RADAR ASTRONOMY

Completion of the upgrading of the Arecibo Telescope in late 1996 will open a new era in planetary radar astronomy, one which will enable a major expansion of our knowledge of asteroids, comets, the terrestrial planets, and planetary satellites. The Arecibo 12.6-cm wavelength system will be by far the dominant instrument in the field, with 20 times the sensitivity of the 3.5-cm wavelength system on the 70-m JPL/DSN Goldstone antenna, and several hundred times more sensitivity than the 6-cm wavelength system on the 70-m deep Russian space tracking antenna in the Crimea.

1. Past Radar Astronomy Milestones

2. Some recent work at Arecibo

   1. Terrestrial Planets and Outer Planet Satellites

   2. Asteroids

3. Current Activities of the Radar Astronomy Group

4. Future Investigations

   1. Inner Solar System

   2. Asteroids and Comets

   3. Planetary Satellites

5. References

Historical Milestones

During the 35 years since the first detection of Venus by Earth-based radar systems in 1960, planetary radar astronomy has had two major stages in its evolution and, with the Arecibo Upgrade is about to enter a third. During the first stage, from 1960 to 1975, the relatively low sensitivity of the early planetary radar systems limited observations to those of the terrestrial planets, Mercury, Venus and Mars, to two small Earth-approaching asteroids, Icarus and Toro, and near its end, the rings of Saturn. Much important work was done during this period including measurements of the rotation periods of Venus and Mercury, refinement of the value of the Astronomical Unit, tests of General Relativity, measurements of the equatorial topography of all three terrestrial planets and the first, low resolution imaging of the surface of Venus. The second stage, from 1975 to the present, was introduced with the first upgrading of the Arecibo 305-m antenna and installation of a 12.6-cm wavelength transmitter. This new system provided the capability to study the Galilean satellites of Jupiter, the rings of Saturn, main belt and Earth-approaching asteroids and comets plus, of course, much more detailed studies of the terrestrial planets. It was the prospect of being able to map the surface of Venus at resolutions down to 2 km, which persuaded NASA to fund the installation of the 420 kw transmitter on the newly upgraded telescope.

Some Recent Work at Arecibo

Terrestrial Planets and Outer Planet Satellites

Discovery of the anomalous backscattering properties of the icy Galilean satellites of Jupiter in 1975/76 with the then new Arecibo 12.6-cm radar, triggered an extended effort to understand the phenomena. Only relatively recently was a convincing, although still controversial, explanation, coherent backscatter, suggested by Hapke (1990), based on observations of similar phenomena in laboratory optical scattering experiments. One by-product of the discovery of the high backscatter radar cross section and high circular polarization ratio of water ice is that the properties serve as a diagnostic for the presence of ice. This was born out by the very unexpected discovery by both the Goldstone and Arecibo systems of apparent ice deposits in the shadowed floors of craters at the poles of Mercury (Slade et al., 1992; Harmon and Slade, 1992). Observations at Arecibo by Harmon et al. (1994) have provided detailed imagery of the distribution of those ice deposits (Figure 1) and, as a by-product, an improved determination of Mercury's pole direction by aligning multiple images taken at different aspect angles.

Figure 1 was destroyed by a cracker and has not yet been regenerated.

Figure 1: The upper figures are depolarized radar images of the north and south poles of Mercury, obtained from Arecibo S-Band observations in 1991-92. The bright spots are probably water ice deposits which induce enhanced volume backscattering of the radar wave. Comparison between these radar images and Mariner-10 photographs (lower figures) shows that many of the radar-bright spots come from caters. The best match is achieved by shifting the Mariner-10 grid down by about 2° in the north and right by 1.5° in the south. These shifts are within the inherent uncertainty of the Mariner-10 grid, which is much less accurate than the radar grid on th upper figures. The north polar image consists of a score of small to medium size spots; some of the most prominent of these (H, J, K) lie on the unphotographed side of the planet. The south pole is dominated by a feature coming from the large crater Chao Meng-Fu. The crater associations offer support for the ice theory, since thermal studies show that permanently-shaped crater floors are cold enough to sustain water ice for billions of years.

