Pluto Is Far in the Rearview. Next Stop: Ultima Thule

NASA’s New Horizons is poised to arrive at the most distant object ever seen up close—and there could be more to come

Pluto Is Far in the Rearview. Next Stop: Ultima Thule
Artist’s rendering of New Horizons as it sped past Pluto in 2015.

“Liftoff of NASA’s New Horizons spacecraft on a decade-long voyage to visit the planet Pluto and then beyond!”

On January 19, 2006 as the powerful 220-foot-tall Atlas V rocket, with the tiny interplanetary craft cradled in its largely empty nosecone, shot into the blue Florida sky, these words boomed out of loudspeakers, stirring the hearts and minds of the many thousands gathered at Cape Canaveral and many more watching over television and the internet. It was the fastest launch from Earth ever, because the payload was headed for the farthest objects ever targeted by a space probe. As the behemoth Atlas cleared the launch tower, those last three words—“and then beyond” —might not have attracted too much attention amongst all the Plutomania, but now, nearly 13 years later, we are all about to find out what that meant.

The Kuiper Belt—the vast, distant repository of millions of frozen, primitive objects, had been theorized but not yet found when a mission to Pluto was first studied by NASA in 1990. Its discovery in the early 1990s showed that Pluto was not just an oddity at the outer edge of the solar system but part of an entirely unexplored territory beyond the orbit of Neptune. This discovery helped to marshal scientific support for a mission that was recast as an expedition to Pluto and the Kuiper Belt.

In our sci-fi fantasy shows, the spaceships of future centuries are always general-purpose, able to travel anywhere their intrepid captains direct. In the 21st century we’re still in an age where our deep space craft are single-use, designed to go only to specific targets, which they often carry barely enough fuel and resources to reach. A few times we’ve been able to send some of our craft on an added “extended” mission to visit another comet or asteroid.

Yet one of the many exceptional features of New Horizons is that it was designed, from the beginning, to keep going after Pluto and to explore further worlds that had not even been identified when it was launched. This was necessary because at liftoff, there was no known Kuiper Belt object that New Horizons could reach after visiting Pluto. However, our statistical knowledge about the Kuiper Belt suggested there should be many such objects, which might be discovered while New Horizons was en route. Once that happened, New Horizons would be re-directed to visit one of them.

This search did not go as planned. It turned out to be much harder than anticipated to find a post-Pluto destination. This was largely because, the way things lined up, the area of the sky that needed to be explored was near the center of the Milky Way galaxy—the worst possible celestial real estate to look for new, faint objects against the dense population of background stars. As New Horizons made its nine-year journey clear across the solar system, years of looking with the best ground-based telescopes failed to reveal a suitable object. It got to the point where the success of the extended Kuiper Belt mission was in real doubt.

Only an unplanned search with the Hubble Space Telescope could plausibly save the day. In June, 2014, just a year before the Pluto encounter, the Hubble was pressed into service and the New Horizons team managed to find two objects their craft could visit with available fuel. Of these, the object MU69, now nicknamed Ultima Thule (a Greek-Latin hybrid meaning “beyond the known world”), was on a more favorable orbit to intercept.

So now, ever since rounding Pluto in July 2015, and revealing that dwarf planet and its 5 moons to us in all their surprising, variegated glory, New Horizons has been making a beeline towards Ultima Thule, which orbits another billion miles farther from Earth than Pluto. On New Year’s Eve, heading into 2019, Ultima Thule will become by far the farthest world ever explored by human craft, as New Horizons, having traveled for three and a half more years through the cold, black yonder of the Kuiper Belt, skims within a few thousand miles of its surface.

The closest approach will come 33 minutes after midnight East Coast time, yet news of success or failure will not reach Earth until the following morning. At four billion miles, the unprecedented distance of this encounter also means a much greater communications delay than any previous flyby: it will take 12 hours for a round trip at the speed of light between Earth and the spacecraft. While it is executing its last maneuvers, the spacecraft will truly be on its own. The synchronization with our calendrical page turning is completely serendipitous but should make for an especially celebratory encounter: Finally, a real reason to stay up past midnight on New Year’s Eve!

Never has there been the prospect of going so quickly from ignorance to relative clarity about an entirely new type of solar system object. Usually we head into a flyby encounter knowing much more about our target than we do now about Ultima Thule. All we know is its rough size (about 20 miles across), that it seems to have a double-lobed shape, that it may even be two separate objects circling one another and that its overall color is a bit redder than Pluto.

Because it is so distant, small and dark, we don’t even have any detailed spectral clues as to its surface composition. And because it is such a small target and New Horizons is moving so quickly, not until a couple of days before the encounter will it even be resolvable into multiple pixels. Even compared to other first flybys, which are always hectic bursts of discoveries, this one will happen quickly.

The same suite of seven precision scientific instruments that three years ago revealed Pluto to us will now be used to make a quick but detailed study of Ultima and its environment. Is it heavily cratered? Uniform or varied? Rough or smooth? Rocky or icy? Soon we’ll have answers and clues, not just to the history of this one mysterious object but toward a new understanding of the solar system’s lost origin story. For the Kuiper Belt is the distant construction warehouse of the solar system, the place where leftover building materials from making the planets have been kept in a cool, dry place. Up until now it has been off-limits, waiting for someone curious and clever enough to make it out there and discover what relics it holds of the original materials and processes with which the planets were built.

With the Pluto encounter, the last of the classically known planets made the transition from speck to world, from telescopic point to finely photographed landscape. With the rapidly approaching flyby of Ultima Thule, another immense realm of our solar system will be transformed from the astronomical study only of distant unresolved dots to the geological study of the surface, shape and history of variegated bodies.

This will be our first close-up observation of such a distant object and may well be our last for many years or even decades. But maybe not. Since the Pluto flyby, interest in possible new Kuiper Belt missions has grown, and the new results from Ultima may add momentum. It may even be possible for New Horizons to visit more objects in its continuing traverse across the Kuiper Belt. Fuel is limited but some redirection is possible. The nuclear batteries are slowly decaying, but the spacecraft should be operable and maintain communications with Earth for well over another decade. New Horizons’ onboard telescopic camera can be used to search for more targets in its path.

