Four bright orange loops brand off of a dark blue blob

A transmission electron micrograph of HIV-1 virus particles (orange) replicating from the plasma membrane of an infected H9 T cell. Credit: NIAID, Flickr

<p>Scientists have cured a handful of people of HIV by piggybacking on treatments they received for blood cancer. But does that bring a widespread cure any closer?</p>

How a select few people have been cured of HIV

Perfectly round ball of brown ice cream on a spoon against white background

<p>Sixty years ago a teenager’s homemade ice cream raised a surprisingly complicated question: Can hot liquids freeze faster than cold ones?</p>

The ice cream that changed physics

A light blue brain scan against a black background with two purple C-shaped regions highlighted in purple

A brain scan from Fred Barrett's lab with the location of the claustrum indicated in purple. Image Credit: GBH/NOVA

<p>The claustrum seems to act as a switchboard, telling different parts of the brain when to turn on and off. But what happens when the switchboard operator steps away?</p>

How do psychedelics work? This brain region may explain their effects

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<p>NOVA Next is NOVA’s award-winning digital publication that provides answers from the cutting edge of science and technology. Launched in 2012, we feature in-depth articles and commentaries from some of the most respected journalists, scientists, and engineers. NOVA Next explores the ideas that are changing the future, from the frontiers of synthetic biology to the politics of personalized medicine.</p><p>Let us know what you think. We’re listening on <a href="http://www.facebook.com/NOVAonline">Facebook</a>, <a href="http://twitter.com/novapbs">Twitter</a>, and email.</p><p>If you’re a journalist and have an idea for a story, send it our way.</p>

NOVA Next is NOVA’s award-winning digital publication that provides answers from the cutting edge of science and technology.

NOVA Next

<p>NOVA Next is NOVA’s award-winning digital publication that provides answers from the cutting edge of science and technology.</p>

A woman in shorts walks across a concrete area surrounded by buildings, carrying an umbrella to block the blazing sun in the background

A pedestrian uses an umbrella to protect herself from the sun during a heatwave in London on July 18, 2022. Image Credit: Bloomberg, Getty Images

Why cities are so hot (and how we can fix it)

