填空题

A project likely to evolve in. the near or intermediate future is space tourism, a purely (36) activity that would not have to rely on unpredictable government decisions. It would (37) put space operations on a much more solid, dependable footing and would enormously (38) man’s space activity. While some people see space tourism as a (39) multibillion-dollar business, difficult technical problems remain to be (40) First, a manned reusable rocket like the Venture Star must be built. Then, a second serious problem must be (41) : even today’s most-advanced space rockets are able to (42) only about 2.5% of their launch weight into orbit-- a ratio that has hardly changed in the last 40 years. This fact is a result of the (43) of material characteristics and chemical power sources, and (44) . Today the price of a ticket to space would have to be at least ＄ 50 000. It is questionable (45) . However, space tourism may become more attractive once space facilities are built (46) .