NEWS AND VIEWS COMETSHOEMAKER-LEVY9------al comet nucleus was fairly small, at most The Big Fizzle is coming 1.5 km in diameter, in agreement with 5 work by Scotti and Melosh . Paul Weissman Past estimates of the bulk density of cometary nuclei have ranged from 0.1 to 3 How will the fragments of comet tions show the nucleus of tightly packed 1.3 g cm- , based on comparisons of the Shoemaker-Levy 9 meet their end - with snowballs being torn apart by Jupiter's predicted effects of gases jetting from the a bang or a whimper? That is the question gravity during the close approach, the sunlit surface of comet Halley with de­ on everyone's mind as the icy fragments hundreds or thousands of snowballs tailed observations of Halley's orbital rush towards their cosmic rendezvous with stretching into a long column in space. But motion6-8. But the many free parameters Jupiter, beginning on 16 July. Will Jupi­ as the column lengthens and moves away in such comparisons make the estim­ ter's atmosphere be torn with massive from Jupiter, the individual snowballs ates highly uncertain. More recently, explosions, each greater than the sum of begin to clump together because of their meteoriticists have measured the density all the nuclear weapons on Earth, or will it own self-gravity. The truly amazing result of microscopic cometary dust grains re­ covered by U-2 aircraft high in the Earth's 9 atmosphere ; these values are typically 3 between 1 and 2 g cm- . Asphaug and Benz's results clearly rule out the lower range of values from the estimates of jetting forces, but may be in conflict with some of the higher values from the comet­ ary dust grains. A question not answered by Asphaug and Benz is whether the individual dirty snowballs in each clump of Shoemaker­ Levy 9 have reaccreted into a single body, or whether they are only gravitationally bound dynamical swarms, like bees buz­ zing around a hive. Several of the clumps have been observed to split, well away from Jupiter's tidal pull, suggesting that within each clump several sub-nuclei may reaccrete, but that a single solid body did not form. Other clumps have dissi­ pated completely with time, suggesting that the snowball swarms don't reaccrete and instead dynamically disperse or subli­ mate away. Four suggested structures for a comet nucleus, adapted from ref. 10. a, The nucleus as an icy What does this say about the coming 1 conglomerate, b a fractal structure, C a 'primordial rubble pile', as Asphaug and Benz assume impacts on Jupiter? As the clumps for comet Shoemaker-Levy 9; and d the 'icy-glue' model. approach for their final plunge into the atmosphere at 60 km S-I, Jupiter's gravity be a giant fizzle? We are about to find out. is that the number of clumps formed will again pull them apart. Rather than Whatever the outcome, the breakup of appears to be a function of the density of hitting as a single solid body, they will comet Shoemaker-Levy 9 has provided the individual snowballs. At a density less probably come in as an elongated shotgun 3 fresh clues as to the structure of cometary than 0.4 g cm- , no clumping occurs; at a blast of smaller pellets. Because of Jupi­ nuclei and their bulk density. One fasci­ density of 2.4 g cm-3 all the snowballs ter's rapid rotation, the impact sites will be nating example is to be found in the paper come back together to form a single body. spread in longitude, like machine gun by Eric Asphau and Willy Benz on page But at intermediate values, in particular bullets lacing into a moving target. Each p 3 120 of this issue . Asphaug and Benz used between 0.4 and 0.9 g cm- , the snowballs snowball will individually ablate and burn a high-speed computer workstation to form 15 to 20 clumps. Comet Shoemaker­ up like a meteor in Jupiter's upper atmos­ model the breakup of Shoemaker-Levy 9 Levy 9 consisted of 21 individual nuclei phere. Lacking the momentum and the when it passed within Jupiter's Roche when it was discovered last year. (Note: structural integrity of a single solid body, limit two years ago. They assumed that the the densities quoted here are those of the they will probably not penetrate deeper comet was a 'primordial rubble pile', a individual snowballs; Asphaug and Benz into the atmosphere, where they might collection of hundreds to thousands of use the bulk density of Shoemaker-Levy 9 explode with many thousands of megatons dirty snowballs, held together only by before it broke up, which is about 27 per of energy. their own self-gravity. cent less because of the voids between the ------This model for comets was indepen­ packed snowballs.) 