<p>You don’t have to live in Phoenix, Arizona, to notice that cities are getting hotter, but it helps. Consistently the hottest major metropolitan area in the country, Phoenix spent more than a third of 2022 in triple-digit temperatures. It has also seen a dramatic uptick in heat-related deaths in recent years, with the Maricopa County public health department reporting more than 300 heat-associated deaths in 2021, an increase of 450% from 2014.</p>
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<p>“It's a really unfortunate, tragic data point that we find ourselves sharing a lot,” David Hondula, director of the city’s Office of Heat Response and Mitigation, said in an interview for the NOVA documentary “<a href="https://www.pbs.org/wgbh/nova/video/weathering-the-future/">Weathering the Future</a>.” People are often surprised to learn that heat is the leading cause of weather-related death, he said. It “lurks as a background hazard and doesn't always get the attention of hazards like thunderstorms, tornadoes, hurricanes, floods.”</p><p>Phoenix did not hire Hondula because the planet in general is getting hotter but because cities are the hottest of all. That’s due to what’s known as the “urban heat island” effect, which refers to the ways that the design and infrastructure of cities tend to make that environment hotter—up to 20 degrees warmer than surrounding rural areas.</p><p>We’ve known about the urban heat island effect for a long time; even the Romans noticed it. But it’s especially important now because of the health risks that come with heat, according Brian Stone, a professor of environmental planning at Georgia Institute of Technology. “Of all the stressors we associate with climate change, heat is the only one that has a physiological threshold that we know will lead to certain death,” he told NOVA. </p><p>There’s a flip side, though. Since “we’ve basically engineered cities to be hot,” he said, “we can reverse engineer them to be cooler.”</p>
<h3><b>What creates an urban heat island?</b></h3><p>Researchers have pinpointed four main factors that drive the urban heat island effect. First is vegetation, or lack thereof. Trees are often the first thing to go when cities expand. (Take Atlanta, which has lost nearly 80,000 trees to development since 2014.) Cutting down trees creates a double-whammy effect, since they not only offer shade but also soak up heat from their environment. When we cut down urban trees, “we reduce the capacity of the natural system to cool itself through evaporation, much like we cool ourselves through perspiration,” Stone said.</p><p>Then, the materials we install in place of that lost greenery tend to aggravate the problem further. Mineral-based materials such as asphalt, concrete, shingles, and other roofing have a high “thermal capacity,” meaning they absorb a lot of heat during the day and then release it slowly back into the environment as they cool, even well into the night. That’s a problem in particular because the atmosphere doesn’t actually heat up very much from direct sunlight; instead, it warms more as heat is released from the ground.</p><h4>WATCH: HOW EXTREME HEAT OVERWHELMS THE HUMAN BODY</h4>
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<p>Next, urban morphology, or the three-dimensional structure of a city, also contributes to increasing temperatures, since tall buildings are particularly good at trapping heat between them. And lastly, there’s waste heat, another double whammy. The process of consuming energy to do work, by anything from a car engine to a lightbulb to a computer, generates heat as a byproduct. Since cities are where most humans live together, they’re also where we consume the most energy: for transit, heating and cooling our homes, and the myriad electrified activities of daily life. And the waste heat that escapes into the environment from those activities measurably affects city temperatures.</p>
<h3><b>Urban heat islands and human health</b></h3><p>As all of those factors accumulate, the consequences for human health add up, too. Human bodies can handle very hot temperatures briefly but are sensitive to heat over the long term. “It's the warm nighttime temperatures that cause the health problems,” Marshall Shepherd, director of the atmospheric sciences program at the University of Georgia, told NOVA. “And that's where the urban heat island comes in, because that's when it tends to peak”— with all those mineral surfaces re-releasing heat after dark.</p><p>Our bodies respond to heat by elevating our heart rate, allowing us to better circulate moisture. That moisture becomes sweat, which we use to cool ourselves down. Most people can handle an elevated heart rate for a few hours a day. But “if you have to elevate your heart rate for 24 hours because it's 85 degrees at night in your home, then you are overly taxing your physiologic capacity to deal with heat,” Stone said. Then things start to get dangerous. </p><p>Research shows that it’s generally the neighborhoods that can least afford it that get hit the hardest by urban heat. That’s in part because of discriminatory urban planning policies like redlining, which ensured that poorer neighborhoods had fewer trees and more industrial development. One recent study found a difference of 7 to 10 degrees between lower and higher income areas of <i>the same</i> cities. In another project, Shepherd and his colleagues compared redlining maps with satellite data tracking heat. The results showed “a clear heat island within the heat island for marginalized groups,” he said.</p>
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<h3><b>What can we do about it?</b></h3><p>With Phoenix’s heat-related death rate doubling in just five years, the city went into crisis mode in 2021. Nighttime temperatures in Phoenix average about 10 degrees hotter than they were half a century ago, hovering around 85 degrees during the hottest part of the year. And there are some days when “we've measured pavement, for example, at 160 Fahrenheit, 170 Fahrenheit,” Hondula said, with similar numbers on, for example, metal playground slides—a temperature that can cause direct burns. </p><p>As he’s stepped into his role managing the city’s heat crisis, Hondula has remained hopeful. Phoenix’s trends aside, “we've seen decreasing heat-associated deaths over the past several decades, as our planet has been warming,” he said. He attributes that to increased understanding of the urban heat island effect, which is beginning to lead to meaningful change in many cities.</p><p>One place to start: more trees. Large cities such as Washington, D.C., Baltimore, Philadelphia, and New York currently average around 25%-40% tree canopy cover but could support up to 60% cover, the U.S. Forest Service has found. Even arid Phoenix has committed over $7 million to planting drought-tolerant trees over the next few years. Similar non-profit organizations all over the world have committed to planting trees in the coming decades: 170,000 in Paris, 1 million in Atlanta, 1 million in São Paulo, 4 million in Houston. </p>
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<img alt="Two adults talk while walking toward the camera through two rows of small trees in a grassy park" class="richtext-image full-width" src="https://www.pbs.org/wgbh/nova/media/images/Phoenix_cool_corridor.width-990.jpg"/>
<figcaption><p>Professor David Sailor (left) and Professor Na'Taki Osborne Jelks (right) walk through a "cool corridor," where trees have been planted to provide relief from the heat in Phoenix, Arizona. Image courtesy of GBH</p>
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<p>But it will take years, maybe decades, for those trees to mature and start making a difference. So Stone also sees great potential in changing the materials we use to build cities. For example, dark-colored slate roof shingles with high thermal capacity might make sense in London, or somewhere else cold and wet, but are much less logical in the southern United States. “We can engineer any color we'd like of roofing material, and so part of what we need is not just a technological change in the way we build, but a cultural shift to be more accepting of lighter, more reflective materials,” he said. Other more immediate solutions include initiatives like Phoenix’s cool pavement program, which encourages the use of alternative road materials, as well as the city’s work prioritizing shade structures at bus stops or in other places where trees might not be suitable.</p><p>The powerful thing about recognizing the heat island effect is recognizing it as a phenomenon within local control. That means it’s possible to make progress without waiting for national or international policy change, Stone said. Research shows “we can substantially cool down cities in a relatively quick period of time without major expenditures.” </p><p>Hondula takes this optimism even further. Even in the context of global warming, he said, efforts to increase tree and other shade cover and promote use of materials with lower thermal capacity mean “we could wind up with a city of the future that's cooler than the one we have today.”</p><h4>WATCH: WEATHERING THE FUTURE</h4>
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<p data-block-key="i0kee">Even the Romans noticed that cities are engineered to be heat islands. But that means we can do something about it.</p>

A curving line of low flames burns in a forest at night, while four fire crew members are silhouetted by the orange glow of fire and smoke

Crews manage a low-intensity fire to burn ground fuels and create a "black line" to help contain a wildfire in New Mexico's Gila National Forest in May 2012. Image Credit: Kari Greer, USFS Gila National Forest

