Status report of the Solar System Working Group F. Mignard 10 June, 2002 Observatoire de la Côte d Azur/CERGA 1 Introduction This text is a compilation of the reports provided by each of the task leaders of the working group. Editing has been kept to minimum meaning that the reports retain the informality of the email exchange on which they are based. This is just provided to keep the members of the working group aware of the current activities and in view of our next meeting in Bordeaux during the second fortnight of October. 2 Asteroids : Detection, Size and Shapes The activities related to this task have been developing in strict collaborations among different groups, due to the intrinsic complementarity of several of the tasks identified by the GAIA SSWG. A simulation of the passages of the Asteroids and NEOs in the fields of view of GAIA has been run by F. Mignard, leading to a realistic estimation of the number of such detections achievable by GAIA as a function of the size and albedo of the objects. The model is based on the most recent assessment of the distribution of the orbital parameters of the NEOs. The group with A. Cellino and A. Dell Oro in Torino, P. Tanga at OCA and D. Hestroffer in Paris is developing a software aimed at computing the signal recorded by the CCDs in the GAIA focal plane, produced by asteroids of different sizes and shapes. The final goal is to obtain estimates of the overall capability of the system to measure asteroid sizes, and to estimate the relative shift of the photocentre of objects observed at different phase angles. Several factors influence the final features of the signal that will be received from asteroidal bodies, including the observing circumstances (phase, aspect and obliquity angles, 1
apparent angular size, scattering of solar light at the surface, overall shape properties). All of the above properties must be extensively modeled. Currently, we are starting to implement different possible scattering laws in the software (which is already capable of producing simulated GAIA signals without taking into account scattering properties), and to test it by means of theoretical results in a number of standard conditions. After this phase is accomplished, and the software is carefully checked, we plan to carry out extensive sets of simulations in which the different parameters affecting the resulting signal will be varied in their reasonable ranges of variation. The final goal is to achieve a satisfactory understanding of the expected performances of GAIA in the measurement of asteroid sizes, and the reconstruction of shapes by means of different observations of the same objects under different observing circumstances. Moreover, the expected photocentre shift will be evaluated, and used by the different GAIA tasks for which this effect is important. In so doing, the Torino team (A. Cellino and A. Dell Oro) is collaborating primarily with Karri Muinonen and Daniel Hestroffer. A meeting between A. Cellino, A. Dell Oro and D. Hestroffer was held in Paris on June 6-7. Further collaborations with M. Kaasalainen and Paolo Tanga are also planned. 3 Photometry and taxonomy Worked out by : Elisabetta Dotto, Vincenzo Zappala, Marco Delbo Reflectance spectroscopy and spectrophotometry provide a powerful tool for determining important aspects of the mineralogical and chemical composition of planetary surfaces and has been the most heavily exploited technique for the characterization of asteroid surface materials. The asteroid population has been classified, depending on the assumed surface composition, by comparing the asteroid reflectance spectra with laboratory spectra of meteorites, terrestrial rocks and known synthetic materials, and by combining this information with some observational parameters (e.g. albedo and color indexes). So far we have several taxonomic classifications. Although the classification of asteroids into the taxonomic groups is not specifically an interpretative procedure, the taxonomic classes do have some general compositional significance, since their distribution varies systematically with heliocentric distance (Gradie et al., 1989): moderate-albedo asteroids are dominant in the inner belt, while low-albedo asteroids are prevalent in the outer belt and beyond. This spatial distribution seems to suggest that the surface composition of asteroids has been altered by primordial processes due to the temperature gradient. We are working to use all our expertise to point out a method to obtain a new taxonomic classification in order to optimize the GAIA scientific output. In our experience it is critical to arrive up to about 1000 nm. In fact, the spectral feature at about 1000 nm, related to the Ca2+ content of pyroxene, as well as olivine-pyroxene abundance ratio, is the most diagnostic in this wavelength range to discriminate about different taxonomic classes. If GAIA spectrophotometric data will arrive just up to 900 nm, we will loose the possibility to discriminate between R, V and Q asteroids. Considering that Vesta, one 2
of the most intriguing main belt asteroid, is a V type, the impossibility to discriminate between R and V asteroids will be a real damage to our knowledge of the whole asteroidal population. Moreover the analysis of the depth of the 1000 nm feature will allow us to investigate the space weathering process which is supposed to be the principal alteration of the physical and spectroscopic properties of asteroid surfaces. Ordinary chondrite meteorites and S- type asteroids, which are supposed to be their parent bodies, show a range of variation on their spectra. Binzel et al. (1996) suggested that this range could arise through a diversity of mineral composition and regolith particle sizes, as well as through a timedependent surface weathering process. This mechanism is supposed to alter the asteroid regolith which exposed to the interplanetary environment, progressively changes color with time. The asteroid surfaces become darker and their reflectance become redder (which much weaker absorption bands) than does the spectrum of its constituent rocks. In this scenario, the S-type asteroids having flatter spectra could be younger and have a surface composition similar to ordinary chondrites. On the basis of these considerations the analysis of the 1000 nm feature is really fundamental to obtain a new taxonomic classification of a wide number of asteroids, and to have some indication of the effects of space weathering processes on the surface of objects belonging to different classes. In conclusion, in order to optimize the scientific output of GAIA in asteroid taxonomy, the best solution should be to have at least 6-7 filters from 300 nm to 1000 nm, at almost constant wavelength distances, and with a band depth of about +-20 nm. 4 Mass determination M. Rappaport (Bordeaux) is gathering data to reflect the state of the art in this area