Figure 2: delay-Doppler image of the South pole of the Moon

Figure 2: This radar image in delay-Doppler coordinates of the south pole region of the Moon was made at Arecibo in a search for ice in permanently shadowed areas. It is 400 km in each coordinate and the original image has a resolution of 500 m in delay (vertical) and 580 m in Doppler (horizontal). The illumination is from the top (so range increases downward), and increasing Doppler frequency is towards the left. The approximate location of the south pole is indicated by the cross. The search was for the characteristic radar signature of ice, high backscatter cross section and high circular polarization ratio. No clear indication of ice was found, although a small number of areas with anomalous radar properties need further investigation. These lunar investigations were done by N. J. Stacy as part of his Cornell Ph.D. thesis.

The discovery of ice at the poles of Mercury triggered a search for ice in the shadowed floors of craters near the lunar poles as part of Cornell graduate student, Nick Stacy's Ph.D. thesis work. Stacy developed sophisticated data acquisition and analysis techniques to image selected regions of the Moon in all four Stokes' polarization parameters at resolutions down to 20 m. No clear evidence of ice was found, although there are a small number of areas of anomalous backscatter at the south pole, which warrant further study. These lunar observations have, for the first time, demonstrated the capability for radar measurements to map, assuming a scattering model for the surface, variations in mare surface dielectric constant and areas with high concentrations of titanium or ion oxides.

Asteroids

Since 1980, Arecibo has investigated the radio wave scattering properties of over sixty main belt and Earth-approaching asteroids and five comets. In 1989, the first good high-resolution images of small asteroid, 4769 Castalia (1989 PB), were obtained (Ostro et al., 1990) revealing a double-lobed object modeled by Hudson and Ostro (1994) as a contact binary. This discovery of the first contact binary asteroid was followed by high-resolution imaging observations of 4179 Toutatis in late 1992 by both the Goldstone and Arecibo radars, revealing that it, too, is a double-lobed object, probably another contact binary (Ostro et al., 1993). These imaging observations of asteroids are a harbinger of many such observations with the new Arecibo system.

Figure 3: A time-series of delay-Doppler images of the asteroid 4763 Castalia

Figure 3: The close approach to the Earth in 1989 of the newly-discovered, small asteroid 1989 PB (now named 4769 Castalia) provided the first good imaging opportunity for the Arecibo 12.6-cm wavelength radar of a small body. While there were hints from measurements of the spectra of reflected radar signals that some asteroids have a bifurcated shape, it was still a surprise that the radar images revealed 1989 PB to consist of two distinct kilometer-sized pieces that appear to be in contact and rotating about a common axis. The photo shows a sequence of radar images made over a period of 2.5 hrs, 60% of the rotation period. Time progresses from left to right in each row and from the top row to the bottom. The radar illumination is from the top of the image so that time delay is from top to bottom, and Doppler broadening from left to right. Color indicates the total radar cross section of each approximately 150 m-sized pixel.

The radar systems resulting from the current upgrading of the telescope will have approximately 20 times the sensitivity of the one used for the 1989 PB measurements allowing many imaging opportunities for asteroids and, hopefully, comets down to resolutions of 10 m to 20 m. Asteroid 1989 PB was discovered in August, 1989 by E.F. Helin at the Palomar Observatory. These radar images were made two weeks later by S.J. Ostro (JPL), C.F. Chandler (CfA), A.A. Hine (NAIC), I.I. Shapiro (CfA), K.D. Rosema, and D.K. Yeomans (JPL).

Current Activities of the Radar Astronomy Group

Since the Arecibo planetary radar system was de-commissioned in December 1993, it has not been possible to carry out new observations. However, there has been considerable activity in reducing and analyzing existing data, occasional use of other telescopes, and preparations for future observations.

Michael Nolan joined the Arecibo staff as a postdoctoral research associate in February 1995. Mike completed his Ph.D. in 1995 at the University of Arizona with a thesis on theoretical studies of asteroid fragmentation. He also has considerable experience in optical spectral reflectance observations of these objects. Since coming to Arecibo, Mike has been finishing up work related to his thesis research and preparing for asteroid investigations with the new radar system.

John Harmon has been working on problems related to both Mars and Mercury. He has been finishing up the analysis and interpretation of Arecibo random-code mapping observations of Mars from the 1990 and 1992-3 oppositions. These are the only radar reflectivity maps of Mars to have been obtained using the delay-Doppler method and are the highest resolution Mars radar maps currently available. Considerable improvements in the quality (signal-to-noise ratio) of the maps has been made by averaging data over multiple observing runs.

