r bulletin 108 Figure 1. Asteroid Ida and its moon Dactyl in enhanced colour. This colour picture is made from images taken by the Galileo spacecraft just before its closest approach to asteroid 243 Ida on 28 August 1993. The moon Dactyl is visible to the right of the asteroid. The colour is ‘enhanced’ in the sense that the CCD camera is sensitive to near-infrared wavelengths of light beyond human vision; a ‘natural’ colour picture of this asteroid would appear mostly grey. Shadings in the image indicate changes in illumination angle on the many steep slopes of this irregular body, as well as subtle colour variations due to differences in the physical state and composition of the soil (regolith). There are brighter areas, appearing bluish in the picture, around craters on the upper left end of Ida, around the small bright crater near the centre of the asteroid, and near the upper right-hand edge (the limb). This is a combination of more reflected blue light and greater absorption of near-infrared light, suggesting a difference Figure 2. This image mosaic of asteroid 253 Mathilde is in the abundance or constructed from four images acquired by the NEAR spacecraft composition of iron-bearing on 27 June 1997. The part of the asteroid shown is about 59 km minerals in these areas. Ida’s by 47 km. Details as small as 380 m can be discerned. The moon also has a deeper near- surface exhibits many large craters, including the deeply infrared absorption and a shadowed one at the centre, which is estimated to be more than different colour in the violet than any 10 km deep. The shadowed, wedge-shaped feature at the lower area on this side of Ida. The moon is not identical in spectral right is another large crater viewed obliquely. The angular shape properties to any area of Ida in view here, though its overall of the upper left limb of the asteroid results from the rim of a third similarity in reflectance and general spectral type suggests that it is large crater viewed edge-on. The bright mountainous feature at basically made of the same rock types. the far left may be the rim of a fourth large crater emerging from (Credit: JPL, Galileo mission) the shadow. The angular shape is believed to result from a violent history of impacts. (Credit: Johns Hopkins University Applied Physics Laboratory, NEAR-Shoemaker mission) Figure 4. The ups and downs of Eros. While NEAR- Shoemaker does not directly measure gravity on Eros’ surface, the spacecraft gathers other data that allow scientists to infer this measurement. Radio tracking has been analysed to determine the asteroid’s gravitational ‘pull’ on the orbiting spacecraft. The many images of the asteroid, plus range measurements Figure 3. This picture of asteroid 951 Gaspra is a mosaic of two images from the laser altimeter, taken by the Galileo spacecraft from a range of 5300 km just before closest measure the body’s shape. approach on 29 October 1991. The resolution is about 54 m/pixel. Gaspra is Comparisons of the latter an irregular body with approximate dimensions of 19 x 12 x 11 km. The with the gravitational pull felt portion illuminated in this view is about 18 km from lower left to upper right. by the spacecraft from The north pole is located at upper left; Gaspra rotates counterclockwise different parts of its orbit show every 7 h. The large concavity on the lower right limb is about 6 km across, that the density of the interior the prominent crater on the terminator, centre left, about 1.5 km. A striking must be nearly uniform. The feature of Gaspra’s surface is the abundance of small craters. More than 600 asteroid’s shape, density and of them, 100-500 m in diameter, are visible here. The number of such small spin combine to create a bizarre craters compared to larger ones is much greater for Gaspra than for pattern of what is ‘uphill’ (red previously studied bodies of comparable size such as the satellites of Mars. areas) and ‘downhill’ (blue areas). Gaspra’s very irregular shape suggests that the asteroid was derived from a In this view, a map of ‘gravitational larger body by nearly catastrophic collisions. Consistent with such a history topography’ has been painted onto a is the prominence of groove-like linear features, believed to be related to shape model. A ball dropped onto one of fractures. These linear depressions, 100-300 m wide and tens of metres the red spots would try to roll across the nearest green deep, are in two crossing groups with slightly different morphology, one area to the nearest blue area. group wider and more pitted than the other. Gaspra also shows a variety of (Credit: Johns Hopkins University Applied Physics enigmatic curved depressions and ridges in the terminator region at left. Laboratory, NEAR-Shoemaker mission) (Credit: US Geological Survey, Galileo Orbiter) 38 iso and asteroids ISO and Asteroids T.G. Mueller & L. Metcalfe ISO Data