1. Asphaug, E. &Benz, W. Nature370, 120-124 (1994), dently proposed nearly a decade ago by Results are modified if the original 2, Weissman, P. R Nature 320, 242-244 (1986), myself2, and by Bertram Donn and David comet nucleus was rotating. Asphaug and 3, Donn, 8. & Hughes, 0, Proc. 20th ESLAB Symposium on 3 the Exploration of Halley's Comet (eds Battrick, B" Rolfe, Hughes , who referred to their idea as the Benz's simulations rule out a retrograde E.), & Reinhard, R) 523--524 (1986). 'fractal model'. An improved description rotation, because the snowballs then form 4, Weidenschilling, S,), Nature 368, 721-723 (1994), 5, Scotti,), V, &Melosh, H,), Nature 365, 733-735 of how such 50-metre-diameter dirty a large central clump and smaller outlying (1993), snowballs (or more aptly, frozen mud­ clumps; this was not observed for 6. Rickman, H, in The Comet Nucleus Sample Return Shoemaker-Levy 9. But if the comet had a Mission (ed, Melita, 0.) ESA SP-249, 19!>-205 (1986), balls) might form in the primordial solar 7, Sagdeev, R Z,' Elyasberg, p, E. & Moroz, V.1. Nature nebula and then come together to form prograde rotation, 15-20 clumps are 308,240-242 (1988), kilometre-sized nuclei was recently pro­ obtained if the density of the snowballs is 8, Peale, S. )./carus82, 36-49 (1989), 4 3 9, Love, S, & Brownlee, D.lcarus(inthe press). vided by Stuart Weidenschilling . higher, perhaps 1.3 g cm- . Asphaug and 10, Weissman, P. R. in The Comet Nucleus Sample Return Asphaug and Benz's dynamical simula- Benz's results also suggest that the origin- Mission (ed, Melita, 0.) ESASP-249,l!>-25 (1986). 94 NATURE . VOL 370 . 14 JULY 1994 NEWS AND VIEWS

Thus the giant impacts will produce a Nevertheless, Shoemaker-Levy 9's tions are correct is debatable; their rele­ spectacular meteor shower of bright legacy will probably be an improved vance here is that they suggest a bigger bolides, but not the massive fireball explo­ understanding of the nature of cometary vaccination problem because cross­ sions that have been predicted by some nuclei. It will provide a dramatic con­ protection implies higher effective Ro researchers. The impacts will be a cosmic firmation of the primordial rubble pile and values. fizzle. The cometary meteors may resem­ fractal models, and will provide the first ble the bolide which exploded harmlessly definitive bounds on the bulk density of Test at 25-34 km altitude over the south Pacific cometary nuclei. Then again, maybe it One test for cross-protection calculates on 1 February this year, with an estimated won't. D the expected number of people in diffe­ yield of 15-20 kilotons. The Shoemaker­ rent age classes who have been exposed to Levy 9 snowball explosions may be closer Paul Weissman is in the Earth and Space different strains, assuming independent to about 30 megatons each, but still far less Sciences Division, Jet Propulsion Labora• transmission and some distribution of re­ than the 100,000 megaton explosions that tory, 4800 Oak Grove Drive, Pasadena, production numbers across strains. Gupta some have predicted. California 91109, USA. et al. 5 found that expected and observed numbers matched quite closely when five EPIDEMIOLOGY -- strains studied in Papua New Guinea were each given the same Ro of about 7. This is a useful numerical demonstration of the A theory of malaria vaccination way in which Ro can appear quite small, but it must be seen as no more than Christopher Dye and Geoffrey Targett illustrative for two reasons. First, if expo­ sure to each strain does not lead to COULD malaria vaccines eradicate Plas­ strain. Immunity here means that an indi­ effective protection, then the question of modium falciparum from its stronghold in vidual should be protected against becom­ cross-immunity is irrelevant. Second, the tropical Africa? While trials of Manuel ing infectious following further exposures supposition that each strain has roughly Patarroyo's S~f66 vaccine l are under way to the parasite; protection of this kind may the same low Ro is not yet backed by 2 in Tanzania , , The Gambia and Thai­ or may not be associated with clinical detailed calculations of the incidence rates land, following partial successes in South immunity. Second, there must be no of a large collection of strains. Parity of Ro 4 5 America , Gupta et al. have published a cross-protection between strains. values is not in any case essential for their stimulating theoretical analysis which sug­ Are there strains of P. falciparum that argument; the strains could have Ro values gests that the eradication of malaria by satisfy these two conditions? Gupta which vary from low to very low. Indeed, 5 6 10 vaccination might be nothing like as diffi­ et al. have identified the neoantigens this is one proposition made • to explain cult as imagined. They also propose that (PIESAs) produced on the surface of how insecticide-impregnated bednets can their theory can explain the distribution of infected erythrocytes as markers of reduce malarial disease without reducing different clinical forms of malaria with specific strains. As they acknowledge, the the prevalence of infection. 