by Andrea Milani.

Presentation to the ESA Solar System Working Group

Nice, 27 April 2000

The private man, the public scientists

You need to be aware that I am certainly not able to speak about Paolo Farinella in an unbiased, detached way. I met him when he was an 18 year old student, a freshman at Scuola Normale Superiore in 1971; I was teaching for the first time in the University of Pisa. Thus I was one of his first teachers, and he was one of my first students. We followed together, Farinella, A. Nobili and myself, the lectures of Giuseppe (Bepi) Colombo at Scuola Normale in 1976/78, and we started working together; our first common paper, a short letter to Nature, was published in 1978. We have worked together often, but not always, since then; we have even shared the same office for more than 20 years. We have also been very close and intimate friends.

Having confessed that I cannot be an impartial observer, I nevertheless feel that I owe to the memory of my friend an effort to be loyal to the ideal of scientific, objective knowledge. Farinella main characteristic was to be uncompromising in his quest for the scientific truth. Thus I will try to present an objective view of some of the innovative scientific ideas he introduced. However, this view will be somewhat biased by the fact that it is easier for me to present the work we did together, although this accounts for a small fraction of his scientific production (we have signed together ``only'' 44 papers, out of the about 300 he published). A balanced reassessment of his total scientific production will have to wait for the publication of his Opera omnia, which I hope to promote.

Before introducing some of his ideas I need, however, to tell a fact of his private life which was not widely known, even among the many scientists from all over the world who knew him and held him in great esteem. Paolo was a so called blue child, that is he was born with a congenital heart defect, which left him an handicapped child until the age of 8. Then he underwent open heart surgery, of a kind which was, at the time, highly experimental. The operation was successful, nevertheless his life expectancy was quite low; actually he has lived much more than originally expected. He always knew he had less time than most of us. This goes a long way to explain some traits of his personality: although he was friendly and extremely tolerant with everybody, there was in his attitude a vein of intolerance for the useless, time-wasting scientific and academic bureaucracy, and even more for the kind of power struggles which claim to be about science and leave to the addicts no time to do real science. He knew he had no time for this; do we, in fact, have such spare time?

The celestial mechanics of the space age

Colombo's course in the late 70's did not consist at all of formal lectures: he would arrive direct from America, and on the way from the airport he would start showing us the most recent results of NASA planetary explorations. These were years of renaissance for celestial mechanics and dynamical astronomy, thanks to the space exploration drive. Subjects such as the rings of Uranus, the peculiar spin-orbit configuration of Pluto and Charon, the internal structure of the satellites of Saturn imaged by Voyager, were new and exciting because allowed to exercise our theoretical preparation -strong, but by far too abstract, as usual in the Italian universities- to solve real problems. Each resonance, either known or conjectured, was the source of a new theory, for which the space probes would sooner or later provide experimental evidence, thus a decisive argument to discriminate the scientific truth. We learned in this way that a modern scientist owes allegiance not to his/her discipline, but to the truth, and that a problem is worth solving not when it is easy to tackle with the available tools, but when it is a challenge, in which others have already failed.

Some of the problems we worked on were fashionable, as the ones listed above, some were about to become fashionable, such as the practical consequences in planetary sciences of the existence of chaotic, unstable orbits. Farinella was ready to the challenge of the fashionable problems, on which there was advantage in publishing a few weeks before the competition, and in writing a paper he was the fastest I knew. However, he was also capable of seeing far ahead of anybody else. A paper he published in 1980 had the title Irregular extrasolar systems and dynamical instability; when extrasolar planetary systems were indeed discovered in the late 90's, many expressed surprise at the high eccentricity of some orbits, not realizing that the theoretical bases had been established that long before. A similar ``prediction'' was contained in a paper we published in 1991, just a few months before the discovery of the first transneptunian small planet (beside Pluto): the stability regions of the transneptunian belt, where most objects would be found, were clearly mapped on the basis of a pure celestial mechanics computation.

Asteroid shapes, collisions, families

Nobody better than Farinella had a global grasp on the evolution of the asteroid belt. This problem cannot be properly understood by the use of only a few specialized tools; as an example the calculation of proper elements is a highly technical challenge, which Farinella would rather leave to more mathematically minded researchers (like myself); the problem is what to do with this tool once it has been made available by complex computations. In the same way knowledge arising from meteorites' chemistry and dating, from telescopic photometry and spectroscopy of asteroids, from ground experiments of hypervelocity impacts, from asteroid radar and from space missions flybys, needed to be integrated in models of the dynamical and collisional evolution of the asteroid population.

It is not possible to summarize in a few lines Farinella's achievements in the modeling of the asteroid belt, but some of the concepts he pioneered, like rubble-piles, equilibrium shapes, binary asteroids, and the relationship between impact rates, solid state strength, asteroid families and sources of meteorites, are standard tools of our present understanding.