This has given a much better picture of enhanced surface roughness over the volcanic provinces of Tharsis and Elysium. The Arecibo map of the Elysium basin/channel complex (a preliminary version of which was published in 1992) is timely given the recent interest in this unusual area (it has even been proposed as a possible landing site for the 1999 Mars Surveyor mission). Another interesting result from the new Arecibo maps is the identification of several backscatter enhancements in the Chryse channel region. The most prominent of these is associated with Maja Vallis, while the most curious one is located on the plateau bordering Simud Vallis. Why there should be enhanced roughness on this plateau is unclear, although one intriguing possibility is that it is associated with volcanism that might have also triggered the water release. The random-code data (along with older CW data) have also been used in a study of the radar echoes from the Martian ice caps. An enhanced backscatter feature from the south polar ice cap has been identified in the 1990 data, although it is more than twice as weak at the Arecibo wavelength of 12.6 cm than at the Goldstone wavelength of 3.5 cm (this feature was first seen in Goldstone/VLA radar images). This strong wavelength dependence (which is not seen in the anomalous radar backscatter from the icy Galilean satellites) indicates that the residual south polar ice cap of Mars is optically thin at the Arecibo wavelength. If the ice has homogeneous scattering properties, then it is hard to see how the ice cap can be more than about 300 meters thick. Arecibo random-code data from late 1992 also reveal a backscatter enhancement from the north polar ice cap which is slightly weaker than the south polar echo at the same wavelength. No such north polar feature has yet been seen in Goldstone/VLA images, and it is planned to compare the Arecibo data with Goldstone CW spectra to determine the wavelength dependence of the north polar feature. All of the above results were reported by J. Harmon at the Spring Meeting of the AGU and a journal article is being prepared in collaboration with R. Arvidson (Wash. U.) and M. Slade (JPL).

The 1997 Mars Pathfinder mission will attempt the first Mars landing since the Viking missions of two decades ago. Arecibo radar observations played a role in the final site selection for the Viking-1 lander. There has been some urgency in obtaining new radar data over the prime Pathfinder landing site in Ares Vallis (Chryse), especially as this site is the same A1 site rejected for Viking-1 on the grounds that it showed potentially hazardous roughness. Although the telescope upgrade prevented us from making radar observations of the Ares Vallis site during the 1995 opposition, J. Harmon has made a study of the site and its environs using Arecibo data from previous (1980, 1982, 1990) oppositions. Arecibo delay-Doppler data from 1980 and 1982 indicate that Ares Vallis is about twice as rough as the Martian average and that the lander site itself has rms slopes of about 8 degrees. This is consistent with radar data from the Viking lander site reconnaissance in 1976. There is also an indication that the Pathfinder site may be rougher than the final Viking-1 lander site. Arecibo random-code reflectivity maps from 1990 indicate that the main channel of Ares Vallis southeast of the lander site is moderately (on rock scales) than the terrain through which it flows, but neither it nor the lander site itself are unusually rough in an absolute sense. All of these results will be presented at the Pathfinder Lander Site Workshop in Spokane at the end of September.

Last year, J. Harmon and M. Slade published high-resolution Arecibo radar maps of Mercury's north and south polar anomalies, radar-bright spots which may be the signatures of water ice deposits in permanently shaded crater floors. More recently they have used the same random-code data set and multi- run averaging technique to study the three most prominent non-polar radar features in Mercury's unphotographed hemisphere (the dark side at the time of the Mariner-10 encounters). Hints of these features were first seen by Goldstein in CW radar data taken 25 years ago, and, in fact, mapping of the "Goldstein features" was one of the goals of the full-disk radar imaging of Mercury initiated at Goldstone/VLA and Arecibo in 1991 (the polar "ice" features coming as an unexpected bonus). One of these features (located at 34S, 348W) has been found to be a fresh Tycho-class impact crater (115-km diameter) with a prominent ejecta blanket and ray system. The large extent of the ejecta and rays explains why this feature appeared to be so large in the original Goldstone/VLA and Arecibo radar images. A second feature (located at 57N, 343W) looks much different, showing no obvious impact structure at all. This feature is roughly circular, with a diameter of about 500 km and a small central dark spot. Its gross radar morphology suggests that it might be a giant shield volcano similar to those seen on Mars and Venus. This is certainly an intriguing possibility given that no volcanos were found in the hemisphere imaged by Mariner 10. The third feature is large and irregular and does not resemble either of the other two features. There is some hint that it may be located in smooth plains, but we have found no obvious diagnostic features in either the random-code images or 4-microsecond normal-code images that would allow us to speculate on its nature. These new results on the major non-polar radar features of Mercury were reported at the Spring AGU meeting and will be presented in more depth at the annual meeting of the AAS-DPS in Hawaii in October.