How Native American traditions control wildfires

<p>In September 2020, the Slater fire roared through the tiny northern California town of Happy Camp, destroying almost 200 homes. Like many of the megafires that have increasingly engulfed the state, the fire burned hot and fierce for days, spitting embers miles ahead of itself and rapidly igniting the space in between. It ultimately consumed more than 200,000 acres. “For about a year in that area, you wouldn't see any birds,” Kathy McCovey, a forester and anthropologist from the area, said in an interview for the NOVA documentary “<a href="https://www.pbs.org/wgbh/nova/video/weathering-the-future/">Weathering the Future</a>.” “You wouldn't see any animals, any insects. It was just devoid of life.”</p>
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<p>Happy Camp is tucked in the Klamath River Valley, in the traditional homeland of the Karuk people, and it is still home to a significant Native American population. But many residents were forced to move after the fire, afraid of what might happen if they rebuilt. “We'd had fires come close to Happy Camp before, but they didn't burn as intensely or as quickly,” McCovey told NOVA.</p><p>Some 24% of California’s forest burned between 2012 and 2021, with blazes like the Slater fire sparking more frequently and putting communities at ever-greater risk. “Even though we have the largest, most well-trained firefighting force on earth, there's simply several months out of the year when no amount of resources are going to stop these wildfires,” Will Harling, director of the local mid-Klamath watershed council, told NOVA.</p><p>It’s a painful reality for the Karuk and other Native American groups in this part of California. Their livelihoods and traditions were closely linked to fire for millennia. But after European contact, they were harassed, jailed, or worse for practicing traditions that involved careful maintenance of the land with fire. Now, California and other Western states are seeking a return to those traditional practices as they try alternative approaches to controlling wildfires, a bittersweet victory for the Native Americans who have long fought for the right to burn.</p>
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<img alt="A bright blue river runs diagonally from the bottom left corner, between two mountains covered in pines" class="richtext-image full-width" src="https://www.pbs.org/wgbh/nova/media/images/Klamath_River_3tH1zBf.width-990.jpg"/>
<figcaption><p>The Klamath River in Northern California, ancestral home to Native American tribes including the Karuk. Image courtesy of GBH</p>
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<p>For thousands of years, the Karuk used fire to maintain forests as a source of food, following an annual burning cycle to create an environment that was optimal for the area’s native plants and animals. Some 80% of the species the Karuk have historically relied upon for food, traditions, and rituals cannot thrive without fire, according to Bill Tripp, the Karuk director of natural resources and environmental policy. Those species include willow used for basket weaving; tanoak trees, whose acorns are a major food staple; and elk, an important source of protein. The types of forest meadows created by regular fires are important for elk populations, Tripp told NOVA. “Maintaining these types of places with fire helps to make sure there's more healthy calves, which means there's more healthy adults.” Similarly, a well-timed fire will cause insect-infested acorns to drop to the ground, while leaving healthy acorns unscathed and ready for gathering. And flames prompt willow shoots to grow back straighter, more pliable, and better suited for weaving.</p><p>“Our place in the world is to manage our piece of the world,” Chook Chook Hillman, a Karuk community organizer and cultural practitioner, told NOVA. "That's part of the reciprocity that has allowed us to live here for so long.” </p><p>But starting in the 1800s, White settlers arriving in the area saw these forests as a source of income and worried that their new towns were vulnerable to the fires that occasionally swept through. One of the first laws after the establishment of the state of California banned intentional burning. “Native people in this place were shot and imprisoned and beaten for trying to maintain their cultural fire practices,” Harling said. Those practitioners who continued Karuk burning traditions risked legal trouble to do so until as recently as the 1990s. McCovey recalled hiding her fire practices even from friends, mindful that they might land her in jail. </p><p>Without regular, low-intensity fires to thin them out, California forests are now twice as dense as they were 200 years ago. “Everything’s dead underneath there because every tree is wall to wall,” Hillman said. “All you have is this brown layer of branches and needles. It's just this bed waiting to really explode.” Add to that the increased drought and heat wrought by climate change, and you get the conditions that produce <a href="https://www.pbs.org/wgbh/nova/video/as-megafires-have-become-more-common-so-have-fire-tornadoes/">the megafires</a> that have plagued the West for the last decade, turning what was once a fire season into what Harling refers to as a “fire year.”</p>
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<img alt="Aerial view of a road curving down the left side of the photo, dividing a large burned patch of land on the right from green vegetation on the left" class="richtext-image full-width" src="https://www.pbs.org/wgbh/nova/media/images/Karuk_prescribed_burn.width-990.jpg"/>
