 1: This is the only day in which we have coverage at both Goldstone and Arecibo. The Arecibo image is much stronger, has higher range and frequency resolution, and shows more detail. Comparison of the Goldstone and Arecibo images indicates that there are no features in the Goldstone image that are not also present in the Arecibo image. The two large crater-like features seen on July 31 are obvious on August 1 and also correspond to the two tear-shaped features seen on July 24. One feature is clearly an impact crater with an apparently radar bright rim near the LE. This suggests either elevated topograph, surface roughness, or both along that rim. There is a narrow region of bright pixels extending from the crater rim just negative of zero Hz toward the positive LE/limb and in the other direction toward the negative limb. Within the large crater just negative of zero Hz there is a group of radar bright pixels at ranges just inside (i.e. closer to the radar) than the far rim. The really obvious crater on the negative side of zero Hz has a range extent of about 850 m in the Arecibo image (using bright pixels along apparent proximal and distal rims) and about 880 m in the Goldstone image. There is a prominent tear-shaped feature along the positive limb that may be a crater viewed obliquely. It has a radar-bright rim and there is a diffuse group of bright pixels about 10 range gates deep and about 8 Doppler cells wide adjacent to it on its side toward the LE. The range extent is indefinite due to the non-circular shape but it appear to be either 1.0 or 1.2 km, depending on which group of bright pixels one chooses to define the distal rim. The receding limb is strikingly angular with a sharp bend about 3 km beyond the LE at the subradar point. Between the sub-Earth point and the bend, much of the receding limb is approximately linear as a function of delay and Doppler, and appears to correspond to the linear LE that was seen at negative frequencies on July 31. There is a column of faint but statistically significant pixels at the receding limb that extend at least ~1 km deeper in range than the rest of the TE. The column is about 0.2 Hz wide. The TE is irregular in shape and it suggests the presence of a large concavity; the shape of the concavity is not obvious but it does not appear to be not circular. AUGUST 2: The perimeter of the August 2 image is strikingly rounded with about 3/4 of a circular perimeter visible. The most prominent interior feature in the echo is a large radar dark region that extends from the TE inward to slightly less than 1/2 of the echo's delay depth. The large dark feature appears to be a substantial concavity that is about 2.4 km wide. Two prominent tear-shaped features are probably the craters that were seen on August 1, but they are rotated by about 45 degrees relative to their position on the previous day. The one near zero Hz has an intriguing radar bright feature that appears inside it near the distal crater rim. This feature may be a topographic high, but it is not clear whether it is actually inside the crater or whether it is a superposition from the opposite hemisphere. The rim of this crater closest to the LE is radar bright, as was seen with the rim of the other crater (that is near the receding limb on Aug 2) that was near zero Hz in the Aug 1 image. The central crater in this image has an apparent range extent of about 800 m. The arc of points on the receding limb near the TE has an irregular distribution of bright and dark pixels suggesting surface structure. AUGUST 3: The echo is rounded and modestly asymmetric as a function of Doppler frequency with points closer to the radar to the negative of zero Hz. The August 3 image has an angular region at the TE that is devoid of pixels with significant echo power, suggesting that it is the concavity seen in the August 2 image. The progression of tear-shaped crater from August 2 is obvious in the August 3 image. The crater near zero Hz on August 2 is close to the receding limb on August 3. The rim nearest the LE is again radar bright, particularly at less negative Doppler frequencies. The tear-shaped structure appears to have a maximum range extent of about 1.2 km, comparable to its extent on August 1 but about 50% more than its extent on August 2. There is another probable impact crater with dimensions of about 560 m x 0.75 Hz that is evident on the negative side of zero Hz near the sub-Earth point. Its proximal rim begins about 150 m into the echo from the LE. A prominent radar bright, narrow linear feature extends from the positive limb toward the LE at zero Hz. There are hints of structure on the approaching limb and TE but the echoes there are not strong enough to distinguish the features. AUGUST 4: The August 4 image is distinctly asymmetric with the LE to the negative of zero Hz. The delay-Doppler distribution is broadly triangular. The image has only 5 looks and