Spacecraft Exploration of Asteroids

Transcription

1 Solar System Research, Vol. 39, No. 1, 2005, pp Translated from Astronomicheskii Vestnik, Vol. 39, No. 1, 2005, pp Original Russian Text Copyright 2005 by Shevchenko, Mohamed. Spacecraft Exploration of Asteroids V. G. Shevchenko 1 and R. A. Mohamed 2 1 Institute of Astronomy of Kharkiv National University, Sumska ul. 35, Kharkiv, Ukraine 2 Department of Physics, Faculty of Science, University of Garyounis, Benghazi, Libya Received February 12, 2004 Abstract The past, current, and planned space missions for asteroid exploration are reviewed. The main results based on observations performed with satellites in near-earth orbits (OAO-2, IUE, FIRSSE, IRAS, HST, Hipparcos, ISO, MSX) and space probes sent to particular objects (Galileo, NEAR, DS1, Stardust) are reported. Future space missions (MUSES-C, Rosetta, DOWN, etc.) and their main goals in asteroid study are considered. The feasibility of using spacecraft for minor-body exploration is discussed. INTRODUCTION Space missions have been providing an ever-growing amount of physical, chemical, mineralogical, and morphological data about asteroids in recent years, although ground-based observations will long remain primarily because of their long-term nature and low cost the main method of asteroid study. This trend has become especially conspicuous in the last decade of the past century and in the beginning of this century. Interest in the study of the asteroid belt has been due not only to the role of asteroids in the solution of fundamental problems (the origin of minor bodies; their role in the formation of the Solar System; relationships between asteroids, comets, meteors; etc.); it is also stimulated by purely applied problems to be solved (danger from asteroids and comets, sources of extraterrestrial mineral resources, future space bases, etc.), all the more so as the solution of these applied problems is already on the agenda. The directions of space-based studies of minor bodies of the Solar System (observations from near-earth orbit or space missions to particular objects) change correspondingly. A space mission to objects assumes the comprehensive study of one or two or three objects (e.g., in the case of Galileo and NEAR), whereas studies from onboard a spacecraft in a near-earth orbit involve observations of several hundred or even several thousand objects (e.g., the IUE, IRAS, and MSX satellites). In this paper we review the tasks already solved by completed space missions and those to be performed in the near future. These missions were carried out by the American, European, and other space agencies and/or in close collaboration between them. The table lists the spacecraft used for asteroid studies, their launch dates, wavelength intervals, and the main results. OBSERVATIONS FROM NEAR-EARTH ORBIT The first spaceborne observations of asteroids were made in 1971 onboard the American OAO-2 satellite (Orbital Astronomical Observatory 2) (Caldwell, 1975). The satellite was put into orbit in December 1968 and was equipped with detectors operating at UV wavelengths. It observed only the three major asteroids (1 Ceres, 2 Pallas, and 4 Vesta) to study the reflectance of their surfaces in four spectral bands centered at 2590, 3075, 3360, and 4300 Å. The results of these observations made it possible to determine the albedos of the above asteroids in the spectral bands considered and found their reflectance to increase linearly with wavelength. Later, spaceborne observations of asteroids were also continued at UV wavelengths by the IUE (International Ultraviolet Explorer) satellite, which was launched into a near-earth orbit in January The satellite was equipped with a 45-cm telescope and spectrographs covering the wavelength interval Å with a resolution of 2 3 Å. During the 18 years of its operation (the satellite was decommissioned in the end of 1996 (Stickland, 1996)), it obtained the spectra of more than 100 asteroids (Veeder et al., 1980; Butteworth and Meadows, 1985; Festou et al., 1991; A Hearn and Feldman, 1992; Brosh, 1995, etc.). It observed not only the major asteroids (e.g., 1 Ceres, 2 Pallas, 3 Juno, and 4 Vesta), but also Earth-approaching asteroids (1566 Icarus, 2201 Oljato, 4015 Wilson Harrington, and 4179 Toutatis) with diameters of 2 4 km. For asteroid 4 Vesta, IUE obtained the light curves in two spectral interval, namely, Å and Å, which covered the entire period of axial rotation (Festou et al., 1991). Each of these light curves shows one maximum and one minimum in the period of axial rotation (like in the visible part of the spectrum) with an amplitude of 0.10 m, thereby corroborating the presence of albedo spots on the surface of Vesta. Roettger and Buratti (1994) determined the albedos of 45 asteroids of various composition types in wavelength bands centered at 2450, 2670, 2950, and 3150 Å. Unlike what we see in the visible part of the spectrum, the mean albedo in these wavelength bands was found to be somewhat higher for M-type (metallic) asteroids /05/ Pleiades Publishing, Inc.

