Dotto et al.: Observations from Orbiting Platforms 219

Observations from Orbiting Platforms

E. Dotto Istituto Nazionale di Astrofisica Osservatorio Astronomico di Torino

M. A. Barucci Observatoire de Paris

T. G. Müller Max--Institut für Extraterrestrische Physik and ISO Data Centre

A. D. Storrs Towson University

P. Tanga Istituto Nazionale di Astrofisica Osservatorio Astronomico di Torino and Observatoire de Nice

Orbiting platforms provide the opportunity to observe without limitation by ’s atmosphere. Several Earth-orbiting observatories have been successfully operated in the last decade, obtaining unique results on physical properties. These include the high-resolu- tion mapping of the surface of 4 and the first spectra of asteroids in the far- wave- length range. In the near future other space platforms and orbiting observatories are planned. Some of them are particularly promising for asteroid science and should considerably improve our knowledge of the dynamical and physical properties of asteroids.

1. INTRODUCTION 1800 asteroids. The results have been widely presented and discussed in the IRAS Minor Survey (Tedesco et al., In the last few decades the use of space platforms has 1992) and the Supplemental IRAS Survey opened up new frontiers in the study of physical properties (Tedesco et al., 2002). This survey has been very important of asteroids by overcoming the limits imposed by Earth’s in the new assessment of the asteroid population: The aster- atmosphere and taking advantage of the use of new tech- oid taxonomy by Barucci et al. (1987), its recent extension nologies. (Fulchignoni et al., 2000), and an extended study of the size Earth-orbiting satellites have the advantage of observing distribution of main-belt asteroids (Cellino et al., 1991) are out of the terrestrial atmosphere; this allows them to be in just a few examples of the impact factor of this survey. operation 24 h per , every day, without moonlight and/or IRAS values, together with color indexes determined weather limitations. Space observations are not affected by by the ECAS asteroid survey (Zellner et al., 1985), have atmospheric absorptions and/or emissions. Also, because of provided important input for classifying asteroids by a the absence of turbulence, they are not limited by the atmos- multivariate statistical method (Barucci et al., 1987). On pheric seeing and they are diffraction-limited in a wide wave- the basis of the IRAS dataset on asteroid and diam- length range. eters, Cellino et al. (1991) found that the size distribution Starting with IRAS and continuing through IUE, ISO, of asteroids cannot be fitted by a single-exponent power MSX, and HST, the spectral range investigated has been con- law: Below a critical value of diameter of ~150 km, the size siderably enlarged and the sample of known asteroid diam- distribution resembles a power law with index α ~ 1, while eters and albedos has been greatly increased. This has led for larger objects the index rises to α ~ 3. Moreover, the to significant advances in our understanding of the physi- size distribution changes completely when different semi- cal properties of asteroids, especially larger ones. major axis regions are considered and when families are The InfraRed Astronomical Satellite (IRAS), which op- taken into account. On the basis of this result, the size dis- erated from January to November 1983, was a joint project tribution of larger bodies seems to be little altered by col- of the , , and . It lisional evolution, while at smaller sizes the apparent dis- performed an unbiased survey of more than 96% of the sky crepancy between the size distributions of family members at four infrared bands (12, 25, 60, and 100 µm), allowing and nonfamily objects seems to be a subject of research that determination of the albedos and the diameters for about is interesting and not yet fully understood.

219 220 Asteroids III

The International Ultraviolet Explorer (IUE) was launched sitivity for an exploration of the universe at infrared wave- in January 1978, and remained operational until September lengths range 2–240 µm. The satellite was a great techni- 1996. During its lifetime IUE acquired ultraviolet spectra cal and scientific success, with of its subsystems (0.23–0.33 µm) of about 50 asteroids between 1978 and operating far better than the specifications and its scientific 1995. Roettger and Buratti (1994) determined the geometric results impacting practically all fields of astronomy. At a albedo of 45 objects at 0.267 µm and obtained the first ultra- wavelength of 12 µm, ISO was 1000× more sensitive and violet phase curves of asteroids. Long-exposure spectra had 100× better angular resolution than its predecessor, the obtained by A’Hearn and Feldman (1992) suggested the all-sky-surveying IRAS. During its routine operational presence of escaping from the surface of 1 . Sta- phase (from February 4, 1996, to April 8, 1998), ISO made tistically significant OH emission has been detected in an ex- over 26,450 scientific and approximately 4000 calibration posure off the northern limb of the asteroid after perihelion, observations, ranging from objects in our own system whereas it was not detected in an exposure off the south- right out to the most distant extragalactic sources. In addi- ern limb of the object before perihelion. The authors argued tion to the dedicated observations, ISO also obtained par- that the presence of a northern polar cap that changes with allel data while other instruments were prime, and seren- time (accumulating during winter and dissipating dur- dipitous data during slews of the satellite. ing summer) is compatible with the obtained results. Other The ISO’s operational orbit had a period of just below results obtained by Festou et al. (1991) for are men- 24 h, an apogee height of 70,600 km and a perigee height tioned in section 4. of 1000 km. The lower parts of this orbit were inside Earth’s The Hipparcos Space Mission was an ESA van Allen belts, which reduced the observing time per orbit project for measuring positions, distances, motions, bright- to about 16 h. The accuracy of the pointing system was at ness, and colors of . Between 1989 and 1993 it ob- the arcsecond level, with an absolute pointing error of 1.4" served 48 asteroids and pinpointed more than 100,000 stars and a short-term jitter of less than 0.5". The tracking of solar with an accuracy 200× better than ever before. These data system objects with ISO was limited to objects with apparent have very good accuracy (~0.01 arcsec) and allowed signi- velocities of less than 120"/h. Geometric constraints limited ficant improvement in the orbital parameters of observed ISO observations to about 15% of the sky at any one time. asteroids (Hestroffer et al., 1998). The analysis of the gravi- The cryogenic system enabled ISO to observe for nearly tational perturbations on the orbits of the 48 minor 29 . The combination of superfluid and normal He observed by Hipparcos allowed the determination of the cooled the infrared detectors, the scientific instruments, and largest asteroids’ (Viateau and Rapaport, 1998; parts of the telescope to of 2–4 K. Bange, 1998). The Midcourse Space Experiment (MSX) (http://sd- 2.1. Infrared Space Observatory (ISO) www.jhuapl.edu) was launched in 1996 and carried out ob- Observations of Asteroids servations in a wide wavelength range from ultraviolet to midinfrared (Mill et al., 1994). Three hundred seventy-five The ISO performed spectroscopic, photometric, imaging, main-belt asteroids have been observed by Price et al. and polarimetric measurements of ~40 different asteroids at (2001). Tedesco et al. (2001a) presented the results of the infrared wavelengths between 2 and 240 µm. All the aster- MSX Infrared Minor Planet Survey (MIMPS), consisting oid observations are summarized in Table 1. More details on of the of 26,791 asteroids and 332 sightings the programs, including the scientific abstracts of the pro- of 169 different asteroids. Among these were 31 asteroids posals, can be found in the ISO Data Archive (http://www. that were not included in the IRAS observations. iso.vilspa.esa.es). In recent the most important space platforms for Several instruments and observing modes were used to the observation of asteroids have been the Infrared Space observe asteroids. The ISOCAM instrument (Cesarsky et Observatory (ISO) and the Hubble (HST). al., 1996; Cesarsky, 1999) performed observations of as- ISO provided a very large sample of infrared data for as- teroids in two different modes designated as CAM01 (gen- teroids, while HST obtained some impressive results in the eral observation) and CAM04 (spectrophotometry). The construction of the geologic maps of some of the biggest long wavelength spectrometer, or LWS (Clegg et al., 1996, asteroids. ISO terminated its operative phase in April 1998. 1999), which covered the wavelength range from 43 to HST is still operational. Since their results over the last 196.7 µm, observed asteroids in mode LWS01 (grating decade have provided a significant improvement on the range scan) and LWS02 (grating line scan). The ISOPHOT knowledge of the physicochemical nature of asteroids, we instrument (Lemke et al., 1996; Lemke and Klaas, 1999) devote the rest of this chapter to a description of their char- with its three subinstruments (PHT-C, PHT-P and PHT-S) acteristics and their principal results on asteroid science. observed asteroids in five different modes: in single pointing and staring raster modes (PHT03, PHT22), 2. INFRARED SPACE OBSERVATORY photometry in scanning/mapping mode (PHT32), spectro- photometry (PHT40), and polarimetry (PHT50). Several The ISO (Kessler et al., 1996; Kessler, 1999), equipped asteroids have been observed in “many OBSMODEs” for with four sophisticated and versatile scientific instruments, calibration purposes (Laureijs et al., 2001). The short wave- provided astronomers with a facility of unprecedented sen- length spectrometer, or SWS (de Graauw et al., 1996; de Dotto et al.: Observations from Orbiting Platforms 221

