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Astronavi di Carta

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Sulle pagine di questa rivista è echeggiata di recente una domanda davvero bella e impegnativa su quale sia lo scopo ultimo dell’astronomia. E’ la ricerca della vita, di altre forme di vita?
I miliardi di organismi che popolano la terra sono solo un’eccezione su scala universale, o dobbiamo invece avere ancora fiducia nella celebre equazione di Frank Drake sulla probabilità di vita extraterrestre?
Ci saremmo aspettati di leggere molte risposte, di vedere sorgere fiori di dibattiti, perché la domanda è realmente essenziale, ed in certa misura è in qualche modo ripetuta, anche se forse inconsapevolmente, ogni volta che uno di

noi posizione suo telescopio per passare una notte in compagnia delle stelle.
Perché appoggiare l’occhio all’oculare è sempre soprattutto un gesto poetico, più che tecnico; e la cattura di fotoni che hanno viaggiato per tempi lunghissimi, talvolta quasi pari all’età stessa dell’universo; particelle che hanno attraversato indenni distanze inimmaginabili, solo per terminare la corsa sulla nostra retina.
Le domande che questa semplice constatazione comporta sono tante e tali che ogni tanto occorrerebbe davvero fermarsi, tirare un sospiro di meditazione, sospendere la meraviglia che ci regala la vista di un ammasso aperto o di una remota galassia e provare a scrivere su un pezzo di carta quali davvero siano le sensazioni, le cognizioni, l’emozione che questa quotidiana meraviglia ci genera.

Un foglio di carta può bastare, perché – anche se siamo fin troppo abituati alla sua presenza – è anch’esso uno strumento potentissimo, pur se non all’avanguardia della tecnologia. Può ospitare sciocchi scarabocchi una lista della spesa, ma anche creatività, fantasia, formule complesse; teorie, emozioni, versi, canzoni. Quasi tutte le cose importanti prodotte dall’uomo prendono forma inizialmente su un banale, trascurabile, ovvio foglio di carta. E anche le “inimmaginabili distanze” di cui parlavamo prima, gli spazi quasi infiniti percorsi dalla luce stellare sono tutti lì, dentro quel foglio, basta riempirlo di scienza o di poesia, o anche solo di fantasia.
Ma, se proprio non ci riuscite, non demordete. Non occorre avere il genio di Einstein o di Leopardi per correre veloci su un’astronave di carta, tutt’altro; basta seguire le poche istruzioni che noi siamo pronti a darvi.

Allora, procuratevi un foglio di carta grande e sottile, iniziate a concentrarvi: state per partire per un viaggio tra le stelle. Immaginate una bella visione astrale, e quando l’avete bene in testa, “blindatela” piegando il foglio a metà.
Adesso tornate a guardare il foglio piegato: è più piccolo, e concentrarvi su di esso sarà ancora più facile: scegliete un’altra immagine dalla memoria, o un verso di poesia che volete portare con voi nel viaggio e fissate in testa anche questo, piegando di nuovo foglio a metà. E poi ancora, e ancora… Il foglio diventa sempre più piccolo in superficie e sempre più spesso…
Procedete, così, passo per passo, sovrapponendo immagini o versi e piegatura del foglio. E fatevi una domanda: per quante volte dovrete continuare a piegarlo affinché il suo spessore (e i vostri pensieri) possano raggiungere, che so, la Luna, come se davvero fosse un’astronave di carta? Ovviamente risponderete che il dato fondamentale da considerare lo spessore iniziale del foglio… Ebbene, ipotizziamo pure uno spessore di 0,1 mm… Qual è allora il numero di piegatura necessario per raggiungere la Luna? State pensando numeri come i miliardi? Milioni? Centinaia di migliaia di piegatura? Siete fuori strada!
Se davvero foste riusciti a piegarlo e ripiegarlo senza le difficoltà che un reale foglio di carta comporterebbe, vi sareste infatti già accorti che una quarantina di piegatura sono più che suffiL’astronautica è sempre alla ricerca del propellente perfetto: deve essere leggero, potente, occupare poco spazio, e liberare rapidamente energia; avete mai pensato che non esistono aerei diesel, proprio per queste ragioni? E se certe esigenze sono presenti già per la costruzione di aerei, pensate quali saranno le richieste per il perfetto combustibile per astronavi: a differenza di quelli destinati ad uso terrestre, il combustibile per lo spazio non deve aver bisogno d’ossigeno o d’aria – o, se proprio è necessaria, dovrà portarsela dietro. Dovrà essere drammaticamente poco pesante (anzi, pardon, massivo) ed eccezionalmente poco voluminoso, se non si vuole che i viaggi interplanetari si facciano su astronavi ridotte in pratica a serbatoi volanti. E’ certo per questo che i sistemi di trasporto immaginati dalla fantascienza non prendono mai in considerazione i combustibili fossili: antigravità, salti spaziali, vento solare… va bene tutto, basta che non si tiri in ballo nulla che assomigli neanche lontanamente alla benzina.
cienti a coprire tutta la distanza Terra-Luna!
Incredibile, dite? No, ve lo assicuriamo: un foglio di carta dello spessore di 1/10 di millimetro è piegabile per 40 volte solo con gli occhi della mente, e non con le dita delle mani, ma se davvero poteste farlo, la progressione geometrica vi mostrerebbe un foglio di carta più alto di voi già con una quindicina di piegature; supererebbe un grattacielo solo con 22, scalerebbe l’Everest con 27, e da lì in avanti proseguirebbe sicuro verso gli spazi siderali. Una vera e propria “astronave” fatta solo da un foglio di carta, da un po’ d’immaginazione e giusto un po’ di matematica.
È a questo punto la domanda facile facile, quella che potrebbe portarvi a vincere il consueto abbonamento semestrale alla rivista, può essere posta in questo modo: quante piegature serviranno per arrivare al centro della Galassia di Andromeda, supposta ad una distanza di 2,5 milioni di anni luce da noi?

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