<figcaption><p>The Karuk Tribe's Tishaniik ceremonial site, which was maintained by prescribed burning. Image courtesy of GBH</p>
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<p>With infernos like the Slater fire becoming the norm, many areas in California are turning toward purposeful burns like those the Karuk practiced for millennia, recognizing that, as Harling said, “it’s how you want your fire. Do you want your fire in the middle of the fire season as an uncontrollable wildfire? Or do you want to put it in intentionally?”</p><p>In much of the West, that has meant mixing what’s known as “cultural fire," with a ritual focus, and “prescribed burns” that incorporate more Western fire science and data. Tripp, like many cultural fire practitioners, was raised on stories and oral tradition, which taught him how to burn complex fires from a young age. “We didn't have electricity, running water, refrigeration, all that kind of stuff when I was little,” he said of his childhood with his great-grandmother, “but I learned how to read the environment, how to know when this is going to burn, but this over here isn't.” This new era allows Tripp and others like him to return to those practices with less anxiety about the consequences.</p>
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<p>One goal in this approach to fire is to achieve a kind of burned mosaic, Hillman said, a patchwork in which some areas are more deeply burned while in others only grasses or undergrowth are singed, clearing fuels away but leaving the forest canopy untouched. Karuk tradition even includes guidance about the ideal timing to burn around specific plant species. </p><p>Meanwhile, prescribed burns tend to apply this approach to more generalized fire management that’s not for explicit cultural or ritual purposes. At a recent prescribed burn near Happy Camp, community members helped lay down a “black line,” a border of burned greenery meant to stop the advance of a wildfire that might come through or a prescribed burn that might burn out of control. That created what practitioners often call a “catcher’s mitt” or “safety net,” a burned buffer zone useful for protecting communities and controlling future prescribed fire. </p><p>With that safety zone in place, they lit fires across a field, keeping a close eye on the level of moisture in the air and the speed and direction of the wind. Water hoses waited nearby to help put down any flames that might leap across the black line. With any fire, escape is a concern, but careful planning allows for burning when the risk is minimal. “What we know is that the relative risk of being organized ahead of time, of choosing your burn window and choosing your resources, is night and day from the situation in a wildfire, when it's coming at the hottest, driest day of the year,” Harling said.</p>
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<img alt="Two men walk on a road that winds between two patches of burned land, with a forested mountain in the background" class="richtext-image full-width" src="https://www.pbs.org/wgbh/nova/media/images/Tishaniik_ceremonial_site.width-990.jpg"/>
<figcaption><p>Chook Chook Hillman (left) and his father Leaf Hillman (right) walk through the Karuk Tribe's Tishaniik ceremonial site, an area maintained by burning. The U.S. Forest Service made headlines in 2022 when a planned burn in New Mexico got out of control. But only .5% of prescribed fires in California escape, and even then those rarely cause severe damage, since they tend to take place when conditions are less volatile. Image courtesy of GBH</p>
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<p>In 2021, the U.S. Forest Service burned nearly 2 million acres of federal land in prescribed fires, more than ever before. And it has promised to continue expanding the practice, managing more than 50 million acres with fire over the next decade. That’s a meaningful shift for Native American fire practitioners like Tripp, who are able to reclaim their tradition and bring “good fire” back to the mountains. </p><p>To start, Harling, Hillman, and others are focusing on more intensively managing existing wildfire footprints, like the ruins of the Slater fire. “It’s an opportunity without any major fuels built up to start to put fire back in as those pieces of that landscape become receptive, so that you immediately get that patchwork of frequent fire,” Harling said.</p><p>As California looks to a climate-altered future, Tripp echoes Harling in emphasizing the fire-dependent nature of the state’s ecosystems. “The fact is, it's going to burn under the conditions that we choose for it to burn in, or it's going to burn in the conditions that nature or a careless match or cigarette butt or power line transformer decides,” he said. That means that part of the task of managing California fire is changing the population’s relationship to it. </p><p>In Happy Camp, both the Native and non-Native communities have engaged in intensive discussions about how to use cultural and prescribed burns better and more frequently. And in recent years, local students have begun visiting sites before and after prescribed burns to understand the process and observe the health of various species. Some have also started attending the prescribed burns themselves, to start getting used to what it’s like to be around flames again. “Fire can be your friend if you can learn to understand it,” Tripp said.</p><h4><b>WATCH: WEATHERING THE FUTURE</b></h4>
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<p data-block-key="uj17i">As wildfires escalate in Western states, authorities are embracing once-outlawed burning practices.</p>