With a little luck, this well-traveled spacecraft may yet find another world to visit before it heads off to wander the galaxy forever, a derelict spacecraft that has completed its mission, a relic of 21st-century human curiosity and daring.


Earthrise, a photo that changed the world

December 24 is the 50th anniversary of Earthrise, arguably one of the most profound images in the history of human culture. When astronaut William Anders photographed a fragile blue sphere set in dark space peeking over the Moon, it changed our perception of our place in space and fuelled environmental awareness around the world.

The photo let us see our planet from a great distance for the first time. The living Earth, surrounded by the darkness of space, appears fragile and vulnerable, with finite resources.


Viewing a small blue Earth against the black backdrop of space, with the barren moonscape in the foreground, evokes feelings of vastness: we are a small planet, orbiting an ordinary star, in an unremarkable galaxy among the billions we can observe. The image prompts emotions of insignificance – Earth is only special because it’s the planet we live on.

As astronaut Jim Lovell said during the live broadcast from Apollo 8, “The vast loneliness is awe-inspiring, and it makes you realise just what you have back there on Earth.”

The Apollo 8 Christmas Eve broadcast.

Earthrise is a testament to the extraordinary capacity of human perception. Although, in 1968, the photograph seemed revelatory and unexpected, it belongs to an extraordinary history of representing the Earth from above. Anders may have produced an image that radically shifted our view of ourselves, but we were ready to see it.

A history of flight

People have always dreamed of flying. As we grew from hot-air balloons to space shuttles, the camera has been there for much of the ride.

After WWII, the US military used captured V-2 rockets to launch motion-picture cameras out of the atmosphere, producing the first images of Earth from space.

Russia’s Sputnik spurred the United States to launch a series of satellites — watching the enemy and the weather — and then NASA turned its attention to the Moon, launching a series of exploratory probes. One (Lunar Orbiter I, 1966) turned its camera across a sliver of the Moon’s surface and found the Earth, rising above it.

The non-human version of Earthrise from Lunar Orbiter in 1966. NASA

Despite not being the “first” image of the Earth from our Moon, Earthrise is special. It was directly witnessed by the astronauts as well as being captured by the camera. It elegantly illustrates how human perception is something that is constantly evolving, often hand in hand with technology.

Earthrise showed us that Earth is a connected system, and any changes made to this system potentially affect the whole of the planet. Although the Apollo missions sought to reveal the Moon, they also powerfully revealed the limits of our own planet. The idea of a Spaceship Earth, with its interdependent ecologies and finite resources, became an icon of a growing environmental movement concerned with the ecological impacts of industrialisation and population growth.

‘Spaceship Earth’ became a powerful rallying cry for environmental groups.

From space, we observe the thin shield provided by our atmosphere, allowing life to flourish on the surface of our planet. Lifeforms created Earth’s atmosphere by removing carbon dioxide and generating free oxygen. They created an unusual mix of gases compared to other planets – an atmosphere with a protective ozone layer and a mix of gases that trap heat and moderate extremes of temperature. Over millions of years, this special mix has allowed a huge diversity of life forms to evolve, including (relatively recently on this time scale) Homo sapiens.

The field of meteorology has benefited enormously from the technology foreshadowed by the Earthrise photo. Our knowledge is no longer limited to Earth-based weather-observing stations.

Satellites can now bring us an Earthrise-type image every ten minutes, allowing us to observe extremes such as tropical cyclones as they form over the ocean, potentially affecting life and land. Importantly, we now possess a long enough record of satellite information so that in many instances we can begin to examine long-term changes of such events.

Tropical Cyclone Owen seen from space. 

The human population has doubled in the 50 years since the Earthrise image, resulting in habitat destruction, the spread of pest species and wildfires spurred by climate warming. Every year, our actions endanger more species.

Earth’s climate has undergone enormous changes in the five decades since the Earthrise photo was taken. Much of the increase in Australian and global temperatures has happened in the past 50 years. This warming is affecting us now, with an increase in the frequency of extreme events such as heatwaves, and vast changes across the oceans and polar caps.


With further warming projected, it is important that we take this chance to look back at the Earthrise photo of our little planet, so starkly presented against the vastness of space. The perspective that it offers us can help us choose the path for our planet for the next 50 years.

It reminds us of the wonders of the Earth system, its beauty and its fragility. It encourages us to continue to seek understanding of its weather systems, blue ocean and ice caps through scientific endeavour and sustained monitoring.

The beauty of our planet as seen from afar – and up close – can inspire us to make changes to secure the amazing and diverse animals that share our Earth.

Zoos become conservation organisations, holding, breeding and releasing critically endangered animals. Scientists teach us about the capacities of animals and the threats to their survival.

Communities rise to the challenge and people in their thousands take actions to help wildlife, from buying toilet paper made from recycled paper to not releasing balloons outdoors. If we stand together we can secure a future for all nature on this remarkable planet.

But is a 50-year-old photo enough to reignite the environmental awareness and action required to tackle today’s threats to nature? What will be this generation’s Earthrise moment?

A NASA probe is going to visit Ultima Thule, the farthest object humanity has ever tried to reach, on New Year’s Day

ultima thule new horizons 2014 mu69 kuiper belt nasa jhuapl swri steve gribben
An illustration of NASA’s New Horizons probe visiting 2014 MU69, a Kuiper Belt object that exists about 1 billion miles beyond Pluto.
  • NASA’s New Horizons probe, which visited Pluto in 2015, is closing in on a mysterious object called Ultima Thule.
  • New Horizons will fly past Ultima Thule, formally known as 2014 MU69, on New Year’s Day.
  • Ultima Thule will be the most distant object humanity has ever visited if the flyby goes as planned.
  • The nuclear-powered spacecraft will take hundreds photos of the space rock.
  • The flyby is “about 10,000 times” more challenging than visiting Pluto, the mission’s leader said.

NASA scientists are about to make history by flying a probe past a mysterious, mountain-size object beyond the orbit of Pluto.

If the flyby goes as planned, it will be the most distant object in space that humanity has ever tried to visit.

NASA’s nuclear-powered New Horizons spacecraft will attempt the maneuver on New Year’s Day. The object the probe is approaching is called Ultima Thule (pronounced “tool-ee”) or 2014 MU 69, as it’s formally known.