with the view of providing input masses for the simulations and also idea of their real accuracy whenever possible. The masses of the largest asteroids is the at present the limiting factor in the accuracy of the solar system ephemeris. Among the big five, Pallas has the most poorly known mass as a result of its significant orbital inclination to the ecliptic. Several close approaches have been identified during the expected period of the GAIA mission that would be very favorable. This work has also shown that multiple close approaches between a large and a small planet will be the rule and that a global treatment will be required. Several criteria to qualify a useful or poor passage have been devised and are being implemented into a full size simulation by F. Mignard. Beyond that, the major issue, is to produce the observation equations with the set of masses and orbital elements as unknown so that one can investigate the conditioning of the system. The problem of mass determination with GAIA is now well understood and must be considered with a completely new look compared to the determination using century-old measurements. In principle, only GAIA based observations should be useful to this purpose. It is known 3
that the merging of accurate spaced based observations covering a limited range with old data of low accuracy is a source of systematic errors difficult to remove. 5 Orbit determination Investigations carried out by : K. Muinonen, M. Carpino, D. Hestroffer, J. Virtanen We have initiated studies of the overall effect of improving astrometric accuracy on orbitalelement probability density functions (p.d.f. s) by examining three types of inverse problems (Muinonen & Virtanen 2002): first, the observations comprise Right Ascension (R.A.) and Declination (Decl.) positions (standard astrometry); second, the observations contain both R.A. and Decl. positions and R.A. and Decl. position differences; and third, the observations include R.A. and Decl. positions and motions, that is, R.A. and Decl. time derivatives are involved (in essence, the GAIA inverse problem). Tentative application of the statistical ranging technique to high-precision short-arc astrometric data suggests that three positions can provide well-confined orbital-element p.d.f. s, whereas two positions yield wide-spread distributions almost independently of the observational error assumed. We note that the current examples contain low-inclination asteroids only, a caveat we will soon be removing. In the coming months, we will be studying the GAIA inverse problem in considerable detail, in particular, for short-arc observations. The comprehensive computation of ephemeris uncertainties for multiapparition asteroids is in progress based on the ideas exchanged at the SSWG meeting in Torino (February 6-7, 2002) and during Mario Carpino s visit to the Astronomical Observatory of Torino (February 17-18). In the coming months, we expect the following activities: Simulation of observations of asteroids by GAIA, including: a. a database of all observations of numbered (possibly also multi-apparition) asteroids with statistical analysis; this is an extension of the study already performed by F. Mignard, but we will do it again in order to have correct input for the next stage; b. a more detailed study of some typical cases (a main belt, an Apollo, a IEO, a Trojan, etc.) in order to show what is to be expected from GAIA in connection with specific classes of objects; A study of the improvement in the accuracy of the orbit of known objects produced by GAIA observations. This will include: a. simulation of ground based astrometric observations of asteroids and of their accuracy from now to the epoch of the launch of GAIA (extrapolation based on the data flow from ongoing asteroid surveys); b. estimate of the accuracy of GAIA observations 1a) We have an approximate error model for the observations supplied by M. Lattanzi et al. some time ago, but it 4
refers to an hypothesis for the instrument which is now obsolete. Therefore this part of the study must be updated when we shall know the final configuration. Another important issue is the error introduced by the offset from the centerof-mass to the photocentre, so also the results of the ongoing study on this topic by A. Cellino et al. have to be included here. c. simulated orbital determination based on the two data sets above and comparison of covariance matrices of the orbital elements in the two cases. We are also testing the possibility of measuring radial velocities of asteroids (in addition to the knowledge of the positions and velocities in the focal plane). If available, these provide additional constraints on the identification and preliminary orbit determination problems. 6 Ground based follow-up (W. Thuillot) Ground based observations will be necessary mainly to avoid the loss of new solar system objects detected by GAIA but also to more accurately characterize their dynamical and physical parameters. Several actions may be performed for this purpose. Mean and long term observational programs could be carried out thanks to a network of observers and the broadcasting of information through an official GAIA follow-up web page linked to the IAU-MPC follow-up pages. Due to the requirements in sensitivity and to the duration of the GAIA mission, only a dedicated and coordinated network appears to be the efficient way to insure CCD astrometric (and photometric) data usable to characterize new objects. But fast new objects (NEA) would require observations on alert and robotic instruments would then be the best method to avoid the loss of these objects. For example, the French project dedicated to the study of transient phenomena in the universe named ARAGO (Advanced Robotic Agile Guest Observatory), a compact telescope in SiC with diameter 1.5m, field of view 2 degrees, limiting magnitude R=21, will allow to combine several observational programs thanks to a fast pointing speed (60 deg./s). It then could perform immediate and efficient follow-up observations of fast objects. The work to be done in the next months is mainly to collect more information on candidates sites and instruments, robotics and not robotics, which could be included in such an international network in order to cover the sky and to avoid meteorological problems thanks to various longitudes and latitudes of the stations. Several questions remain also to be studied, in order to get a more precise definition of this task, for example: - how many new objects do we expect to be detected by GAIA, taking account the recent new estimates of the numbering of asteroidal objects. - how to deal with detections of objects difficult to observe from the ground, for example objects very close to the Sun, or objects moving in very reach stellar fields. What kind of instrumentation or process will allow us to get astrometric measurements in this case. 5