Pluto and beyond / Michael D. Lemonique

For the first time, spacecraft will be able to look closely at comets, asteroids and dwarf planets found in the Kuiper Belt. These spacecraft are supposed to reveal how the solar system was born

January 20, 2014 was a fateful day for the men and women who worked on the robotic spacecraft Rosetta. The 3,000-kilogram spacecraft had been launched by the European Space Agency nearly 10 years earlier and was on its way to rendezvous in August with an unknown comet bearing the unwieldy name 67P/Churyumov-Grasimenko (or 67P for short). Recently, on November 12, Rosetta did an unprecedented thing: it entered a tight orbit around the comet in a loop maneuver and landed a lander named Philae on its surface. Rosetta will continue to accompany the frozen celestial body as a shadow in its orbit when the heat of the sun will awaken it to activity.

(See comprehensive coverage of the Rosetta/Philae mission on the Science website)

But in order for at least one of these things to happen, Rosetta first had to wake up. She was put into an energy-conserving "sleep mode" more than two years before. Her internal alarm clock was set to go off on January 20 at 11 a.m. CET. The scientists and engineers waiting in a situation room at the European Space Agency's Operations Center (ESOC) in Darmstadt, Germany, were confident that the spacecraft would report back as planned. However, they also remembered the Mars Observer spacecraft with which radio contact was lost without a trace in 1993. A few minutes it seems that something similar may happen again.
"I saw a lot of pale faces in the room," recalled Holger Zirx of the Max Planck Institute for Solar System Research in Göttingen, Germany, who was responsible for the spacecraft's optical and infrared cameras. The time that passed seemed like an eternity, although in reality only about 15 minutes had passed, but finally an electronic beep reached Darmstadt from the spaces beyond the planet Jupiter. "She said, 'Here I am again,'" Zirks says, "and it was a huge relief."
In the weeks that have passed since then, it became clear that Rosetta not only woke up, but also functions flawlessly and is ready to answer crucial questions about the structure, composition, behavior and origin of comets: icy celestial bodies that have not changed almost since the formation of the solar system, about 4.6 billion years ago. As mentioned, on November 12, Rosetta released its lander. Due to malfunctions, she made a series of uncontrolled landings and jumps before getting stuck in the shadow of a cliff. For about 60 hours, until her battery ran out, she made measurements to extract from the comet the very history of the solar system.
Rosetta is not alone in deep space. In July 2015, after its own nine-year journey, NASA's New Horizons spacecraft will perform another unprecedented mission: a close flyby of Pluto and its five known moons. "The spacecraft is in remarkably good shape," reports principal investigator Alan Stern from the Southwest Research Institute's office in Boulder, Colorado. And even though the two tasks are independent of each other, it cannot be said that there is no connection between them. Astronomers now understand that Pluto and 67P belong to the Kuiper Belt, a vast, mostly unmapped swarm of billions of celestial bodies beyond Neptune that range in size from a few meters to 2,000 kilometers in diameter.
These encounters will be the highlight of a series of discoveries over the past twenty years that, to borrow Stern's words, "fed us dust and wrote down, literally, everything we thought we knew about the architecture of the solar system." In fact, just over twenty years ago no one even knew the Kuiper Belt existed. During these years, planetary scientists discovered a handful of icy worlds that are close to the size of Pluto and even equal to it. They found evidence of violent activity that long ago disrupted the orbits of Jupiter, Saturn, Uranus and Neptune, and perhaps even the existence of a lost giant fifth planet. They analyzed the sizes and orbits of the approximately 1,500 objects known to us in the Kuiper belt (KBOs)) to learn about how the belt itself was formed. The researchers also wondered if it was chunks of ice that crashed during the formation of the Kuiper Belt that, in ancient times, supplied water to the oceans on the young, dry Earth.
Each of these observations opened a small window into the origin and development of the Kuiper Belt. But as in the folk legend about the blind men and the elephant, they began to paint a more comprehensive picture of its structure, composition and development. And thanks to these two encounters, Rosetta's encounter with the comet and New Horizons' future encounter with Pluto, which are the first ever encounters with two very different objects originating in the Kuiper Belt, this picture is going to become infinitely clearer.
rediscovery
In 1930, when a young astronomer named Clyde Tombaugh discovered a new body beyond Neptune, he and the rest of the astronomical community had no doubt that he had discovered "Planet X", the ninth planet of the solar system, which had long been suspected to exist. At first, the calculations showed that the new object, named Pluto after the suggestion of Vinycia Barney, an 11-year-old British schoolgirl, had a mass similar to that of Earth. But in the 70s it was already clear that Pluto is smaller than the Earth's moon and less massive. What Tombaugh actually discovered was the brightest object in the Kuiper Belt.
