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[19.0] Missions To The Kuiper Belt

v2.0.0 / chapter 19 of 20 / 01 jul 16 / greg goebel

* In 1930, a young man named Clyde Tombaugh who was working as what amounted to a technician at an astronomical observatory became the world's most famous astronomer by discovering the world "Pluto", which became known as the "ninth planet". Little was known about Pluto at the time beyond the parameters of its orbit. Over the following decades, astronomers learned more details about Pluto, and then discovered that it wasn't alone in the outer reaches of the Solar System, being simply a large member of the Kuiper Belt objects.

Pluto & Charon


[19.1] DISTANT PLUTO
[19.2] THE DISCOVERY OF PLUTO
[19.3] THE REDISCOVERY OF PLUTO
[19.4] MISSION TO PLUTO
[19.5] ANOTHER PLANET X?
[19.6] WORLDS OF THE KUIPER BELT

[19.1] DISTANT PLUTO

* Although Pluto is no longer seen as unique, more is known about it than any other large KBO, and so it is worth discussing in detail. Pluto actually turned out to be a "system" of two worlds, consisting of Pluto proper, with a diameter of 2,320 kilometers, about two-thirds that of the Earth's Moon, and a moon named Charon, with a diameter of about 1,200 kilometers. Pluto and Charon have a separation of only 19,570 kilometers and are tidally locked, always keeping the same face towards each other. Charon orbits around Pluto once every 6.4 Earth days, the same length as Pluto's day. Pluto itself has an axial tilt of 122 degrees relative to the plane of its orbit, or in other words it roughly orbits on its "side".

The Pluto system orbits the Sun at an average distance of 5.9 billion kilometers, about 39 astronomical units. Its orbit has a relatively high degree of eccentricity, and actually comes closer to the Sun than Neptune for part of its orbit. Pluto cannot collide with Neptune, however, due to orbital dynamics and also because Pluto's orbit has a high inclination of 17.2 degrees to the ecliptic and never crosses Neptune's path. Pluto was inside the orbit of Neptune from 1979 to 1999.

Total mass of the Pluto system is only 0.2% that of the Earth, with Charon only being about 15% of the mass of Pluto itself. Pluto has a density of about two grams per cubic centimeter, or about twice that of water, and is thought to have a very tenuous atmosphere of methane and other gases. The atmosphere is believed to be temporary, since Pluto is currently relatively close to the Sun and comparatively warm. The atmosphere is expected to freeze sometime in the near future.

A visitor to Pluto would see the Sun as no more than a point of light, 900 times fainter than as seen from Earth -- though far brighter than any other star, and still brighter than the full Moon as seen from Earth. Temperatures on the surface of Pluto average about 50 degrees Kelvin, while surface gravity is only about 5% that of Earth. Although Charon is much smaller than our Moon, it is so much closer to its primary that it spans 8 degrees of arc, meaning it appears eight times bigger in Pluto's sky than the Moon does as seen from Earth.

This small amount of information was almost all that is known about Pluto and Charon at the beginning of the 21st century. In fact, it was largely due to a series of lucky accidents that we even knew this much about the Pluto system.

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[19.2] THE DISCOVERY OF PLUTO

* The discovery of Pluto in 1930 was the first of the lucky accidents. Leverrier's discovery of the planet Neptune in 1846 though an analysis of irregularities in the orbit of Uranus suggested that the same approach might be used to find other, more distant planets in orbit around the Sun. However, after the orbit of Neptune was determined with what was believed to be high precision, astronomers found that its irregularities were so small as to be noise. Few astronomers believed that any meaningful results could be extracted from such meaningless data. A few were still willing to give it a try, particularly Percival Lowell. Although his major focus at his Flagstaff observatory was the study of Mars, he also had a significant interest in the hunt for a "trans-Neptunian" planet.

His first search for the ninth planet began in 1905, with Lowell continuously revising his calculations and changing the agenda. By 1907, 440 photographic plates had been obtained and examined, but Lowell found nothing. Lowell was initially secretive about his efforts, but he found that he had little competition in such a difficult and unrewarding job, and finally began to publicize his efforts to track down what he called "Planet X".