Another Mercury project currently in progress involves using the Arecibo north polar images of Mercury to make a more accurate determination of Mercury's pole direction. Although here are strong theoretical grounds for believing that the polar obliquity is essentially zero, the existing observational constraints (though consistent with zero obliquity) are fairly loose. Since the recently published Arecibo image of Mercury's anomalous north polar features is composed by summing images obtained over a sub-Earth longitude span of 180 degrees (assuming zero obliquity), any true non-zero component of polar obliquity would show up as a smearing of the crater "ice" features. This project, which is nearing completion, involves C. DeVries (past undergraduate at Cornell; now at U. Mass.), J. Harmon, and M. Slade.

Earth-based radar (including Arecibo) observations of Mercury are not only providing some of the most important new information about this planet, but are also providing a motivation for proposals of new spacecraft missions and (non-radar) observing programs. Both of the recently proposed Mercury spacecraft missions for the Discovery program (Hermes Global Orbiter and Mercury Polar Flyby) had polar ice detection as an important mission goal (in addition to the obvious primary goal of completing the photoimaging of the planet). As it happened, neither of these missions was selected for funding by NASA, placing the burden of future Mercury studies on new and improved observations from Earth and Earth-orbit. We can, of course, expect improved Mercury radar imaging capabilities as a result of the Arecibo upgrade (including possible bistatic imaging with VLBA). The time would also appear to be ripe to look at the unimaged hemisphere of Mercury with the Hubble Space Telescope. For example, a giant volcano (such as that suggested above) would be easily resolvable by the HST-WFPC and might even cast an observable shadow or highlight when observed near the Mercury terminator. J. Harmon has recently argued the case for HST observations of Mercury (as a member of the Mercury committee of the Terrestrial Planets Science Working Group) and is a co-investigator on a new HST proposal for Mercury observations by A. Stern (U. Colorado).

Don Campbell is working with Cornell graduate student Greg Black on the analysis of 430 MHz radar observations of the icy Galilean satellites, with Cornell graduate student Jean-Luc Margot on VLA radio thermal observations of the Moon aimed at mapping out variations in surface dielectric constant, with former student Nick Stacy on continuing analysis of the S-Band radar observations of the Moon, and with Bruce Campbell of the Air and Space Museum and Chris DeVries, a former Cornell undergraduate who is now at the University of Massachusetts, on the analysis of the polarization properties of the reflected radar signal from the surface of Venus.

Greg Black has reduced the 70-cm data for the icy Galilean satellites and, tentatively, found that the total cross sections are smaller than at 3.5 and 12-cm wavelengths, and that, at least for Callisto and Europa, the circular polarization ratio may be larger. He is currently looking into possible causes for this wavelength dependence.

Jean-Luc Margot and Bruce Campbell used the VLA in the D-configuration to map the thermal emission of the Moon in all four Stokes' parameters at 6, 20 and 90-cm wavelengths, while Don Campbell used the 140-ft. Green Bank to measure the "zero spacing" emission at 6 and 20 cm. Jean-Luc has reduced the 20-cm VLA data to obtain an initial image of the dielectric constant variations over the surface. Part of the rationale for this work is to combine the dielectric constant measurements with Nick Stacy's 12-cm radar determinations to get a better handle on the radar scattering model for the lunar regolith.

The New System

Completion of the upgrading of the 305-m antenna and the installation of the 1.0MW transmitter will result in a planetary radar system which will, averaging over Arecibo's declination range, be approximately 20 times more sensitive than the system that it is replacing. With about 60 times the sensitivity of the JPL/DSN Goldstone system, Arecibo will be very much the dominant instrument in the field although the limitated declination coverage of the Arecibo antenna means that the Goldstone system will continue to play an important role in providing both coverage at declinations outside Arecibo's coverage and measurement of wavelength dependent properties. Figure 4a shows the relative overall sensitivities of the new and old Arecibo systems and the Goldstone system as a function of declination.

