Giotto Spacecraft in Danger High-Power Laser Bursts Will Be the Best Way of Simulating Hypervelocity Impacts

23 1983 659 _NA_ru_RE_v_o_L._JO_~- -JUNE--- - -----------NEWSANDVIEWS------------------- Astronomy sufficient to penetrate 0.5-mm aluminium sheets. If the scaling factors relating them to solid particles can be established, the Giotto spacecraft in danger high-power laser bursts will be the best way of simulating hypervelocity impacts. from Max K. Wallis Another feature of Giotto is that its shield and exposed instruments present asymmetric surfaces to the oncoming dust. The ESA scientists decided that because the spacecraft rotates, the net effect of unbal anced impacts would be zero. I have chal lenged this as conceptually flawed. The situation is analogous to an unbalanced spinning top, or more accurately, to an un balanced gyroscope in a head wind: the asymmetric torque induces a precession of the rotation axis, the precession angle growing with time. For the spacecraft spin ning in free space, the mathematical prob lem is a classical demonstration of the use of Euler's equations: the angular momen tum vector wanders in inertial space at the Reference trajectory of Giotto spacecraft same time as the spacecraft axis precesses. While Giotto rotates once every 4 seconds, ESA's interplanetary spacecraft Giotto counter, and indeed will only be accurately the precession which depends on will fly by comet Halley in March 1986 on a determined by the Giotto mission itself. anisotropy in inertial moments has a 35-s trajectory that will bring the spacecraft A second difficulty is that impulses due period. I calculate that on the planned within a few hundred kilometres of the to the 69 km s· 1 impacts cannot be spacecraft trajectory through comet comet nucleus. It is intended that it should simulated in practice. When ESA scientists Halley, the angular momentum vector tilts pass through the scientifically interesting, Coupe and Dean first studied the problem by up to 1.5 degrees and combines with a relatively dense part of the dust cloud sur (ESA J. 7, 15; 1973), they assumed that im precession of ±0.5 degrees, depending on rounding the nucleus. This will inevitably pacting particles simply give up their the actual momentum factor. Thus the mis disturb the spacecraft's attitude, and could momentum to the spacecraft. Totalling the sion appears at risk not only from possible lead to a loss of information. An important impulses over a theoretical distribution in milligramme-sized impacts, but also from consideration in the design of Giotto has size, they found that the rare largest par therefore been to identify the exact cause of ticles cause the greatest perturbation. They the disturbance so that appropriate therefore devised a Monte-Carlo calcu countermeasures can be taken. lation to simulate stochastic impacts rang Giotto will be spinning nominally at 15 ing over size and position and found tilts of r.p.m. and is equipped with a two-element the axis of around 1-2 degrees and infre shield to protect its vital equipment from quently of several degrees. damage by dust particles impacting at a But impacts at 69 km s·1 do not simply relative velocity of 69 km s· 1• The outer deliver momentum. As shown by experi 1-mm aluminium 'bumper' shield causes ments with dust particles moving a factor small particles to fragment and vaporize, of l 0 more slowly, they cause vaporization while the inner Kevlar sheets, some 25 em of a much greater mass of material which, behind it, protect against the expanding if it escapes anisotropically, can exert many debris. times greater an impulse than the initiating It was first thought that impacts of momentum. Tony McDonnell of the University of Kent conducts such experi 10-100-mg grains would present the Geometry of the Giotto encounter with Halley. greatest danger, forat69kms·1 these might ments and, extrapolating from his group's only partially fragment, smash through and others' results, has pointed out that a the 'rain' of non-penetrating several millimetres of shielding and multiplication factor of around 40 is in microgramme dust. perhaps disrupt crucial components. But it order. This perhaps applies only to par The scientific controversy needs to be is now realized that the mission is en ticles of less than a microgramme in mass, resolved quickly. As the spacecraft design dangered by still smaller grains causing tilts for larger ones penetrate to the inner shield is well advanced, it will be necessary to of the axis by more than l degree, the width or even further, where the vapour they decide whether to meet the problem, for ex of the transmitted radio-beam. There is no release does not penetrate primarily ample, by attaching counterbalancing sec rapid re-orientation mechanism, so once forwards. tions to the shield to improve symmetry. contact with Earth is lost the spacecraft As technical factors limit experimental The higher-mass grains that penetrate the would pass far through the comet and lose particle speeds to about lO km s· 1 or excep outer bumper and generate puffs of vapour much of the scientific data before the trans tionally 20 km s-, laser impacts are being between the shield sections have not yet mission link could be restored. studied as a way to simulate the 69 km im been taken into account, as their modelling The present strategy is to fly as close to pacts. Recent laser experiments at SERC's and calculation present difficulties - it the comet as feasible without substantial Rutherford-Appleton Laboratory (W.M. may be that laser impacts on a scale model risk of exceeding the !-degree pertur Burton Adv. Space Res. 2, from the can help quantify and test practical solu bation. So it is important to calculate that COSPAR Symposium on 'Impact pro tions. D risk carefully. One aspect of the calculation cesses on solid bodies', 1982) and France's is inevitably uncertain, since the actual dust Ecole Polytechnique (J -P. Bib ring eta/. J. Max K. Wallis is Co-investigator on plasma and magnetic field experiments on Giotto and is in particle fluxes and sizes will only be known Phys. 44, Ll89; 1983) have demonstrated the Department of Applied Mathematics and from studies carried out once comet Halley crater formation analogous to that from Astronomy, University College Cardiff, Car has brightened in the months before the en- dust impacts. The impulse in some cases is difjCFJJXL. 0028-0836/83/250659-02$01.00 © 1983 Macmillan Journals ltd .

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