TABLE 1. ISO and HST asteroid observations.

Asteroid ISO Program ISO Instrument HST Program HST Instrument 1 Ceres THENCRE GALSAT PHT03 PHT40 1268 Albrecht FOC (UV) imaging* ASALAMA AST_MIN SWS06 4545 A’Hearn FOS spectroscopy* HLARSON ASTEROID PHT40 SWS06 5175 Albrecht FOC (UV) imaging LWS_CAL LWS01 LWS02 LWS99 5842 Stern FOC (UV) imaging BSCHULZ PHT_CAL many OBSMODEs 8583 Storrs WFPC2 (unaber.) imaging SWS_CAL SWS06 SWS99 THENCRE GALSAT PHT03 PHT40 8583 Storrs WFPC2 (unaber.) imaging ASALAMA AST_MIN SWS06 HLARSON ASTEROID PHT40 SWS06 LWS_CAL LWS01 LWS02 LWS99 BSCHULZ PHT_CAL many OBSMODEs SWS_CAL SWS06 SWS99 3 THENCRE GALSAT PHT03 PHT40 BSCHULZ PHT_CAL many OBSMODEs SWS_CAL SWS06 SWS99 4 Vesta THENCRE GALSAT PHT03 PHT40 5489 Zellner WF/PC1 (aber.) imaging BESCHULZ VESTALC1 PHT03 PHT22 LWS02 5175 Albrecht FOC (UV) imaging LWS_CAL LWS01 LWS02 LWS99 6481 Zellner WFPC2 (unaber.) imaging BSCHULZ PHT_CAL many OBSMODEs 7433 McCarthy NICMOS (IR) imaging SWS_CAL SWS06 SWS99 5 Astraea 8583 Storrs WFPC2 (unaber.) imaging TMUELLER AST_POL1/2 PHT50 8583 Storrs WFPC2 (unaber.) imaging 8583 Storrs WFPC2 (unaber.) imaging 8 Flora 6559 Storrs WFPC2 (unaber.) imaging 9 Metis TMUELLER AST_POL1/2 PHT50 4521 Zellner WF/PC1 (aber.) imaging THENCRE GALSAT PHT03 PHT40 6559 Storrs WFPC2 (unaber.) imaging MBARUCCI ASTEROID PHT03 PHT40 MBARUCCI ASTEROI2 SWS06 CAM_CAL CAM01 CAM04 LWS_CAL LWS01 LWS02 LWS99 BSCHULZ PHT_CAL many OBSMODEs SWS_CAL SWS06 SWS99 6559 Storrs WFPC2 (unaber.) imaging HLARSON ASTEROID PHT40 SWS06 8583 Storrs WFPC2 (unaber.) imaging 8583 Storrs WFPC2 (unaber.) imaging 7488 Zappalà FGS 8583 Storrs WFPC2 (unaber.) imaging 4764 Weiss WF/PC1 (aber.) imaging 5238 Westphal WF/PC1 (aber.) imaging 8583 Storrs WFPC2 (unaber.) imaging 4521 Zellner WF/PC1 (aber.) imaging 20 Massalia CAM_CAL CAM01 CAM04 8583 Storrs WFPC2 (unaber.) imaging 6559 Storrs WFPC2 (unaber.) imaging 38 8583 Storrs WFPC2 (unaber.) imaging 7488 Zappalà FGS 7488 Zappalà FGS 8583 Storrs WFPC2 (unaber.) imaging 8583 Storrs WFPC2 (unaber.) imaging 46 Hestia CAM_CAL CAM01 CAM04 8583 Storrs WFPC2 (unaber.) imaging 49 Pales 8583 Storrs WFPC2 (unaber.) imaging 8583 Storrs WFPC2 (unaber.) imaging 52 THENCRE GALSAT PHT03 PHT40 8583 Storrs WFPC2 (unaber.) imaging ASALAMA AST_MIN SWS06 54 Alexandra BSCHULZ PHT_CAL many OBSMODEs 6559 Storrs WFPC2 (unaber.) imaging 56 Melete CAM_CAL CAM01 CAM04 7488 Zappalà FGS THENCRE GALSAT PHT03 PHT40 8583 Storrs WFPC2 (unaber.) imaging CAM_CAL CAM01 CAM04 BSCHULZ PHT_CAL many OBSMODEs 222 Asteroids III

TABLE 1. (continued).

Asteroid ISO Program ISO Instrument HST Program HST Instrument

77 Frigga MBARUCCI ASTEROID PHT03 PHT40 8583 Storrs WFPC2 (unaber.) imaging 89 Julia 6559 Storrs WFPC2 (unaber.) imaging 8583 Storrs WFPC2 (unaber.) imaging 106 BSCHULZ PHT_CAL many OBSMODEs 8583 Storrs WFPC2 (unaber.) imaging 8583 Storrs WFPC2 (unaber.) imaging 109 Felicitas 4521 Zellner WF/PC1 (aber.) imaging 111 Ate 3744 Zellner FOS spectroscopy* 114 Kassandra MBARUCCI ASTEROID PHT03 PHT40 MBARUCCI ASTEROI2 SWS06 121 Hermione 8583 Storrs WFPC2 (unaber.) imaging 130 Elektra 8583 Storrs WFPC2 (unaber.) imaging 6559 Storrs WFPC2 (unaber.) imaging 146 Lucina 4764 Weiss WF/PC1 (aber.) imaging 150 Nuwa CAM_CAL CAM01 CAM04 4784 Schenk FOS spectroscopy* 216 Kleopatra 4764 Weiss WF/PC1 (aber.) imaging 7488 Zappalà FGS 224 Oceana 4784 Schenk FOS spectroscopy 243 Ida 5633 Zellner WFPC2 (unaber.) imaging 8583 Storrs WFPC2 (unaber.) imaging 308 Polyxo MBARUCCI ASTEROID PHT03 PHT40 MBARUCCI ASTEROI2 SWS06 AFITZSIM DTYPE PHT40 313 Chaldaea BSCHULZ PHT_CAL many OBSMODEs 336 Lacadiera AFITZSIM DTYPE PHT40 8583 Storrs WFPC2 (unaber.) imaging 4669 Whipple FGS* 434 Hungaria 4521 Zellner WF/PC1 (aber.) imaging 8583 Storrs WFPC2 (unaber.) imaging 498 Tokio AFITZSIM DTYPE PHT40 MBARUCCI ASTEROID PHT03 PHT40 BSCHULZ PHT_CAL many OBSMODEs 4764 Weiss WF/PC1 (aber.) imaging 5238 Westphal WF/PC1 (aber.) imaging MBARUCCI ASTEROID PHT03 PHT40 4764 Weiss WF/PC1 (aber.) imaging MBARUCCI ASTEROI2 SWS06 7488 Zappalà FGS 4521 Zellner WF/PC1 (aber.) imaging 702 Alauda 3744 Zellner FOS spectroscopy 709 Fringilla 4521 Zellner WF/PC1 (aber.) imaging 8583 Storrs WFPC2 (unaber.) imaging CAM_CAL CAM01 CAM04 899 Jokaste 4784 Schenk FOS spectroscopy MBARUCCI ASTEROID PHT03 PHT40 914 Palisana MBARUCCI ASTEROID PHT03 PHT40 944 Hidalgo 4784 Schenk FOS spectroscopy* 1137 Raissa 4521 Zellner WF/PC1 (aber.) imaging* 1144 Oda 3744 Zellner FOS spectroscopy 1172 Aneas MBARUCCI ASTEROID PHT03 PHT40 1220 Crocus 6559 Storrs WFPC2 (unaber.) imaging 1437 Diomedes MBARUCCI ASTEROID PHT03 PHT40 1980 Tezcatlipoca AHARRIS ASTEROID PHT03 PHT32 PHT40 CAM01 2062 Aten CAM_CAL CAM01 CAM04 4784 Schenk FOS spectroscopy* 2411 Zellner 3153 Buie WF/PC1 (aber.) imaging* 2703 Rodari CBARBIER ROSAST PHT03 AHARRIS ASTEROID PHT03 PHT32 7884 Campins NICMOS (IR) imaging PHT40 CAM01 and spectroscopy Dotto et al.: Observations from Orbiting Platforms 223