2 74 SHEVCHENKO, MOHAMED Spacecraft used to study asteroids Spacecraft Launch date Wavelength interval, µm Main results OAO UV albedos are determined for three asteroids IUE UV albedos are determined for 48 asteroids and the UV light curve is obtained for 4 Vesta FIRSSE , 27, 85 Fluxes are measured at the given wavelengths and thermal models of asteroids are refined IRAS , 25, 60, 100 Albedos and diameters are determined for 2228 asteroids Hipparcos Orbits of selected asteroids are refined Galileo The albedos, sizes, shapes, and coordinates of the rotation axes are determined for Gaspra and Ida. A satellite of Ida is discovered HST The albedos and sizes of some asteroids are determined and detailed images of Ceres and Vesta are obtained ISO Far-IR fluxes are measured and the thermal inertia of asteroids is estimated MSX Fluxes are measured in a wide wavelength interval and the albedos and diameters are determined for 168 asteroids NEAR The albedos, sizes, shapes, pole coordinates, and densities of Mathilde and Eros are measured. Spacecraft landed on the surface of Eros DS The size and albedo of asteroid Braille are determined Stardust The albedo, size, and shape of the asteroid Annefrank are determined compared to S-type (silicate) asteroids; i.e., the reflectance of M-type asteroids increases more slowly with wavelength than that of S-type asteroids. On January 23, 1982, the FIRSSE (Far-Infrared Sky Survey Experiment) satellite observed 20 main-belt asteroids in the IR (Levan and Price, 1984). Thermal radiation of these asteroids in spectral bands centered at 20 and 27 µm agreed with the gray -body model; N Albedo Fig. 1. Histogram showing the distribution of asteroid albedos (based on the data of Tedesco et al. (2002)). however, the measured 85-µm fluxes were found to be two to three times lower than is implied by an extrapolation of Planck s law. These data were later used to develop a more reliable thermophysical model of asteroids. IRAS (Infrared Astronomical Satellite) was the next satellite to observe asteroids in the infrared. It operated in orbit from January 25 through November 28, 1983, making observations in bands centered at 12, 25, 60, and 100 µm (Bender and Tedesco, 1986; Matson and Tedesco, 1992). During its operation the satellite transmitted data for 3318 numbered and 135 unnumbered asteroids. The analysis of this extensive data allowed the diameters and albedos to be determined for a total of 2228 asteroids (Tedesco et al., 2002a). Figure 1 shows the histogram of the distribution of asteroid albedos based on the IRAS data. As is evident from the figure, the maximum of the distribution is in the domain of low-albedo asteroids. This is so far the most extensive set of data on asteroid albedos and diameters, which exceeds that of the results of ground-based determinations by more than one order of magnitude. These data will long be used as the principal source of main physical parameters of asteroids, despite the fact that they are affected by systematic errors (Lupishko, 1998) resulting from an unfortunate choice of the asteroid thermal model and a use of absolute magnitude that is not entirely correct. Gaffey (1989) analyzed IRAS data in order to estimate the surface metal content of S asteroids from the 12/25 µm flux ratios. He showed that the flux ratios for S and M asteroids are systematically higher than those for asteroids of other composition types. The flux ratios

3 SPACECRAFT EXPLORATION OF ASTEROIDS 75 Fig. 2. Images of asteroid 4 Vesta during the opposition of 1996 (Thomas et al., 1997). for S asteroids span the same interval as those of M-type asteroids. It follows from this that an analysis of the above fluxes alone does not make it possible to identify purely metallic asteroids, and additional data in other wavelength intervals are required to this end. The Hipparcos astrometric satellite observed asteroids in order to refine their orbits and to test the methods used for orbit computation. Hipparcos data allowed the astrometric positions for 48 asteroids to be obtained with errors no greater than arcsec (Hestroffen and Morando, 1995). These results showed that, when determining asteroid positions with such accuracy, one has to take into account the displacement of the photocenter relative to the geometric center of the photometric model in the cases of observations performed at nonzero phase angles (Lupishko et al., 2002). HST (Hubble Space Telescope) carries out extensive observations of asteroids. This telescope was put into a near-earth orbit on April 24, Although its primary targets are distant astrophysical objects, HST has also been used successfully to study Solar-System objects. Thus, HST observed 12 major asteroids (Storrs et al., 1994) in order to reveal their eventual satellites. No