A large sphere with varied blue and white bands fills most of the frame, with sections of its golden rings visible on either side

A false-color image of Saturn based on near-infrared data from the Cassini spacecraft, which orbited the planet for 13 years. Image Credit: NASA/JPL-Caltech/SSI/CICLOPS/Kevin M. Gill

8 mind-blowing space documentaries to watch now on NOVA

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<p>For almost 50 years, NOVA has explored the cosmos, taking viewers across our solar system, into distant galaxies, and right up to the “edge” of a black hole. From a Mars rover swooping down to the red planet, to a probe’s daring encounter with an asteroid, we’ve followed NASA and other space missions as they’ve revealed the universe to humanity. Now we present a curated selection of space documentaries from the past five years so you can explore the universe alongside the scientists who make the journey possible.</p>
<h3>New Eye on the Universe (2023)</h3><p>In July 2022, NASA’s James Webb Space Telescope released its first images, looking further back in time than ever before to show our universe in <a href="/wgbh/nova/video/how-nasa-colors-images-of-the-universe/">stunningly beautiful detail</a>. But that was just the beginning: With tons of new data and spectacular images flooding in, Webb is allowing scientists to peer deep in time to try to answer some of astronomy’s biggest questions. When—and how—did the first stars and galaxies form? And can we see the fingerprints of life in the atmospheres of distant worlds–or even within our own solar system?</p>
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<h3><a href="https://www.pbs.org/wgbh/nova/video/star-chasers-of-senegal/"><b>Star Chasers of Senegal</b></a><b> (2023)</b></h3><p>A NASA spacecraft named <a href="/wgbh/nova/article/nasa-lucy-mission-trojan-asteroids/">Lucy blasts off</a> from Cape Canaveral on a mission to the Trojans, a group of asteroids over 400 million miles from Earth thought to hold important clues about the origins of our solar system. Just hours before, in Senegal, West Africa, a team of scientists sets out to capture extraordinarily precise observations vital to the success of the Lucy mission—crucial data needed to help NASA navigate Lucy to its asteroid targets across millions of miles of space. The team’s leader, Senegalese astronomer Maram Kaire, takes viewers on a journey to investigate his nation’s rich and <a href="/wgbh/nova/video/how-to-calculate-time-with-an-islamic-astrolabe/">deep history of astronomy</a>, reaching back <a href="/wgbh/nova/video/these-ancient-stones-helped-track-seasons/">thousands of years</a>—and the promising future ahead.</p>
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<h3>Ultimate Space Telescope (2022)</h3><p>How did NASA engineers build and launch the most ambitious telescope of all time? Follow the dramatic story of the James Webb Space Telescope—the most complex machine ever launched into space. If it works, scientists believe that this new eye on the universe will <a href="/wgbh/nova/video/nasa-reacts-to-the-james-webb-space-telescope-images/">peer deeper back in time and space</a> than ever before to the birth of galaxies, and may even be able to “sniff” the atmospheres of exoplanets as we search for signs of life beyond Earth. But getting it to work is no easy task. The telescope is far bigger than its predecessor, the famous <a href="/wgbh/nova/article/hubble-space-telescope-best-images/">Hubble Space Telescope</a>, and it needs to make its observations a million miles away from Earth—so there will be no chance to go out and fix it. That means there’s no room for error; the most ambitious telescope ever built needs to work perfectly. Meet the engineers making it happen and join them on their high stakes journey to uncover new secrets of the universe. </p>
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<h3><a href="https://www.pbs.org/wgbh/nova/video/nova-universe-revealed-alien-worlds/"><b>NOVA Universe Revealed: Alien Worlds</b></a><b> (2021)</b></h3><p>In the five-part series <a href="https://www.pbs.org/wgbh/nova/series/nova-universe-revealed/">NOVA Universe Revealed</a>, we delve into the vastness of space to capture moments of high drama when the universe changed forever. In this episode, we tackle an age-old question: Are we alone? Or do other lifeforms and intelligences thrive on worlds far beyond our own? Ultra-sensitive telescopes and dogged detective work are transforming <a href="/wgbh/nova/video/how-to-find-an-exoplanet/">alien planet-hunting</a> from science fiction into hard fact. Join NOVA on a visit to exotic worlds <a href="/wgbh/nova/video/planets-two-stars/">orbiting distant suns</a>, from puffy planets with the density of Styrofoam to thousand-degree, broiling gas giants. Most tantalizing of all are the Super-Earths in the “Goldilocks zone,” just the right distance from their sun to support life, and with one of them signaling life’s essential ingredient, water, in its atmosphere. Are we on the brink of answering that haunting question?</p>
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<h3>Looking For Life on Mars (2021)</h3><p>Follow along as NASA launches the Mars 2020 Mission, perhaps the most ambitious hunt yet for <a href="/wgbh/nova/video/nasa-mars-perseverance/">signs of ancient life</a> on Mars. In February 2021, the spacecraft blazes into the Martian atmosphere at some 12,000 miles per hour and lowers the Perseverance Rover into the rocky Jezero Crater, home to a dried-up river delta scientists think could have harbored life. Perseverance will comb the area for signs of life and collect samples for possible return to Earth. Traveling onboard is a four-pound helicopter that will conduct a <a href="/wgbh/nova/article/nasa-ingenuity-first-powered-flight-mars/">series of test flights</a>—the first on another planet. During its journey, Perseverance will also test technology designed to produce oxygen from the Martian atmosphere, in hopes that the gas could be used for fuel—or for humans to breathe—on future missions.</p>
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<h3><a href="https://www.pbs.org/wgbh/nova/video/touching-the-asteroid/"><b>Touching the Asteroid</b></a><b> (2020) </b></h3><p>In October 2020, a NASA spacecraft called OSIRIS-REx attempts to reach out and <a href="/wgbh/nova/video/inside-osiris-rexs-mission-to-tag-asteroid-bennu/">grab a piece of an asteroid</a> named Bennu to bring it back to Earth. The OSIRIS-REx team has just three chances to extend its spacecraft’s specialized arm, touch down for five seconds, and collect material from the surface of Bennu. But if they can pull it off, scientists could gain great insight into Earth’s own origins—and even learn to defend against rogue asteroids that may one day threaten our planet.</p>
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<h3>Back to the Moon (2019)</h3><p>On the 50th anniversary of the historic Apollo 11 Moon landing, NOVA looks ahead to the hoped-for dawn of a new age in lunar exploration. This time, governments and private industry are working together to reach our nearest celestial neighbor. <a href="https://www.pbs.org/wgbh/nova/article/moon-water-human-space-exploration/">But why go back?</a> The Moon can serve as a platform for basic astronomical research; as an abundant source of rare metals and hydrogen fuel; and ultimately as a stepping stone for human missions to Mars and beyond. Join the next generation of engineers that aim to take us to the Moon, and discover how our legacy of lunar exploration won't be confined to the history books for long.</p>
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<h3>Black Hole Apocalypse (2018)</h3><p>Black holes are the most enigmatic and exotic objects in the universe. They’re also the most powerful, with gravity so strong it can trap light. And they’re destructive, swallowing entire planets, even giant stars. Anything that <a href="/wgbh/nova/video/whats-inside-a-black-hole/">falls into them</a> vanishes…gone forever. Now, astrophysicists are realizing that black holes may be essential to how our universe evolved—their influence possibly leading to life on Earth and, ultimately, us. In this two-hour special, astrophysicist and author Janna Levin takes viewers on a journey to the frontiers of black hole science. Along the way, we meet leading astronomers and physicists on the verge of finding new answers to provocative questions about these shadowy monsters: Where do they come from? What’s inside? <a href="/wgbh/nova/video/take-a-ride-into-a-supermassive-black-hole/">What happens if you fall into one?</a> And what can they tell us about the nature of space, time, and gravity?</p>
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<p data-block-key="z1ikv">Check out some of NOVA’s best space documentaries available for streaming.</p>