However, until the 80s, no one realized that the Kuiper Belt existed at all, not even Gerard Kuiper, the Dutch-American astronomer whose name it bears. In the 50s, Kuiper raised the possibility that the region just beyond Neptune was once full of icy bodies. But he believed that the gravity of the "massive" Pluto scattered them towards deep space. This part of the solar system, he wrote, should be mostly empty. "It was basically an anti-prediction," says astronomer David K. Jewitt of the University of California, Los Angeles, one of the pioneers of observations of the outer solar system.
At about the same time, Kuiper's compatriot Jan Oort hypothesized that the widely spaced objects might form a spherical cloud of protocomets orbiting up to a light-year in diameter from the Sun. Every once in a while, he suggested, one of them would work its way out of the cloud and fall into the inner solar system, where it would come to life as a comet. This scenario provided a good explanation for the existence of long cycle comets, which fall into the solar system from all directions and take at least 200 years to complete one revolution.
But this hypothesis failed to explain the short-cycle comets, which tend to fly inward across the relatively flat plane where the planets reside. Ort thought they were simply long-period comets diverted into shorter orbits by close encounters with the giant planets, and no one had a better idea. (Or, in fact, almost to no one; in the 40s, the Irish astronomer Kenneth Edgeworth raised the possibility that the short-cycle comets came from a swarm of small bodies that resided much closer in. But he made this suggestion generally and only casually. "If You think that counts as a prediction, okay," says Michael A. Brown, the astronomer from the California Institute of Technology who in 2005 discovered Eris, another large object in the Kuiper Belt, following the discovery of which Pluto was downgraded to a "dwarf planet" a year later. Brown does not believe that it is possible See this as a prediction, and in any case no one paid attention to Edgeworth's idea at the time.)
Most planetary scientists now agree that the first legitimate prediction of the existence of the Kuiper Belt is credited to Uruguayan astronomer Julio Fernandez. In his 1980 paper, "On the Existence of a Belt of Comets Beyond Neptune", he made the argument presented by Edgeworth but in much more careful detail. In 1988, Scott Tremaine, then working at the University of Toronto, and his colleagues Martin Duncan and Thomas Quinn showed that the swarm of bodies that Fernandez envisioned could explain the frequency of short-cycle comets and their orbits. They were the first to use the term "Kuiper belt," although according to Tremaine, who now works at the Institute for Advanced Research in Princeton, New Jersey, "it's probably not the right term. The truth is that Fernandez is the man whose name we should have given her."
While Tremain, Duncan, and Kevin worked to polish the Kuiper Belt argument, Jewett and Jane "X" Lu, who was then a student of his at MIT [and added the letter X as a middle name following Jewett's suggestion ], to look for evidence in the field. It wasn't the predictions that prompted them to start looking: Jewitt and Lu had not heard of Fernandez's paper, and they started looking in 1986, two years before Tremain and his colleagues published their results. "What encouraged us and motivated us," says Jewitt, "was the simple idea that it was quite strange that the outer solar system would be empty."
And of course, it turned out that it was not empty. In August 1992, during the search they called the "Slow Object Survey", which they conducted using a 2.2 meter diameter telescope placed on the summit of the extinct volcano Mauna Kea in Hawaii, Jewitt and Lowe discovered the first object in the Kuiper Belt, which received the designation 1992 QB1. Six months later they found the second object, and even though in those days Jewitt and Lou were the only astronomers looking, "the community of astronomers quickly got involved," says Jewitt. To date, astronomers have already identified about 1,500 such objects; Based on this number, they estimate that the Kuiper Belt is home to 100,000 objects larger than 100 kilometers in diameter and up to 10 billion objects larger than two kilometers in diameter. “For every asteroid in the main asteroid belt,” says Jewett, “there are 1,000 objects in the Kuiper belt. This is a shocking fact for me."