NASA didn’t know Ultima Thule existed when New Horizons launched toward Pluto in 2006. There wasn’t even a reliable way to detect it until after astronauts flew out to the Hubble Space Telescope in May 2009 and plugged in an upgraded camera.

Hubble first definitively photographed Ultima Thule in June 2014 — about a year before New Horizons flew past Pluto. Now, 4 billion miles away from Earth, New Horizons has the object in its sights.

The deep uncertainty about Ultima Thule makes planetary science researchers like Alan Stern, who leads the New Horizons mission, all the more excited about the flyby.

“If we knew what to expect, we wouldn’t be going to Ultima Thule. It’s an object we’ve never encountered before,” Stern told Business Insider. “This is what what exploration is about.”

Where and what the heck is Ultima Thule?

kuiper belt objects kbos pluto new horizons flight path ultima thule 2014 mu69 alex parker jhuapl swri
An illustration of Kuiper Belt Objects (dots) with New Horizons’ flight path (yellow), Pluto, and Ultima Thule/2014 MU69.

New Horizons is coasting through a zone called the Kuiper Belt, a region where sunlight is about as weak as the light from a full moon. That far away, frozen leftovers of the solar system’s formation — Kuiper Belt Objects, or KBOs — lurk in vast numbers (including Pluto).

Ultima Thule is one of these pristine remnants. It has presumably remained in its distant and icy orbit for billions of years, and it is not a planet that has deformed under its own mass and erased its early history. This means studying it may help reveal how the solar system evolved to form planets like Earth, Stern said.

“Ultima is the first thing we’ve been to that is not big enough to have a geological engine like a planet, and also something that’s never been warmed greatly by the sun,” he said. “It’s like a time capsule from 4.5 billion years ago. That’s what makes it so special.”

Stern added that the flyby will be the astronomical equivalent of an archaeological dig in Egypt.

“It’s like the first time someone opened up the pharaoh’s tomb and went inside, and you see what the culture was like 1,000 years ago,” he said. “Except this is exploring the dawn of the solar system.”

asteroids asteroid field star nasa jpl 717846main_pia16610_full
An artist’s rendering of planetesimals.

Stern considers Ultima Thule to be a “planetesimal” or seed that might have formed a planet if it had acquired enough material.

“It’s a building block of larger planets, or a planetary embryo,” Stern said. “In that sense, it’s like a paleontologist finding the fossilized embryo of a dinosaur. It has a very special value.”

In New Horizons’ first images, researchers will pay close attention to the outward appearance of Ultima Thule. Learning whether the surface is relatively smooth or features a mix of pebbles, huge boulders, cliffs, and other features will yield clues about how planets form.

Each bit of image data from New Horizons, moving at the speed of light as radio waves, will take about six hours to reach antennas on Earth.

Journey into the unknown

In June, New Horizons woke up from half a year of hibernation to begin zeroing in on Ultima Thule.

After a series of checks, mission managers in October fired the probe’s engine to put it on a more precise path to Ultima Thule.

This week, researchers finished confirming that there are no obvious moons, debris fields, or other objects floating in the flight path of New Horizons (and that it might slam into), so they kept the robot on-course for its historic encounter.

The flyby is slated to begin late on New Year’s Eve. New Horizons will start taking hundreds of photos in a highly choreographed, pre-programmed sequence.

“Rendezvousing with something the size of a large, filthy mountain covered in dirt, a billion miles away from Pluto, and honing in on it is about 10,000 times harder than reaching Pluto,” Stern said. “That’s because it’s about 10,000 times smaller. The achievement of getting to it is unbelievable.”

The target of New Horizons’ cameras and other instruments won’t just be Ultima Thule itself, either.

“We’re plastering all of the space around it for moons, rings, and even an atmosphere,” Stern said. “If any of those things are there, we’ll see them.”

new horizons rtg NASA

At 12:33 a.m. ET on New Year’s Day, the space probe will be its closest point — about 2,175 miles— to the mountain-size object. New Horizons will also turn around to photograph its exit at a speed of 35,000 mph.

Stern said the initial images will each take two hours to transmit, and the first ones will be released early in the day on January 2.

However, those early photos will be small (as they were for Pluto). It will take months to receive the most detailed, full-resolution images due to the power, antenna, and other physical limitations of the spacecraft. The first full-resolution images won’t arrive until February.

Stern, who recently helped write a book titled “Chasing New Horizons: Inside the Epic First Mission to Pluto,” said Ultima Thule got its name from a Norse phrase that means “beyond the farthest frontiers.” He shied away from making any predictions about what the images might show, citing how shocking the first close-up pictures of Pluto were.

“I don’t know what we’re going to find,” he said. “If it’s anything as surprising as Pluto, though, it will be wonderful.”

How to watch live coverage of New Horizons’ flyby of Ultima Thule

New Horizons control room
Members of the New Horizons science team react to seeing the spacecraft’s last and sharpest image of Pluto before closest approach

Those interested in watching expert commentary about the flyby and seeing if it succeeded can tune into a live broadcast on New Year’s Day.

Michael Buckley, a spokesperson for Johns Hopkins University’s Applied Physical Laboratory (which hosts the New Horizons mission for NASA), said the lab’s YouTube channel will stream a video feed of the moment scientists learn that the spacecraft made it past Ultima Thule.

The show will go on even if President Donald Trump’s government shutdown over border wall funding silences NASA TV into 2019.

“We’re still planning one going ahead with the programming that we’ve scheduled,” Buckley told Business Insider. “The biggest change is that we wouldn’t be using any NASA platforms.”

He said live coverage is expected to begin on January 1 around 9:30 a.m. EST, and the “ok” signal from New Horizons should arrive after 10 a.m. EST.

Navigating NASA’s first mission to the Trojan asteroids

This diagram illustrates Lucy’s orbital path. The spacecraft’s path (green) is shown in a frame of reference where Jupiter remains stationary, giving the trajectory its pretzel-like shape.

In science fiction, explorers can hop in futuristic spaceships and traverse half the galaxy in the blink of a plot hole. However, this sidelines the navigational acrobatics required in order to guarantee real-life mission success.

