The Kuiper belt is a concentration of asteroid-like objects in orbit around the Sun, beyond Neptune. Since the belt was discovered in 1992, the number of known Kuiper belt objects (KBOs) has increased to over a thousand. And computed projections indicate that there are likely to be more than 70,000 of them. But, running contrary to the calculations, observations show that the number of KBOs dramatically falls off at about 47.8 AU from the Sun. There is no agreed explanation.