 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 km 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 this 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 twenty minutes by the searing heat of the sun's corona. They 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 Kwawa, was found. In 2004, KBO Sedna was discovered. And we have seen that Pluto and Maki Maki are dwarf planet KBOs. It is estimated that the Kuiper Belt contains a hundred million comets. In 1950, in order to explain long period orbits, the Dutch astronomer Jan Ort proposed the existence of a cloud of comets between 5,000 and 55,000 astronomical units from the sun. Other estimates have it going out much further. All estimates put the Ort cloud well outside the heliosphere. Ort estimated that this reservoir contains a hundred billion comets. Maybe a trillion. Sighting spring, ale bop, and lubejoy 2014 are three of them. No space probe has yet been sent to the area of the Ort 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 Ort cloud. And over 12,000 years to pass through it. So its existence will remain a theory for some time to come.