In 2021, the feat of navigation that is the Lucy mission will launch. To steer Lucy towards its targets doesn’t simply involve programming a map into a spacecraft and giving it gas money – it will fly by six asteroid targets, each in different orbits, over the course of 12 years.

Lucy’s destination is among Jupiter’s Trojan asteroids, clusters of rocky bodies almost as old as the Sun itself, and visiting these asteroids may help unlock the secrets of the early solar system. Lucy will encounter a Main Belt asteroid in 2025, where it will conduct a practice run of its instruments before encountering the first four Trojan targets from 2027-2028. In 2033, Lucy will end its mission with a study of a binary system of two Trojans orbiting each other.

Getting the spacecraft where it needs to go is a massive challenge. The solar system is in constant motion, and gravitational forces will pull on Lucy at all times, especially from the targets it aims to visit. Previous missions have flown by and even orbited multiple targets, but none so many as will Lucy.

Scientists and engineers involved with trajectory design have the responsibility of figuring out that route, under Flight Dynamics Team Leader Kevin Berry of NASA’s Goddard Space Flight Center in Greenbelt, Maryland. One such engineer is Jacob Englander, the optimization technical lead for the Lucy mission. “There are two ways to navigate a mission like Lucy,” he said. “You can either burn an enormous amount of propellant and zig-zag your way around trying to find more targets, or you can look for an opportunity where they just all happen to line up perfectly.” To visit these aligned targets, the majority of Lucy’s high-speed lane changes will come from gravity assists, with minimal use of fueled tweaks.

Though Lucy is programmed to throw itself out into a celestial alignment that will not occur for decades, it cannot be left to its own devices. Once the spacecraft begins to approach its asteroid targets, optical navigation is the next required step.

“OpNav,” as optical navigation technical lead Coralie Adam refers to it, is the usage of imagery from the on-board cameras to determine Lucy’s position relative to the . This is a useful measurement used by the navigation team to tweak Lucy’s route and ensure it stays on the nominal flyby path. Adam works in Simi Valley, California, with KinetX, the company NASA selected to conduct Lucy’s deep space navigation.

By using the communications link from the spacecraft to Earth, Adam said, the Lucy team gets information about the spacecraft’s location, direction and velocity. The spacecraft takes pictures and sends them down to Earth, where Adam and other optical navigators use software to determine where the picture was taken based on the location of stars and the target. The orbit determination team uses this data along with data from the communications link to solve for where the spacecraft is and where it is expected to be, relative to the Trojans. The team then designs a to get Lucy on track. “The first maneuver is tiny,” said navigation technical lead Dale Stanbridge, who is also of KinetX. “But the second one is at 898 meters per second. That’s a characteristic of Lucy: very large delta V maneuvers.” Delta V refers to the change in speed during the maneuver.

Communicating all of these navigation commands with Lucy is a process all on its own. “Lockheed Martin sends the commands to the spacecraft via the Deep Space Network,” Adam said. “What we do is we work with Lockheed and the Southwest Research Institute, where teams are sequencing the instruments and designing how the spacecraft is pointed, to make sure Lucy takes the pictures we want when we want them.”

“The maneuvers to correct Lucy’s trajectory are all going to be really critical because the spacecraft must encounter the Trojan at the intersection of the spacecraft and Trojan orbital planes,” Stanbridge said. “Changing the spacecraft orbital plane requires a lot of energy, so the maneuvers need to be executed at the optimal time to reach to next body while minimizing the fuel cost.”

While Lucy is conducting deep space maneuvers to correct its trajectory toward its targets, communications with the spacecraft are sometimes lost for brief periods. “Blackout periods can be up to 30 minutes for some of our bigger maneuvers,” Stanbridge said. “Other times you could lose communications would be when, for example, the Sun, comes between the Earth tracking station and the spacecraft, where the signal would be degraded by passing through the solar plasma.”

Losing contact isn’t disastrous, though. “We have high-fidelity predictions of the spacecraft trajectory which are easily good enough to resume tracking the spacecraft when the event causing a communication loss is over,” Stanbridge said.

What route will Lucy take once its mission is complete, nearly 15 years from now? “We’re just going to leave it out there,” Englander said. “We did an analysis to see if it passively hits anything, and looking far into the future, it doesn’t.” The Lucy team has given the a clear path for thousands of years, long after Lucy has rewritten the textbooks on our solar system’s history.

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Mystery of coronae around supermassive black holes deepens

This computer-simulated image shows a supermassive black hole at the core of a galaxy. The black region in the center represents the black hole’s event horizon, where no light can escape the massive object’s gravitational grip.

Researchers from RIKEN and JAXA have used observations from the ALMA radio observatory located in northern Chile and managed by an international consortium including the National Astronomical Observatory of Japan (NAOJ) to measure, for the first time, the strength of magnetic fields near two supermassive black holes at the centers of an important type of active galaxies. Surprisingly, the strengths of the magnetic fields do not appear sufficient to power the “coronae,” clouds of superheated plasma that are observed around the black holes at the centers of those galaxies.

It has long been known that the that lie at the centers of , sometimes outshining their host galaxies, have coronae of superheated plasma around them, similar to the corona around the Sun. For black holes, these coronae can be heated to a phenomenal temperature of one billion degrees Celsius. It was long assumed that, like that of the Sun, the coronae were heated by magnetic field energies. However, these magnetic fields had never been measured around black holes, leaving uncertainty regarding the exact mechanism.

In a 2014 paper, the research group predicted that electrons in the plasma surrounding the black holes would emit a special kind of light, known as , as they exist together with the magnetic forces in the coronae. Specifically, this radiation would be in the band, meaning electromagnetic waves with a long wavelength and low frequency. And the group set out to measure these fields.

They decided to look at data from two “nearby,” in astronomical terms, active galactic nuclei: IC 4329A, which is about 200 million light-years away, and NGC 985, which is approximately 580 million light-years away. They began by taking measurements using the ALMA observatory in Chile, and then compared them to observations from two other : the VLA observatory in the United States and the ATCA in Australia, which measure slightly different frequency bands. The team found that indeed there was an excess of radio emission originating from synchrotron radiation, in addition to emissions from the “jets” cast out by the black holes.

