NASA's Mission to Pluto and the Kuiper Belt
In 1992, Dave Jewitt and Jane Luu at the University of Hawaii discovered a small object, designated 1992QB1, orbiting the Sun beyond Neptune at a distance of about 40 AU. Since then, more than 3,100 similar objects with orbits beyond Neptune have been discovered, and scientists estimate there are several hundred thousand objects bigger than 20 miles across waiting to be discovered in that vast region of the solar system.
We call this swarm of bodies the Kuiper Belt, in honor of Dutch-American astronomer Gerard Kuiper, who speculated about the existence of small bodies beyond Neptune in the 1950s. Some call this the Edgeworth/Kuiper Belt, sharing the honor with Irish scientist Kenneth Edgeworth, who published a similar idea in the 1940s. The inhabitants of this realm are called Kuiper Belt objects (KBOs), Edgeworth/Kuiper Belt objects, or simply Trans-Neptunian objects (TNOs). It is very likely that most of the short-period comets found in the inner solar system come from the Kuiper Belt, being held there in cold storage until random gravitational tugs from Neptune nudge them inward.
The Minor Planet Center of the IAU at Harvard/Smithsonian web page shows the latest information about KBOs, including plots of their current locations and properties of various distant minor bodies.
We have learned a lot about the Kuiper Belt since its discovery — enough to classify its numerous inhabitants based on their orbits. The main categories are:
This animation displays a view of the outer solar system. The New Horizons spacecraft is represented by the white triangle, and the line it took through the solar system is shown with the white path. The gas giants are shown by the four large colored dots, while the small colored points depict the known Kuiper Belt objects, with color indicating the dynamical class. Red points signify Cold Classical objects, which are found in a tight ring, on low-inclination orbits. Blue points depict members of the scattered disk. These objects are on highly eccentric orbits. Some of their full orbits are presented by the light blue dashed lines. Magenta points and yellow points depict objects in the 3:2 and 2:1 mean-motion resonances with Neptune. The two New Horizons flyby targets, Pluto and Arrokoth (2014 MU69), are circled in white. Scale is marked by white dashed circles at 5, 30, 50, and 100 astronomical units.
Credit: Wes Fraser, National Research Council of Canada
"Cold Classical" KBOs:"Cold" here refers not to temperature but to the circular uninclined orbits of these objects. Cold Classical KBOs occupy a narrow region about 6 AU wide, between 42 and 48 AU from the Sun, and about 3 AU thick. There aren't any large (500 miles across or larger) KBOs in this group and they tend to be redder than other KBOs, so they might have a different origin. These objects are called "classical" KBOs since they are on the sorts of orbits that Kuiper talked about. The New Horizons Kuiper Belt flyby target, Arrokoth, is a member of the Cold Classical category. The Cold Classicals appear to be the gravitationally unperturbed and original material of the Kuiper Belt.
"Hot Classical" KBOs: Here, "Hot" refers to the non-circular and inclined orbits of these objects. Though they have a similar average distance from the Sun as Cold Classical KBOs, their eccentric and inclined orbits cause them to stray much farther from that average position. Like most KBOs, their sizes and colors vary, and they include larger and grayer objects than Cold Classicals. The Hot Classicals are in a disk that is about 12 AU thick.
Resonant KBOs are locked in an orbital dance with Neptune – that is, they orbit in resonant with that planet. The 3:2 resonant objects, which include Pluto, make two orbits around the Sun for every three of Neptune's orbits. These are sometimes called "Plutinos" or "little Plutos." The 2:1 objects are farther from the Sun and orbit once for every two orbits of Neptune. There are dozens of resonance groups in the Kuiper Belt.
Scattered KBOs have probably wandered too close to Neptune in the past, and Neptune's gravity has knocked them onto unstable orbits, sometimes taking them hundreds of AU from the Sun at their most distant point and bringing them closer to the Sun than Neptune at their closest.
The last category, sometimes called the Extreme TNOs, is so new that so far it has only a few known members, such as Sedna and 2012 VP113, and possibly 2015 TG387. These objects may end up not being considered part of the Kuiper Belt at all. Sedna never gets closer than 76 AU and reaches 1,000 AU at the most distant point of its 12,000-year orbit. Sedna is at least half the size of Pluto, and is probably one of the largest members of a huge population of undiscovered objects in this distant region of the solar system.