With his characteristic ability in applying the same basic physical rules to different situations, he was the first to perceive the analogy between the problems of the dynamical and collisional evolution of the asteroid belt and the evolution of the cloud of Earth orbiting space debris. This allowed him to give an essential contribution also in this field; some of his recent works on the estimation of the impact risk for the space station are likely to be remembered as warnings the space agencies should have listened more carefully.

Space is not empty, and meteorite transport

The celestial mechanics model of planetary orbits as an N-body problem has the property that the accuracy of the correspondence between the theoretical equations of motion and the real problem is of extraordinary accuracy. This, however, does not imply that the range of physical phenomena resulting from the interaction of a solid body in interplanetary space is limited to the long range gravitational interaction. In fact space is not empty, and every object moving in space, be it either natural or artificial, experiences interaction of its surface with the particles and the electromagnetic field and radiation. For a human manufactured spacecraft the area-to-mass ratio is such that such interactions can significantly change the orbit with respect to the purely gravitational one.

With the availability of spacecraft tracking data, including the very accurate ones of satellite laser ranging, the comparatively small push of non gravitational perturbations became the main limitation for accurate orbit determination, especially when the goal was to solve for geodetic and geophysical parameters. In the 80's we successfully argued that this is a fundamental limitation, which cannot be removed by modeling the interaction of each small piece of the spacecraft surface with the radiation and particle environment, because such complex modeling would result in illusory precision; now this is understood to the point that space-borne accelerometers are considered an essential monitoring tool to achieve top accuracy in gravimetry and geodesy. Farinella continued this line of research until the 90's, when he was able to propose the first self consistent model, based on physical laws and not just empirical parameter fitting, capable of explaining the long term orbital perturbations of the satellites of the LAGEOS class, a long standing open problem. For such model he needed to take into account the interactions with the magnetic field of the Earth, as well as a full thermal model resulting from the interaction of LAGEOS with both sunlight and radiation reflected/emitted from the Earth, and of course the celestial mechanics of the resulting orbital changes; this is a combination of knowledge not accessible to that many scientists.

The difference between natural and artificial bodies with respect to the relevance of non gravitational perturbation is mostly a matter of area-to-mass ratio; thus, for the orbits of the very small asteroids, which are believed to be the parent bodies of meteorites, the same non gravitational effects can be relevant. On the long run, million of years, a small push by non gravitational forces can result in transport between the main asteroid belt and Earth crossing orbits, passing through some unstable resonance. Models of this kind, especially the ones based upon the so called Yarkovsky effect, have been successful in accounting for unexplained cosmic rays exposure ages of the meteorites. Farinella was not afraid to study a new subject, if this was needed to grasp the scientific context of a problem, and he studied the evidence from meteorites with his usual commitment. A paper on Science on the Yarkovky effect was among his last, published while he was already hospitalized.

A concerned scientist

This brief outline of the cultural profile of Paolo Farinella would not be complete without mentioning his activity as a scientist committed to peaceful uses of knowledge. In the mid 80's the arms race was raging, and the pressure to make the scientific research subservient to the needs of the strategic confrontation was increasing. Farinella was not only concerned, but committed to act among fellow scientists and among all citizens for the prevalence of reason, of the quest for peace, of a disarmament bilateral and stable in time. In these years we felt that it was indeed possible that nuclear weapons would be massively used in war in our lifetimes; now we believe this is less likely, but do we really know this?

One work we did together left an impression on me which I will not forget. We computed, by using basic physical laws and non classified information, the effect a nuclear attack, of the kind most likely in a global war, would have had on our home town of Pisa. I was shocked by the results, which figured in my worst nightmares for many years after. But Paolo was less afraid of the truth, and he could continue much longer than me to use his scientific knowledge to increase the public awareness of the dangers of unlimited military aggression. He also was very clear in his opposition to the limitations that the technological-military competition was introducing in the scientific research and in the freedom of information. He studied carefully not only the rules to be introduced by arms limitation treaties to ensure proper verification, but also the economics of the necessary reconversion of the military-industrial complex.

Last but not least, as the people who sat with him on the SSWG know, he was influential among the specialists of space missions dedicated to the exploration of the solar system, even though he had never taken part to the experimental, instrument-building side of such missions. Both his influence and his aloftness arose from the same basic conviction; namely the need to give absolute priority to science, without accepting compromises with other needs, important in themselves but of lesser value in the long run, such as economic, political and industrial interests. His detachment notwithstanding, his advice was often sought when an interpretation of the results of space exploration was needed. In this, as in many other scientific and academic environments, he was not looking for immediate recognition, for positions of power, but his intransigence was his strength, making it difficult to ignore him. Some nevertheless ignored him, especially in his own country, and we should be sorry for them.

There are other aspects of the personality and of the influence of Paolo Farinella which would be worth mentioning, such as his dedication to teaching and popular science, and his great human warmth which gave him a very large number of friends. Both those who knew him well and since a long time, and others who met him only in a few scientific meetings, will feel his absence. Those, like myself, who have been very close to him for many years, will certainly struggle to keep his ideas and his contributions alive.