TABLE 1. (continued).

Asteroid ISO Program ISO Instrument HST Program HST Instrument 3361 Orpheus 5483 Storrs WF/PC1 (aber.) imaging* 3671 Dionysus AHARRIS ASTEROID PHT03 PHT32 PHT40 CAM01 3840 Mimistrobell CBARBIER ROSAST PHT03 MFULCHIG AROSETTA PHT03 PHT40 4015 Wilson- Harrington AHARRIS ASTEROID PHT03 PHT32 2432 Zellner WF/PC1 (aber.) imaging PHT40 CAM01 7884 Campins NICMOS (IR) imaging and spectroscopy AHARRIS ASTEROID PHT03 PHT32 4748 Noll WF/PC1 (aber.) imaging PHT40 CAM01 *Bad data (missed target, too weak, saturated).

Graauw, 1999), covered the wavelength range from 2.38 to at 12 µm, and (3) the first polarimetric measurements of the 45.2 µm. SWS observations of asteroids were taken in mode disk-integrated thermal emission of asteroids at 25 µm. SWS06 (grating wavelength range scan). The ISO modes 99 (LWS99 and SWS99) included a manual setting of the 2.2. Disk-integrated Polarized Thermal instrument commands. Data taken with these modes are Emission of Asteroids nonstandard, used mainly for special calibration purposes. The ISO observations underwent careful data process- Depending on wavelength, refractive index, and viewing ing and calibration. This data processing includes several geometry, the thermal-infrared emission from asteroids orig- corrections due to the properties of the detector, instrumen- inates from some depth below the surface. The radiation tal effects by electronics and optics, and external effects propagates in the porous regolith toward the surface, where such as cosmic rays. The effects and their handling in the it is refracted according to Fresnel’s equations. As a con- data processing and calibration procedures are described in sequence, the thermal emission becomes polarized. Due to the ISO Handbooks (2001) (http://www.iso.vilspa.esa.es). symmetry, however, there can be no net polarization in the Photometric and spectroscopic uncertainties were caused disk-integrated flux from a circular symmetric mainly by the difficulties in modeling the instrument be- distribution. But the nonzero thermal inertia together with havior in space. Detector transients, nonlinearities, and fre- the rotation of the object cause an asymmetry in the tem- quent glitches from high-energy particles limited the final perature distribution. The effect increases with phase angle accuracy. In the relatively unexplored far-infrared wave- and for elongated asteroids. The contribution of subsurface length range the structured and bright infrared background, emission to the total emission is wavelength dependent, with along with uncertainties from celestial calibration standards, the largest contribution at peak wavelength and beyond. influenced the final data quality. The standards included Observing the polarized thermal emission is an unex- stars with confidence in the absolute flux level of about 3% plored and potentially very promising technique for studies in the 2.5- to 35-µm wavelength range and better than 10% of the surface regolith and its influence on the thermal emis- at longer wavelengths up to 300 µm (Cohen et al., 1999). sion. Additionally, it facilitates the interpretation of infra- Asteroids were used in the far-infrared between 35 and red spectra of asteroids. Asteroids 6 Hebe and 9 Metis have 200 µm as photometric and spectroscopic references. Based been observed at a wavelength of 25 µm with ISOPHOT on Müller and Lagerros (1998), the accuracy of the pre- (Lagerros et al., 1999). Due to the observing geometry and dicted asteroid fluxes is better than 10% in the wavelength the elongated shapes of both objects, the temperature distri- range from 24 to 200 µm for a few large and well-known bution on the surface was expected to be asymmetric. Con- asteroids. The outer planets and Uranus were used sequently it was expected to measure polarized thermal for calibration at the longer wavelengths in the high flux den- emission, which could then be attributed to surface prop- sity range up to 1000 Jy. The model predictions are based erties. Unfortunately, for this technique the ISO targets were on Griffin and Orton (1993) and are accurate to approxi- limited due to strong visibility and brightness constraints. mately 5% for the continuum flux. The scientific interpretation was done on the basis of the Among the ISO data on asteroids there are (1) complete polarization code of the ThermoPhysical Model (TPM) de- spectra from 2 to 200 µm of bright objects with spectral veloped by Lagerros (1996a,b, 1997, 1998). Critical for po- resolutions of 1000–2000 in the SWS range and about 200 larization analysis is the modeling of the small-scale surface in the LWS range, (2) imaging and photometric asteroid roughness, which was approximated by hemispherical seg- measurements with sensitivities 1000× higher than IRAS ment craters covering a smooth surface. Analytical solutions 224 Asteroids III

