Life in the future（Period 4　Reading and Writing）

the third goal is to return to the moon by 2020, as the launching point for missions beyond. beginning no later than , they will send a series of robotic missions to the lunar surface to research and prepare for future human exploration.using the crew exploration vehicle, they will undertake extended human missions to the moon as early as , with the goal of living and working there for increasingly extended periods.with the experience and knowledge gained on the moon, man will then be ready to take the next steps of space exploration: human missions to mars and to worlds beyond.yet the human thirst for knowledge ultimately cannot be satisfied by even the most vivid pictures, or the most detailed measurements. and only human beings are capable of adapting to the inevitable uncertainties posed by space travel.

galaxyany of the systems of stars and interstellar matter makes up the cosmos.many such assemblages are so enormous that they contain hundreds of billions of stars.nature has provided an immensely varied array of galaxies, ranging from faint, diffuse dwarf objects to brilliant, spiral-shaped giants.virtually all galaxies appear to have been formed soon after the universe began, and they pervade space, even into the depths of the farthest reaches penetrated by powerful modern telescopes.galaxies usually exist in clusters, some of which in turn are grouped into larger clusters measuring hundreds of millions of light-years across.(a light-year is the distance traversed by light in one year, traveling at a velocity of 300 000 kilometers per second, or 650 000 000 miles per hour.) these so-called super clusters are separated by nearly empty voids, causing the gross structure of the universe to look somewhat like a network of sheets and chains of galaxies.

教學(xué)資源，完全免費(fèi)，無(wú)須注冊(cè)，天天更新！" type="#_x0000_t75">galaxies differ from one another in shape, with variations resulting from the way in which the systems were formed.depending on the initial conditions in the pregalactic gas some 15 000 000 000 years ago, galaxies formed either as slowly turning, smoothly structured, round systems of stars and gas or as rapidly rotating pinwheels of such entities.other differences between galaxies have been observed and are thought to reflect evolutionary changes.some galaxies are rife with activity: they are the sites of star formation with its attendant glowing gas and clouds of dust and molecular complexes.others, by contrast, are quiescent, having long ago ceased to form new stars.perhaps the most conspicuous evolutionary changes in galaxies occur in their nuclei, where evidence suggests that in many cases super massive objects—probably black holes—formed when the galaxies were young.such phenomena occurred several billion years ago and are now observed as brilliant objects called quasars.