The yellowing, line sketch that Herge made for the 1942 book was bought by a European investor, Marina David of Petits Papiers-Huberty-Breyne, specialist comic book dealers, told AFP.
David declined to give further details, saying only "the buyer is neither Belgian nor French," with the sale showing just how strong the market is for comic-book art.
The published, colour version of the cover shows Tintin and his faithful dog Snowy looking on in astonishment as a huge, red and white-capped mushroom swells up out of a barren rocky beach, part of a meteorite, hence the title which has fallen into the sea.
Since then, investors and collectors alike have looked more and more seriously at comics as potentially valuable works of art in their own right.
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In May, a two-page spread depicting Tintin in various risky escapades against the villains of his day sold for 2.65 million euros in Paris, at the time a world record equal to USD 3.58 million.
In June, a near-flawless edition of the first book featuring Superman dating from June 1938 fetched USD 3.2 million, according to auction site eBay.
David said that apart from the Tintin cover, the gallery also found buyers for about 60 pieces by Philippe Geluck, the Belgian designer behind the Le Chat series, while works by Francois Avril and Dominique Corbasson who use the comic book format for their paintings also did well.
The "Shooting Star" cover design was put up for sale by a private collector.
Most of Herge's work is held by a family foundation which jealously guards his legacy and so increases the rarity value of any work coming to market.