were used for the multiple scattered solar and thermally 1000 emitted radiation inside the craters. A higher surface rough- ness generally lowers the polarization due to a randomizing of the scattering planes. The predicted degree of linear po- larization increases with higher refractive index and with 100 SWS LWS higher absorption coefficient of the surface material. Oct. 30, 1997 Dec. 3, 1997 The model flux densities for 6 Hebe and 9 Metis were in good agreement with the photometric results. No linear po- Flux (Jy) 10 larization was detected, but useful constraints on the prop- erties of the regolith of these objects have been obtained (Lagerros et al., 1999) on the basis of the obtained upper limits and the extended model. The derived detection limits 1 restricted the possible parameter space in surface roughness, 10 100 refractive index, and thermal inertia. For Hebe the obser- Wavelength (µm) vations were inconclusive since they coincided with a mini- Fig. 1. The asteroid 1 Ceres observed by ISO’s LWS and SWS. mum in the polarization curve. The Metis observations LWS and SWS observations have been taken at different epochs favored a low refractive index and high surface roughness. with different flux level for Ceres. The TPM predictions are over- plotted as solid lines. 2.3. Fundamental Thermal Emission Parameters of Main-Belt Asteroids inertia taken into account, a rougher surface is needed in The thermophysical emission aspects of several asteroids order to produce the same amount of beaming. were analysed on the basis of a large uniform pre-ISO data- Figure 1 shows SWS and LWS observations of the aster- base of about 700 individual observations, in the wavelength oid 1 Ceres from two different epochs. The corresponding range 7–200 µm. The TPM developed by Lagerros (1996a, TPM predictions are overplotted. 1997, 1998) (see preceding section) was applied to investi- Further comparisons with ISO observations of asteroids gate surfaces roughness, heat conduction, and the emissivity belonging to different taxonomic classes show that the ther- over the thermal wavelength range (Müller and Lagerros, mal parameters of Ceres are also valid for other main-belt 1998). The model included aspects of observing geometry, asteroids (Müller and Lagerros, 2002). This means that the illumination conditions, shape, and spin vector and, if avail- TPM parameters allow a reliable prediction of the thermal able, physical size and the albedo [more details about the emission for main-belt asteroids when the shape and H-G thermal models are reported and discussed in Harris and values are known. The TPM and the derived thermal pa- Lagerros (2002)]. rameters are also useful in understanding thermal asteroid The investigations indicated very low levels of heat con- spectra where the wavelength-dependent emissivity and the duction, expressed in thermal inertias between 5 and 25 J beaming effect are clearly visible. The thermal inertia to- m–2 s–0.5 K–1. Based on the Ceres observations alone, a ther- gether with the spin state cause different fluxes before and mal inertia of Γ = 15 J m–2 s–0.5 K–1 was found. This value after opposition due to the morning/afternoon effect. Here is much smaller than the lunar value of 50 J m–2 s–0.5 K–1 again, the TPM allows combined analysis and interpreta- (Spencer et al., 1989). This is mainly due to the differences tion of observations. Note that the model does not provide between the environment of the asteroid regolith (lower a perfect reproduction of the spectrum of an asteroid. Limi- temperature, lower density) and the lunar environment. tations are introduced by the necessity to model all the Infrared observations show that the thermal emission is physical characteristics of the body with the minimum num- directed more strongly toward the solar direction at the ber of free parameters. The agreement on all flux levels over expense of the emission at larger phase angles. This beam- the full thermal emission spectrum of different asteroids at ing effect is largest at midinfrared wavelengths. For ex- different epochs confirms that this kind of thermal model ample, the 10-µm flux is 20–40% higher than predicted for is plausible. a smooth Lambertian surface. The thermal infrared beaming parameters for Ceres have been determined together with 2.4. Asteroid Size-Frequency Distribution the thermal inertia, from a least-squares method including all observational infrared data. The derived values are ƒ = During the ISO mission six deep maps of a 12-arcmin 0.6 (fraction of surface covered by craters) and ρ = 0.7 square field on the were obtained through the ISO (r.m.s. of the surface slopes). Lagerros (1998) compared 12-µm “IRAS” filter (ISOCAM LW10 band) (Tedesco and these values with the default beaming parameter (η = 0.756) Desert, 1999). These regions were sampled to an 8-µm of the Standard Thermal Model (STM) (Lebofsky and Spen- equivalent depth of ~0.6 mJy at S/N ratio of 5. Thus, most cer, 1989), derived by Lebofsky et al. (1986) from obser- of the asteroids detected by ISO were more than 200× vations of 1 Ceres and 2 Pallas. A nice agreement has been fainter than those detected by IRAS, whose limiting sensi- found when heat conduction is neglected. With the thermal tivity was about 150 mJy at 12 µm. Dotto et al.: Observations from Orbiting Platforms 225

The ISO survey results (160 ± 40 asteroids per square physical parameters, such as density, mineralogy, particle degree) were modeled with the Statistical Asteroid Model size, packing, and other effects (Logan et al., 1975). (Tedesco et al., 2001b). This resulted in a value of ~160 ± Spectroscopic and spectrophotometric observations of 10 asteroids per square degree with diameters greater then asteroids have been carried out by ISO with PHT-P (filters ~1.7 km. The ISO data seem to suggest that the actual num- from 3.29 to 100 µm), PHT-C (filters from 50 to 200 µm), ber of kilometer-sized asteroids is significantly greater than PHT-S (low-resolution spectroscopy in the ranges 2.5– expected by some size-frequency distribution models (e.g., 4.9 µm and 5.8–11.6 µm), SWS (high-resolution spectros- Farinella et al., 1992), and in reasonable agreement with copy in the range 2.38–45.2 µm), and LWS (spectroscopy the Statistical Asteroid Model. in the range 43–196.7 µm with a resolution of about 200). STM and TPM (see preceding section) have been applied 2.5. Asteroids Serendipitously Seen by the to the data. Subsolar and black-body temperatures have Infrared Space Observatory (ISO) been computed for several observed asteroids (Barucci et al., 1997; Dotto et al., 1999, 2000). Further ISOPHOT data Many surveys and large observing programs have been allowed the determination of albedo and diameter of some conducted by ISO. Surveys close to the ecliptic plane, per- near-Earth asteroids (Harris and Davies, 1999; Harris and formed by ISOCAM and ISOPHOT, include many asteroids Lagerros, 2002). Heras et al. (2000) published SWS rota- (Müller, 2001). The detection of asteroids at thermal wave- tionally resolved spectra of 4 Vesta in the wavelength range lengths has many applications. For well-known objects ther- between 19.4 and 27.6 µm. A short discussion of these re- mal model predictions can be tested against the measured sults is reported in section 4. Dotto et al. (2000) presented infrared brightness. In all cases radiometric diameters and and discussed the spectra obtained by PHT-S for five bright albedos can be determined and compared to direct size deter- asteroids: 1 Ceres, 2 Pallas, , 4 Vesta, and . minations and/or IRAS results, if available. The midinfrared Since these objects are among the brightest and best-known ISOCAM observations allow further studies of beaming and main-belt asteroids, with spin vector, shape, and size com- surface structure properties, while the far-infrared measure- puted with a good precision, their thermal continuum has ments give clues about the as-yet poorly known emissivity been modeled using TPM. The spectral features that remain behavior of asteroids. after division for the modeled thermal continuum have been A first analysis of the approximately 40,000 CAM par- analyzed and discussed in terms of surface composition of allel observations between 6 and 15 µm revealed infrared the objects. fluxes of about 50 asteroids, most of them not seen by IRAS The main features observable in the midinfrared spectral (T. G. Müller, personal communication, 2002). Many addi- range, which are diagnostic of the mineralogical composi- tional objects are expected to be included in other CAM tion of the surface of the observed asteroids, can be cate- surveys, with some of the deep observations being sensitive gorized into three classes: reststrahlen, Christiansen, and enough to detect 1-km-diameter objects in the asteroid transparency features. Reststrahlen bands are fundamental midbelt. stretching and bending vibration bands that in the case of ISOPHOT performed observations at 170 µm while the silicates occur in the 8–25-µm region. The Christiansen fea- satellite was slewing from one target to the next one. This ture is associated with the principal molecular vibration so-called ISOPHOT Serendipity Survey covered approxi- band, where the refractive index changes rapidly, and occurs mately 15% of the sky with a limiting sensitivity of 1 Jy. at a wavelength that for silicates is just short of the Si-O The ISO database can now be systematically searched stretching vibration bands. A volume scattering feature of for objects with known orbits. The flux deter- fine particulates is the transparency feature, which forms an mination allows thermophysical investigations and the deri- emissivity trough between the fundamental stretching and vation of surface parameters. But ISO also saw moving bending vibration bands of silicates. In the spectral region targets where so far no counterpart in the Minor Planet where the absorption coefficient decreases, grains become Database is known. For these objects only a statistical analy- more transparent and volume scattering comes to dominate sis is possible at the moment, at least as long as the orbits the scattering process as the particle size is reduced. A de- and the visual brightnesses are not known. tailed description of these spectral features is given in sev- eral papers (e.g., Salisbury, 1993). 2.6. Spectroscopic and Spectrophotometric Results In order to investigate the surface composition of the five brightest asteroids observed by ISO, Dotto et al. (2000) The interpretation of spectral features in mid- and far- compared the observed spectra with the emissivity of me- infrared is not easy, as asteroid surfaces are composed by teorites and minerals available in literature and new labo- mixtures of minerals whose absorption features are com- ratory spectra of a selected sample of minerals. The thermal bined following nonlinear paths. This means that the spec- emissivities of all the five asteroids show a strong signa- tral features of a mineral present even at a few percent level ture around 10–11 µm, suggesting the presence of silicates on the surface of an asteroid can dominate the asteroid on the surface of these bodies. As an example, the observed spectrum. Moreover, asteroid spectra are affected not only emissivity of the C-type asteroid 52 Europa and of pyrox- by the surface composition but also by several unknown enes and olivines obtained by laboratory experiments is 226 Asteroids III