Through the observations, the team deduced that the coronae had a size of about 40 Schwarzschild radii, the radius of a black hole from which not even light can escape, and a strength of about 10 gauss, a figure that is a bit more than the magnetic field at the surface of the Earth but quite a bit less than that given out by a typical refrigerator magnet.

“The surprise,” says Yoshiyuki Inoue, the lead author of the paper, published in the Astrophysical Journal, “is that although we confirmed the emission of radio synchrotron radiation from the corona in both objects, it turns out that the magnetic we measured is much too weak to be able to drive the intense heating of the around these black holes.” He also notes that the same phenomenon was observed in both galaxies, implying that it could be a general phenomenon.

Looking to the future, Inoue says that the group plans to look for signs of powerful gamma rays that should accompany the radio emissions, to further understand what is happening in the environment near supermassive .

Beyond the black hole singularity

Artist representation of a black hole. The bottom half of the image depicts the black hole which, according to general relativity, traps everything including light. Effects based on loop quantum gravity, a theory that extends Einstein’s …more

Our first glimpses into the physics that exist near the center of a black hole are being made possible using “loop quantum gravity”—a theory that uses quantum mechanics to extend gravitational physics beyond Einstein’s theory of general relativity. Loop quantum gravity, originated at Penn State and subsequently developed by a large number of scientists worldwide, is opening up a new paradigm in modern physics. The theory has emerged as a leading candidate to analyze extreme cosmological and astrophysical phenomena in parts of the universe, like black holes, where the equations of general relativity cease to be useful.

Previous work in loop quantum gravity that was highly influential in the field analyzed the quantum nature of the Big Bang, and now two new papers by Abhay Ashtekar and Javier Olmedo at Penn State and Parampreet Singh at Louisiana State University extend those results to black hole interiors. The papers appear as “Editors’ suggestions” in the journals Physical Review Letters and Physical Review on December 10, 2018 and were also highlighted in a Viewpoint article in the journal Physics.

“The best theory of gravity that we have today is , but it has limitations,” said Ashtekar, Evan Pugh Professor of Physics, holder of the Eberly Family Chair in Physics, and director of the Penn State Institute for Gravitation and the Cosmos. “For example, general predicts that there are places in the universe where gravity becomes infinite and simply ends. We refer to these places as ‘singularities.’ But even Einstein agreed that this limitation of general relativity results from the fact that it ignores .”

At the center of a black hole the gravity is so strong that, according to general relativity, space-time becomes so extremely curved that ultimately the curvature becomes infinite. This results in space-time having a jagged edge, beyond which no longer exists—the singularity. Another example of a singularity is the Big Bang. Asking what happened before the Big Bang is a meaningless question in general relativity, because space-time ends, and there is no before. But modifications to Einstein’s equations that incorporated quantum mechanics through loop quantum gravity allowed researchers to extend physics beyond the Big Bang and make new predictions. The two recent papers have accomplished the same thing for the black hole singularity.

“The basis of loop quantum gravity is Einstein’s discovery that the geometry of space-time is not just a stage on which cosmological events are acted out, but it is itself a physical entity that can be bent,” said Ashtekar. “As a physical entity the geometry of space-time is made up of some fundamental units, just as matter is made up of atoms. These units of geometry—called ‘quantum excitations’—are orders of magnitude smaller than we can detect with today’s technology, but we have precise quantum equations that predict their behavior, and one of the best places to look for their effects is at the center of a black hole.” According to general relativity, at the center of a black hole gravity becomes infinite so everything that goes in, including the information needed for physical calculations, is lost. This leads to the celebrated ‘information paradox’ that theoretical physicists have been grappling with for over 40 years. However, the quantum corrections of loop quantum gravity allow for a repulsive force that can overwhelm even the strongest pull of classical and therefore physics can continue to exist. This opens an avenue to show in detail that there is no loss of information at the center of a blackhole, which the researchers are now pursuing.

Interestingly, even though continues to work where general relativity breaks down—black hole singularities, the Big Bang—its predictions match those of general relativity quite precisely under less extreme circumstances away from the singularity. “It is highly non-trivial to achieve both,” said Singh, associate professor of physics at Louisiana State. “Indeed, a number of investigators have explored the quantum nature of the black hole singularity over the past decade, but either the singularity prevailed or the mechanisms that resolved it unleashed unnatural effects. Our new work is free of all such limitations.”

Gravity is mathematically relatable to dynamics of subatomic particles

Gravity, the force that brings baseballs back to Earth and governs the growth of black holes, is mathematically relatable to the peculiar antics of the subatomic particles that make up all the matter around us.

Albert Einstein’s desk can still be found on the second floor of Princeton’s physics department. Positioned in front of a floor-to-ceiling blackboard covered with equations, the desk seems to embody the spirit of the frizzy-haired genius as he asks the department’s current occupants, “So, have you solved it yet?”

Einstein never achieved his goal of a unified theory to explain the natural world in a single, coherent framework. Over the last century, researchers have pieced together links between three of the four known physical forces in a “,” but the fourth force, gravity, has always stood alone.

No longer. Thanks to insights made by Princeton faculty members and others who trained here, gravity is being brought in from the cold—although in a manner not remotely close to how Einstein had imagined it.

Though not yet a “theory of everything,” this framework, laid down over 20 years ago and still being filled in, reveals surprising ways in which Einstein’s theory of gravity relates to other areas of physics, giving researchers new tools with which to tackle elusive questions.

The key insight is that gravity, the force that brings baseballs back to Earth and governs the growth of , is mathematically relatable to the peculiar antics of the that make up all the matter around us.

This revelation allows scientists to use one branch of physics to understand other seemingly unrelated areas of physics. So far, this concept has been applied to topics ranging from why black holes run a temperature to how a butterfly’s beating wings can cause a storm on the other side of the world.

This relatability between gravity and subatomic provides a sort of Rosetta stone for physics. Ask a question about gravity, and you’ll get an explanation couched in the terms of subatomic particles. And vice versa.

“This has turned out to be an incredibly rich area,” said Igor Klebanov, Princeton’s Eugene Higgins Professor of Physics, who generated some of the initial inklings in this field in the 1990s. “It lies at the intersection of many fields of physics.”