the SNR relatively weak so that fewer surface details are visible than on other days. The approaching limb has three distinct linear segments, two of which are approximately parallel. There is a short segment between the two parallel segments. The approximately linear radar-bright feature seen on August 3 (between the approaching limb and the sub-radar point) is more obvious on August 4, although it has rotated by close to 50 degrees. There are hints of several circular features but none has sufficient detail to be confident that it is an impact crater. The best plausible crater candidate is just inside the bend in the approaching limb and at range gates just beyond the linear radar bright feature mentioned above. AUGUST 5: The LE of this echo is the most nearly uniform in time delay of any day that we observed 1999 JM8. The LE is not remotely parabolic, as would be expected of a spheroidal target, but is approximately flat. The image is very strong and shows the most detail of any Arecibo image of this object. Numerous approximately circular patches of pixels dot the surface and are probably impact craters. The two largest craters are just past the LE near zero Hz and about half-way between zero Hz and the receding limb. They have dimensions of about 470 m x 0.14 Hz and 450 m x 0.17 Hz. There is another smaller crater-like feature near the positive limb about half way between the LE and the TE that has dimensions of about 420 m x 0.11 Hz. A fourth comparably-sized feature exists slightly closer to the TE and near the receding limb. The August 5 image gives the impression that the portion of the surface illuminated is heavily cratered. Clearly more features that appear to be impact craters are visible. The linear feature seen on the previous two days is evident close to the receding limb but it isn't as prominent as it was on August 4. There is an indentation in Doppler frequency on the approaching limb. Pixels on the trailing edge at this feature are radar bright, suggesting that surface facets have more projected area oriented normal to the line of sight than is present on adjacent regions. The brightest pixels are on the positive side of the approaching LE. AUGUST 6: The nearly flat surface that formed the LE in the August 5 image forms the receding limb on August 6. The echo is asymmetric and angular and there are more pixels at positive frequencies than at negative frequencies. The brightest pixels are on an approximately flat region positive of zero Hz. Numerous dark circular features are present and are probably impact craters. Fewer are visible in the August 6 image, which has only about 1/3 the number of looks relative to the August 5 image. The craters appear to be up to several hundred meters in diameter. AUGUST 7: We resumed observing at Goldstone for two days. The echoes have an asymmetric delay-Doppler distribution. The echo is more rounded than on the previous three days. There is a radar-bright "step" on the negative limb that is the rotated "indentation" that was seen on the approaching limb on August 5. The image does not show any obvious detail inside the echo other than an arcuate collection of pixels at the positive limb. AUGUST 8: The delay-Doppler distribution of echo power illustrates that this side of 1999 JM8 is approximately spheroidal, in stark contrast with the distribution seen between August 4 and 6. The August 8 images span more than 7 hours and show the most compelling evidence of rotation among any of the 1999 JM8 images. However, the rotation is subtle. In general the image is similar to the August 1 image: the two prominent craters close to 1 km in diameter are both obvious, with what appears to be a smaller crater between them, although there is some rotation between the Aug 1 and 8 images. This is the best example of a nearly circular delay-Doppler distribution for the tear-shaped feature that was seen on August 1, 2, and 3. That feature on August 8 has a visible range extent of about 0.9 km, which is substantially less than suggested by the August 1 and 3 images but is closer to the extent from the August 2 image. The large concavity seen previously seems to have more of its interior illuminated on August 8. The concavity appears to be somewhat angular and it is about 1.5 km across and close to 1.4 Hz wide. The distal arc of points at the negative limb is on the negative limb and is now seen to be the rotated "indentation" that was prominent on the approaching limb on August 5. AUGUST 9: This is the highest resolution view of 1999 JM8. The delay-Doppler distribution is parabolic and indicates that the portion of the asteroid that was imaged is rounded. Nevertheless, the image is asymmetric: there are more distal points at negative frequencies than at positive frequencies. One of the two prominent craters that is evident in the August 8 images is located just negative of zero Hz on August 9. The crater has dimensions