1.2 must be dimmed by the camera’s filter systems to avoid saturating the detectors. With a broadband filter,

1.0 the brightest object that can be accurately measured is 20th magnitude. The Wide Field Planetary Camera (WFPC2), which replaced the original Wide Field/Planetary Camera 0.8 (WF/PC1) in December 1993, is a spare instrument devel- oped in 1985 by the Jet Propulsion Laboratory in Pasadena, 0.6 enstatite California. The “heart” of WFPC2 consists of an L-shaped

Relative Obs/Mod trio of wide-field CCD sensors and a smaller, high-resolu-

0.4 forsterite tion (“planetary”) camera tucked in the square’s remaining corner. The relay mirrors in WFPC2 are designed to correct for the spherically aberrated primary mirror of the obser- 0.2 6 7 8 9 10 11 12 vatory. The Faint Object Spectrograph (FOS), which was Wavelength (µm) one of the original four axial instruments on HST removed during the second servicing mission in February 1997, was Fig. 2. Relative PHT-S OBS/MOD of 52 Europa compared with used to make spectroscopic observations of astrophysical the emissivity of a sample of olivine (forsterite) and a sample of sources from the near-ultraviolet to the near-infrared (0.115– pyroxene (enstatite) obtained by laboratory measurements. The 0.8 µm). The Near Infrared Camera and Multi-Object Spec- spectra are vertically offset for clarity (from Dotto et al., 2000). trometer (NICMOS) provides the capability for infrared imaging and spectroscopic observations of astronomical tar- gets between 0.8 and 2.5 µm. NICMOS exhausted its on- shown in Fig. 2. The spectral behavior between 8 and 11 µm board cryogen load in January 1999. It is hoped that the suggests the presence of a mixture of pyroxenes and oliv- installation of the NICMOS Cooling System in servicing ines on the surface of 52 Europa. In particular, the 8.8-µm mission 3B, performed in March 2002, will allow the restor- maximum in the spectrum of 52 Europa seems to be consis- ation of at least some of the capability. More details are re- tent with the Christiansen peak of olivine, which occurs at ported at the STScI Web site. The Fine Guidance Sensors a distinctively long wavelength (Salisbury et al., 1991a). At (FGSs) are used to point the telescope and track moving tar- longer wavelengths, olivine and pyroxene exhibit two major gets, but can also be used as interferometers to determine the reststrahlen bands separated by a band gap that seem to be size and separation of multiple systems of stars or asteroids. consistent with the ISO spectrum of Europa even though the error bars in this region are high. 3.2. Image Restoration

3. Image restoration requires high signal-to-noise images and a good knowledge of the point spread function (PSF), Hubble Space Telescope (HST) is a cooperative program of and only if these are available can the effects of the PSF be the National Aeronautics and Space Administration (NASA) at least partially removed from the original image. Some of and the European Space Agency (ESA). It is a 2.4-m UV/ the most popular techniques to do this are the Richardson- VIS/NIR telescope in a 96-min orbit (about 600 km above Lucy formulation (Richardson, 1972; Lucy, 1974) and the ground level). Operating since 1990, HST has provided, up Maximum Entropy formulation (Gull and Daniell, 1978; to February 2002, images of more than 60 asteroids Wu, 1994). These have been used quite successfully to (Table 1). HST is a queue-scheduled robot, with little real- “clean up” the aberrated HST images and to improve the time interactive capability. It has an array of instruments spatial resolution when the S/N ratio was adequate. These that can be (and have been) changed out during refurbish- algorithms work quite well in the standard astronomical ment visits by astronauts. A complete presentation of the case of point sources with perhaps a slowly varying back- up-to-date information on the instrument complement and ground, but in the planetary case of a bright disk with a capabilities is given at the Space Telescope Science Institute sharp limb and features on the disk, they can cause trouble. (STScI) Web site (http://www.stsci.edu) or the Web site of The problem is that the iterative algorithms do a good job the European Coordinating Facility (ECF) (http://www. of fitting the limb (where most of the digital information is) stecf.org). The “HST Primer” provides a good overview of but overshoot at the edge, causing a “ringing” effect. the history and current capabilities of the observatory. In the A new algorithm [“MISTRAL” (cf. Conan et al., 2000)] following sections some general characteristics are given. gets around this problem. In contrast to the “blind” deconvo- lution programs that keep information about the real image 3.1. Hubble Space Telescope (HST) and the PSF separately, this method is “myopic” in allowing Observations of Asteroids some information exchange. HST has a well-characterized and stable PSF, however, so this aspect of the algorithm is Several HST instruments have been used to observe aster- not vital to the reconstruction of planetary images. What is oids. The Faint Object Camera (FOC), built by the European of great interest is that the input parameters can be varied to Space Agency, is so sensitive that objects brighter than 21st eliminate the “ringing” effects at the edge of the restoration. Dotto et al.: Observations from Orbiting Platforms 227

