填空题 Space was explored as early as the fourth century BC, through ancient astronomy. It was only in the twentieth-century that man sent out {{U}} {{U}} 1 {{/U}} {{/U}}to explore space. Space exploration, then, can be {{U}} {{U}} 2 {{/U}} {{/U}}: astronomy, unmanned probes, and manned probes. Although {{U}} {{U}} 3 {{/U}} {{/U}}just drawn, man is the explorer in all of these categories; it is {{U}} {{U}} 4 {{/U}} {{/U}}, technology, and understanding of science that forms the basis of all forms of space exploration. The exploration of space {{U}} {{U}} 5 {{/U}} {{/U}}, that is, man has "reason" to send men to the moon and to {{U}} {{U}} 6 {{/U}} {{/U}}, just to name a couple of such values. From ancient times to well into the twentieth-century, the {{U}} {{U}} 7 {{/U}} {{/U}}to explore space was astronomy, the studying of the millions of {{U}} {{U}} 8 {{/U}} {{/U}}, which invade night sky, as they have done for billions of years. The {{U}} {{U}} 9 {{/U}} {{/U}}and the ebbing of stars across the sky had originally {{U}} {{U}} 10 {{/U}} {{/U}}, but as man's understanding of the science of astronomy increased, {{U}} {{U}} 11 {{/U}} {{/U}}, and not dogma, took form. And, as a solid foundation was laid with {{U}} {{U}} 12 {{/U}} {{/U}}, man walked resolutely into the Space Age, upon the advent of {{U}} {{U}} 13 {{/U}} {{/U}}. Given this stepping stone of the liquid fueled rocket, man was able to enter the cosmic "ocean". {{U}} {{U}} 14 {{/U}} {{/U}}, during the Cold War era, allocated millions of dollars to the exploration of space, but {{U}} {{U}} 15 {{/U}} {{/U}}in the later part of the twentieth-century. {{U}} {{U}} 16 {{/U}} {{/U}}, as a function of government and public support, came {{U}} {{U}} 17 {{/U}} {{/U}}, with the Apollo program. The public has generally been more supportive of {{U}} {{U}} 18 {{/U}} {{/U}}, but the costs and the values at risk are malignant to the support of space exploration as a whole. Today, economic resources for space exploration are scarce and public, and thus {{U}} {{U}} 19 {{/U}} {{/U}}. The glorious Apollo missions are impossible to reconstruct, and instead there has been a steady trend {{U}} {{U}} 20 {{/U}} {{/U}}.