required.At each viewing opportunity,manydozens of meticulous measurements areneeded over the typical eight-hour rotationperiod of the asteroid to define its light curve;light curves must be measured repeatedlyover several weeks to determine the aster-oid’s spin period precisely.Now multiply thiseffort for one asteroid by at least ten, to get areasonable statistical sample, and… you getthe idea.

But someone was up to the challenge.Using the Wallace Astrophysical Observa-tory’s 0.6-m telescope for nearly a decade,postgraduate student Stephen Slivan and acadre of undergraduate students amassedthousands of measurements for ten Koronisfamily asteroids9,10. Decidedly nonrandomspin vectors were found1. Four prograde(anticlockwise) rotators have nearly alignedrotation axes with very similar spin periodsof about eight hours, and the remaining six retrograde (clockwise) rotators are alsonearly co-aligned together (but on a differentheading) with a dispersion of spin periodsbetween 3 and 30 hours (Fig. 2). The align-ments almost defied belief — how could acollisionally dominated family have alignedspin vectors? But this tour de force data set1

(confirmed by an independent analysis ofspin vectors by Mikko Kaasalainen10,11)called out for explanation.

Vokrouhlicky et al.2 have responded to thecall, with a model that invokes the YORPeffect and the influence of Solar System planets. For the 20–40-km-diameter mem-bers of the Koronis asteroid family (the samesize range and location as the Slivan sample),they find that prograde rotators should havetheir spins slowed and their spin axes cast intoa slow precession like a top.When the preces-sion rate matches the rate at which the planeof Saturn’s orbit undergoes a slow twist, theasteroid becomes caught in an equilibriumstate that Vokrouhlicky et al. call a ‘Slivanstate’. The spin vector and spin rate becomefixed, and their calculated values closelymatch those for the observed sample. Retro-grade rotators don’t seem to be influenced bySaturn’s twist or to land in an equilibriumstate, but thermal torques (turning forces)turn their spin vectors upside down withrotation rates diverging towards either fast or slow values — again just as is observed.According to Vokrouhlicky and colleagues’model, it takes typically 2–3 billion years forthe Koronis family asteroids to evolve to theirend states, with smaller and more irregularlyshaped objects proceeding more quickly.

How well this YORP-based model2 holdsup can be tested through further obser-vations. Most asteroids smaller than 40 km in diameter should be influenced by theeffect, with those in the outer asteroid beltbeing most likely to land in observable Slivan states. Measuring an abundance ofupside-down spin vectors for fast or slowlyrotating asteroids would be another tell-tale

fingerprint of the YORP effect. If it can beshown to dominate asteroid spins, the impli-cation is that geologically frequent collisionsare highly inefficient in transferring rota-tional angular momentum. Collisions thatfracture but do not destroy asteroids mightleave them so weak that a hard push has littleeffect. In contrast to what has been longthought, a light touch might be the best wayto influence asteroid spins. ■

Richard P. Binzel is in the Department of Earth,Atmospheric and Planetary Sciences,Massachusetts Institute of Technology,Cambridge, Massachusetts 02139, USA.e-mail: rpb@mit.edu

1. Slivan, S. M. Nature 419, 49–51 (2002).

2. Vokrouhlicky, D., Nesvorny, D. & Bottke, W. F. Nature 425,

147–151 (2003).

3. Öpik, E. J. Proc. R. Irish Acad. 54, 165–199 (1951).

4. Hartmann, W. J. et al. Meteoritics Planet. Sci. 34,

161–167 (1999).

5. Bottke, W. F., Vokrouhlicky, D., Rubincam, D. P. & Broz, M. in

Asteroids III (eds Bottke, W. F. et al.) 395–408 (Univ. Arizona

Press, Tucson, 2002).

6. Rubincam, D. P. Icarus 148, 2–11 (2000).

7. Binzel, R. P. Icarus 73, 303–313 (1988).

8. Binzel, R. P. in Asteroids, Comets, Meteors III (eds Lagerkvist,

C. I. et al.) 15–18 (Uppsala Universitet, 1990).

9. Slivan, S. M. & Binzel, R. P. Icarus 124, 452–470 (1996).

10.Slivan, S. M. et al. Icarus 162, 285–307 (2003).

11.Kaasalainen, M., Mottola, S. & Fulchignoni, M. in Asteroids III

(eds Bottke, W. F. et al.) 139–150 (Univ. Arizona Press, Tucson,

2002).

