TOKYO, June 5 (Reuters) - Shares of Japanese space junk removal startup Astroscale Holdings rose 51% in their debut on Tokyo's growth market on Wednesday.

The stock first traded at 1,281 yen per share, compared with an initial public offering (IPO) price of 850 yen, valuing the company at 145 billion yen ($934 million).

Astroscale sold 12.49 million shares in Japan and 9.68 million shares overseas, after pricing the IPO at the top of its indicated 750-850 yen range. That doesn't include a greenshoe option - allowing security underwriters to sell more if there is additional demand - of 3.12 million shares.

The startup has won government backing in Japan, the United States and Britain as it develops technology to remove orbital debris seen as a collision risk and to extend the life of satellites.

Astroscale follows suit of moon explorer ispace and radar satellite maker iQPS, which went public last year as investors, especially retail traders, pay growing attention to Japanese space companies backed by the government's policy.

Japan has pledged billion-dollar space grants and contracts for private firms to boost its national security and to double the size of the domestic space industry to 8 trillion yen by the early 2030s. ($1 = 155.2700 yen) (Reporting by Kantaro Komiya; Editing by Christopher Cushing and Gerry Doyle)