of 1.1 km x 0.22 Hz. The big concavity seen on August 8 is largely in radar shadow on August 9. As was the case on August 2, the crater has a distinct group of bright pixels inside it near the distal rim. The pixels look like a topographic high. There is another large region just positive of zero Hz that looks like another large concavity. It has dimensions of 1.4 km x 0.21 Hz. SIZE, ALBEDOS, and COMPOSITION ============================== The visible range extents listed in Table DELAY-DOPPLER DISPERSIONS have a mean of 3.3 km for the Goldstone data and 4.2 km for the Arecibo data, a difference that is probably due to the greater sensitivity at Arecibo. The variation in the range extents and the asymmetric distribution of delay-Doppler echo power indicate that JM8 has an irregular shape, but since the Arecibo observations cover close to one full rotation, let us assume that the mean range extent of 4.2 km (which is effectively rotationally-averaged) equals the effective diameter of JM8 and explore the implications. If JM8 has an effective diameter of 4.2 km, then the absolute magnitude of 15.2 (Table OPTICAL PROPERTIES) corresponds to an optical geometric albedo pv = 0.08 that establishes that JM8 is an optically dark object. This is probably a conservative upper bound given that experience with modeling other asteroid echoes (Hudson and Ostro 1994, 1995) indicates that the true effective diameter is about a factor of two larger than the visible range extent. The nominal optical albedo of 0.08 is consistent with C- and BFGP-type taxonomies. If JM8 has an effective diameter of 8.4 km, then the optical albedo is pv = 0.02, which is among the lowest optical albedo estimates ever obtained for any near-Earth or main-belt asteroid. Radar albedo ------------ The mean X-band radar cross section is 1.89 km^2 from observations between July 18-August 1. Those observations appear to sample more than one full rotation, so although there is considerable variation in the radar cross sections that may be real due to the target's irregular shape, let us adopt the mean of 1.9 km^2 and explore its implications. If JM8 has an effective diameter of 4.2 km, then the mean X-band radar cross section corresponds to a radar albedo of ~0.14, which overlaps the radar albedos for C-, S-, and BFGP-type main-belt and near-Earth asteroids. If the effective diameter is much larger, say 8.4 km, then the radar albedo decreases to 0.034, which is among the lowest asteroid radar albedos ever obtained. However, systematic errors in the radar cross section indicate that the true radar albedo of JM8 could be a factor of two-three larger than this estimate, but even so, it is clear that JM8 is not a radar bright object, so we can rule out E- and M-class taxonomies. SC/OC images ------------ Figure "RATIO IMAGES" shows Arecibo SC, OC, and SC/OC images from Aug 1, 2, 3, and 5. The images cover days with the strongest echo strengths and for which there are at least 25 looks, so that the noise statistics are approximately Gaussian. The images show only those pixels in which the echo power in both polarizations exceeds a threshold of 3-standard deviations. We also explored thresholds of 5 and 10 standard deviations and found that although the number of points decreases with each increase in the threshold, the overall patterns in the distribution of SC/OC did not change. The OC and SC images look very similar except for features that are present in the OC images but are not evident in SC due to differences in the SNR. In each ratio image there is a region of low SC/OC (~0.1) at the LE and a general pattern of increasing SC/OC as a function of increasing range toward the TE. Many points near the TE are characterized by SC/OC > 0.5. This pattern is similar to that seen with the 1996 Toutatis data, except that the variation in SC/OC for Toutatis is more subtle and the ratio for Toutatis along the LE is close to 0.2. The low SC/OC at the LE probably indicates that the subradar point of 1999 JM8 has smooth facets that are oriented orthogonal to the radar line of sight, in direct analogy with the specular reflections seen from the sub-Earth points in radar echoes from Mercury, Venus, the Moon, and Mars. Ultimately the shape model will clarify the situation. Regions of low SC/OC appear at the LE on each day, generally at the sub-Earth region. A notable exception is the image from August 5: lowest SC/OC occurs in two regions on either side of zero Hz. The August 2 image has an ellipsoidal region with low SC/OC near zero Hz and close to the apparent TE. We filtered the image with a 10 x 10 pixel boxcar and found that SC/OC = 0.08+/-0.002 within the "feature" region and SC/OC = 0.24+/-0.01 in the adjacent region at more positive Doppler frequencies (but covering the same span of range gates). This constitutes the first significant circular polarization ratio feature observed on an asteroid. The low SC/OC