On going public, Lowell found that he wasn't completely alone in his search. William H. Pickering of Harvard was conducting an analysis of his own to hunt down what Pickering called "Planet O". Pickering suspected there was more than one trans-Neptunian planet, and came up with a series of planets designated "Planet O" through "Planet U". Lowell was apprehensive about competition from Pickering, until Lowell examined Pickering's research in detail and judged it to be casual, no real threat.

Lowell redoubled his efforts to find "Planet X". He needed more powerful instruments and better ways of searching photographic plates. Various telescopes and telescopic cameras were bought, begged, or borrowed to take care of the first problem, and a new invention, the "blink comparator", was eventually obtained to take care of the second. The blink comparator was an optical device that allowed two photographic plates to be placed side by side and then examined under an eyepiece. The comparator switched the view rapidly back and forth between the plates, and anything that was different between the two plates would appear to "jump" slightly.

The Lowell Observatory in Flagstaff began the search for "Planet X" in earnest in 1912, and for the next four years Lowell cycled between excitement and depression as he conducted the hunt. Ironically, "Planet X" was actually photographed -- but since Lowell was expecting a larger and brighter object, the tiny dot went unnoticed.

* Lowell died of a stroke at age 61, on 12 November 1916. The Lowell Observatory had been his passion, so much so that he was interred in a mausoleum on the observatory's grounds. He had intended that the search continue after his death, specifying a million-dollar endowment for the research in his will, but Lowell's widow challenged the will in court and the litigation wasn't resolved until 1927. As soon as the estate was settled, the observatory begin implementing another search, obtaining a 33 centimeter photographic refractor through a grant of $10,000 USD from the president of Harvard, Abbott Lawrence Lowell, Percival's brother.

The new instrument was installed in 1929. The next step was to find someone to perform the tedious search. The observatory's director, Vesto M. Slipher, had received a letter from a Kansas boy named Clyde W. Tombaugh, who was an enthusiastic amateur astronomer and was seeking employment. Tombaugh was only a high-school graduate, but the search for "Planet X" did not require extensive scientific training, and so Slipher asked Tombaugh to come out to Flagstaff for a trial period. The 22-year-old Tombaugh arrived on 15 January 1929. It is common for idealistic young people to take a leap into the unknown, and though such ventures often don't pan out, for Tombaugh it was to pay off beyond his wildest dreams.

* Tombaugh quickly found the entire search for "Planet X" dumped into his lap. He conducted the photographic survey and the tedious work at the blink comparator. The use of the blink comparator was painfully difficult. The comparator provided a large number of false leads. Given that the search covered very faint objects, any flaw in the photographic plates could lead to a false sighting. Variable stars that fluctuated in and out of visibility were also a difficulty. Asteroids were a major problem, particularly when the faster movement of the Earth along its orbit caused an asteroid to slow and then reverse its apparent motion along the sky for a short period of time.

Even without such distractions, the work was hideously tiresome. Each plate took an hour to expose, and blinking required three days of work to cover each 12-by-14-degree field of view. The average star count per plate was 160,000, and the dreaded Milky Way regions gave counts of over a million. Tombaugh was extraordinarily dedicated, but even he could not stay at the comparator for more than 3 to 6 hours at a time. Worse, he had a sense that he was pursuing a dead end.

He wasn't. On 18 February 1930, two weeks after his 24th birthday, Tombaugh noted with extreme excitement the faintest of dots flickering back and forth in the blink comparator. He carefully examined his work, doublechecking with older plates, and when he was sure that he had something, he went to Vesto Slipher and said: "I have found your Planet X."

Tombaugh was not an exciteable young man, not inclined to make hasty statements, and Slipher knew Tombaugh might well be on to something. Further observations were made to verify the discovery, and it was announced on 13 March 1930, Percival Lowell's 75th birthday, and also the date of the discovery of Uranus, 149 years previously. "Planet X" had been found within six degrees of the location predicted by Lowell from his calculations.