Looking down on a black 4-burner induction cooktop with a boiling pot of pasta. A hand pushes a button; raw shrimp and vegetables sit on either side of the stove.

A pot of pasta boils on an induction cooktop. Image Credit: Juan Algar, Getty Images

How do induction stoves work?

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<p>Some 110 million U.S. households use a range–or cooktop and oven–for cooking. About <a href="https://www.eia.gov/consumption/residential/data/2020/hc/pdf/HC%203.1.pdf">two-thirds of those</a> are powered by electricity, while one-third use natural gas or other fuels. But of the electric-powered majority, only a tiny percentage use what’s called induction. Though induction stoves might look like other electric stovetops, there’s some fascinating science that sets them apart from both gas and traditional electric stoves. </p><p>In a gas stove, natural gas or another fuel like propane is ignited to create a flame that can quickly be turned to the desired level. Electric ranges, on the other hand, come in a variety of styles, but all rely on a heating coil. Once the power is turned on, alternating current sends a rush of electrons around the coil. But the electrons aren’t on a slip-n-slide: The coil itself is designed to resist the electron flow, priming the particles to release their energy as heat. This process of building up the heat means it takes longer for an electric burner to reach the desired temperature than an instantly adjustable gas burner.</p><p>At a glance, induction cooktops may resemble traditional electric ones—both can have smooth, glass tops with coils powered by electricity. But induction cooktops use electricity in a slightly different way to heat up your pans.</p>
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<p>In contrast to traditional electric stoves, the coil inside an induction cooktop does not itself generate heat. In fact, the coil is designed with metals that conduct electricity rather than resist it. As the alternating current flows from the power socket into the metal coil, it forms a magnetic field. This magnetic field can interact with ferromagnetic materials, like the iron in a <a href="https://www.pbs.org/wgbh/nova/article/season-cast-iron-skillet-polymerization/">cast iron skillet</a> or the alloys in a stainless steel pan, to create small currents in those metals. The pan itself then acts like a traditional heating coil, resisting those currents and causing the electrons to release their energy as heat.</p><p>This method of heating is possible because ferromagnetic materials have magnetic domains, or groups of atoms that are aligned in such a way that they can be collectively influenced by a magnetic field. This is why induction stovetops only work with pots and pans made of magnetic materials—and they won't directly heat up other things, like aluminum pans or the skin on your hand.</p><p>Like a gas stove, an induction stove can be adjusted up and down immediately, heating food quickly and efficiently. According to <a href="https://www.energystar.gov/about/2021_residential_induction_cooking_tops#:~:text=The%20per%20unit%20efficiency%20of,times%20more%20efficient%20than%20gas">Energy Star</a>, roughly about 85% of the electricity consumed by induction stovetops actually goes toward heating your food. By contrast, traditional electric stoves have an efficiency of 75-80%, and gas stoves are 32% efficient.</p>
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<p data-block-key="o5d6c">Here’s how a magnetic field can heat up your pans.</p>

Bright stars in a royal blue triangular opening in deep red and purple dust clouds.

The Tarantula Nebula’s chemical composition is remarkably like the conditions in the Milky Way billions of years ago, during peak star formation. This composite of X-ray and infrared observations uses chromatic ordering to show both supernova explosion remnants (royal blue and purple) and clouds of gas (red and orange). Image Credit: X-ray: NASA/CXC/Penn State Univ./L. Townsley et al.; IR: NASA/ESA/CSA/STScI/JWST ERO Production Team