Figure 4: Sensitivity of the upgraded Arecibo

Figure 4: The signal-to-noise ratios shown are those expected using various instruments to target an unresolved (in plane-of-sky) object, normalized to the best Arecibo pre-upgrade sensitivity. For systems under construction (Arecibo and the GBT), the expected system parameters have been used. Monostatic systems are shown in panel (a) and bistatic systems shown in panel (b) compared to upgraded Arecibo. The transmitting antenna is indicated first for the bistatic systems. Configurations involving Arecibo operate at a wavelength of 12 cm, those involving Goldstone at 3.5 cm. The projected Arecibo-Goldstone curve is for a wavelength of 12 cm, although the reverse, Goldstone-Arecibo at 3.5 cm would in fact be a similar curve.

Symbols used:

Symbol	Instrument
A	Arecibo
Aold	pre-1995 Arecibo
G	Goldstone
GBT	Green Bank 100m antenna (under construction)
VLA	Very Large Array (27, 25m antennas)
VLBA	Very Long Baseline Array (10, 25m antennas)

With the completion of the new planetary radar system, radar astronomy will finally move away from a "target of opportunity" dominated approach to planetary studies to one based more on systematic studies of classes of objects or phenomena. Up to the present sensitivity or, more correctly, the lack of it, has very much dictated which bodies can be studied at what resolution. While there have been tremendous successes from this forced approach, they have, with a few exceptions, been relatively isolated contributions to our knowledge of the solar system. The prime exception was the 28-year history of Venus studies where sensitivity improvements kept Earth-based radar observations competitive with, and complementary to, spacecraft measurements until the Magellan mission. With sensitivity often no longer a major limiting factor, the new system will be used for systematic studies such as the radio wave scattering properties, shapes, surface roughness, and sizes of large numbers of main belt and Earth-approaching asteroids and topical studies such as ice in the inner solar system and in the Jupiter and Saturn systems.

Virtually all radar astronomy observations have been carried out in a monostatic mode; that is, the same antenna is used to transmit and receive with approximately half the time devoted to each. This was primarily because the transmitter is normally installed on the largest antenna available and the monostatic mode maximized sensitivity. With the upgraded Arecibo system, sensitivity will no longer always be the factor limiting imaging resolution, and we will be driven to the use a variety of antenna systems for signal reception. The Goldstone system is already being used in conjunction with a nearby 34-m antenna and with the Very Large Array (VLA) in New Mexico. Arecibo will be used with the new 100-m Green Bank Telescope (GBT), the 70-m Goldstone antenna, and with the Very Long Baseline Array (VLBA) supplemented, hopefully, with some larger antennas. Figure 4b shows the relative sensitivity of some of these bistatic systems. For slowly rotating asteroids or comets approaching within less than about 0.02 AU of the Earth, sensitivity is not an issue, and matching the 10 to 20 m range resolution of the system in the Doppler or azimuth direction will require use of a separate receiving antenna to obtain long enough data spans to generate the very fine frequency resolution required. The delay-Doppler images obtained from either monostatic or bistatic observations are difficult to interpret due to the two fold or, for irregular objects, possibly larger, ambiguities inherent in this technique. Modeling the shape of the body from multiple aspect angle delay-Doppler images as used very successfully by Hudson and Ostro (1994) to obtain the bifurcated shape of the small Earth-approaching asteroid 4769 Castalia (1989 PB), will not provide unambiguous images of the surface at the 10 to 20-m resolution capability of the system. Techniques for direct inversion of the delay-Doppler images to surface imagery still need to be developed. Use of long baseline interferometers, such as the VLBA for direct synthesis of images of the radar illuminated target, is one alternate approach to solving this problem which will be investigated in the near future, but many problems need to be solved. This technique will also provide precise plane-of-sky positions to complement radar measured distance and velocity measurements for determining precise orbital elements for future orbit predictions.