satellites were found for imaged asteroids with resolved disks. High-angular-resolution images of asteroid 1 Ceres were obtained in photometric bands centered at 1621, 2795, and 3636 Å, and the albedo distribution over the surface of this asteroid was mapped (Landis et al., 1998; Parker et al., 2002). These observations revealed a large formation on the surface of Ceres in the vicinity of the central meridian. It appears to be a crater (called Piazzi) with a diameter of about 250 km. The albedo in the three photometric bands mentioned above was found to be 0.090, 0.029, and 0.056, respectively. HST also observed asteroid 4 Vesta during the oppositions in 1994 and 1996 (Binzel et al., 1997; Thomas et al., 1997; Zellner et al., 1997). On the images taken in 1994, Vesta appears as an irregular-shaped object with a large albedo spot on its surface (now referred to as Olbers) with a diameter of about 200 km. The contrast between the spot and the main surface of Vesta reaches 20%. The period of axial rotation of the asteroid was found to be 5.34 h, thereby corroborating the results of ground-based observations. The HST observations also allowed the size of the asteroid to be refined (the semiaxes of the approximating ellipsoid are equal to km), yielding an estimate of 3.8 ± 0.6 g/cm 3 for its mean density. Maps of albedo and color distributions over the surface of Vesta were constructed, which allowed the identification of 19 gradations of mineralogical formations. It is possible that the western hemisphere is composed of pyroxene enriched in iron and calcium. Observations made by HST in 1996 (see Fig. 2), when Vesta was closest to the Earth, resolved landscape details including the giant crater in the circumpolar region of the asteroid with a diameter of 460 km (Binzel et al., 1997; Thomas et al., 1997). It is assumed that Vesta may owe its fast rotation to the impact process that produced this crater. The Hubble Space Telescope also observed two Centaur asteroids 2060 Chiron and 5145 Pholus (Meech et al., 1994; Meech and Veawer, 1996). These asteroids in the aphelion are beyond the orbit of Saturn. One of the aims of these observations was to determine whether these objects are comets or asteroids. For Pholus, the images taken on April 28, 1992 revealed no coma. HST observed Chiron on February 22 and 23 and March 8, 1993 (Meech et al., 1994). Analysis of these images revealed azimuthal structures in the outer coma at a distance of 0.2 arcsec from the nucleus. Subsequent

4 76 SHEVCHENKO, MOHAMED 243 Ida 951 Gaspra Fig. 3. Images of the asteroids 243 Ida and 951 Gaspra obtained by the interplanetary space probe Galileo (Veverka et al., 1994; Belton et al., 1996). observations of Chiron made on January and April 13, 1996 in the wavelength interval Å found no cometary emissions in the coma. Observations also yielded a 4-h-long UV light curve and an albedo estimate of p = 0.08 ± 0.01 for the adopted diameter of D = 180 km. Thus, the question of whether Centaur asteroids are cometary objects remains open and requires further investigation. As a whole, HST observed more than 60 asteroids during its operation (Dotto et al., 2002), and its observing programs aimed at the study of asteroids are still in progress. On November 17, 1995, the European Space Agency launched the ISO (Infrared Space Observatory) satellite (Müller, 2002). This satellite was equipped with several instruments (photopolarimeter, high- and low-resolution spectrometers, etc.) covering the wavelength interval µm. The ISO satellite observed about 40 asteroids during the three years of its operation (Müller, 2002). Observations were performed within the framework of several programs. Several asteroids (1 Ceres, 2 Pallas, 3 Juno, 4 Vesta, 6 Hebe, 9 Metis, 10 Hygiea, etc.) were observed for subsequent use as photometric and polarimetric standards in the far infrared (Cohen et al., 1998; Müller and Lagerros, 1998; Lagerros et al., 1999). Asteroids 1980 Tezcatlipoca, 3200 Phaethon, 3671 Dionysus, etc. were investigated for their eventual cometary activity (Harris and Davies, 1999). For some of the asteroids, thermophysical models were developed and the thermal inertia of the regolith was estimated, which was found to be 5 25 cal m 2 s 0.5 K 1. The thermal inertia of the surface of Ceres is equal to 15 cal m 2 s 0.5 K 1, which is almost three times less than that of the Moon (Müller and Lagerros, 1999). The MSX (Midcourse Space Experiment) satellite was launched on April 24, 1996 (Price et al., 1997, 2001). Instruments onboard this satellite make it possible to perform observations in a wide wavelength interval ranging from the ultraviolet to the far infrared. The satellite observed more than 1000 asteroids both in the wavelength interval µm and in the UV part of the