The orbit for the object was worked out by several astronomers in 1930 and 1931. This process was simplified by the fact that the object showed up, if very faintly, on well over a hundred archival photographic plates going back to 1914. The name of the new world was provided by an 11-year-old English schoolgirl from Oxford named Venetia Birney, who suggested the name Pluto, god of the underworld and darkness. The first two letters were also the initials of Percival Lowell, and the shorthand symbol became a merged "P" and "L", Percival Lowell's monogram.

Tombaugh became a celebrity. He received a scholarship to the University of Kansas, obtaining a master's degree in astronomy between 1932 and 1939. The department head was appalled when Tombaugh, one of the world's best-known astronomers, proposed taking a freshman astronomy class. Tombaugh went on to a prominent career as a professional astronomer.

* The discovery of Pluto led to many questions. One of the most puzzling was the fact that Lowell had predicted that "Planet X" would have a mass 6.6 times that of Earth. The tiny dot that was actually discovered could not possibly be that big, unless it was extraordinarily dark. After the discovery of Charon in 1978, which permitted determination of the mass through a few simple calculations, the mass of the Pluto system was determined to be about 500 times smaller than that of the Earth. If the discovery of Pluto had been a lucky accident, it still had to be credited to Percival Lowell, since it was his funding and vision that led to it. The finding of Pluto was a matter of luck, skill, and persistence.

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[19.3] THE REDISCOVERY OF PLUTO

* Pluto is so distant that even in the largest telescopes it is barely more than a dot of light, and so for decades almost nothing was known about it other than its orbital parameters. Estimates of its diameter, for example, ranged from 6,000 to 14,000 kilometers, with a comparable range of estimates for mass. In 1955, analysis of the cycle of changes of Pluto's brightness using a sensitive photomultiplier tube demonstrated that it rotated once every 6.4 days.

In the 1970s, observations provided a new and much improved picture of the Pluto system. In 1976, careful spectroscopic analysis confirmed the presence of methane ice on its surface, which suggested that Pluto was more reflective than expected and so probably smaller than had been assumed to that time. Instead of being a big object that was very dark, it was a small object that was very bright. The size estimates dropped so steadily over the years that some astronomers constructed a curve of declining size estimates to show that in 1980, it would disappear completely.

They were being silly, of course. However, the silliness was then replaced by excitement. The breakthrough took place in 1978. James W. Christy of the US Naval Observatory in Washington DC was performing precise measurements of the orbit of Pluto, using photographic plates taken by the 1.5-meter telescope of the Naval Observatory in Flagstaff, not far from the Lowell Observatory. On one plate, Christy noticed that the image of Pluto seemed to have a tiny bump on it. This had been noted on several plates generated by the observatory, but the astronomers had assumed it was a flaw in the plates, and had marked them as "defective". Christy took a more careful look and observed that images of the stars on the plate looked fine. He examined other plates, and found that the bump seemed to move around Pluto. Either Pluto had one hell of a big mountain on it or ...

Christy told his colleague Robert S. Harrington: "Bob, Pluto's got a moon!" Harrington replied: "Jim, you're crazy." Christy did some more homework, inspecting archival plates from 1970, and found that the bump moved around Pluto every 6.4 days, in synch with Pluto's rotation rate. Harrington finally decided that Christy wasn't crazy after all, and helped him verify the discovery.

The discovery was announced on 7 July 1978. As the official discoverer, Christy had the right to name the new object, and called it "Charon", after the boatman who ferried the dead across the river Styx into Pluto's domain of the underworld. Christy also wanted to honor his wife, Charlene, but quickly found out that "Charon" is pronounced "Karen", not "Sharon". The International Astronomical Union didn't formally recognize the name until 1985, after observational data had finally reached a mass where no one could possibly doubt that the bump Christy had seen was a moon, and not a ridiculously big Plutonian mountain.

* That was the second lucky accident in the history of Pluto exploration. Astronomers back to Clyde Tombaugh had looked for a moon of Pluto and had come up empty-handed, partly because Pluto was farther away at that time. Otto Franz of the Lowell Observatory had actually seen the elongation of Pluto's image on a plate in 1965, but to his later embarrassment, he shrugged it off at a photographic error.

