Sex in space, specifically human sexual activity in the weightlessness of outer space, presents difficulties 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. The discussion of sex in space has also raised the issue of conception and pregnancy in space.