Because they are small and far away, KBOs look like faint stars even through the world's largest telescopes. They are so hard to see that the first classical Kuiper Belt object was just discovered in 1992. Astronomers can find Kuiper Belt objects among the myriad stars because KBOs slowly move relative to those stars. Because of the changing orbital positions of Earth and the Kuiper Belt objects, very detailed images of the sky taken many hours or days apart will show faint points of light that have slightly changed position compared to the distant stars that appear stationary. Those whose positions change slowly must be very far from the Sun — these are the Kuiper Belt Objects. (More easily discovered are faster moving asteroids much closer to the Sun, residing in the main asteroid belt between Mars and Jupiter.)
Modern astronomers use extremely sensitive digital cameras — highly specialized versions of the digital cameras now in wide use by shutterbugs around the world – to discover KBOs. Digital cameras used by astronomers are so sensitive that they must be operated at extremely cold temperatures, around minus-58° to minus-148° Fahrenheit (minus-50° to minus-100° Celsius).
The largest two Kuiper Belt objects are Pluto and Eris, with Pluto being slightly larger at a diameter of 2,377 kilometers (1,477 miles). There are between three and seven other known KBOs with diameters that are approximately 950-1,500 kilometers (600-900 miles), including Pluto's largest moon, Charon. Scientists believe additional KBOs in the 950-2,000 kilometer size range will be found, but most KBOs are much smaller.
Kuiper Belt objects exhibit different reflectivity and colors. Pluto is very bright, with an average reflectivity of 0.5. For comparison, Earth's Moon's reflectivity is only about 10%. The high reflectivity of Pluto was known from ground- and space-based telescopic measurements before New Horizons flew by in 2015. New Horizons' images of Pluto revealed the surface brightness was due to both geologic activity (convective overturn of nitrogen ice in Pluto's Sputnik Planitia) and the deposition of fresh ice, from condensation of atmospheric volatiles.
Other KBOs have darker surfaces with reflectivity in the 4-20% range. The darkest bodies may be covered by chemically complex carbon-rich polymers. The wide range of KBO colors – from grey to red – suggests a wide diversity in the composition and evolution of these bodies. Due to the faintness of KBOs, it has been difficult to obtain infrared spectra that indicate the existence of specific minerals and ices. The faint signature of water ice has been detected on the surface of several KBOs from ground-based telescopic measurements, and also on Pluto and Charon from instruments on New Horizons.
Roughly 125 KBOs have companions, and more are being discovered all the time. They are often called "binary KBOs" because the objects have similar size, so it's not clear which is the "KBO" and which is the "moon!" The best-known pair is Pluto and Charon; Charon orbits Pluto every six days at a distance of about 17,000 kilometers (10,000 miles). Some pairs have much slower orbits (up to 17 years) and greater separation (up to 100,000 kilometers, or 60,000 miles), while others are very close. We don't yet know how these binary KBOs form, but collisions or close encounters with other objects are likely involved. Only the largest KBOs are expected to have atmospheres, and a tenuous atmosphere was detected on Pluto from ground-based telescopic measurements before New Horizons arrived at Pluto. Pluto's atmosphere – now known from New Horizons measurements to have a density about 100,000 times smaller than Earth's – could potentially freeze out on the surface as Pluto continues to recede from the Sun over the next several decades.
Comets are usually classified into two main families depending on their orbits around the Sun. Oort Cloud comets come from a roughly spherical-shaped region between 10,000 and 100,000 AU from the Sun and typically have orbital periods of about a million years. Jupiter family comets have orbits strongly influenced by Jupiter's gravity and usually need less than 20 years to travel once around the Sun.
When the solar system formed nearly 4.6 billion years ago, about 10% of the comets that formed near the giant planets (perhaps a trillion total) were ejected into the Oort Cloud. Occasionally, gravitational perturbations by the galactic tide and nearby passing stars sent some Oort Cloud objects toward the Sun, creating the "new comets" we see today. In contrast, Jupiter family comets are thought to come from the Kuiper Belt, pulled into the inner solar system by Jupiter's and Neptune's gravity. Nearly 150 short-period comets are now classified as "JFCs."