The discovery of Charon meant that the mass of Pluto could finally be calculated with reasonable precision. Newton's laws of gravity provide a simple relationship between the distance between two bodies in orbit and their combined mass. The distance was determined with high precision by optical interferometric techniques, and the resulting estimate of 0.2 Earth masses for the system was an order of magnitude smaller than previous estimates.

* By a third lucky accident, the discovery of Charon occurred at almost exactly the right time. Twice during Pluto's 248-year orbit around the Sun, the orbital plane of Charon is directly edge-on to a line-of-sight from Earth, and the moon will "eclipse" its Pluto as it crosses in front, and be occulted in turn by Pluto as it passes behind. Timing these "transit events" could reveal the diameter of the two bodies, and also implicitly allow determination of their density and albedo. Other benefits provided by the transit events were information about gross features of the surface of Pluto and Charon, the tilt of Pluto's axis, and improved spectroscopic analysis of the two worlds.

The first observable transits were calculated by Leif Andersson -- a Swedish-born researcher at the University of Arizona's Lunar & Planetary Science Laboratory in Tucson -- to begin in the early 1980s. This was remarkably fortunate, since the last time when eclipses could have been observed was during the American Civil War, and had Charon been discovered just a few years later, astronomers would have had to wait for generations for another period of transits.

Transit observations began in 1982, but nobody succeeded in observing a transit until 1985, with total occultations of Charon beginning in 1987 and the transits ending in 1990. Brightness changes were measured to within a few tenths of a percent. The transits provided a flood of information on Pluto and Charon, and after decades of frustration astronomers quickly learned Pluto's mass, density, axial tilt, and other details. Unfortunately, Andersson had died of lymphatic cancer in 1979 at age 35 and never had the honor of seeing the results of his work.

* A stellar occultation by Charon in July 2005 provided more details about the moon, showing it had no atmosphere and pinning its diameter down to about 1,206 to 1,212 kilometers in diameter. By this time, it was known that Charon wasn't alone; much to everyone's surprise, Pluto turned out to have more than one moon:

Styx turned out to be about twice as far away from Pluto as Charon, with Nix about 6,000 kilometers further out. Kerberos is about three times as far away, Hydra about four times.

Pluto & its moons

* Pluto turned out to have an unusually high albedo, reflecting half the light that fell on it, which is why its size and density had been overestimated for so long. Charon reflected about 40% of the light that fell on it. Spectral analysis showed that Pluto seemed to be covered by a frost of methane, which had been suggested by the observations in the 1970s, while Charon appeared to be covered by water ice, making it less reflective. It is believed that Charon's very low gravity prevented it from retaining methane, resulting in a different appearance from Pluto.

Density estimates for Pluto gave a result of a little more than two grams per cubic centimeter. Pluto had been earlier thought to be basically a huge iceball, but the high density figures imply that it has a substantial rocky core.

* Another issue for astronomers was whether Pluto had an atmosphere. Traditionally, the best evidence for an atmosphere was to observe the occultation of a star by an object and see how quickly the star "winked out". Predicting a stellar occultation by Pluto was tricky because it is so small, and the influence of Charon introduces a wobble into the orbit. Astronomers attempted to observe stellar occultations by Pluto in 1965, 1980, and 1985, and failed all three times. The predicted 1965 and 1980 occultations didn't happen, though interestingly the 1980 observations measured a dead-on occultation of Charon. The 1985 occultation did take place, but one of the few places where it was observable was in northern Israel, and the photographic plates were ruined by a night dogfight between Israeli and Syrian fighter jets.

Another stellar occultation was finally predicted for 9 June 1988, with the "shadow" falling on parts of Australia, New Zealand, and the South Pacific. Planning for the event was precise, with the occultation to be tracked by ground observatories in Australia and New Zealand, and by NASA's Kuiper Airborne Observatory. The occultation wasn't going to be missed this time.