Storrs et al. (2000) used this method to restore WF/PC1 The small numbers in this sample may not be significant, (aberrated) images of asteroid 216 Kleopatra to show the though. rotation of the asteroid during the time of the observations. One of the major projects Zellner et al. (1989) expected to be addressed by HST was the search for asteroidal com- 3.3. Hubble Space Telescope (HST) Results panions. The absence of atmospheric effects on HST images allows diffraction limited operation over a very large field More than 20 observational programs have generated of view. The results to date of searches for companion ob- asteroidal data in the HST archive (Table 1); however, some jects to asteroids are discussed in Merline et al. (2002). of these are only useful for engineering or missed the tar- The best-known asteroidal companion is Dactyl, orbit- get entirely. The study of asteroids with HST has centered ing 243 Ida. It was discovered by the spacecraft mainly on imaging programs, searching for companion ob- during its flyby and HST observations were made shortly jects and surface features, although some searches for com- thereafter, to check for companions farther from Ida and etary emissions (e.g., comae, OH) have been done as well. to constrain Dactyl’s orbit. No companions were observed, All HST data is available through the HST data archive, but the observations put constraints on the orbit and thus which can be accessed through the STScI Web site cited the and bulk density of Ida as published by Belton et above. Care should be taken in the analysis of data from al. (1995, 1996). the archive, particularly data from early in the program. The Storrs et al. (1999b) carried out a wide program to look for “HST Data Handbook” (also available from the STScI Web asteroidal companions. No binary systems were detected. site) is an invaluable guide to understanding HST data. A limiting case for an asteroidal companion is when the 3.3.1. Imaging. Many asteroid observations have been companion is of comparable size to the primary and orbits performed by HST using WF/PC1 or the new WFPC2. The it very closely, perhaps even touching it. This contact bi- method for many of these programs was to image the object nary situation has been suggested for a variety of asteroids. with the Planetary Camera using four or five filters. These Noll et al. (1995) report HST images of 4179 Toutatis. Their are chosen to define the blue continuum slope, the width observation was made at a maximum in the asteroid light- and depth of the 1-µm “silicate feature,” and the 0.7-µm hy- curve yet it shows no extension, indicating that the elon- dration feature. gated object [as seen in radar delay- images (Hudson Binzel et al. (1997) and Storrs et al. (1998) used image and Ostro, 1995)] must have been nearly end-on to Earth ratios to study the mineralogical variation on the asteroid at the time. surfaces. For 4 Vesta, Binzel et al. (1997) had enough data Recently, observations of 49 Pales showed no compan- to make albedo maps in each filter, which could then be ion. The image is elongated, however, and subsequent pro- ratioed. These maps are discussed in section 4. cessing may reveal whether or not this object is a contact Storrs et al. (1998) attempted a broad imaging analysis, binary, as suggested by Tedesco (1979). Observations dur- looking for surface variegation in asteroids 8 Flora, 10 Hy- ing the same program confirmed the recently discovered giea, 11 Parthenope, 29 Amphitrite, 54 Alexandra, 89 Julia, companion of 87 Sylvia (IAUC 7588), providing visible 144 Vibilia, and 1220 Crocus. Because only one HST orbit colors for the companion object that appear slightly less red was devoted to each asteroid (16 were used for the studies than the primary object (IAUC 7590). A companion of 107 of 4 Vesta described in section 4), only one hemisphere of Camilla (see Merline et al., 2002) has also been detected. each of these bodies was imaged. This raises the possibility 3.3.2. Fine Guidance Sensor results. The Fine Guid- of phase effects affecting the mineralogical map. No sur- ance Sensors (FGS) are used to point HST to the extreme face variegation was reported, and this was confirmed by a degree of accuracy necessary for long exposures of faint reanalysis of the data presented by Storrs et al. (1999a). targets. In normal operation, two of the FGSs are used for These papers used the maximum entropy image restoration spacecraft attitude control. The third FGS is free to carry method, however, which causes some loss of information out astrometric and photometric observations, including due to edge effects (see section 3.2). (1) measuring the relative positions of sources to a preci- et al. (1998) report an unusual use of serendipi- sion of a few milliarcseconds, (2) measuring the separations tous asteroidal observations with HST. As part of a program and magnitude differences of binary stars, and (3) meas- to monitor the performance of WFPC2, they noted the paths uring stellar angular diameters. Technical details are avail- of 96 asteroids that were in the field of view of long expo- able in the FGS handbook (available at http://www.stsci. sures of fixed targets. HST’s orbital motion during these edu). Observations with FGS, in TRANSfer mode, are po- long exposures causes the trails to be curved and an analy- tentially the most sensitive way of picking up close com- sis of the size and shape of the curve gives a good estimate panions of asteroids. The technique was pioneered by a pro- of the distance to the asteroid. Most of their objects are too gram targeting 433 Eros. The technical implementation of faint to turn up in groundbased surveys, and are probably such observations is daunting, however, and only one pro- only a few kilometers in diameter. Evans et al. report a sig- gram has been successful so far (Tanga et al., 1999). The nificantly shallower slope to the population distribution photometric measurements of the beams resulting from in- (N ∝ H0.25 vs. N ∝ H0.46 from Tedesco et al., 2001b) for terference, repeated for different values of inclination of the these small objects than is seen for larger bodies, similar incoming wavefront, provide the response function of the to the result of Cellino et al. (1991) discussed in section 1. instrument. This produces a characteristic “S-curve” whose 228 Asteroids III features are related to the shape and the size of the target, Even if an accurate calibration of the absolute scale of as projected on the sky plane (Fig. 3). In the case of aster- the FGS has not been performed, the engineering specifi- oid observations, the S-curve was sampled with a stepsize cations of the instrument guarantee an error smaller than of 1–1.5 milliarcsec, over a total length corresponding to 10% (Tanga et al., 2001). This characteristic makes the FGS 2 arcsec. one of the most precise tools available for measurement of In order to retrieve shape and size parameters, the S- asteroid diameters and shapes. Due to sensitivity limitations, curve observed by FGS is compared with a calibration re- however, the FGS gives the best results with bodies whose sponse curve obtained by the observation of pointlike stars. brightness is not lower than V ~ 13. This requires an aver- The simplest model, supported by various theoretical argu- age of 4–8 scans in order to reach a S/N ratio better than ments about the equilibrium figures of asteroids (see, e.g., 10. A larger number of averaged response functions allows Leone et al., 1984), is to assume that the observed bodies in principle the observation of fainter bodies, but it implies are composed of one (or two) triaxial ellipsoids. System- that the orientation of the asteroid has not changed signifi- atic fit residuals can give hints about other effects (com- cantly during each sequence of scans. plex shape, albedo variations) not taken into account. The The limit for useful data is probably represented by 624 success of the observation, in terms of shape reconstruc- Hektor, which at magnitude V ~ 14 required about 15 min of tion and measurement, depends upon (1) a careful choice observations to reach, by scan averaging, a S/N ratio around 3. of the observing , together with a good knowledge In the case of 63 Ausonia, using average pole coordi- of the spin axis orientation; (2) a compromise between the nates derived from those reported in literature (http://www. number of scans that have to be averaged (depending upon astro.uu.se/), it has been shown that the scan shape can be magnitude) and the spin rate of the asteroid; and (3) a suf- exceedingly well fit at all epochs using an ellipsoid of sizes ficiently long observation, at several rotational phases. The 151 × 66 × 66 km, with a residual uncertainty of the order result of the fit process constrains the model parameters (in of 1 km on axis a and b. The third axis is more poorly con- this case, the semiaxes of the ellipsoids and their on-sky strained due to the nearly pole-on geometry. Furthermore, orientation), with a level of uncertainty determined by the since the projected on-sky shape is observed, the residual geometry of the observation. The extent of the model can ambiguity on the pole longitude is easily resolved (Hes- be constrained with a typical 1-σ deviation that can be as troffer et al., personal communication, 2002). Of course, low as 1 milliarcsec, an exceedingly good value correspond- given the high level of precision reached, the orientation ing to about 1 km at ∆ = 1.2 AU. and amount of the phase effect must be taken into account. Theory based on shape stability criteria for fluid bodies pre- dicted a possible binary object (Leone et al., 1984; Cellino et al., 1985), while Hestroffer et al. (personal communica- Single φ = 0.1 arcsec tion, 2002) find a single but very elongated shape. 0.2 Another interesting case is that of 216 Kleopatra. Despite φ = 0.15 being another candidate binary, this asteroid was revealed

φ = 0.2 to be a very elongated — but probably single — body by 0 means of radar observations (Ostro et al., 2000). FGS data strongly support this result, suggesting that 216 Kleopatra can be modeled as a shape made up of two ellipsoids, –0.2 aligned along their semimajor axis and slightly superim- Double (φ = 0.1) sepa = –0.015 arcsec posed (Fig. 4). The best fit is obtained with ellipsoids of size sepa = 0.0 152 × 75 × 36 km and 143 × 70 × 51 km, having a center- 0.1 sepa = 0.015 to-center separation of 125 km (Tanga et al., 2001). 3.3.3. Spectrographic observations. Only a limited 0 amount of spectrographic work has been done on asteroids Signal Signal with HST. This is primarily because of the small apertures –0.1 of the spectrographic instruments and the relatively poorly known ephemerides of most asteroids. To acquire an ob- –0.2 0 0.2 ject in an HST spectrograph, its position must be known to Abscissa (arcsec) within 2 arcsec, as the acquisition is completely robotic. A good solar spectrum is necessary for determining the reflectance spectrum of asteroids. This has proven to be Fig. 3. Synthetic FGS S-curves obtained by convolution of the more problematical than originally thought. The “solar response curve obtained on a pointlike source with the brightness distribution of a single (upper panel) and double (lower panel) type” spectra used in calibrating NICMOS and other instru- object. Each curve represents the signal as measured along one ments are not solar in detail — the stars were selected for of the FGS axes. The single body is represented by a uniform disk their broad color match to the . For a solar-type to of size Φ. The double one is composed by two uniform disks be faint enough to be a good standard for the sensitive HST (placed along the axis) at different separations (sepa). instruments, however, it must be far enough away that its Dotto et al.: Observations from Orbiting Platforms 229