Future Investigations

Major areas to be addressed at Arecibo are the investigation of the recently-discovered ice deposits at the poles of Mercury, the mapping of the hemisphere of Mercury not photographed from the Mariner 10 spacecraft, studies of the surface morphology of volcanic lava flows on Venus, and the low emissivity surfaces found at high altitudes on the planet, surface composition and topography for the Moon, and the study of the polar ice caps on Mars.

Conditions will be very favorable for observing the north polar region of Mercury in the summers of 1998, 1999, and 2000. The next good opportunities for observing the south pole will be in March of 1998 and 1999, although some observations could be made earlier. It is also planned to continue radar imaging of other parts of Mercury, with particular emphasis to be given to the half of the planet that was not photographed by Mariner 10. Of particular interest are the equatorial and mid-latitude bright features recently seen in the full-disk depolarized images from Arecibo and Goldstone-VLA. The discovery of water at the planet's poles has helped to rekindle interest in spacecraft missions to Mercury. The European Space Agency is actively discussing a possible mission and there are proposals for U.S. Discovery missions.

For Venus, it is now clear that answers to a number of questions arising from the examination of Magellan data, such as the nature of the high-reflectivity/low-emissivity regions on Maxwell Montes and other elevated areas and the relationship of Venus lava flows to terrestrial flow types, would benefit by knowledge of the polarization properties of the backscattered signal. Magellan transmitted and received a single linear polarization. The improved Arecibo radar sensitivity should provide very high quality imagery at about 1 km resolution in both senses of received circular or linear polarization. Using Arecibo in conjunction with the Green Bank Telescope (GBT) as a radar interferometer, it will also be possible to address the detailed height distribution of the low-emissivity material on Maxwell Montes by making 3-D images of Maxwell in all Stokes' parameters with approximately 1 km spatial resolution and 50 m height resolution, a significant improvement over the Magellan altimeter in spatial resolution.

For Mars, the sub-radar point track will be near its northern latitude limit during the first observable opposition with the new system in 1997. High priority will be given to observations of the north polar radar anomaly associated with the polar ice cap at both 12-cm and 3.5 cm wavelength to understand some puzzling differences in the wavelength behavior of the echoes from the two polar regions. Again, these observations will be relevant to proposed spacecraft and lander investigations of the Martian poles.

Arecibo will continue to be used to make high-resolution, multi-polarization images of the Moon at both S-Band and 430 MHz. Since signal strength is not a limitation for lunar work, the Upgrade itself should not have a major impact on the type of work done. Rather, we expect the emphasis to be given to improving the quality and resolution of lunar radar images using special techniques and to extending the utility of radar images as a tool in geologic interpretation. On the issue of possible ice deposits at the poles, one of the major problems in the interpretation of both the Arecibo and Clementine data is lack of knowledge of which areas have been permanently shadowed for time scales of hundreds of millions of years. The Clementine mission did not provide detailed topography for the poles. Radar interferometry could provide high spatial and height resolution topography for the areas visible from the Earth, which may be enough to infer much of the overall topography.

Asteroids and Comets

Amazing results are expected for observations of Earth-approaching asteroids with the new system. An average of three numbered asteroids and, probably, several other newly- discovered asteroids and comets will be imaged each year with resolutions as small as 10 to 20 m. While spacecraft will examine a very small number of asteroids and comets in great detail over the next ten years, Arecibo will provide imagery at 10's of meters resolution of a relatively large number of objects over the same time. As demonstrated by the recent observations of Castalia (1989 PB), Toutatis, and Geographos by the Goldstone and Arecibo systems, such observations can provide detailed information on their shape, size, and surface morphology. Table 1 gives a list of numbered asteroids which can be imaged by Arecibo between 1996 and 2006, including the theoretical number of pixels which the sensitivity will allow. With improved inversion techniques or direct imaging with interferometer systems, it should be possible to also obtain detailed images of the surfaces of asteroids and comets, something which has not been possible to date.

Figure 5: Asteroids detectable by the post-upgrade Arecibo Radar

Figure 5: This figure plots as white dots the orbital eccentricity and semimajor axis for the ~6000 numbered asteroids and a number of unnumbered Earth-crossing asteroid. The green dots are those asteroids which are detectable with a minimum single day signal-to-noise ratio of 10 during the period 1996-2006. The dashed blue curve corresponds to orbits whose perihelia occur at 1 AU; objects above this curve have perihelia interior to the Earth's orbit, perihelia of objects below are exterior. There are 315 main belt objects and 30 near-Earth objects detectable, or roughly 30 per year and 3 per year, respectively. The entire main asteroid belt will be well sampled. Numerous unnumbered near-Earth objects are also detectable, given sufficiently accurate ephemerides.

