Short period comets visit the sun so often that they quickly evaporate, vanishing in only a few hundred thousand years. Here's one that evaporated in the sun's corona in July 2011. It was traveling at 644 kilometers per second. That's 400 miles per second. Here we see it in time lapse from the solar dynamics observatory. The comet enters on the right in this video. In the second pass, I've marked it along the way. It starts out with the mass of an aircraft carrier and it's completely evaporated in 20 minutes by the searing heat of the sun's corona. We evaporate so quickly compared to the age of the solar system that we shouldn't see any left at all, yet we routinely track dozens of them every year. In 1951, Gerard Kuiper proposed that there must be a belt of icy bodies orbiting beyond Neptune that is the source for new comets. It is much further away, much larger and far less dense than the asteroid belt. It starts at 30 astronomical units from the sun and is 20 astronomical units wide. 1992, QB1 was the first Kuiper belt object. In 2002, a large KBO, hundreds of kilometers in diameter, named Kuawa was found. In 2004, KBO Sedna was discovered. And we have seen that Pluto and Machi Machi are to work planet KBOs. It is estimated that the Kuiper belt contains 100 million comets. In 1950, in order to explain long period orbits, the Dutch astronomer Jan Orte proposed the existence of a cloud of comets between 5,000 and 55,000 astronomical units from the sun. Other estimates haven't going out much further. All estimates put the Orte cloud well outside the heliosphere. Orte estimated that this reservoir contains 100 billion comets. Maybe a trillion. Spring, Elbop and Lovejoy 2014 are three of them. No space probe has yet been sent to the area of the Orte cloud. Voyager 1 in the heliosheath is traveling at 1.6 million kilometers per day. That's a million miles per day. It will take over 1,200 years just to reach the Orte cloud. It's been over 12,000 years to pass through it. So its existence will remain a theory for some time to come.