However, many astronomers are more suitable in view of what is not in the Kuiper Belt. According to their best models of planet formation, it should boast objects as large as Earth and even larger. But while Pluto has been joined by objects that rival it in size, worlds such as Make-Make, Umea, Kwa-War and Eris, nothing even approaching the size of one of the planets has yet been discovered. "There's a huge number of bodies out there," says Jewitt, "but all together it amounts to a tenth of the mass of Earth. It's really pretty lame."
Something must have happened early in the history of the solar system that wiped out the largest inhabitants of the Kuiper belt. Planetary astronomers have been debating for years what might have happened. Rosetta and New Horizons should finally begin to provide some answers.
emission model
By the time the Kuiper belt was discovered, physicists already had a neat theory about how the solar system formed. It began in a vast interstellar cloud of gas and dust, which collapsed to form a swirling disc. Gravity pulled the disk's core and created a mass of material so compressed and hot that it ignited in thermonuclear fire and thus created the sun.
The heat and radiation of the sun tried to drive out most of the gases and some of the dust; Closer to the center, the dust crystallized into small stones, then into rocks, then into asteroid-like bodies called planetesimals. The hypothesis is that finally, in the last stages of the formation of the planets, hundreds of Mars-sized objects flew back and forth, crashed into pieces, collided with each other and stuck together again and finally formed the eight planets that we see today - not only the rocky inner planets but also Jupiter, Saturn , Uranus and Neptune, which are essentially lumps of rock that exert gravity strong enough to suck in huge amounts of the surrounding gas.
Beyond Neptune, the "dust" will probably be mostly ice particles, which were supposed to form on planet-sized objects in a similar process. There are two problems with this scenario. One, astronomers simply don't see objects this size (though, Brown says, for all we know, there may be some objects as large as Mars somewhere in the distant Oort cloud, where they cannot be detected with current technology).
The second problem is that there is not enough material in the Kuiper Belt to explain the existence of any object, of any size. If all the matter in all the objects in the belt started out as a primordial cloud of glacial dust, that cloud was too diffuse to ever form anything from it.
It therefore seems that the very existence of the Kuiper belt is inconsistent with the way theorists describe its formation. "The solution that everyone agrees on," says Jewett, "is that in the beginning there was much more matter in the Kuiper belt, 30, 40 or even 50 times the mass of the Earth." This material did belong to a huge swarm of objects, but this swarm somehow thinned out.
The most plausible mechanism for that "somehow", first proposed by Renu Malhotra, a physicist from the University of Arizona, is that the four giant planets of the solar system, Jupiter, Saturn, Uranus and Neptune, were once much closer to each other than they are today.
Malhotra and some of her colleagues argued that the gravitational interactions between these densely packed planets and the primordial bustle of the Kuiper belt objects pushed Saturn, Uranus and Neptune outward. At the same time Jupiter, which reacted both on these objects and on asteroids, moved inward.
According to this hypothesis, these gravitational encounters were supposed to not only mix the planets but also throw many distant objects outward, to the far fringes of the Sun's gravitational influence, creating the distant Oort cloud, as well as throw many asteroids inward, towards the inner solar system. Moreover, Jupiter and Saturn had to find themselves for a certain period of time in their migration in a state of resonance with each other, that is, in a state where Saturn made exactly one revolution while Jupiter made two revolutions.
The gravitational shock created when the two planets were so precisely aligned along a straight line scattered the Kuiper belt itself with such vigor that more than 99% of them drifted away. Some of them finally found themselves in the Oort cloud. Others crashed into the inner planets in a catastrophe known as the Late Heavy Bombardment (LHB). "The solar system must have taken a wild beating," says Jewitt.
At least one physicist, David Nesborny of the Southwest Research Institute, is taking this idea a step further. He argues that the solar system may once have boasted a gas giant planet Jupiter, thrown into interstellar space in this violent shuffle.
If the mixing of the giant planets did occur, it could explain why the Kuiper belt has no truly large objects: the material that built them was swept away prematurely. Furthermore, the resulting objects were probably very similar to planetesimals: small protoplanets that later merged to form planets. From this perspective, the Kuiper Belt is like a frozen snapshot in time of the rocky inner solar system just a few million years after the planet formation process got underway.