6 age , and the diversity and dynamics of observation that these antigens gener­ Many malariologists with control ex­ different parasite strains?, ate a long-lasting antibody response does perience will doubt that Ro in highly The argument about vaccination builds not imply durable protective immunity. endemic areas is as low as Gupta et al. on the logic of the campaign against The only evidence so far that PIESAs suggest. Notwithstanding the effort being measles, mumps and rubella. Simple stimulate a protective response comes devoted to vaccine development, attack­ epidemiological theory says that the critic­ from work by Marsh et al. S in The Gam­ ing the mosquito population continues to al fraction (P) of people to be immunized bia. They found that clinical episodes be a mainstay of malaria control. The with a combined vaccine (MMR) to en­ were fewer, although protection was criterion for eradication by vector control sure eradication of all three pathogens is neither complete nor proven to be dur­ is essentially the same as that for vaccina­ determined by the infection that spreads able. They also did not address directly tion; p can also be interpreted as the most quickly through a population; that is, the more relevant but difficult problem of critical fraction by which the mosquito by the one with the largest basic case protection against becoming infectious population must be reduced. If Ro = 7, reproduction number, Ro. In the case of (producing viable gametocytes which are say, then p > 0.86. Magesa et al. II satisfied MMR, this is measles, with Ro of around transmissible to mosquitoes), although this condition in a DDT trial in Tanzania, 15, which implies thatp > 1 -ljRo "" 0.93. there was no evidence in cross-sectional but their success was accompanied by no Gupta et al. 5 point out that, if a population data that antibodies to PIESAs prevented more than a small reduction in P. falcipar­ of malaria parasites consists of a collection parasitaemia. um incidence, and no detectable reduc­ of pathogens or strains that have the same Although a degree of clinical protection tion in prevalence over the following 12 properties as common childhood viruses, may be associated with strain- or variant­ months. The implication is that Ro was then vaccine coverage would be deter­ specific antibody responses, these anti­ effectively much higher than 7 in this mined by the strain with largest Ro, rather body responses may in turn be associated Tanzanian village. than by the Ro of the whole parasite with common or cross-reacting T-cell Even if a P. falciparum population does population. While estimates of the latter epitopes that progressively enhance the Y have been as high as 100, the former could speed of response to new variants • Gupta 5 1. Patarroyo. M. E. eta/. Nature 332, 158--161 (1988). be much lower. and colleagues do actually acknowledge 2. Alonso. P. L. eta/. Vacclne12,181-186 (1994). that cross-protection may have a part to 3. Teuscher, T. etal. Vaccine 12, 328--336 (1994). 4. Valero, M. V. etal. Lancet341, 705-710 (1993). Analogy play, and they use the idea in their other 5. Gupta, S., Trenholme, K .. Anderson, R. M. & Day, K. P. The MMR analogy is a useful one in the recent papers to explain why children with Science 263, 961-963 (1994). context of this theory because it makes cerebral malaria tend to be older than 6. Gupta, S. et al. Proc. natn. Acad. Sci. U.S.A. 91, 3715-3719 (1994) clear thc defining characteristics of a children suffering from severe malarial 7. Gupta, S .. Swinton, J. &Anderson, R. M. Proc. R. Soc. 6 'strain' of P. falciparum. Strains are not anaemia , and to show how oscillations in B256, 231-238 (1994). 8. Marsh, K. etal. Trans. R. Soc. trap. Med. Hyg. 83, merely antigenic types; rather they must malaria incidence could be driven by 293-303 (1989). fulfil two particular conditions. First, in­ immunity rather than by fluctuations in 9. Brown, K. N. Nature 230, 163-167 (1971). 10. Glynn, J., lines,J. D. & Bradley, D. J. Parasito/. Today 10, fection with each strain should generate external factors, such as mosquito 279-281 (1994). fully protective, lifelong immunity to that densiti. Whether or not these explana- 11. Magesa, S. M. etal. Acta trap. 49, 97-108 (1991). NATURE· VOL 370 . 14 JULY 1994 95