From tiny bits of string

The seeds of this correspondence were sprinkled in the 1970s, when researchers were exploring tiny subatomic particles called quarks. These entities nest like Russian dolls inside protons, which in turn occupy the atoms that make up all matter. At the time, physicists found it odd that no matter how hard you smash two protons together, you cannot release the quarks—they stay confined inside the protons.

One person working on quark confinement was Alexander Polyakov, Princeton’s Joseph Henry Professor of Physics. It turns out that quarks are “glued together” by other particles, called gluons. For a while, researchers thought gluons could assemble into strings that tie quarks to each other. Polyakov glimpsed a link between the theory of particles and the theory of strings, but the work was, in Polyakov’s words, “hand-wavy” and he didn’t have precise examples.

Meanwhile, the idea that fundamental particles are actually tiny bits of vibrating string was taking off, and by the mid-1980s, “string theory” had lassoed the imaginations of many leading physicists. The idea is simple: just as a vibrating violin string gives rise to different notes, each string’s vibration foretells a particle’s mass and behavior. The mathematical beauty was irresistible and led to a swell of enthusiasm for string theory as a way to explain not only particles but the universe itself.

One of Polyakov’s colleagues was Klebanov, who in 1996 was an associate professor at Princeton, having earned his Ph.D. at Princeton a decade earlier. That year, Klebanov, with graduate student Steven Gubser and postdoctoral research associate Amanda Peet, used string theory to make calculations about gluons, and then compared their findings to a string-theory approach to understanding a black hole. They were surprised to find that both approaches yielded a very similar answer. A year later, Klebanov studied absorption rates by black holes and found that this time they agreed exactly.

That work was limited to the example of gluons and black holes. It took an insight by Juan Maldacena in 1997 to pull the pieces into a more general relationship. At that time, Maldacena, who had earned his Ph.D. at Princeton one year earlier, was an assistant professor at Harvard. He detected a correspondence between a special form of gravity and the theory that describes particles. Seeing the importance of Maldacena’s conjecture, a Princeton team consisting of Gubser, Klebanov and Polyakov followed up with a related paper formulating the idea in more precise terms.

Another physicist who was immediately taken with the idea was Edward Witten of the Institute for Advanced Study (IAS), an independent research center located about a mile from the University campus. He wrote a paper that further formulated the idea, and the combination of the three papers in late 1997 and early 1998 opened the floodgates.

“It was a fundamentally new kind of connection,” said Witten, a leader in the field of string theory who had earned his Ph.D. at Princeton in 1976 and is a visiting lecturer with the rank of professor in physics at Princeton. “Twenty years later, we haven’t fully come to grips with it.”

Two sides of the same coin

This relationship means that gravity and subatomic particle interactions are like two sides of the same coin. On one side is an extended version of gravity derived from Einstein’s 1915 theory of general relativity. On the other side is the theory that roughly describes the behavior of subatomic particles and their interactions.

The latter theory includes the catalogue of particles and forces in the “standard model” (see sidebar), a framework to explain matter and its interactions that has survived rigorous testing in numerous experiments, including at the Large Hadron Collider.

In the standard model, quantum behaviors are baked in. Our world, when we get down to the level of particles, is a quantum world.

Notably absent from the standard model is gravity. Yet quantum behavior is at the basis of the other three forces, so why should gravity be immune?

The new framework brings gravity into the discussion. It is not exactly the gravity we know, but a slightly warped version that includes an extra dimension. The universe we know has four dimensions, the three that pinpoint an object in space—the height, width and depth of Einstein’s desk, for example—plus the fourth dimension of time. The gravitational description adds a fifth dimension that causes spacetime to curve into a universe that includes copies of familiar four-dimensional flat space rescaled according to where they are found in the fifth dimension. This strange, curved spacetime is called anti-de Sitter (AdS) space after Einstein’s collaborator, Dutch
astronomer Willem de Sitter.

The breakthrough in the late 1990s was that mathematical calculations of the edge, or boundary, of this anti-de Sitter space can be applied to problems involving quantum behaviors of subatomic particles described by a mathematical relationship called conformal field theory (CFT). This relationship provides the link, which Polyakov had glimpsed earlier, between the theory of particles in four space-time dimensions and string theory in five dimensions. The relationship now goes by several names that relate gravity to particles, but most researchers call it the AdS/CFT (pronounced A-D-S-C-F-T) correspondence.

Tackling the big questionsThis correspondence, it turns out, has many practical uses. Take black holes, for example. The late physicist Stephen Hawking startled the physics community by discovering that black holes have a temperature that arises because each particle that falls into a black hole has an entangled particle that can escape as heat.

Using AdS/CFT, Tadashi Takayanagi and Shinsei Ryu, then at the University of California-Santa Barbara, discovered a new way to study

entanglement in terms of geometry, extending Hawking’s insights in a fashion that experts consider quite remarkable.

In another example, researchers are using AdS/CFT to pin down chaos theory, which says that a random and insignificant event such as the flapping of a butterfly’s wings could result in massive changes to a large-scale system such as a faraway hurricane. It is difficult to calculate chaos, but black holes—which are some of the most chaotic quantum systems possible—could help. Work by Stephen Shenker and Douglas Stanford at Stanford University, along with Maldacena, demonstrates how, through AdS/CFT, black holes can model quantum chaos.

One open question Maldacena hopes the AdS/CFT correspondence will answer is the question of what it is like inside a black hole, where an infinitely dense region called a singularity resides. So far, the relationship gives us a picture of the black hole as seen from the outside, said Maldacena, who is now the Carl P. Feinberg Professor at IAS.

“We hope to understand the singularity inside the black hole,” Maldacena said. “Understanding this would probably lead to interesting lessons for the Big Bang.”

The relationship between gravity and strings has also shed new light on quark confinement, initially through work by Polyakov and Witten, and later by Klebanov and Matt Strassler, who was then at IAS.

Those are just a few examples of how the relationship can be used. “It is a tremendously successful idea,” said Gubser, who today is a professor of physics at Princeton. “It compels one’s attention. It ropes you in, it ropes in other fields, and it gives you a vantage point on theoretical physics that is very compelling.”

The relationship may even unlock the quantum nature of gravity. “It is among our best clues to understand gravity from a quantum perspective,” said Witten. “Since we don’t know what is still missing, I cannot tell you how big a piece of the picture it ultimately will be.”

