This week’s green light signal from New Horizons tells us that it’s continuing its mission through the Kuiper Belt. At the present time, the spacecraft is 40.55 astronomical units from the Sun and just under 3 AU from its next target, 2014 MU69. It’s moving at 14.19 kilometers per second and at that rate will get to MU69 on December 31, 2018. To give you some perspective on where it is in the Kuiper Belt, the inner edge of this distant region is about 30 AU and the outer edge is around 50 AU. So, the spacecraft is well within the third “regime” of the solar system. (Note: an astronomical unit is the distance between Earth and the Sun, 150 million kilometers.)

Deducing the Kuiper Belt

Planetary scientists haven’t always known the Kuiper Belt existed. However, the discovery of Pluto suggested that there might be more bodies beyond Neptune, populating a region between that planet and the Oort Cloud (the most distant reaches of the solar system). That got people to thinking about a theoretical population of objects dubbed “Trans-Neptunian Objects”, or TNOs. Today, some are also referred to as Kuiper Belt Objects (KBOs). Several astronomers suggested that such objects were out there, but of course, their distance, size, and likely dim surfaces would make them hard to find from Earth.

Eventually, astronomer Kenneth Edgeworth suggested that materials left over from the original proto-solar system nebula of gas, dust, and ice existed “out there”. He theorized they’d be too thinly spread out over the region to form large planets. Instead, this area could be populated by smaller worlds, like Pluto. In addition, some of its materials would be cometary nuclei. They occasionally get knocked out of their orbits and sent sunward. Another astronomer, Gerard Kuiper, also suspected a disk of material once orbited “out there”, but questioned whether it still existed. He thought that gravitational influences from a once Earth-sized Pluto would have knocked all the region’s bodies out of orbit.

Both men were partially right. There is a huge torus-shaped belt of material “out there”. It turns out there are also a number of fairly large worlds (although none so far are Earth-size). However, the idea that Pluto was once an Earth-sized world is wrong. Further discoveries, particularly in studies of short-period comets, confirmed the existence of bodies in this region. Eventually, it was named the Kuiper-Edgeworth Belt for the two men. Another astronomer, Julio Fernández, suggested a “comet belt” between 30 and 50 AU could account for many of the comets observed from Earth. That region coincides with the Kuiper Belt, and some astronomers suggest that the region really should include his name, as well.

The Kuiper Belt Today

Today, the region of space beyond Neptune is called the Kuiper Belt. It is now known to be home to a handful of dwarf planets. Some astronomers suspect there is at least one larger, massive world out there, but so far nothing has been directly seen. They have only gravitational influences on other objects to go by to suggest it even exists. Continuing observations will undoubtedly uncover it, not only from the ground, but by space-based observatories.

Exploring the Kuiper Belt In Situ

Due to its distance, the Kuiper Belt is not well-studied by spacecraft. Much of what we know about it comes from ground-based observations. Astronomers David Jewitt, Jane Luu, and more recently Michael Brown and others have made many observations. Their efforts paid off in the discovery of such worlds as Quaoar, Eris, Haumea, and others in the trans-Neptunian region. Further studies of cometary bodies now suggest that the Kuiper Belt is not exactly the home to all short-period comets. Instead, they come from a region called the “scattered disk” that lies outside of the orbit of Neptune as well. Some of its comets have approached the Sun, others remain out in this region of interplanetary space.

While some spacecraft (notably the Voyagers and Pioneers) are beyond Neptune, no missions explored the Kuiper Belt until New Horizons got there. As we all know, it imaged and studied Pluto and its moons in 2015 in a spectacularly successful flyby. That was one of its main goals. The others focus it on the conditions in the Kuiper Belt as it flies along to its next target. This is the kind of “scouting” mission needed to understand what the Belt is really like. The data it sends back will, one day, inform a mission that will circle Pluto and study that world extensively. And, who knows, maybe other missions will traverse the Kuiper Belt at different points, charting this otherwise unknown territory of the solar system.