spectrum (Tedesco et al., 2001). So far, albedos and diameters have been determined for a total of 168 asteroids (Tedesco et al., 2002b). SPACE MISSIONS TO ASTEROIDS Galileo, which was launched from the Atlantis space shuttle to study the system of Jupiter (Yeomans et al., 1993), was the first mission to asteroids. The trajectory of the spacecraft was chosen so as to make it twice approach the Earth (in 1990 and 1992) and reach and cross the asteroid belt after the first and second gravity-assist maneuvers, respectively. Two intermediatealbedo S-type asteroids 243 Ida and 951 Gaspra were selected as the mission target objects. The first encounter occurred on October 29, 1991 with asteroid 951 Gaspra. The spacecraft took a total of 57 images

5 SPACECRAFT EXPLORATION OF ASTEROIDS Mathilde 433 Eros Fig. 4. Images of asteroids 253 Mathilde and 433 Eros obtained by the interplanetary space probe NEAR-Shoemaker (Cheng, 2002). with a best resolution of 54 m per pixel. Veverka et al. (1994) analyzed the images to determine the size ( km), surface albedo (0.23), pole coordinates, and the right ascension of the rotation axis of the asteroid. The asteroid appears as a very irregularly shaped object (see Fig. 3) with craters and grooves on its surface, and it may be a fragment of a bigger body that broke into pieces as a result of a collision. An analysis of Gaspra images also yielded a depth-to-diameter ratio of 0.14 for the craters of this asteroid. This is lower than the corresponding ratio for the Moon, Mars, and Phobos (0.2). This result is indicative of greater regolith depth on the surface of Gaspra compared to other bodies. The regolith layer of Gaspra soil is composed of olivine and orthopyroxene in a proportion of 9 : 1 (Veverka et al., 1994). The encounter between the Galileo spacecraft and Ida took place on August 28, Galileo passed within km of Ida at the time of closest approach, and the highest resolution of images obtained was 25 meters per pixel. Analysis of the images (Belton et al., 1996) yielded the size of the asteroid ( km), pole coordinates, surface albedo (0.21), mass (4.2 ± g), density (2.6 ± 0.5 g/cm 3 ), and some other physical properties of the body. The object was found to have an irregular shape (see Fig. 3) with numerous craters on its surface indicating the considerable impact history of the asteroid. Unlike Gaspra, Ida has recently formed craters. The greatest crater, Lasnaux, has a diameter of 11.8 km. Ida was found to possess a satellite, which was called Dactyl. It is a small body, with a size of only 1.4 km, that is located at a distance of 85 km from the main asteroid. Dactyl s albedo, 0.20, is almost the same as that of Ida; however, the color indices of the two asteroids differ. Dactyl was the first asteroid satellite discovered using cosmic methods. The second spacecraft sent to asteroids was called NEAR (Near-Earth Asteroid Rendezvous) and was launched on February 17, 1996 (Cheng et al., 1997; Cheng, 2002). This spacecraft was later (upon reaching Eros) named NEAR-Shoemaker after Eugene Shoemaker ( ), a famous American planetologist. The primary aim of launching this spacecraft was to study the Earth-approaching asteroid 433 Eros, with an additional goal of investigating the low-albedo mainbelt asteroid 253 Mathilde. The trajectory of the spacecraft was chosen so as to make it first encounter asteroid 253 Mathilde on July 26 27, 1997 (see Fig. 4). The spacecraft passed within 1212 km of the asteroid and took more than 500 images of the body. Analysis of these images (Veverka et al., 1999; Clark et al., 1999) yielded the albedo (0.036), size ( km), and shape of Mathilde. The color indices of the surface of the asteroid are similar to those of CM-type carbonaceous chondrites. No albedo or color variations over the surface were found. The images also illustrate the importance of collision processes in the asteroid belt and the role these processes play in shaping asteroids. The surface of Mathilde was found to host more than four craters with diameters exceeding the mean radius of the asteroid (Thomas et al., 1999). This is so far the largest number among the asteroids studied from space.