Still, the AdS/CFT correspondence, while powerful, relies on a simplified version of spacetime that is not exactly like the real universe. Researchers are working to find ways to make the theory more broadly applicable to the everyday world, including Gubser’s research on modeling the collisions of heavy ions, as well as high-temperature superconductors.

Also on the to-do list is developing a proof of this correspondence that draws on underlying physical principles. It is unlikely that Einstein would be satisfied without a proof, said Herman Verlinde, Princeton’s Class of 1909 Professor of Physics, the chair of the Department of Physics and an expert in string , who shares office space with Einstein’s desk.

“Sometimes I imagine he is still sitting there,” Verlinde said, “and I wonder what he would think of our progress.”

Stellar corpse reveals clues to missing stardust

Stellar corpse reveals clues to missing stardust
The Butterfly Nebula, also known as the Twin Jet Nebula, is an example of a so-called bipolar planetary nebula. The object of this study, K4-47, is much less known, but may be similar in appearance. Having nothing to do with planets.

Everything around you – your desk, your laptop, your coffee cup – in fact, even you – is made of stardust, the stuff forged in the fiery furnaces of stars that died before our sun was born. Probing the space surrounding a mysterious stellar corpse, scientists at the University of Arizona have made a discovery that could help solve a long-standing mystery: Where does stardust come from?

When stars die, they seed the cosmos around them with the elements that go on to coalesce into , planets, asteroids and comets. Most everything that makes up Earth, even life itself, consists of elements made by previous stars, including silicon, carbon, nitrogen and oxygen. But this is not the whole story. Meteorites commonly contain traces of a type of stardust that, until now, was believed to form only in exceptionally violent, explosive events of stellar death known as novae or supernovae – too rare to account for the abundance preserved in meteorites.

Researchers at the UA used radio telescopes in Arizona and Spain to observe gas clouds in the young planetary nebula K4-47, an enigmatic object approximately 15,000 light-years from Earth. Classified as a nebula, K4-47 is a stellar remnant, which astronomers believe was created when a star not unlike our sun shed some of its material in a shell of outflowing gas before ending its life as a white dwarf.

To their surprise, the researchers found that some of the elements that make up the nebula – carbon, nitrogen and oxygen – are highly enriched with certain variants that match the abundances seen in some meteorite particles but are otherwise rare in our solar system: so-called heavy isotopes of carbon, nitrogen and oxygen, or 13C, 15N and 17O, respectively. These isotopes differ from their more common forms by containing an extra neutron inside their nucleus.

Fusing an additional neutron onto an atomic nucleus requires in excess of 200 million degrees Fahrenheit, leading scientists to conclude those isotopes could only be formed in novae – violent outbursts of energy in aging binary star systems – and supernovae, in which a star blows itself apart in one cataclysmic explosion.

“The models invoking only novae and supernovae could never account for the amounts of 15N and 17O we observe in ,” said Lucy Ziurys, senior author of the paper, which is published in the Dec. 20 issue of the journal Nature. “The fact that we’re finding these isotopes in K4-47 tells us that we don’t need strange exotic stars to explain their origin. It turns out your average garden variety stars are capable of producing them as well.”

In lieu of cataclysmic explosive events forging heavy isotopes, the team suggests they could be produced when an average-size star such as our sun becomes unstable toward the end of its life and undergoes a so-called helium flash, in which super-hot helium from the star’s core punches through the overlaying hydrogen envelope.

“This process, during which the material has to be spewed out and cooled quickly, produces 13C, 15N and 17O,” explained Ziurys, a professor with dual appointments in the UA’s Steward Observatory and Department of Chemistry and Biochemistry. “A helium flash doesn’t rip the star apart like a supernova does. It’s more like a stellar eruption.”

Stellar corpse reveals clues to missing stardust
At 15,000 light-years, object K4-47 is about seven times farther away than the Twin Jet nebula, making it much more difficult to image. Based on what scientists have learned about K4-47 so far, it may have a similar structure of two lobes .

The findings have implications for the identification of stardust and the understanding of how common stars create elements such as oxygen, nitrogen and carbon, the authors said.

The discovery was made possible through a collaboration between disciplines that traditionally have remained relatively separate: astronomy and cosmochemistry. The team used at the Arizona Radio Observatory and Institut de Radioastronomie Millimetrique (IRAM) to observe rotational spectra emitted by the molecules in the K4-47 nebula, which reveal clues about their mass distribution and their identity.

“When Lucy and I started collaborating on this project, we realized that we could reconcile what we found in meteorites and what we observe in space,” said co-author Tom Zega, associate professor of cosmochemistry, planetary materials and astrobiology in the UA’s Lunar and Planetary Laboratory.

The researchers are eagerly awaiting the discoveries that lie ahead for NASA’s OSIRIS-REx asteroid sample return mission, which is led by the UA. Just two weeks ago, the spacecraft arrived at its target asteroid, Bennu, from which it will collect a sample of pristine material in 2020. One of the mission’s major goals is to understand the evolution of Bennu and the origins of the solar system.

“You can think of the grains we find in meteorites as stellar ashes, left behind by stars that had long died when our formed,” Zega said. “We expect to find those pre-solar grains on Bennu – they are part of the puzzle of the history of this asteroid, and this research will help define where the material on Bennu came from.”

“We can now trace where those ashes came from,” Ziurys added. “It’s like an archeology of stardust.”

Mars Express beams back images of ice-filled Korolev crater

Trapped layer of cold air keeps water frozen in 50-mile-wide impact crater

A composite picture of the Korolev crater in the northern lowlands of Mars, made from images taken by the Mars Express High Resolution Stereo Camera overlaid on a digital terrain model.
A composite picture of the Korolev crater in the northern lowlands of Mars, made from images taken by the Mars Express High Resolution Stereo Camera overlaid on a digital terrain model.

The stunning Korolev crater in the northern lowlands of Mars is filled with ice all year round owing to a trapped layer of cold Martian air that keeps the water frozen.

The 50-mile-wide crater contains 530 cubic miles of water ice, as much as Great Bear Lake in northern Canada, and in the centre of the crater the ice is more than a mile thick.