region appears inside a concavity that is probably an impact crater, but whether the feature is physically in the crater or whether it is superposed from the opposite hemisphere is not yet clear. Careful inspection of OC images on Aug 1, 2, and 9 shows the feature on all three days and suggests that it is a topographic high, but a pronounced region of low SC/OC is present only on August 2. There could be a lack of small-scale structure in the low-SC/OC region, causing a deficit of SC power, which seems plausible, or perhaps the radar scattering law there could be different from other parts of the surface. The August 2 image also has a narrow, arcuate low SC/OC feature between the ellipsoidal region described above and the echo's LE. This feature is only a few range gates deep and it appears to follow, at least in part, the contours on the apparent rim of the crater that encloses the larger ellipsoidal low SC/OC feature described above. The arcuate feature also seems to extends toward the negative limb along what may be the rim of another crater. There are also hints of similar structures adjacent to apparent crater rims on August 1. SHAPE INVERSION =============== DISCUSSION ========== 1. Surface bulk density -------------------- The X-band circular polarization ratio of JM8 indicates that most of the echo power comes from single back reflections. Consequently, we can relate the OC radar albedo to the normalized reflection coefficient and thus back out constraints on the surface bulk density (refs). In the absence of a 3-D shape model we do not know the gain g, but from previous shape reconstructions we have found that g does not usually differ from unity by more than several tens of percent, which is less than the adopted systematic uncertainty in the radar cross section, so, to first order, let us assume that g = 1. Then R = the radar albedo (+/- 100%) and the bulk density d (R) = 3.2 ln[(1 + sqrt(R))/(1 - sqrt(R))]. d (R = 0.034) = 1.2 g/cm^3 ...similar to Mathilde... d (R = 0.14) = 2.5 g/cm^3 ...similar to Phobos... ==> We need the shape model <== 2. Compare the radar properties of JM8 with those of other radar-dark asteroids ---------------------------------------------------------------------------- 1998 KY26, 1986 JK, 1580 Betulia, 2100 Ra-Shalom (?), main belt asteroids It's clear now that JM8 is an optically and radar dark object. It's the best look with radar that we've ever had at such an object, but there are still others with which we can compare it: Mathilde, Phobos, Deimos, and possibly Ra-Shalom, 1986 JK, Betulia, and 1998 KY26. KY26 is so drastically different due to its small size, rapid rotation, and weak gravity that a comparison may not be very meaningful. For KY26 SC/OC ~ 0.5 and the radar albedo is between 0.012 and 0.11. The radar albedo estimates overlap very preliminary estimates for JM8. We don't know the radar albedo, effective diameter, or rotation period of 1986 JK. SC/OC = 0.26+/-0.02. That's close to estimates for JM8, but it's also close to the mean for ALL radar-detected NEAs, so it doesn't say much (yet). Correction: Steve estimated the radar albedo by adopting the best available estimates of the effective diameter from optical observations of H and the taxonomy, suggesting that the radar albedo is between 0.0053 and 0.067. The upper limit is consistent with that of JM8. 1580 Betulia was observed at both 3.5 cm and 13 cm in 1989. SC/OC is 0.18+/-0.03 at 3.5 cm and 0.16+/-0.01 at 13 cm. In the 1986 DA paper Steve reported that the radar albedo of Betulia is about 0.09, which is also consistent with the preliminary estimates. How well does the radar albedo of JM8 compare with the mean and rms dispersion observed among radar-detected main-belt targets? Chris Magri found that the main-belt C- and S-class radar albedos are indistinguishable. The lowest is 0.11 (Daphne) and a few others are about 0.13. Some, however, are as large as 0.23. Perhaps it would be more appropriate to compare the radar albedo of JM8 with the BFGP objects. Most of their radar albedos cluster at less than 0.1, although one (554 Peraga, type FC) is about 0.2. Conclusion: radar albedo of JM8 is consistent with other radar-dark C- and BFGP-type objects. SC/OC is less diagnostic due to the substantial observed range of values. What about Phobos? Steve mentioned that he's almost sure that the radar cross section and radar albedos that he published for Phobos are too small: he didn't trust the Goldstone calibration, which he later discovered were reliable. Steve's nominal estimated albedo is 0.093+/-0.035, which is consistent with results for JM8, although the consistency isn't as good if Steve's estimate is too small by ~20%. SC/OC = 0.09+/-0.10, so the decimeter-scale surface is smooth. This is all based on a ~9 sigma detection. SC/OC is about a factor of two smaller for Phobos than it is for JM8. Again, I don't think that SC/OC has reliable correlations