6 78 SHEVCHENKO, MOHAMED The greatest crater (with a diameter of 33 km) was named Karoo. The deviations of the spacecraft trajectory caused by the attraction of the asteroid were used to estimate the mass ( g) and density (1.3 ± 0.2 g/cm 3 ) of Mathilde (Yeomans et al., 1997). This asteroid is now considered to be one of the darkest and lowest density objects in the Solar System. However, the principal aim of this space mission was one of the biggest Earth-approaching asteroids, Eros. The first encounter between the spacecraft and Eros took place on December 22 23, The spacecraft passed within 4100 km of Eros and transmitted more than 1000 images of the asteroid, with a best resolution of 500 m. On the images, Eros appears as an elongated irregularly shaped object and is likely to be a fragment of a bigger body that was disrupted as a result of an impact. These are the first images of an Earthapproaching asteroid that crosses Earth s orbit and poses a potential threat of colliding with the Earth. The main encounter with Eros took place on February 14, On February 17, the spacecraft began to orbit the asteroid, and since then it studied the object in detail for a year. During its operation, the spacecraft was at a height of km above the surface of Eros. It determined the main physical properties of the asteroid: its mass ( g), density (2.67 ± 0.03 g/cm 3 ), size ( km), pole coordinates (λ = 17.24, β = 11.35), period of axial rotation ( h), and albedo (0.29) (Cheng, 2002; Domingue et al., 2002; Miller et al., 2002; Thomas et al., 2002). The average density of Eros is lower than the average volume density of ordinary chondrites and, thus, indicates that the asteroid is made of more porous material. A small offset of the center of mass relative to the center of figure points to the presence of a regolith layer with a depth of up to 100 m. On the whole, Eros is a single solid body with a thermally heated surface, in contrast to the earlier suggested hypothesis that this asteroid may consist of small gravitationally bound bodies ( rubble piles ). The images transmitted (Fig. 4) show that, despite its irregular shape Eros has, unlike other asteroids, a round-shaped surface with surprisingly smooth portions (Fig. 3). The surface of Eros was found to be covered not only by craters but also by various ridges, troughs, depressions, individual stone blocks, and stones. The largest (10-km wide, saddle-shaped) depression was named Himeros, and the greatest crater (with a diameter of 5 km) was called Psyche. Craters on the surface of Eros are shallower than lunar craters of the same diameter. According to the results of x- and gamma-ray spectroscopy, the surface of Eros is composed of material whose meteorite analogs are ordinary chondrites. The spacecraft determined the elemental ratios Fe/Si, Al/Si, Mg/Si, Fe/O, Si/O, etc., which confirm the silicate mineralogy of Eros. On February 12, 2001, NEAR-Shoemaker transited from a circular orbit to a descent trajectory and, after 4.5 h, landed successfully on the surface of Eros. The last image of the surface was taken from a distance of 120 m. After landing, the spacecraft transmitted for seven days the results of gamma spectrometry directly from the surface of Eros. It was the first landing of a space probe on the surface of such a small cosmic body. The enormous amount of data transmitted by the space probe will be long used by specialists in various fields of science for detailed analyses. The DS1 (Deep Space 1) spacecraft was launched on October 24, 1998 toward comet Borelly (Huntress, 1999; Farquhar et al., 2002). On July 1999, the spacecraft passed within km of the asteroid 9969 Braille (1992 KD) and took images and spectra of this object. The spectrum of the asteroid is similar to that of the asteroid 4 Vesta, its maximum size corresponds to 2.1 km, and its albedo is equal to 0.34 (Soberblom et al., 1999; Buratti et al., 2004). The Stardust mission was launched on February 7, 1999 toward comet 81P/ Wild-2 with the aim of collecting a sample of the comet s coma and returning it to the Earth (Huntress, 1999). On November 2, 2002, the spacecraft passed within km of the asteroid 5535 Annefrank. The images obtained made it possible to determine the size (average diameter, 5 km), albedo (0.24), and the magnitude phase dependence in the phase-angle interval (Newburn et al., 2003). It is the first phase dependence for asteroids obtained for such large phase angles. PLANNED SPACECRAFT ASTEROID STUDIES Currently, several space projects aimed at the investigation of asteroids are at the final stage of preparation. These are, primarily, the joint Japanese American MUSES-C project, the Rosetta project of the European Space Agency, and the American project DOWN. The MUSES-C satellite was launched in March The Institute of Space and Astronautical Science (Japan) in cooperation with the Jet Propulsion Laboratory (USA) sent this probe to the asteroid Itokawa (1998 SF36) (Huntress, 1999; Farquhar et al., 2002; Yano et al., 2002). It is a small asteroid with a diameter of about 500 m, which belongs to the group of Earth-approaching asteroids. The space probe will encounter the asteroid in the summer of 2005 and will remain in its vicinity for two months in order to thoroughly investigate the body (determine its shape, size, volume, density, and rotational properties; measure the elemental and mineralogical composition of its surface; study the geology and morphology of the surface). After its closest approach with the asteroid, the space probe will bombard the surface and collect the ejected soil samples. The space probe will then start moving toward the Earth. The return to Earth is planned for the summer of 2007, when the samples, packed into a capsule, will be dropped to Earth for subsequent analysis in laboratory conditions.