Results of the occultation observations showed that Pluto does in fact have an atmosphere, and the atmosphere seems to have two layers, an upper layer that is clear and a lower layer that is more opaque. The atmosphere is extremely thin, no more than a hundred-thousandth of the density of the Earth's at sea level. Methane is the only component of the atmosphere that is known for certain to exist, but other relatively heavy gases such as argon, nitrogen (N2), carbon monoxide (CO), and oxygen (O2) are probably present. Since Pluto is relatively close to the Sun at present, as mentioned the atmosphere is likely a temporary phenomenon, and is expected to freeze out between 2010 and 2020. Infrared observations of Pluto by Earth orbiting observatories indicate that the surface temperature is very cold, probably about 58 degrees Kelvin in the equatorial regions.

Refined observations of Pluto and Charon were performed by the NASA Hubble Space Telescope in 1994. While the images were of very low resolution, they showed that Pluto was patterned with highly contrasting light and dark areas. The bright areas may be fields of nitrogen ice, while the dark areas may be methane ice fields, or valleys or impact craters. The Hubble observations also allowed astronomers to obtain improved estimates of the sizes of the two worlds.

* One of Pluto's enduring mysteries is its origin. Some astronomers have speculated that it formed in the inner solar system and migrated to its present orbit. Current observations show that Pluto's orbit is "chaotic", or unpredictable on a long-term basis.

Pluto appears to be similar to Neptune's big moon, Triton. Triton has a diameter of 2,700 kilometers, a density of 2.08 grams per cubic centimeter, and an atmosphere that seems to be similar to that of Pluto's. Triton was probably captured soon after its formation, in an event that resulted in its unusual orbit, at a high inclination to the equator of Neptune and in a retrograde direction.

In 1936, a British astronomer named Raymond Lyttleton suggested that Pluto had once been a moon of Neptune as well, but had been ejected by a collision or near-miss with Triton that gave Triton its current bizarre orbit. This theory remained popular for decades, but it was deflated when the mass of Pluto proved to be too small to accommodate this theory, and also when computer simulations of Pluto and Neptune's orbit showed they were never close to each other. Pluto could have been captured by Neptune like Triton or ejected by it from the Solar System, which was probably the fate of many other Pluto-like bodies. Instead, Pluto managed to settle into a stable "3:2" orbital resonance with Neptune, meaning that for every two orbits of Neptune around the Sun, Pluto completes three. The dynamics of Pluto's orbit prevents it from ever coming any closer than 18 AU to Neptune. In fact, Pluto gets closer to Uranus than it ever does to Neptune.

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[19.4] MISSION TO PLUTO

* There was substantial interest in sending a probe to Pluto for a long time, but the effort to do was almost as torturous as the Galileo mission to Jupiter. A Pluto probe was considered by NASA in the 1970s, but was never funded. It was followed by a series of concepts that didn't do any better:

This blow to the PKE project led to a strong lobbying effort by the planetary science community. There was a certain belief that JPL had mishandled the whole Pluto exploration effort, making it out to be harder than it was. Suggestions were made that a Pluto fly-by could be performed by modifying space probes already being designed and built for other missions by outside organizations. The Bush II Administration did not provide any funding for Pluto / Kuiper Belt exploration in the NASA 2002 budget proposal, but in response to the lobbying, in the fall of 2001 the US Congress added $30 million USD for development of a Pluto / Kuiper Belt mission, and NASA and the Southwest Research Institute (SWRI) formed a team to begin preliminary studies.

New Horizons Kuiper Belt probe

The concept got yet another name: "New Horizons" -- probably in hopes of dissociating itself from the dead-end concepts that had preceded it. In the spring of 2003, the Bush II Administration caved in and approved the program for full development, with SWRI and the Applied Physics Laboratory of Johns Hopkins University to build the probe. With a certain amount of justice, NASA selected two astronomers from Lowell Observatory to be on the New Horizons science team.

As it emerged, the New Horizons probe was a modest-sized spacecraft with a launch mass of 478 kilograms and a payload consisting of:

The entire instrument payload consumed only 28 watts of power. Science data was dumped to dual 8-gigabyte solid-state recorders. The spacecraft provided the necessary power, communications, and control systems. Of course, the spacecraft was powered by an RTG.

