TOKYO—“Space now,” was what Japanese billionaire Yusaku Maezawa wanted to writer on Twitter for years. He finally really did it, from the International Space Station. “The space market holds so much potential,” he said Friday at the Foreign Correspondents’ Club in Tokyo, his first news conference in Japan after returning to earth before Christmas. Maezawa, who heads a company called Start Today, is preparing to invest in various businesses which may develop from the ongoing research by NASA, the Japanese equivalent called JAXA, and others. But he wants first to recover from his recent celestial adventure: returning to life with gravity has proved heavier than he’d expected, he said. Maezawa, 46, blasted off in a Russian Soyuz spacecraft along with a Russian cosmonaut on Dec. 8, becoming the first self-paying tourist to visit the station since 2009. He returned to earth after spending 12 days at the orbiting outpost, where …