7 SPACECRAFT EXPLORATION OF ASTEROIDS 79 The European space probe Rosetta was launched in early March The principal aim of this mission is to study the Churyumov Gerasimenko comet, which the probe is to encounter in After transiting to a circumcometary orbit, a landing module will be dropped onto the surface of the comet. This landing module will analyze the nucleus of the comet (determine the elemental, molecular, mineralogical, and isotopic composition of the surface, as well as its density, porosity, and thermal properties). Additional targets to be encountered during the asteroid-belt flyby include asteroids 2867 Steins (diameter 10 km, encounter in September 2008) and 21 Lutetia (diameter 96 km, encounter in July 2010). The planned mission program includes the determination of the global parameters of asteroids (shape, size, density, etc.), as well as their dynamic properties, surface morphology, and composition (Hechler, 1997; Huntress, 1999; Farquhar et al., 2002). The preparation of the DOWN space probe is now in progress for its scheduled launch in May 2006 towards two major asteroids: 1 Ceres and 4 Vesta (Farquhar et al., 2002; Russell et al., 2002). The space probe is to reach Vesta in July 2010, orbit it for 11 months, and then fly to Ceres (to be reached by August 2014). Research programs include the thorough investigation of these two asteroids, the determination of their main physical parameters (mass, size, density, etc.) and the elemental and mineralogical composition of their surfaces, and an analysis of their tectonic and impact histories and the early histories of their evolution. Several other space programs aimed at the investigation of asteroids have been discussed intensely in recent years. First and foremost is a project aimed at the investigation and discovery of Earth-approaching asteroids and comets that pose potential collision threats. This project has been called SIMONE (Smallsat Intercept Missions to Objects Near Earth) (Ball et al., 2002). Another project under development is a mission aimed at investigating the internal structure of Earthapproaching asteroids, including a possible landing of research modules (ISHTAR Internal Structure High- Resolution Tomography by Asteroid Rendezvous, D Arrigo et al., 2002). The next project, named BERING, envisages the study of small (sub-kilometersized) asteroids of the main belt (Haack et al., 2002). The particular targets of these missions will be chosen at the final stage of their development. CONCLUSIONS Asteroids have thus been studied for more than 35 years using 12 spacecraft. Most of these studies were observations made from satellites in near-earth orbits with the aim of measuring the reflectance of asteroid surfaces in the spectral intervals where groundbased observations are difficult or impossible to perform because of atmospheric absorption. These observations yielded, in particular, far-uv and IR albedos for selected asteroids (1 Ceres, 2 Pallas, 4 Vesta, etc.), which allowed the researchers to make assumptions about the mineral composition and thermal properties of the surfaces of these bodies. It goes without saying that long-term observations of asteroids (aimed at determining their rotation periods, pole coordinates, phase variations of light and polarization) will long remain ground-based. However, space probes must be used to obtain data on diameters and reflectance over the widest possible wavelength interval for the greatest possible number (several thousands) of objects. The data collected by IRAS, MSX, ISO, and other satellites corroborate the feasibility of such investigations. Although these satellites were not specially launched to study minor bodies, and though the latter account only for a small fraction of their operation, the results obtained have significantly expanded our knowledge of the physical properties of these bodies. The data obtained by space probes sent directly to asteroids have yielded very important results. These results contributed to the solution of both purely applied problems (finalization of the methods of gravitational maneuvers for sending space probes to any Solar-System objects, testing new navigation systems, approach and landing of spacecraft on small space bodies, search for objects for future space bases, etc.) and elucidating fundamental scientific issues (formation and