New Horizons was launched by an Atlas 5 booster from Cape Canaveral on 17 January 2005. In another fitting gesture, the probe carried Clyde Tombaugh's ashes. There were a few protesters in attendance objecting to the flight of the nuclear-powered probe, but they made no attempt to disrupt the launch. The Atlas 5 was NASA's most powerful booster at the time, fitted with five solid-rocket boosters -- far more than was needed to put such a small spacecraft into space, but necessary to give as big a push as possible to the probe.

New Horizons flew by Jupiter in late February 2007 to get a gravity assist. The spacecraft performed observations of the planet and its moons during the flyby, obtaining pictures of the Jovian Little Red Spot and a spectacular volcanic eruption on Io.

The probe flew by Pluto in July 2015, closest approach during the flyby being on the 14th, the high-payoff observation period lasting a total of nine days. The probe's observations were backed up by more from Earth- and space-based observatories, working together to obtain a matrix of data to enhance what New Horizons found. Although select images were released immediately to the public, they were only a very small fraction of the haul. The encounter filled up the probe's two mass storage units with about 50 gigabits of data, and given a data rate of no more than 2 kilobits per second at best -- even using NASA's biggest receiving antennas -- it took a total of 16 months to complete the download.

Early results showed Pluto features surprisingly diverse terrain, with broad plains and young mountains, suggesting an active geology; there were also streaks that suggest winds or geysers. New Horizons is now en route to a flyby of another KBO, "2014 MU69", which was discovered by the Hubble Space Telescope, with the flyby to take place on 1 January 2019. The KBO is approximately the size of Pluto's mini-moons, about 25 to 55 kilometers (15 to 34 miles) in diameter. Focusing on Pluto had the drawback that region of sky through which possible trajectories could be taken is relatively sparse of Kuiper Belt objects.

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[19.5] ANOTHER PLANET X?

* After Clyde Tombaugh discovered Pluto using the blink comparator in 1930, he was left with the question of what to do next. The answer was obvious: look for another planet. He blinked for 13 more years, until he was drafted by the US Navy in 1943 to teach navigation. He never returned to the search, since he was completely burned out on the tedious blinking work and couldn't stand to do it any more. He had accomplished a great deal by that time, having not only detected Pluto but new star clusters, clusters of galaxies, comets, and hundreds of asteroids. The search also allowed Tombaugh to determine that there was no planet with a visual magnitude of more than 16.5 in the areas that he searched. This ruled out, say, a Jupiter-like planet any closer than 470 AU, or 12 times the distance of Pluto from the Sun; or a Pluto-sized planet at 60 AU, or 1.5 times the distance of Pluto from the Sun.

There was still the issue of the discrepancies in the orbit of Uranus, and from the 1970s a number of astronomers tried to see if they could use them to track down another Planet X. One interesting concept put forward in the 1980s was that the Solar System actually had an unseen star, a small and dim red dwarf, with an orbital period of 28 million years. At perihelion, or closest approach to the Sun, this "Nemesis" star would disturb the Oort Cloud and send comets hurtling in towards the Sun and the Earth. However, the problem with Nemesis was that it was so far away that a passing star would have pulled it out of its orbit long ago.

After completion of the IRAS infrared astronomy satellite mission in the 1980s, the data returned was examined by Thomas J. Chester and Michael Melnyk of JPL to hunt for dim stars, and incidentally see if there was any evidence of Planet X glowing faintly in the infrared. They found no such evidence. In 1993, Myles Standish, also of JPL, announced that he had recomputed the orbits of Uranus and Neptune with refined accuracy. The flyby of the Neptune by the Voyager 2 deep-space probe in 1989 showed that the mass of Neptune was about half a percent smaller than estimated. He also demonstrated that there were blatant inconsistencies in some of the historical records of the orbits of the two planets. He concluded there were no real discrepancies in their orbits, and all the calculations performed to find the tenth planet, from Percival Lowell on, had been a waste of time.

Two University of Louisiana astronomers, John Matese and Dan Whitmire, have continued to investigate a possible Planet X. They base their idea not on the perturbations of the orbits of the planets, but from the fact that Oort Cloud comets seem to have a statistical bias towards one section of the sky, as if something big and distant was disturbing them. They suggest that it could be a planet from one to four times the size of Jupiter, more than a thousand times more distant than Pluto. The two astronomers are careful to say the data is no more than suggestive.