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132 NATURE | VOL 425 | 11 SEPTEMBER 2003 | www.nature.com/nature

Ageing

A toast to long lifeToren Finkel

Reducing food intake increases lifespan in many species. A smallmolecule that occurs naturally in plants seems to mimic the beneficialeffects of caloric restriction and extend longevity in yeast.

In the spring of 1512, three wooden vesselscommanded by the Spanish explorerPonce de León left the warm Caribbean

waters off Puerto Rico in search of thefountain of youth. Fuelled by his desire forimmortality, Ponce de León was convincedthat drinking from this legendary springwould confer a state of eternal youth. Locallegend suggested that the fountain could befound in the lands to the north and that itwas surrounded by magnificent floweringplants. Over the next few months, theexplorers travelled from island to island,tasting rivers and lakes (Fig. 1) until shortsupplies and hostile natives forced them toabandon their quest.

Today, the once desolate shores of Florida

— the ‘land to the north’ that Ponce de Leóndiscovered inadvertently on his voyage — arecrowded with ageing retirees who yearn justas passionately for the magical elixir ofyouth. Although the past two decades haveseen an explosion in our understanding ofthe molecular regulation of ageing, a simplepotion or chemical that could slow the age-ing process seems as elusive today as it was500 years ago. But on page 191 in this issue,Howitz et al.1 present a study of longevity inyeast that could represent the first steptowards creating such a hypothetical elixir.

It is well established that reducing foodintake (caloric restriction) extends lifespanin a wide range of species2. But given the cur-rent epidemic of obesity in industrialized

Figure 1 A quest for longevity. Five hundred years ago, the Spanish explorer Ponce de León drank hisway around the Florida coast during his expedition to find the legendary fountain of youth.

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© 2003 Nature Publishing Group

societies, eating less for the sake of longevityis not likely to gain widespread compliance.More palatable would be a drug that couldmimic the effects of caloric restriction — achemical that would allow the individual toeat normally, while tricking the body torespond as though food were in short supply.Such mimetics might be thought of as thepharmaceutical equivalent of ‘eating yourcake without having it’.

Studies of longevity in simple organismshave greatly expanded our understanding ofhow caloric restriction might increase life-span. In the budding yeast Saccharomycescerevisiae, nutrient withdrawal extendslongevity through a pathway that requiresthe enzyme Sir2 (ref. 3). Overproducing thisenzyme can prolong the life of yeast grownunder normal nutrient conditions4. Simi-larly, in the evolutionarily more advancedworm Caenorhabditis elegans, increasedexpression of the worm’s version of Sir2 hasalso been shown to extend lifespan5.

The Sir2 enzyme belongs to a large familyof evolutionarily conserved moleculestermed sirtuins. In lower organisms, such asyeast and worms, these enzymes regulate a

wide range of cellular activities that affectlifespan, including modulating how tightlyDNA is packaged inside cells. In mammaliancells, sirtuins act as regulators of pro-grammed cell death and differentiation (cellmaturation)6. Sirtuins exert their effects onthese cellular processes by removing acetylgroups from specific target proteins. Inter-estingly, this ‘deacetylase’ function dependson the intracellular concentration of a mole-cule involved in metabolism — nicotin-amide adenine dinucleotide (NAD). Thismolecule can exist in two states,oxidized andreduced, and it is the oxidized form thatgreatly enhances Sir2 activity. It seems that inyeast, caloric restriction may regulate Sir2activity, and hence prolong life, by subtlyshifting the ratio of oxidized to reduced NADor by altering the level of the NAD derivativenicotinamide7,8. Together, these findingssuggest a potential mechanism by whichmetabolic activity and lifespan might con-verge (Fig.2).

Building on the knowledge that caloricrestriction prolongs longevity through Sir2,Howitz et al.1 searched for a small moleculethat could activate this enzyme directly.Using several chemical ‘libraries’, these inves-tigators discovered two related compoundsthat each stimulated Sir2 activity. Both com-pounds belong to a family of moleculescalled polyphenols — products of metabo-lism in plants. One of the most widely stud-ied of these compounds is resveratrol,a plantpolyphenol that is abundant in red wine andis reputed to underlie many of wine’s health-related benefits. Interestingly, resveratrolseemed to be the most potent Sir2 activatorof all of the plant polyphenols tested. Theauthors showed that this chemical pro-longed the lifespan of yeast by approximately70%. The extension of longevity was entirelydependent on Sir2 — yeast strains deficientin this enzyme did not benefit from resvera-trol treatment.