While detailed imaging of, mainly, small Earth-approaching objects will provide much of the excitement of the asteroid program, the system's capability (as indicated in Figure 5) to measure the radio wave backscattering properties of literally hundreds of main belt objects will provide much statistical information about their sizes, shapes, surface morphology and, in some cases, composition. Over 30 numbered asteroids, which have not previously been observed with the radar system, will be observable each year. Given the rate of new discoveries this number is very much a lower bound, so that limitations on antenna time will be the major constraint on the program.

Only four known periodic comets will be observable over the next ten years, with two of them being marginally close enought to image. Good imagery of a cometary nucleus will depend on the passage of new comets through the inner solar system similar to that of IRAS-Aracki-Alcock in 1983.

Planetary Satellites

The holy grail of radar observations of planetary satellites is a solid detection of Titan. Measurements of the radar albedo as a function of longitude would narrow the range of the theories regarding the nature of it surface. Such measurements would also be of assistance to the Cassini mission in planning its radar observations of the satellite. Weak detections have been obtained by Muhleman (1990) using the Goldstone 70-m antenna to transmit, and the VLA to receive. Interpretation of those measurements seems to require a non-synchronous rotation, although recent observations in the near infrared of surface features by P.H. Smith et al. indicate that Titan is in synchronous rotation.

The Saturn system will re-enter Arecibo declination range in 1996, but tracking time will be less than the round trip time to the planet for the first three years requiring bistatic observations. In 1996, the only possible receiving antenna is the 70-m Goldstone antenna. For 1997 and 1998, hopefully, the new 100-m Green Bank antenna will be available.

The smaller Saturnian satellites, Iapetus, Rhea, Dione, and Hyperian are potentially detectable with the new system with Iapetus being of considerable interest due to the large disparity in visual brightness between its hemisphere.

The Jupiter system is currently at southern declinations, and it will not be possible to observe the Galilean satellites until late 1999. By that time, techniques such as long baseline interferometry need to be developed to provide two-dimensional imaging capability. Only then will it be possible to examine the scattering properties of different terrain types and albedo features.

One of the more intriguing non-detections of recent years was that of Diemos, the smaller of the two small satellites of Mars. Despite being smaller than Phobos, its slower rotation rate should make it roughly equally detectable. Since Phobos was detected, the absence of a detection for Diemos implies a very low radar albedo and distinctly different properties for its regolith than Phobos. The new system should have no trouble obtaining a detection allowing comparative studies of the two satellites and comparison with the asteroid population.

References

Muhleman, D.O., A.W. Grossman, B.J. Butler, and M.A. Slade, "Radar Reflectivity of Titan," Science 248, 975 (1990).

Slade, M.A., B.J. Butler, and D.O. Muhleman, "Mercury Radar Imaging: Evidence for Polar Ice," Science 258, 635 (1992).

Harmon, J.K. and M.A. Slade, "Radar Mapping of Mercury: Full Disk Images and Polar Anomalies," Science 258, 640 (1992).

Ostro, S.J., J.F. Chandler, A.A. Hine, I.I. Shapiro, K.D. Rosema, and D.K. Yeomans, "Radar Images of Asteroid 1989 PB," Science 248, 1523 (1990).

Hudson, R.S. and S.J. Ostro, "Shape of Asteroid 4769 Castalia (1989 PB) from Inversion of Radar Images," Science 263, 940 (1994).

Hapke, B., "Coherent Backscatter and the Radar Characteristics of Outer Planet Satellites," Icarus 88, 407, (1990).

Harmon, J.K., M.A. Slade, R.A. Velez, A. Crespo, M.J. Dryer, and J.M. Johnson, "Radar Mapping of Mercury's Polar Anomalies," Nature 369, 213 (1994).

Ostro, S.J., et al., "Radar Imaging of Asteroid 4179 Toutatis," Bull. Amer. Astron. Soc. 25, 1126 (1993).

Hudson, R. S. and S. J. Ostro, Science 270, 84-86 (1995)

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