填空题

Do you have any knowledge of the space station A space station is a place 【21】 people can and work on space 【22】 long periods. It orbits the earth, usually about 200 to 300 miles high. A space station may 【23】 as an observatory, laboratory, factory, workshop, warehouse, and fuel depot. Space stations are much larger than manned spacecraft, 【24】 they provide more comforts. Manned spacecraft may 【25】 people between the earth and the space station. Unmanned spacecraft may supply the station 【26】 food, water, equipment, mail and so on. Small space stations can be 【27】 on the earth and launched orbit by 【28】 rockets. Larger stations are 【29】 in space. Rockets or space shuttles 【30】 modules of the station into space, where astronauts assemble them. Old modules can be replaced, and new modules can be 【31】 to expand the station. A space station has 【32】 one docking port 【33】 which a visiting spacecraft can attach itself. Most docking ports 【34】 of a rimmed doorway called a hatch that can connect 【35】 the hatch on the visiting spacecraft to form an airtight seal. When the two hatches open, they form a pressurized tunnel between the station and the visiting spacecraft. The main tasks of a space station crew involve scientific research. For example, they might analyze the 【36】 of micro gravity on various materials, 【37】 the earth’’s surface, or study the stars and planets. Astronauts 【38】 a space station also 【39】 much of their time to the assembly of equipment and the expansion of the station’’s facilities. This includes 【40】 beams, connecting electrical and gas lines, and welding permanent joints between sections of the station. The crew must also fix or replace broken equipment.