evolution of the asteroid belt, Kuiper belt, and the Solar System as a whole). The images of several asteroids obtained by space probes confirmed the hypothesis concerning the crucial role played by collisional evolution in the formation of the surface of asteroids, their shape, and rotation. The density estimates for the S-type asteroids 243 Ida and 433 Eros and the C-type asteroid 253 Mathilde led researchers to conclude that S-type asteroids are monolithic objects that have undergone thermal heating, whereas C-type asteroids may be rubble-pile type objects. The samples taken from the surfaces of asteroid bodies will play a crucial role in our understanding of both the processes that took place during the early stages of the formation of the Solar System and those that take place now. All of this emphasizes the importance and feasibility of studying asteroids with spacecraft. ACKNOWLEDGMENTS We are deeply grateful to Prof. D.F. Lupishko from the Institute of Astronomy of Kharkiv National University for his valuable comments and advice. REFERENCES A Hearn, M.F. and Feldman, P.D., Water Vaporization on Ceres, Icarus, 1992, vol. 98, no. 1, pp Ball, A.J., Green, S.F., and Wells, N.S., SIMONE: Near- Earth Asteroid Rendezvous Microsatellites with Solar- Electric Propulsion, Proc. Asteroids, Comets, Meteors (ACM 2002), Berlin, 2002, pp

8 80 SHEVCHENKO, MOHAMED Belton, M.J.S., Chapman, C.R., Klaasen, K.P., et al., Galileo s Encounter with 243 Ida: An Overview of the Imaging Experiment, Icarus, 1996, vol. 120, no. 1, pp Bender, D.F. and Tedesco, E.F., IRAS Asteroid and Comet Ground-Based Data File, Lunar Planet. Sci. Conf. XVII, 1986, vol. 17, pp Binzel, R.P., Gaffey, M.J., Thomas, P.C., et al., Geologic Mapping of Vesta from 1994 Hubble Space Telescope Images, Icarus, 1997, vol. 128, no. 1, pp Brosh, L., The First UV Spectrum of 2060 Chiron, Mon. Not. R. Astron. Soc, 1995, vol. 286, pp Buratti, B.J., Britt, D.T., Soderblom, L.A., et al., 9969 Braille: Deep Space 1 Infrared Spectroscopy, Geometric Albedo, and Classification, Icarus, 2004, vol. 167, no. 1, pp Butteworth, P.S. and Meadows, A.J., Ultraviolet Reflectance Properties of Asteroids, Icarus, 1985, vol. 62, no. 2, pp Caldwell, J., Ultraviolet Observations of Small Bodies in the Solar System, Icarus, 1975, vol. 25, no. 3, pp Cheng, A.F., Santo, A.G., Heeres, K.J., et al., Near-Earth Asteroid Rendezvous: Mission Overview, J. Geophys. Res., 1997, vol. 105, pp Cheng, A.F., Near-Earth Asteroid Rendezvous: Mission overview, in Asteroids III, Bottke, W.F., Cellino, A., Paolicchi, P., and Binzel, R.P., Eds., Tucson: Univ. Arizona Press, 2002, pp Clark, B.E., Veverka, J., Helfenstain, P., et al., NEAR Photometry of Asteroid 253 Mathilde, Icarus, 1999, vol. 140, pp Cohen, M., Witteborn, F.C., Roush, T., et al., Spectral Irradiance Calibration in the Infrared. VIII Micron Spectroscopy of the Asteroids Ceres, Vesta, and Pallas, Astron. J., 1998, vol. 115, pp D Arrigo, P., Barucci, M.A., and Lagercvist, C.-I., The ISHTAR Mission, Proc. Asteroids, Comets, Meteors (ACM 2002), Berlin, 2002, pp Domingue, D.L., Robinson, M., Carcich, B., et al., Disk- Integrated Photometry of 433 Eros, Icarus, 2002, vol. 155, no. 1, pp Dotto, E., Barucci, M.A., Müller, T.G., et al., Observation from Orbiting Platforms, in Asteroids III, Bottke, W.F., Cellino, A., Paolicchi, P., and Binzel, R.P., Eds., Tucson: Univ. Arizona Press, 2002, pp Farquhar, R., Kawaguchi, J., Russell, C., et al., Spacecraft Exploration of Asteroids: the 2001 Pespective, in Asteroids III, Bottke, W.F., Cellino, A., Paolicchi, P., and Binzel, R.P., Eds., Tucson: Univ. Arizona Press, 2002, pp Festou, M.C., Stern, S.A., and Tozzi, G.P., Asteroid 4 Vesta: Simultaneous Visible and Ultraviolet IUE Observations, Icarus, 1991, vol. 94, pp Gaffey, M.J., The Abundance of Metal on S-Asteroid Surfaces: Indications from IRAS 12 and 25 Micron Flux Ratios, Lunar Planet. Sci. Conf. XX, 1989, vol. 20, pp Haack, H., Michelsen, R., Andersen, A.C., and Jorgensen, J.J., BERING a Deep Space Mission To Study the Smallest Asteroids, Proc. Asteroids, Comets, Meteors (ACM 2002), Berlin, 2002, pp Harris, A.W. and Davies, J.K., Physical Characteristics of Near-Earth Asteroids from Thermal Infrared Spectrophotometry, Icarus, 1999, vol. 142, no. 2, pp Hechler, M., ROSETTA Mission Design, Adv. Space Res., 1997, vol. 19, no. 1, pp Hestroffen, D. and Morando, B., Observations of Minor Planets by Hipparcos, Planet. Space Sci., 1995, vol. 43, pp Huntress, W.T., Mission To Comets and Asteroids, Space Sci. Rev., 1999, vol. 90, pp Lagerros, J.S.V., Müller, T.G., Klaas, U., and Erikson, A., ISOPHOT Polarization Measurements of the Asteroids 6 Hebe and 9 Metis at 25 µm, Icarus, 1999, vol. 142, no. 3, pp Landis, R.R., Stern, A.S., Wood, C.A., and Storrs, A.D., Observations