Many people believe aliens visit Earth, but scientists give strong reasons why this is unlikely. The first reason is that space is incrediblyincredibly/ɪnˈkrɛdɪbli/L2难以置信地；非常地extremely; in a way that is hard to believe vastvast/væst/L2广阔的；巨大的very large in area or size. The nearest star to our Sun is 40 trillion kilometers away, which is 268,000 times farther than the Sun. This distance is so great that our fastest spacecraft would need over six thousand years to reach it.
High-speed travel also creates a problem called time dilation. Because time moves slower for objects in motion, passengers on a fast spaceship would return to a much older planet. In addition, the energy needed for an interstellarinterstellar/ˌɪntərˈstɛlər/L2星际的between or among stars journey is impossibly high. This is because an object's mass increases with speed, demanding more and more energy to accelerate.
Another issue is Earth's unique biospherebiosphere/ˈbaɪoʊsfɪr/L2生物圈the part of Earth where living things exist, which is filled with oxygen. Oxygen is highly reactivereactive/riˈæktɪv/L2反应性的；易反应的likely to react chemically with other substances and could be very dangerous for alien life forms. This is why scientists continue to search for alien life using powerful telescopes and radio signals. As a famous 1959 scientific paper stated, "If we don't search, the chance drops to zero."