In 2014, astronomers Chad Trujillo and Scott Sheppard suggested, from clusterings of KBO orbits, and projected a new Planet X, with an estimated mass of ten Earths, a diameter two to four times that of Earth, and a highly elliptical orbit with a period of about 15,000 years. Given the extreme distance, if it exists, it's not going to be easy to find; further observations of KBO orbits may help.

Incidentally, in 1995 an American woman named Nancy Lieder, who claimed to be in telepathic contact with aliens, said that the Earth would have a close encounter with a roving Planet X named "Nbiru" in 2003 that would have catastrophic results. When nothing happened in 2003, Lieder claimed she hadn't revealed the actual date of the encounter, the intent being to fool the "Establishment". 2012 then became popular as an encounter date, though nothing happened then, either. Nbiru appears to have taken some wrong turns in the course of its journeys, but believers say it will certainly arrive, one of these days.

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[19.6] WORLDS OF THE KUIPER BELT

* The hunt for a tenth planet would actually prove successful, but only in an ironic way that would actually end up reducing the numbers of planets from nine to eight.

The discovery of the first Kuiper Belt object in 1992 led not only to a reconsideration of the hunt for a tenth planet, but also to a changed understanding of the nature of Pluto itself. Astronomers began to believe that Pluto, Charon, and Triton are simply the largest known members of the Kuiper Belt, and as such had insights for the multitude of other icy objects that also populate the belt.

The hunt for KBOs uncovered a large number of bodies in a wide range of sizes. A large KBO that was designated "28978 Ixion" was discovered in 2000, followed by "20000 Varuna" the next year, both being about 500 kilometers across. One named "5000 Quaoar" was discovered in 2001, being determined to be about 1,200 kilometers in diameter, about the size of Charon.

In early 2005, a team of astronomers using the big 1.22-meter Oschin Schmidt camera at Palomar observatory in California hit the jackpot. The orbit of the object, initially designated "2003 UB313" -- it had originally been imaged in 2003, but it took until 2005 to determine its nature -- was at an angle of 45 degrees to the ecliptic, ranged from 97 to 38 AU (within the orbit of Pluto), and with a period of 560 years. The high angle to the ecliptic was why it hadn't been picked up in any of the tenth planet searches, which focused on the ecliptic.

It was at the outlying regions of its orbit when it was discovered; at its closest approach, it will be nearer to the Sun than Pluto. More powerful telescopes were used to observe the new object, showing it to be a fairly typical Kuiper belt object. Preliminary observations showed it to be substantially larger than Pluto, with a diameter of about 3,000 kilometers, but later observations by the Hubble Space Telescope made it out to be just slightly larger than Pluto, with a diameter of 2,400 kilometers, plus or minus a hundred kilometers, and about 27% more massive. It is unusually bright, about as reflective as fresh-fallen snow.

Astronomers nicknamed it "Xena" after the TV swords-and-sorcery heroine. When the Keck II telescope in Hawaii, which had just been fitted with a laser-calibrated adaptive optics system that improved its resolution, discovered a small moon of Xena on 10 September 2005, it was nicknamed "Gabrielle" after Xena's sidekick. In the fall of 2006, Xena was more formally named "Eris" after the Greek goddess of discord, while Gabrielle became "Dysnomia", Eris's daughter and a spirit of discord. The names were regarded as appropriate because they led to controversy, as discussed below. Dysnomia was estimated to be about 150 kilometers in diameter, and orbited the Eris at a distance of 37,000 kilometers with a period of 16 days.

Two other large KBOs were also found in the same timeframe. The first was discovered on 28 December 2004 and was given the seasonally appropriate name of "Santa". Officially it was assigned the temporary designation of "2003 EL61"; the date of the designation suggests that it was spotted earlier but not confirmed. It was later given the formal name of "Haumea", after the Hawaiian goddess of childbirth, since the discovery was confirmed at the Mauna Loa observatory.