"The biggest uncertainty about how the existing planets formed," says MIT planetary researcher Hilka Schlichting, "is the formation of the planetesimals: how they came into the world and how big they were." This information has long been unavailable in the inner solar system, but with the help of a combination of observations and models, she and her colleagues showed that it is possible to explain the size distribution of the Kuiper belt bodies if the icy planetesimals from which they were formed were characterized by a diameter of about a kilometer, an insight that may be applicable to the inner planets as well. "We are beginning to learn," she says, "after decades of speculation, about the initial conditions for the formation of planets."
Pluto - a closer look
Models and remote observations have provided planetary scientists with enormous information about the structure of the Kuiper Belt and its likely history. But these cannot replace close observation, as we have already learned from dozens of spacecraft sent to all the planets and dozens of moons and asteroids. "A picture of Pluto, taken by the Hubble Space Telescope, is cool," says Stern, "but it's only a few pixels wide." In June 2015 "Pluto will run towards us, as a real world."
This world was still a planet in January 2006, when New Horizons launched; His demotion to a dwarf planet didn't happen until the following summer. But whatever you call it, Stern and his research partners will try to learn as much as they can as the spacecraft accelerates towards Pluto and its moon Charon, passing them at a speed of almost 40,000 km/h and reaching an altitude of no more than 10,000 km above its icy surface.
One of the goals will be to count the craters that most likely scar the icy surface of Pluto and note both their total number and how many of them there are of any given size. This information will provide astronomers with an independent measurement of the sizes of objects in the Kuiper belt, because they must have crashed into Pluto according to their relative number in the belt.
"But it's even better than that," says Stern. Over time, Pluto's craters have been eroded by the processes that create its tenuous atmosphere: the repeated heating and cooling of its surface that occurs as the dwarf planet moves in its elongated orbit. But Charon has no atmosphere, and this means that all the collisions that happened on its surface were preserved. "You can compare these two," says Stern, "and find out how the history of collisions has changed, what is the size range of these slingshots today compared to what was in the early Kuiper Belt."
New Horizons will also look for signs of a subsurface ocean. Planetary scientists have already found such hidden oceans under the few thick glaciers of some of the moons of Jupiter and Saturn: Europa, Ganymede, Enceladus and Titan. If Pluto has geysers of ice or volcanoes, this may indicate that the interior is hot and watery, perhaps due to radioactive decay in a rocky core. And even if there are no external signs of heat, the spacecraft's infrared cameras can detect hot spots on the surface. The idea that life could exist inside Pluto is a hypothetical hypothesis, but since liquid water is considered a necessary component of biology, as we know it, the discovery of such water would make such a hypothesis legitimate, at the very least.
The spacecraft will do all this and more for just five months, and the most penetrating observation will be made during the day, more or less, during which it will pass by the dwarf planet. But it will take about 16 months for the data to travel, bit by bit, the nearly five billion kilometers back to Earth.
dance with a comet
Rosetta will spend a similar amount of time orbiting close to 67P. Unlike New Horizons, which will pass over Pluto very quickly, Rosetta will be above its target for 15 months, which will allow it to answer all kinds of questions about the exact chemical composition of 67P and about its internal structure, valuable clues for understanding the nature of the gas and dust that initially built the Kuiper Belt and for understanding The way the objects were assembled in it. The current understanding of scientists is so basic that there is no conclusive evidence that can plausibly corroborate one theory and put an end to the competition. Rosetta's future findings could help researchers put together a compelling theory for the first time.
The journey will also give Rosetta and its lander Philae (if she wakes up from her hibernation, imposed on her by the constant shadow cast by the cliff next to which she got stuck), a front-row seat as the comet heats up as it approaches the Sun. "We will be by the comet's side throughout the summer of 2015, when the activity will be at its peak and the nucleus will emit 1,000 kilograms of material per minute," says Matt Taylor of the European Space Agency, who is the principal investigator of the mission as a whole. The researchers still do not know if this material will come from all over the surface of the comet or if it will be sprayed from small active points. In a year's time, the question will receive an answer that will help planetary scientists understand how and why comets eventually lose their ice and go out.
