Sex in space is human sexual activity in the weightlessness of outer space. It presents difficulties for the performance of most sexual activities due to Newton's third law. According to the law, if the couple remain attached, their movements will counter each other. Consequently, their actions will not change their velocity unless they are affected by another, unattached, object. Some difficulty could occur due to drifting into other objects. If the couple have a combined velocity relative to other objects, collisions could occur. There have been suggestions that conception and pregnancy in off-Earth environments could be an issue.