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-Planck-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 asteroids without limitation by Earth’s atmosphere. Several Earth-orbiting observatories have been successfully operated in the last decade, obtaining unique results on asteroid physical properties. These include the high-resolu- tion mapping of the surface of 4 Vesta and the first spectra of asteroids in the far-infrared 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 Planet Survey (Tedesco et al., In the last few decades the use of space platforms has 1992) and the Supplemental IRAS Minor Planet 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 day, every day, without moonlight and/or IRAS albedo 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 albedos 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 United States, United Kingdom, and Netherlands. 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 most 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 water escaping from the surface of 1 Ceres. 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 solar 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 frost 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 4 Vesta 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 Astrometry 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 stars. 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 planets 29 months. 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’ masses (Viateau and Rapaport, 1998; parts of the telescope to temperatures 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 orbital elements 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 years 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 Space Telescope (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: photometry 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).