N 2

1 FGS X 0.2 Residuals 0.1 0 E 0

–1 –0.1 0.1

–2 –0.4 –0.2 0 0.2 0.4 FGS Y +0.068 " 2 10–1 –2

0

–0.1

–0.2 Yh Residuals

Signal 0.1

0.1 0

–0.1

–0.4 –0.2 0 0.2 0.4 0

–0.1

–0.2

–0.4 –0.2 0 0.2 0.4 Abscissa (arcsec)

Fig. 4. An example of the response curve obtained in the case of 216 Kleopatra (dots) and model fit (line). The larger separation of the two lobes along the FGS-X direction clearly shows a signature in the center similar to that of a double body (Fig. 3). Small residu- als appear on the FGS-Y, and are visible over several scans, thus hinting of the presence of albedo or shape effects. On the left is the model obtained for 216 Kleopatra and its on-sky orientation at the beginning of the observation. The orientation of the FGS-X and FGS-Y axis is also plotted.

spectrum is significantly reddened. Thus the stars discussed in asteroids. A preliminary analysis is reported by Campins in Colina et al. (1996) are generally of an earlier type than et al. (1999), indicating the presence of some hydrated min- the Sun, and are reddened to solar colors. This confusion, erals on 3200 Phaethon. coupled with potential instrumental artifacts not removed in the standard processing, leads to the recommendation that 4. THE SPECIAL CASES OF 4 VESTA the observers use a real solar analog spectrum observed by AND 10 HYGIEA the instrument in the configuration in which they have taken their data. These can be obtained from the HST data archive Two large asteroids have been thoroughly observed by (http://archive.stsci.edu/). both ISO and HST: 4 Vesta and 10 Hygiea. One program attempted to observe the region just off the 4 Vesta is the third largest asteroid, with an IRAS diam- poles of 1 Ceres. Unfortunately, limitations in HST target eter of 468 km and an albedo pV = 0.42 (Tedesco et al., acquisition and tracking capabilities allowed too much scat- 1992). Its history is discussed in Keil (2002). Here we will tered light in these spectra. No results were published. Sev- point out the results obtained on the basis of observations eral programs looked for comae and/or surface variegation carried out by IUE, HST, and ISO. Great interest in the 230 Asteroids III geology of Vesta followed the discovery that howardite, into the southern crater as showing increased Ca content , and diogenite (HED) meteorites could be samples deeper in the crater. On the western hemisphere the HST excavated from its surface. These meteorites have spectral data indicate a predominance of eucritelike assemblage, features common to Vesta and to a cluster of small (<10 km) with the spectra characteristic of single pyroxene with a V-type asteroids extending from the region surrounding modest Ca component. On the eastern hemisphere the spec- Vesta to the edge of the 3:1 resonance at 2.5 AU (Binzel tra indicate the presence of a diogenite-like component (a and Xu, 1993). Impacts may have excavated enough crustal low-Ca pyroxene) and an olivine component (Binzel et al., materials to form this widely scattered family of “vestoids”: 1997). The discovery of substantial impact excavation on Some of them approach the chaotic region associated with Vesta (the crater excavated about 1% of the volume of ν the 3:1 and 6 resonances from where fragments can be rap- 4 Vesta) is consistent with the idea for the origin of basaltic idly transferred to Earth-crossing orbits (Migliorini et al., achondrite HED meteorites. There are thus convincing ob- 1997). servational and dynamical arguments to suggest that Vesta Festou et al. (1991) obtained simultaneous visible (near is the actual parent body for the suite of these meteorites, B filter) and UV (0.17–0.32 µm range) observations of Vesta which represent about 6% of all meteorites falling on Earth. with IUE. They found at all rotational phases small-ampli- As such, Vesta would represent one of the few known solar tude colored features interpreted as part of a progressive system bodies for which actual rock samples are available extended color change that renders the hemisphere before in terrestrial laboratories. the lightcurve maximum bluer than the other one. Vesta was also observed by ISO with SWS (Heras et al., The most impressive asteroidal work done with HST has 2000) and PHT-S (Dotto et al., 2000). Heras et al. (2000) been the observation of 4 Vesta. A trio of papers by Zellner analyzed rotationally resolved SWS spectra between 19.4 et al. (1997), Thomas et al. (1997a), and Binzel et al. (1997) and 27.6 µm and identified spectral features associated with introduced a systematic analysis of Wide Field Planetary the presence of olivine and pyroxene silicate on the surface Camera data that showed a small dark spot followed by a of this object. They concluded that olivines are dominant on generally darker region that moved across the disk as the the eastern hemisphere, while pyroxenes, which are evident asteroid rotated. The spin pole, size, and shape of the aster- at every rotational aspect, become dominant on the west- oid were constrained, and images across the visible wave- ern hemisphere. Dotto et al. (2000) conducted a detailed length region and into the near-infrared were analyzed to investigation of the spectral features of the 5.8–11.6-µm show mineralogical variegation. Interestingly, this variega- PHT-S spectrum. To model the thermal continuum they tion did not closely match the albedo variation (compare used the TPM described earlier. The comparison between the upper and lower panels of Plate 3): The 0.439-µm map the obtained spectrum and laboratory spectra of known shows a dark spot and a dark region that follows it in rota- minerals and meteorites shows that the observed structure tion, while the mineralogical map shows that these are part around 9.1 µm seems to be compatible with the presence of a larger region with a depressed visible continuum and of olivines on the surface of Vesta. This result is compat- a fairly wide 1-µm silicate band. The data came from HST ible with the data obtained by Gaffey (1997), who suggested observations carried out with the original (aberrated) HST that the surface of Vesta is composed by a dark howardite optics, which smearing when coupled with the changes in or polymict eucrite assemblage. Bright regions on one hemi- resolution over the factor of 2.4 wavelength range may ac- sphere would be produced by impacts that exposed brighter count for the lack of a definitive correlation between albedo diogenite and olivine. and mineralogy. Coupled with the time of observation (the Asteroid 10 Hygiea is the fourth biggest asteroid with subobserver latitude was +20°), albedo and compositional an IRAS diameter of 407 km and an albedo pV = 0.07 maps were made of only the region between +50° and –10° (Tedesco et al., 1992). Hygiea rotates around its principal latitude. Observations of 4 Vesta at perihelion were made axis in a retrograde sense with a period of 27.63 ± 0.02 h using WFPC2. This correction, coupled with a more equa- (Michalowski et al., 1991; Erikson and Magnusson, 1993; torward subobserver latitude, gave us the first good view of López-González and Rodríguez, 2000). Ragazzoni et al. the southern hemisphere, reported by Thomas et al. (1997b). (2000) found a shape with an average diameter of 444 ± On the basis of these images, Thomas et al. (1997a) derived 35 km and a semimajor axis ratio a/b = 1.11 using speckle a shape fit by an ellipsoid with semiaxes of 289, 280, 229 interferometry. Hygiea is classified as a C-type asteroid. (±5) km. Deconvolution of these images, as well as shape Aqueous alteration products have been detected on its sur- fitting to the raw images, showed a large mountain on the face by Jones et al. (1990) and Vilas (1994) based on ab- south pole of the asteroid, surrounded by a ring of moun- sorption features at 3.0 µm and 0.7 µm. Other observations tains over halfway to the equator from there (see Plate 7). in the visible region, performed at different epochs, did not This was interpreted as a huge crater (diameter 460 km) detected the 0.7-µm absorption feature (Bus, 1999; For- with a large central peak [6 km above the reference spher- nasier et al., 1999). These results may imply composition oid used by Thomas et al. (1997b) or about 18 km above variation in the surface, not confirmed by Mothé-Diniz et the bottom of the crater]. A few smaller depressions in the al. (2001). topography may be craters as well. Thomas et al. (1997b) HST observations of Hygiea have been presented and interpret trends in the brightness ratios across the rim and discussed by Storrs et al. (1999a). They saw only one side Dotto et al.: Observations from Orbiting Platforms 231

