As more and more nations put satellites into space,the risk of collision can only increase. Measures are already being taken to control the growth of orbital debris. The United States has always required its astronauts to bag their wastes and return them to Earth. The United States Air Force has agreed to conduct low-altitude rather than high-altitude tests of objects it puts into space so debris from tests will reenter the Earth's atmosphere and burn up. Extra shielding will also reduce the risk of damage. For example,2,000 pounds of additional shielding is being considered each of six space-station crew modules. Further,the European Space Agency,an international consortium, is also looking into preventive measures.