Haumea also has two tiny moons, which were originally nicknamed "Rudolph" and "Blitzen", but later formally renamed "Hi'iaka" and "Namaka", after Haumea's children. Haumea is somewhat smaller than Pluto and has a rapid spin rate, a bit under four hours, that distorts it well away from a spherical form, with an equatorial diameter (about 2,200 kilometers) twice that of its polar diameter (about 1,100 kilometers). It is extremely bright, thanks to a surface layer of crystalline water ice, marred by a peculiar rusty patch. The outer Solar System is just full of surprises.

The second large KBO found at the time was discovered on 1 March 2005 and was nicknamed "Easterbunny", though officially it was designated "2005 FY9". It was later given the formal name of "Makemake", after the fertility god of the Easter Islanders. It is about 1,400 kilometers in diameter, making it bigger than Charon, but well smaller than Pluto. A moon of Makemake, nicknamed "MK 2", was discovered by the Hubble Space Telescope in 2015. It is about 160 kilometers in diameter and is dark as charcoal.

* All the KBOs discovered so far are beyond the orbit of Neptune. The biggest, as mentioned, are the size of Pluto or bigger. The smallest observed have a diameter of about 50 kilometers. There are estimated to be about 100,000 KBOs larger than a hundred kilometers in diameter in orbit from 30 to 50 AU around the Sun. The Kuiper Belt, then, is like a second asteroid belt, but it may have a mass hundreds of time larger, in the range of half to one Earth mass.

KBOs are classified by their orbits:

KBOs are all generally dark, with albedos from 5% to 15%, though they range widely in color, from slightly bluish to deep red. Since none of the KBOs have been examined in detail, it is impossible at present to determine if they have structural classes, as do asteroids. Astronomers believe the KBOs are mostly made up of water ice and rock, with some organic compounds and complex molecules. Interestingly, moons seem to be common among KBOs.

Although Pluto is clearly a member of the KBOs, it is unusual in some ways, being lighter in color than most KBOs -- as mentioned, Eris is even lighter than Pluto -- and because its orbit takes it closer to the Sun than Neptune. Lumping Triton in the KBOs makes the matter even murkier, since Triton almost certainly originated in the Kuiper Belt, but as a moon of Neptune the name "Kuiper Belt Object" becomes downright confusing.

Whatever the case, the unusually large size of Pluto among the KBOs does yield some interesting insights. Astronomers believe that in the beginning, the mass of the Kuiper Belt was at least 50 times greater than it is now, and so at first glance a "Planet X" should have been formed there. It appears now that the formation of Neptune disrupted the accumulation of large bodies in the Kuiper Belt, and may have resulted in the eviction of many large bodies into interstellar space. Pluto was protected from this fate by its orbital resonance with Neptune.

In the aeons since the origin of the Solar System, the Kuiper Belt has been eroded by such gravitational disruptions, with bodies either thrown out or pulled in, as well by the effects of collisions whose fragments may suffer a similar fate. The solar wind has also swept out fine dust from the Kuiper Belt, further reducing its mass. It still remains a fascinating place for astronomers, who are eagerly looking forward to the see what the New Horizons probe will reveal about this mysterious region.

* The fact that Pluto is clearly not unique among the bodies of the outer Solar System led some astronomers to suggest that Pluto is not really a planet any more than is Ceres, the biggest asteroid, and should be downgraded accordingly. Initial attempts to push the issue led to strong public resistance, but the discovery of Eris in 2005 was the last straw. It was either demote Pluto, or start adding other small bodies to the list of the planets.

A draft proposal was put in front of the International Astronomical Union in the summer of 2006. The proposal originally stated that a planet had to orbit the Sun instead of another body, and be big enough to pack itself into a sphere. The second constraint would give a minimum diameter of about 800 kilometers. That meant that both Pluto and Charon; Eris; and even Ceres would become planets.

However, a third constraint was added, that the planet should dominate its orbit, and the resolution was passed in that form. Pluto was then referred to as a "dwarf planet", which was just another way of calling it a "minor planet" like the asteroids. That was not seen as satisfactory since the large KBOs are clearly distinct from asteroids, and so in 2008 the decision was made to refer to them as "plutoids". This also helped address the distress over the downgrading of Pluto's status by making it the archetype of an entire class of cosmic bodies.

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