Could plant polyphenols be the long-

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sought elixir of youth? Previous studies havehinted that these compounds have severalpotential health benefits, especially in pro-tecting against age-related maladies such as cancer, neurodegeneration and athero-sclerosis9. Interestingly, caloric restriction isalso thought to protect against these diseases.But caution is warranted before endorsing a strict Cabernet Sauvignon-based regimen.First, the concentration-dependent effects ofresveratrol as observed by Howitz et al. werecomplicated. At relatively low doses thesemolecules stimulated sirtuin activity, but, atleast in certain assays, higher doses had theopposite effect.This is not an ideal character-istic for a pharmaceutical drug. Second, andmore importantly, life extension in yeast is along way from life extension in higher organ-isms. Indeed, how sirtuins function in mam-malian ageing is not yet known.

Further unravelling of the molecular signalling pathways that accompany caloricrestriction should provide clues to otherpotential targets for drug development. In astrange way, however, the study by Howitz etal. suggests that Ponce de León’s misbegottenquest for a fountain of youth surrounded byflowering plants was not so delusional afterall. The explorer’s only mistake was that hekept sampling the waters, when he shouldhave been testing the plants. ■

Toren Finkel is at the Cardiovascular Branch,National Heart, Lung and Blood Institute,National Institutes of Health, Bethesda,Maryland 20892-1622, USA.e-mail: finkelt@nih.gov1. Howitz, K. T. et al. Nature 425, 191–196 (2003).

2. Masoro, E. J. Exp. Gerontol. 35, 299–305 (2000).

3. Lin, S. J., Defossez, P. A. & Guarente, L. Science 289, 2126–2128

(2000).

4. Kaeberlein, M., McVey, M. & Guarente, L. Genes Dev. 13,

2570–2580 (1999).

5. Tissenbaum, H. A. & Guarente, L. Nature 410, 227–230 (2001).

6. Denu, J. M. Trends Biochem. Sci. 28, 41–48 (2003).

7. Anderson, R. M. et al. Nature 423, 181–185 (2003).

8. Lin, S.-J. et al. Nature 418, 344–348 (2002).

9. Tapiero, H., Tew, K. D. & Matthe, G. Biomed. Pharmacother. 56,

200–207 (2002).

Altered metabolism/O2 consumption

Activation of Sir2

Altered nicotinamide concentrationand/or NAD:NADH ratio

Prolonged lifespan

Activation of stress pathways

Caloric restriction

Polyphenols

Figure 2 The pathway to long life. When yeastcells are deprived of food (caloric restriction),stress pathways are activated and the cells are forced to derive energy from alternativesubstrates. This produces alterations in oxygenconsumption, which in turn affects the ratio of oxidized to reduced forms of nicotinamideadenine dinucleotide (NAD:NADH) or theconcentration of its derivative nicotinamide.NAD stimulates the activity of Sir2, which in turn chemically modifies several proteins that are involved in cellular processes affectinglongevity. Howitz et al.1 have found that plant polyphenols directly activate Sir2 andseem to mimic the beneficial effects of foodrestriction. Related pathways may exist in higher organisms.

Condensed-matter physics

Vortices and heartsJohn Clarke

A single vortex of flux, formed inside a superconducting Josephsonjunction, has been detected undergoing quantum tunnelling — afeature that could be developed into a quantum bit.

Vortices are ubiquitous — from themythical whirlpool of Charybdis inthe Strait of Messina, to the putative

cosmic strings that were frozen into space-time shortly after the Big Bang. In super-conductors, too, swirling currents generatevortices of flux. On page 155 of this issue, Wallraff et al.1 show that a single fluxvortex can undergo macroscopic quantum

tunnelling2 to escape from a potential wellinside a long Josephson junction; they alsoshow that the vortex’s energy in the control-lable well is quantized. These observationsof uniquely quantum phenomena meanthat this system becomes another entranton the growing slate of candidates to make superconducting ‘qubits’ for quantumcomputing.

NATURE | VOL 425 | 11 SEPTEMBER 2003 | www.nature.com/nature 133© 2003 Nature Publishing Group