with composition. Mathilde Mathilde is about an order of magnitude larger, but like JM8, is is optically dark (a C-type) and it has several very large craters relative to its diameter. Mathilde actually has *more* craters large compared with its diameter than JM8 does. Perhaps a better analogy with large craters would be Ida... Is 1999 JM8 a rubble pile? -------------------------- Rotation period is extremely slow and is consistent with a rubble pile, but does not prove it. Irregular shape will probably provide the best evidence. Abundance of large impact craters isn't necessarily an indicator: we don't know the shape yet or Deff. However, if the four concavities close to or greater than 1 km in diameter are impact craters, then that suggests that a significant portion of JM8's interior is fractured. Comparison of relative crater sizes on Ida seems to show more that are relatively larger, but Ida has a density of 2.6+/-0.5 g/cm^3 that does not strongly suggest a porous object, plus grooves on Ida are more consistent with an object that is structurally intact, at least in part. Surface bulk density may or may not be consistent with a rubble pile...as that is sensitive to the upper few decimeters. Comet nucleus? -------------- Is JM8 an inactive comet nucleus? JM8's orbit is similar to those of some Jupiter family comets. 1999 JM8's Tisserand parameter (def), which is < 3 for ~90% of numbered Jupiter family comets and > 3 for nearly 100% of numbered asteroids with a < 3.3 AU, has a value of 2.988. However, out of ~890 NEAs discovered through the end of January, 2000, 49 have T less than that of 1999 JM8. Furthermore, it is known that the orbits of NEAs and Jupiter-family comets can intermingle (Valsecchi et al. 1995), so by itself the orbit of 1999 JM8 does not constitute compelling evidence either for or against a cometary origin. 1999 JM8's low optical and radar albedos are consistent with those estimated optically for comet Halley (value; ref) and by radar for comet IAA (0.04; Harmon et al. 1989), but they are also consistent with estimates for radar and optically dark near-Earth (Ostro et al. 1991) and main-belt asteroids (Magri et al. 1999). Nor is there evidence of a coma or tail from the very close encounter in 1999. Comet Halley is strongly suspected of being an NPA rotator (Belton et al.), but the abundance of NPA rotators among short-period comets is not known, and NPA rotation has been confirmed with one NEA (Toutatis, Hudson and Ostro 1995) and is strongly suspected for several others (Harris 1994), so it is unclear whether NPA rotation can be regarded as diagnostic of a comet. Is 1999 JM8 a small version of Mathilde? ---------------------------------------- composition is similar: optically dark and primitive object rotation states are both very slow and apparently NPA abundance of large craters, although really large craters are more common on Mathilde irregular shapes...but we could say that about other objects too. Surface bulk density compared with Mathilde's density. What are plausible explanations for JM8's shape? ------------------------------------------------ Non-principal axis rotation --------------------------- Is 1999 JM8 a non-principal axis rotator? Pravec et al. obtained a best-fit rotation period of 6.76 days, but they note that the lightcurves do not appear to be strictly periodic. A rotation period of close to 7 days is also consistent with delay-Doppler images obtained on July 24 and August 1. Impact simulations by Asphaug and Scheeres (1999) suggest that excitation of NPA rotation may be common among fragments of catastrophic disruptions; if so, then the question is whether enough time has elapsed for damping into a principal axis rotation state. A simple expression for estimating the damping timescale from non-principal to principal axis rotation is tau ~ (P/C)^3/(Deff^2), where tau is in 10^9 y, P is the rotation period in h, Deff is the effective diameter in km, and C is a constant that depends on the material properties of the asteroid. Using Deff = 8.4 km, P = 6.76 d (162.2 h), and C = 17+25-10, we obtain tau ~ 1.2 +16.4 -1.2 x 10^10 y, an estimate so long that unless 1999 JM8 originated in uniform rotation, then it probably is a non-principal axis rotator. Several other asteroids, notably 288 Glauke, 887 Alinda, 1220 Crocus, 1689 Floris-Jan, 3102 Krok, and 3288 Seleucus are strongly suspected of being NPA rotators (Harris 1994). The only confirmed NPA rotator among the NEA population, 4179 Toutatis (Hudson and Ostro 1995), has an estimated damping time of ~6 E 10 y. How did 1999 JM8 acquire its spin state? ---------------------------------------- The nominal ~7-day rotation period of 1999 JM8 exceeds more than 99% of all reported asteroid rotation periods (Pravec and Harris 1999). How did the slow, non-principal axis rotation state of 1999 JM8 originate? 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