Images beamed back from the red planet show that the lip around the impact crater rises high above the surrounding plain. When thin Martian air then passes over the crater, it becomes trapped and cools to form an insulating layer that prevents the ice from melting.

The latest picture is a composite of five strip-like images taken from the European Space Agency’s Mars Express probe, which swung into orbit around the planet on Christmas Day 2003. On the same day, the orbiter released the Beagle 2 lander, a British probe built on a shoestring budget, which touched down but failed to fully open on the surface.

Mars Express photographed the Korolev crater with its high-resolution stereo camera, an instrument that can pick out features 10 metres wide, or as small as 2 metres when used in super-resolution mode.

Colour-coded topographic view
A colour-coded topographic view showing the relative heights of the terrain in and around the crater.

Evidence from orbiting spacecraft, rovers and landers reveals ancient water courses and lake beds on Mars. Vast quantities of frozen water have been found at the planet’s poles. In July, astronomers used Mars Express radar measurements to find what appeared to be a 12-mile stretch of briny water beneath the planet’s surface.

The Korolev crater is named after Sergei Korolev, the Russian rocket engineer and spacecraft designer known as the father of Soviet space technology. Korolev worked on the Sputnik programme that sent the first artificial satellites into space in the 1950s, and later on the Vostok programme that carried Yuri Gagarin into the history books as the first man to orbit Earth.

A Short History of the Missing Universe

Astronomers have known where the universe’s missing matter has been hiding for the past 20 years. So why did it take so long to find it?

Art for "A Short History of the Missing Universe"

Simulations of the universe’s large-scale structure hinted at where the universe’s missing matter might be.

The cosmos plays hide-and-seek. Sometimes, though, even when astronomers have a hunch for where their prey might hide, it can take them decades of searching to confirm it. The case of the universe’s missing matter — a case that appears to now be closed, as I reported earlier this month — is one such instance. To me, it is a fascinating tale in which clever cosmological models drew a treasure map that took 20 years to explore.

Scientists knew back in the 1980s that they could observe only a fraction of the atomic matter — or baryons — in the universe. (Today we know that all baryons taken together are thought to make up about 5 percent of the universe — the rest is dark energy and dark matter.) They knew that if they counted up all the stuff they could see in the universe — stars and galaxies, for the most part — the bulk of the baryons would be missing.

But exactly how much missing matter there was, and where it might be hiding, were questions that started to sharpen in the 1990s. Around that time, astronomer David Tytler of the University of California, San Diego, came up with a way to measure the amount of deuterium in the light of distant quasars — the bright cores of galaxies with active black holes at their center — using the new spectrograph at the Keck telescope in Hawaii. Tytler’s data helped researchers understand just how many baryons were missing in today’s universe once all the visible stars and gas were accounted for: a whopping 90 percent.

These results set off a firestorm of controversy, fanned in part by Tytler’s personality. “He [insisted] he was right in spite of, at the time, a lot of seemingly contradictory evidence, and basically said everyone else was a bunch of idiots who didn’t know what they were doing,” said Romeel Dave, an astronomer at the University of Edinburgh. “Turns out, of course, he was right.”

Then in 1998, Jeremiah Ostriker and Renyue Cen, Princeton University astrophysicists, released a seminal cosmological model that tracked the history of the universe from its beginnings. The model suggested that the missing baryons were likely wafting about in the form of diffuse (and at the time undetectable) gas between galaxies.

As it happens, Dave could have been the first to tell the world where the baryons were, beating Ostriker and Cen. Months before their paper came out, Dave had finished his own set of cosmological simulations, which were part of his Ph.D. work at the University of California, Santa Cruz. His thesis on the distribution of baryons suggested that they might be lurking in the warm plasma between galaxies. “I didn’t really appreciate the result for what it was,” said Dave. “Oh well, win some, lose some.”

Dave continued to work on the problem in the years to follow. He envisioned the missing matter as hiding in ghostly threads of extremely hot and very diffuse gas that connect galaxy pairs. In astro-speak, this became the “warm-hot intergalactic medium,” or WHIM, a term that Dave coined.

Many astronomers continued to suspect that there might be some very faint stars in the outskirts of galaxies that could account for a significant chunk of the missing matter. But after many decades of searching, the number of baryons in stars, even the faintest ones that could be seen, amounted to no more than 20 percent.

More and more sophisticated instruments came online. In 2003, the Wilkinson Microwave Anisotropy Probe measured the universe’s baryon density as it stood some 380,000 years after the Big Bang. It turned out to be the same density as indicated by the cosmological models. A decade later, the Planck satellite confirmed the number.

With the eventual failure to find hidden stars and galaxies that might be holding the missing matter, “attention turned toward gas in between the galaxies — the intergalactic medium distributed over billions of light years of low-density intergalactic space,” said Michael Shull, an astrophysicist at University of Colorado, Boulder. He and his team began searching for the WHIM by studying its effects on the light from distant quasars. Atoms of hydrogen, helium and heavier elements such as oxygen absorb the ultraviolet and X-ray radiation from these quasar lighthouses. The gas “steals a portion of light from the beam,” said Shull, leaving a deficit of light — an absorption line. Find the lines, and you’ll find the gas.

The most prominent absorption lines of hydrogen and ionized oxygen are at very short wavelengths, in the ultraviolet and X-ray portions of the spectrum. Unfortunately for astronomers (but fortunately for the rest of life on Earth), our atmosphere blocks these rays. In part to solve the missing matter problem, astronomers launched X-ray satellites to map this light. With the absorption line method, Shull said, scientists eventually “accounted for most, if not all, of the predicted baryons that were cooked up in the hot Big Bang.”

Other teams took different approaches, looking for the missing baryons indirectly. As my story from last week shows, three teams, including Shull’s, are now saying that all the baryons are accounted for.

But the WHIM is so faint, and the matter so diffuse, that it’s hard to definitely close the case. “Over the years, there have been many exchanges among researchers arguing for or against possible detections of the warm-hot intergalactic medium,” said Kenneth Sembach, director of the Space Telescope Science Institute in Baltimore. “I suspect there will be many more. The recent papers appear to be another piece in this complex and interesting cosmic puzzle. I’m sure there will be more pieces to come, and associated debates about how best to fit these pieces together.”

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