1.6 teorite types. From the comparison of the Hygiea spectrum with the meteorite emissivities it has been found that the feature of Hygiea at about 9.3 µm seems to be consistent with the Christiansen peak of Ornans and Warrenton. The Hygiea analogy with these meteorites is also supported by the com- 1.4 parison of the transparency features around 13 and at 26 µm. These CO3 meteorites seem to show the presence of aqueous alteration processes (Zolensky and McSween, 1988), even if the possible products of aqueous alteration within Warrenton CO have received limited attention. In fact, al- 1.2 by ASTER though present in these meteorite classes, they are not as pervasive as in the CIs and CMs. If 10 Hygiea is really compatible with CO meteorites, Emissivity this would imply that it is a “primitive” object that has Ornans undergone some metamorphism. 1.0 by Barucci et al. (2002) 5. CONCLUSIONS AND FUTURE WORK

In the last decade orbiting platforms have provided im- portant information about the physical properties of aster- 0.8 oids. The data on asteroids collected by ISO, because of their uniqueness and ISO’s wide spectroscopic possibilities, have provided important information on the asteroid sur- 5 10 15 20 25 face composition. A large sample of infrared data of aster- Wavelength (µm) oids up to 200 µm has been collected and several diameters and albedos have been computed. Fig. 5. Comparison between PHT-S and SWS spectra of 10 Hy- Important results have been obtained by HST, even if giea and laboratory spectra of Ornans (by Barucci et al., 2002) and many of the possibilities mentioned by Zellner et al. (1989) Warrenton (by the ASTER database at http://speclib.jpl.nasa.gov). have not yet been accomplished. For example, the high- The spectra are vertically offset for clarity (from Barucci et al., resolution mapping discussed by Zellner et al. has only been 2002). done for 4 Vesta. Programs are currently underway to study the largest main-belt asteroids and transneptunian objects. Several instruments that may be useful for asteroidal obser- of the asteroid (during one HST visibility period of about vations (e.g., the Advanced Camera for Surveys, the Wide 45 min). They observed no surface markings, and did not Field Camera 3, and the NICMOS Cooling System) are detect any companion down to a limit seven magnitudes planned for installation in the near future. In particular, the fainter than the primary asteroid. The WFPC2 images were Wide Field Camera 3, scheduled to replace WFPC2 in 2003, circular, and some significant brightness changes during the will have both CCD detectors for the UV and visible wave- sequence of exposures suggest rotation in agreement with length region, as well as infrared sensitive detectors to give the published rotational solutions. Further analysis of the it coverage from 0.2 to 1.8 µm. Combination studies with data is in progress. both the visible and infrared channels will allow complete The ISO spectroscopic observations of Hygiea have been coverage in both imaging and low-resolution spectroscopy performed with PHT-S and SWS up to 11.6 and 45 µm re- over the entire 1-µm “silicate” feature, and part of the 2-µm spectively (Barucci et al., 2002). TPM has been used to feature. model the thermal continuum. There were no matches be- Orbiting platforms, whose advantage is the ability to ob- tween the observed spectra and those of minerals obtained serve outside the terrestrial atmosphere, gave us access to a in the laboratory. The comparison with meteorite spectra wide range of wavelengths and provided the possibility of shows a similarity to spectra of the carbonaceous better analyzing the physical characteristics of asteroids. meteorites. This confirms previous work in which C-type The identification of minerals on the surface of an asteroid is asteroids were associated with this type of meteorite. The made easier by the analysis of their spectral features occurr- comparison with all the available laboratory spectra (Barucci ing at different wavelengths. Although this analysis is nei- et al., 2002) shows some similarities between Hygiea and ther easy nor unique and further uncertainties are introduced Ornans and Warrenton (CO3 meteorites) at small grain size by thermal models and laboratory experiments, the increase (Fig. 5). In this spectral range the most diagnostic feature in the amount of collected data obliges us to improve our is the Christiansen feature. As shown by Salisbury et al. techniques to interpret the obtained information, and this (1991b), the Christiansen feature of carbonaceous chon- is a challenging task for solving the puzzle of physical prop- drites occurs at a longer wavelength than for the other me- erties of asteroids. 232 Asteroids III

Several new space platform projects (e.g., SIRTF, of asteroid 243 Ida from Dactyl’s orbit. Nature, 374, 785–788. ASTRO-F, FIRST-Herschel, NGST, and ) are planned Belton M. J. S., Mueller B. E. A., D’Amario L. A., Byrnes D. V., to be operational within the near future and will allow us Klaasen K. P., Synnott S., Breneman H., Johnson T. V., Tho- to update our knowledge of the dynamical and physical mas P. C., Veverka J., Harch A. P., Davies M. E., Merline W. J., properties of asteroids. The Space Facil- C. R., Davis D., Denk T., Neukum G., Petit J.-M., Greenberg R., Storrs A., and Zellner B. (1996) The discovery ity (SIRTF) (http://ssc.ipac.caltech.edu), which is the final and orbit of 1993 (243)1 Dactyl. Icarus, 120, 185–199. element in NASA’s “Great Observatories” program, is Binzel R. P. and Xu S. (1993) Chips off of asteroid 4 Vesta — scheduled for launch in December 2002. It will perform Evidence for the parent body of basaltic achondrite meteorites. spectroscopy and radiometry of small bodies of the solar Science, 260, 186–191. system from the asteroid main belt, through the Trojan Binzel R. P., Gaffey M. J., Thomas P. C., Zellner B. H., Storrs clouds, to the Kuiper Disk and . The Herschel Space A. D. and Wells E. N. (1997) Geologic mapping of Vesta from Observatory (http://astro.estec.esa.nl/), the European Space 1994 Hubble Space Telescope images. Icarus, 128, 95–103. Agency’s fourth “Cornerstone Mission,” will perform pho- Bus B. (1999) Compositional structure in the : Results tometry and spectroscopy in the 60–670-µm range. Its of a spectroscopic survey. Ph.D. thesis, Massachusetts Institute launch is scheduled for early 2007 and it is planned as a of Technology, Cambridge.. three- mission. The Next Generation Space Telescope Campins H., McCarthy D., Kern S., Weaver H. A., and Brown R. H. (1999) The 1–2.5 micron spectrum of 3200 Phaethon (NGST) (http://www.ngst.nasa.gov/) is scheduled to be observed with HST’s NICMOS. Bull. Am . Soc., 31, launched in 2009 in a L2 orbit to replace HST. It will be a 1121. 6-m-class telescope equipped with cameras and spectro- Cellino A., Pannunzio R., Zappalà V., Farinella P., and Paolicchi graphs working between 0.6 and 28 µm. A very promising P. (1985) Do we observe light curves of binary asteroids? future space platform is the ESA space astrometry mission Astron. Astrophys., 144, 355–362. GAIA (http://astro.estec.esa.nl/), which is scheduled to be Cellino A., Zappalà V., and Farinella P. (1991) The size distribu- launched around 2010–2012 and will be operation for five tion of main-belt asteroids from IRAS data. Mon. Not. R. years. Present estimates are that GAIA will detect between Astron. Soc., 253, 561–574. 105 and 106 asteroids, compared with about 20,000 cur- Cesarsky C. J. 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