Rosetta should be able to address the questions that concern us, and especially the question of where the Earth's water came from. Many planetary scientists believe that it was a comet storm at the dawn of the solar system that first delivered water to Earth. To test this hypothesis, Rosetta will test whether the H2O molecules trapped in the ice of 67P are chemically identical to the H2O molecules on Earth. Evidence has already been obtained from the Herschel Space Observatory that at least some comets have water whose proportion of hydrogen to its heavier isotope, deuterium, is the same as that which can be found in Earth's oceans. However, thanks to Rosetta's instruments, we will be able to observe more closely and more thoroughly the water of the comet and its other components, including organic compounds rich in carbon, which may have played a role in the creation of life. [During the preparation of the article for printing in Hebrew, first results were obtained indicating that the ratio of hydrogen isotopes in the water vapor emitted from 67P is different from the ratio found in the Earth's oceans - the editors.]
Philae and Rosetta will also work together to answer the question of whether comets are simply chunks of dirty ice, or are groups of smaller chunks held together relatively loosely by their own gravity. When the Rosetta orbiter and the Philae lander were on opposite sides of the comet, Rosetta transmitted a radio signal that passed through the comet's body to Philae and was returned by it. This procedure is similar to what is done in a CT scan, and it will show scientists for the first time the internal structure of a comet. [The experiment was carried out, but its full results are not in our hands - the editors.]
Unfortunately for most of us, 67P will never be visible to the naked eye. Just like with Pluto and the vast majority of objects in the Kuiper belt, you'll need artificial magnification to even know the comet is there. No wonder then that only recently have astronomers come to the conclusion that the Kuiper belt exists and that it may have played a decisive role in the history and architecture of the solar system.
Thanks to two spacecraft that launched almost ten years ago, towards the end of 2015 we will understand more about the history of the solar system - much more.

in brief
The Kuiper Belt beyond the planet Neptune is a band of billions of icy asteroids that serve as near-pristine examples of the components of the Solar System.
Two spaceships are in the middle of missions aimed at rummaging through the secrets of the belt. One of them, Rosetta, orbits a comet born in the Kuiper Belt. The other, New Horizons, is making its way to Pluto, the region's largest resident.
These missions may lead to finding the key to the origins of the solar system.

Orbits - two missions to one destination: the Kuiper Belt
A pair of spacecraft, New Horizons and Rosetta, are taking different approaches to examine the vast ring of icy asteroids called the Kuiper Belt. The billions of small bodies that are said to swarm there, beyond the orbit of Neptune, are remnants of the formation of the solar system and could teach us how our planets appeared. New Horizons is heading straight to the Kuiper Belt to study Pluto, its largest resident. Rosetta, on the other hand, entered orbit around a comet that originated there.
We're Going to the Kuiper Belt: The New Horizons Mission
NASA's New Horizons spacecraft was launched in 2006 and since then it has been on its journey, which is still underway, to Pluto. The spacecraft crossed Neptune's orbit in August 2014, and is due to fly past Pluto in July 2015. During this close encounter, New Horizons will analyze the composition of the dwarf planet's highly reflective surface and investigate how its thin atmosphere is constantly escaping from it. The escape process may explain how Earth's atmosphere lost a significant amount of its hydrogen when the planet was still young. The spacecraft will also search the surface of Pluto for organic compounds, such as frozen methane. The Kuiper Belt itself may have injected such compounds, which are the materials required to create life, onto Earth long ago when they wandered into the inner solar system.

The Kuiper Belt is coming to us: Rosetta observes Comet 67P
In August 2014, after a ten-year flight, the European Space Agency's Rosetta spacecraft arrived at comet 67P/Churyumov-Grasimenko. The explanation is that 67P, like most comets in its region, originated in the Kuiper Belt. A collision that happened a long time ago, or a gravitational pull from another body, may have kicked it into the inner solar system. Rosetta will orbit 67P as it reaches its closest point to the Sun, and its icy surface will melt to form a glowing tail. On November 12, 2014, Rosetta released its Philae lander, which made a deep landing on the comet and sent Earth surface analysis images and data until its batteries were depleted.
More on the subject
Discovery of a Planetary-Sized Object in the Scattered Kuiper Belt. ME Brown et al. in Astrophysical Journal Letters, Vol. 635, no. 1, pages L97-L100; December 10, 2005.
The Pluto Files: The Rise and Fall of America's Favorite Planet. Neil deGrasse Tyson. WW Norton, 2009.

About the author
Michael D. Lemonick is a reporter for Climate Central, a non-profit news site, and the author of "Mirror to Earth: The Search for a Twin Planet" (Walker Press, 2012). He was a scientific reporter for "Time" magazine for 21 years.