of 1 Ceres with HST Faint Object Camera, Lunar Planet. Sci. Conf. XXIX, 1998, vol. 29, Abstract Levan, P.D. and Price, S.D., 85-µm Fluxes from Asteroids: 2 Pallas, 7 Iris, 15 Eunomia, and 45 Eugenia, Icarus, 1984, vol. 57, no. 1, pp Lupishko, D.F., Improved IRAS Albedo and Diameters of Asteroid, Solar System Research, 1998, vol. 32, no. 2, pp Lupishko, D.F., Shevchenko, V.G., and Tungalag, N., Asteroid Photocentre Displacement: Influence of the Scattering Low, Mem. Soc. Astron. Ital., 2002, vol. 73, no. 3, pp Matson, D.L. and Tedesco, E.F., History, in Infrared Astronomical Satellite Minor Planet Survey Catalog, Tedesco, E.F., Ed., Phillips Laboratory Technical Report, PL-TR , Hanscom Air Force Base, MA, 1992, pp Meech, K.J., Bui, M.W., Samarsinha, N., et al., HST Observations of Chiron s Inner Coma. A Possible Bound Atmosphere, Bull. Am. Astron. Soc., 1994, vol. 26, no. 3, pp Meech, K.J. and Weaver, H.A., Unusual Comets (?) As Observed from the Hubble Space Telescope, Earth, Moon, and Planets, 1996, vol. 72, pp Miller, J.K., Konopliv, A.S., Antreasian, P.G., et al., Determination of Shape, Gravity, and Rotational State of Asteroid 433 Eros, Icarus, 2002, vol. 155, no. 1, pp Müller, T.G. and Lagerros, J.S.V., Asteroids As Far-Infrared Photometric Standards for ISOPHOT, Astron. Astrophys., 1998, vol. 338, pp Müller, T.G. and Lagerros, J.S.V., Fundamental Properties and Thermophysical Modelling Asteroids After ISO, Bull. Am. Astron. Soc., 1999, vol. 31, no. 4, p Müller, T.G., ISO and Asteroids, in Proc. Asteroids, Comets, Meteors (ACM 2002), Berlin, Germany, 2002, pp Newburn, R.L., Duxbury, T.C., Hanner, M., et al., Phase Curve and Albedo of Asteroid 5535 Annefrank, J. Geophys. Res., Ser. E, 2003, vol. 108, no. 11, p Parker, J.Wm., Stern, S.A., Tomas, P.C., et al., Analysis of the First Disk-Resolved Images of Ceres from Ultraviolet Observations with the Hubble Space Telescope, Astron. J., 2002, vol. 123, no. 1, pp Price, S.D., Paxton, L.J., Tedesco, E.F., and Walker, R.G., MSX Observations of the Solar System, Bull. Am. Astron. Soc., 1997, vol. 29, no. 3, pp

9 SPACECRAFT EXPLORATION OF ASTEROIDS 81 Price, S.D., Egan, M.P., Carey, S.J., et al., Midcourse Space Experiment Survey of the Galactic Plane, Astron. J., 2001, vol. 121, no. 5, pp Roettger, E.E. and Buratti, B.J., Ultraviolet Spectra and Geometric Albedos of 45 Asteroids, Icarus, 1994, vol. 112, no. 3, pp Russell, C.T., Coradini, A., Feldman, W.C., et al., DOWN: A Journey To the Beginning of the Solar System, Proc. Asteroids, Comets, Meteors (ACM 2002), Berlin, 2002, pp Soberblom, L., Boice, D., Britt, D., et al., Deep Space 1 MICAS Observations of 9969 Braille, Bull. Am. Astron. Soc, 1999, vol. 31, no. 4, p Stickland, D., Farewell To the IUE, Astron. Now, 1996, vol. 10, no. 10, p. 25. Storrs, A.D., Zellner, B., Wells, E.N., et al., Imaging Observations of Asteroids from the Hubble Space Telescope (HST), Bull. Am. Astron. Soc., 1994, vol. 26, no. 3, p Tedesco, E.F., Price, S., and Egan, M.P., MIMPS, Bull. Am. Astron. Soc., 2001, vol. 33, no. 3, Abstract Tedesco, E.F., Noah, P.V., Noah, M., and Price, S.D., The Supplemental IRAS Minor Planet Survey, Astron. J., 2002a, vol. 123, pp Tedesco, E.F., Egan, M.P., and Price, S.D., The Midcourse Space Experiment Infrared Minor Planet Survey, Astron. J., 2002b, vol. 124, pp Thomas, P.C., Binzel, R.P., Gaffey, M.J., et al., Vesta: Spin Pole, Size, and Shape from HST Images, Icarus, 1997, vol. 128, no. 1, pp Thomas, P.C., Veverka, J., Bell, III J.F., et al., Mathilde: Size, Shape, and Geology, Icarus, 1999, vol. 140, no. 1, pp Thomas, P.C., Joseph, J., Carcich, B., et al., Eros: Shape, Topography, and Slope Processes, Icarus, 2002, vol. 155, no. 1, pp Veeder, G.J., Nelson, R.M., Lane, A.L., et al., Observations of Selected Asteroids with the International Ultraviolet Explorer (IUE), Bull. Am. Astron. Soc., 1980, vol. 12, no. 3, p Veverka, J., Belton, M., Klaasen, K., and Chapman, C., Galileo s Encounter with 951 Gaspra: Overview, Icarus, 1994, vol. 107, no. 1, pp Veverka, J., Thomas, P., Harch, A., et al., NEAR Encounter with Asteroid 253 Mathilde: Overview, Icarus, 1999, vol. 140, no. 1, pp Yano, H., Hasegava, S., Abe, M., and Fujivara, A., Asteroidal Surface Sampling by the MUSES-C Spacecraft, in: Proc. Asteroids, Comets, Meteors (ACM 2002), Berlin, 2002, pp Yeomans, D.K., Chodas, P.W., Keesey, M.S., and Owen, W.M., Targeting An Asteroid: The Galileo Spacecraft s Encounter with 951 Gaspra, Astron. J., 1993, vol. 105, no. 4, pp Yeomans, D.K., Barriot, J.-P., Dunham, D.W., et al., Estimating the Mass of Asteroid 253 Mathilde from Tracking Data During the NEAR Flyby, Sci., 1997, vol. 278, no. 5346, pp Zellner, B., Albrecht, R., Binzel, R.P., et al., Hubble Space Telescope Images of Vesta in 1994, Icarus, 1997, vol. 128, no. 1, pp