Centre, Space Science Department, ESA Directorate of Scientific Programmes, Villafranca, Spain We are currently at the end of ISO’s 3.5 year Post-Operations Phase Introduction* and the start of the 5 year Active Archive Phase. Final automatic bulk ISO was the world’s first true orbiting infrared processing of the observations has concluded and all observations observatory. Equipped with four sophisticated are publicly available from the ISO archive. and versatile scientific instruments, it provided astronomers with a facility of unprecedented Here, in addressing ISO’s observations of asteroids we attempt a sensitivity and capability for the exploration of complete overview, from the original scientific ideas – to the current the Universe at infrared wavelengths from 2.5 results – to what can be expected in the future. ISO has delivered a to 240 microns. The Solar System programme wealth of new and unexpected results concerning asteroids, but the comprised only about 1.4% of all science archive still has many hidden treasures. In the near future, ISO’s mid- observations made by the ISO mission, and and far-infrared data, in combination with other observations, will the astronomical community’s expectations provide new advances in our understanding of the Solar System and were not very high in relation to asteroids, new insights into the interrelations between meteorites, asteroids, comets and planets. During the last decades comets and interplanetary material, all of which play fundamental many satellites have studied several of our roles in the Solar System’s history and evolution. neighbouring objects in great detail, both in-situ and during close flybys – so what could be expected from a 60-cm telescope circulating just outside the atmosphere? Now, however, the ISO results are on their way to making a big impact on our knowledge of objects on our celestial ‘doorstep’. Spectacular *Units used in this article: results regarding the ‘life-element’ water in the AU = Astronomical Unit, the Universe – not least its unexpected discovery in average distance between Sun and Earth; large amounts in the higher atmospheres of all 1 AU=149 597 870 km. giant planets – were already presented in ESA Jy = Jansky, flux density, Bulletin No. 104, in November 2000. Here we 1 Jy = 1x10-26 Wm-2 Hz-1. Micron: wavelength unit, maintain the Solar System theme, presenting 1 micron = 1x10-6 m. ISO’s results on asteroids. ISO was an ESA spacecraft launched on 19 November 1995 and operated in orbit until 16 May 1998, carrying instruments funded by the Agency’s Member States (especially the countries of the instrument Principal Investigators (PIs): France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA. Additional information on ISO, including results galleries and how to retrieve data, can be found by following links from the ISO Home Page (www.iso.vilspa.esa.es); news items are also posted at www.sci.esa.int. Previous ESA Bulletin articles have addressed: - The ISO Mission – A Scientific Overview (ESA Bulletin No. 84, November 1984) - The First Results from ISO (ESA Bulletin No. 86, May 1996) - Looking Back at ISO Operations (ESA Bulletin No. 95, August 1998) - The ISO Data Archive (ESA Bulletin No. 98, June 1999) - ISO’s Astronomical Harvest Continues (ESA Bulletin No. 99, September 1999) - ISO and Cosmic Water (ESA Bulletin No. 104, November 2000). 39 r bulletin 108 — november 2001 These small pieces of rock (by celestial observations (plus about 4000 calibration standards) have diameters of up to several observations) of targets ranging from objects in hundred kilometres, with the majority orbiting our own Solar System right out to the most between Mars and Jupiter. Some asteroids distant extragalactic sources. In addition to the collisionally and gravitationally scattered into dedicated observations, ISO obtained other Earth-crossing orbits have made it in recent serendipitous data in parallel during slews of the years into the newspaper headlines. The ISO satellite or during long pointed observations. results are of less immediate journalistic interest, but appear more and more in scientific The accuracy of ISO’s pointing system was at journals and at specialised conferences, by the arcsecond level and allowed the tracking of virtue of the fundamental contributions that Solar System targets up to apparent velocities they make to a deeper understanding of these of 120 arcsec/hour with respect to equatorial important constituent bodies of the Solar coordinates. This covered most Solar System System. targets: only a few near-Earth asteroids and comets in certain constellations were too fast Asteroids receive light from the Sun and what and had to be excluded. we see of them through amateur telescopes (and under certain circumstances also with the The data processing and calibration methods naked eye) is purely reflected light.