How NASA makes those spectacular space images

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<p>Over the past year, we’ve seen some of the most <a href="/wgbh/nova/video/how-scientists-created-the-james-webb-telescopes-first-test-image/">spectacular images</a> of space yet, courtesy of the new <a href="https://www.pbs.org/wgbh/nova/video/ultimate-space-telescope/">James Webb Space Telescope</a> (JWST). But JWST only observes infrared light, which has wavelengths longer than the human eye can see. So how do NASA’s data image developers turn those observations into colorful, <a href="https://www.pbs.org/wgbh/nova/article/james-webb-space-telescope-first-images-nasa/">awe-inspiring images</a>? The trick is in “chromatic ordering,” shifting those wavelengths down the spectrum, just as a musician might play a piece of music in a different key. That means keeping the differences <i>between</i> the wavelengths consistent but moving them into the part of the electromagnetic spectrum that we can see. </p><p>When that transfer is complete and the images combined, the result is a style that visuals developers like Joe DePasquale of the Space Telescope Science Institute call “representative color”: an image that is aesthetically pleasing and also essentially a data visualization. One reason JWST records particular wavelengths of light is because they can indicate the presence of specific elements. Because these images are created using chromatic ordering, it’s possible to “read” them a bit like a map: a red area is rich in hydrogen or sulfur, while blue contains oxygen.</p><p><b>New Eye on the Universe, premiering February 22 at 9 pm on PBS:</b></p>
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<p>Some shades might look muddy next to one another or create an image that’s too bright, almost lurid, so DePasquale might make an adjustment, or raise the contrast to make the colors pop. Still, in every stage of the process, “we’re always respecting the data,” <a href="https://www.pbs.org/wgbh/nova/video/new-eye-on-the-universe/">he told NOVA</a>. “We're not trying to introduce things that weren't there in the data to begin with, and we're not trying to remove things that are there.” </p><p>Here are just a few examples of the beautiful images created from the observations of the most powerful space telescope humanity has ever built—with a little help from chromatic ordering.</p>
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<img alt="A gauzy pale blue cloud against the night sky, orange and white stars at the lower left" class="richtext-image full-width" src="https://www.pbs.org/wgbh/nova/media/images/molecular_cloud.width-990.jpg"/>
<figcaption><p>Image Credit: NASA, ESA, CSA, and M. Zamani (ESA).</p>
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<p><b>Molecular Cloud</b></p><p>This moody watercolor-like image shows what’s called a molecular cloud, where stars and planets are born. The remarkable thing about this cloud is that it’s so cold that many different kinds of molecules are frozen and suspended within it—not just water, but also more complicated molecules like methane and ammonia. These molecules were detectable thanks to light from the background stars shining through the cloud.</p>
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<img alt="Two merging galaxies form a white and red bean-like shape against a black sky. The cores of the galaxies are colored blue, and one is above and to the left of the other." class="richtext-image full-width" src="https://www.pbs.org/wgbh/nova/media/images/galaxies_merging.width-990.jpg"/>
<figcaption><p>Image Credit: ESA/Webb, NASA &amp; CSA, L. Armus, A. Evan</p>
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<p><b>Merging Galaxies</b></p><p>This pair of merging galaxies, known as II ZW 96, was chosen as a target to help test JWST because it’s so bright at infrared wavelengths. This image shows how the gravity of the merger is slowly twisting both galaxies out of shape, their luminous cores connected by bright tendrils of star-forming regions.</p>
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<img alt="A glowing white ball with a few extra bright spots and thin luminescent rings against a black background" class="richtext-image full-width" src="https://www.pbs.org/wgbh/nova/media/images/Neptune_rings.width-990.jpg"/>
<figcaption><p>Image Credit: NASA, ESA, CSA, STScI</p>
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<p><b>Neptune's Rings</b></p><p>This image of Neptune represents our clearest view of the planet's rings since Voyager 2’s flyby in 1989. The telescope’s near-infrared camera gives us amazing detail, showing both the brighter, stronger belts and more subtle bands of dust. This view of Neptune is also unique because the planet’s methane gas registers very dark in infrared, rather than the cool blue we often see, and is broken up by high-altitude methane-ice clouds which reflect sunlight and appear bright white. </p>
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<img alt="Gray cobweb-like formations form a spiral against a black background with blue and red dots throughout" class="richtext-image full-width" src="https://www.pbs.org/wgbh/nova/media/images/Galaxy_IC_5332jpg.width-990.jpg"/>
<figcaption><p>Image Credit: ESA/Webb, NASA &amp; CSA, J. Lee and the PHANGS-JWST and PHANGS-HST Teams<br/> </p>
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<p><b>Galaxy IC 5332</b></p><p>Here, we see the “bones” of galaxy IC 5332, a structure that is cloaked in dust. But by observing in mid-infrared light, JWST can look <i>through</i> that dust to see the fingers of gas fanning out within IC 5332. The telescope’s specialized mid-infrared instrument must be kept much colder than its other sensors—only a few degrees above absolute zero. </p>
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<img alt="A detailed spiral galaxy against a crowded field of galaxies interspersed with bright 8-pointed stars on a dark background." class="richtext-image full-width" src="https://www.pbs.org/wgbh/nova/media/images/LEDA_2046648_Galaxy.width-990.jpg"/>
<figcaption><p>Image Credit: ESA/Webb, NASA and CSA, A. Martel</p>
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<p><b>Galaxy Field</b></p><p>This photo was taken during JWST’s calibration phase, to test the telescope’s ability to find the most ancient galaxies. It was designed specially to find galaxies that are so far away that, as their light travels toward us and the universe expands, that light has been stretched all the way into the infrared range. Those more orange-red galaxies can be compared to LEDA 2046648, the spiral galaxy toward the bottom of the image. Even at a billion light years away it is closer to Earth.</p>
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<p data-block-key="r128x">The James Webb Space Telescope only captures infrared light, but imaging developers can convert the invisible into something both beautiful and scientifically accurate.</p>

A man in a white shirt and black pants, standing knee-deep in water, seen from behind as he looks toward a dolphin with is upper body above the water, nose pointed up

A dolphin gives a cue to a fisher at Praia da Tesoura in Laguna, Brazil. Image Credit: Bianca Romeu, Universidade Federal de Santa Catarina

When wild dolphins help humans fish, both benefit

<p>Along the coast of Santa Catarina in southern Brazil, fishing is a way of life. But not just any kind of fishing. For the last 150 years, the fishers of the city of Laguna have cultivated a unique relationship with the local Lahille’s bottlenose dolphins, working together with their cetacean comrades to catch the schools of mullet that frequent their lagoon. </p>
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<p>The dolphins herd the fish toward human fishers who wait by the shore, hip deep in water. Then, at exactly the right moment, the dolphins dive, a signal for the fishers to cast their nets. Working together, the fishers catch more fish, and the dolphins have a chance to snag a few from the net as well.</p><p>“The experience of fishing with dolphins is unique,” Laguna fisher Wilson F. Dos Santos <a href="https://www.nytimes.com/2023/01/30/science/dolphins-brazil-fishermen.html">told The New York Times</a>. Both the humans and the dolphins have passed the practice down for generations, with the fishers giving their dolphin counterparts <a href="https://www.worldnomads.com/stories/discovery/fishing-with-dolphins-in-laguna-brazil">names like Scooby, Caroba</a>, and Porquinho. Dos Santos learned the tradition from his father, bringing him coffee and food by the shore and watching the nets unfurl, he told the Times.</p><p>Now, in <a href="https://www.pnas.org/doi/abs/10.1073/pnas.2207739120">a study published this week</a> in Proceedings of the National Academy of Sciences, scientists have zeroed in on just how the fishers and dolphins synchronize their hunts, and how both sides benefit in the short and long term. The team, led by marine biologist Mauricio Cantor of Oregon State University, concluded that the situation is one of true mutualism: The fishers catch four times more fish when they have dolphin help, and the dolphins that cooperate with them enjoy higher survival rates. But without intervention, they predict, both the interspecies partnership and the dolphins themselves are at risk. </p><p>To understand the underlying mechanisms at work in Laguna, Cantor and his colleagues spent more than 10 years measuring the synchronized fishing from all angles. They tracked mullet schools with sonar cameras; documented dolphin behavior with photography, drones, and audio recordings; and captured the movements and success of 177 fishers with GPS wristbands, interviews, and regular weighing of the catch. The team then categorized the dolphins as “cooperative,” “occasional cooperative,” and “non-cooperative,” and analyzed the data for each.</p>
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<img alt="View from behind of a man waist-deep in water, with a white shirt and hat, casting a fishing net that splays out in a circle in the air" class="richtext-image full-width" src="https://www.pbs.org/wgbh/nova/media/images/dolphin_net_fishing.width-990.jpg"/>
<figcaption><p>A fisher casts a net in Laguna, Brazil, one of the last places where human-dolphin cooperative fishing still survives. Image Credit: Fabio G. Daura-Jorge, Universidade Federal de Santa Catarina</p>
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<p>Of the 65 dolphins observed, more than 40 collaborated to some degree. While helping the fishers, the cooperative dolphins would drive the mullet toward shore, then dive to signal to their human counterparts. And the data showed that the dolphins would stay underwater longer and modify their echolocation clicks and buzzes in tandem with the sinking nets–but only when the fishers had responded to their diving cues correctly. </p><p>“We knew that the fishers were observing the dolphins' behavior to determine when to cast their nets, but we didn't know if the dolphins were actively coordinating their behavior with the fishers," Cantor said in a statement.</p><p>The scientists hypothesized that the nets swishing through the water disrupted the fish schools, making individual mullet easier to catch. And underwater imaging captured what many fishers could feel: the dolphins snagging a few fish directly from the nets, a quick payoff for their collaboration. “The dolphins know what they are doing,” coauthor Fábio Daura-Jorge of the Universidade Federal de Santa Catarina told The New York Times. “They’re taking advantage of the fishers’ actions to actively forage.”</p><p>The dolphins seem to be the true experts here, according to Simon Ingram, a marine biologist at the University of Plymouth who has studied the human-dolphin dynamic in Laguna but was not involved in this study. Over the decades, the dolphins have shown the fishers “where to stand and when to get ready to throw their nets” in murky water where they might not be able to see otherwise, <a href="https://www.science.org/content/article/dolphins-and-humans-team-catch-fish-brazil">he told Science</a>. “It’s almost as if the dolphins are training the humans.”</p>
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<p>But really, everyone wins. The various tracking techniques showed that 86% of the mullet caught by human fishers came from successfully synchronized interactions with dolphins. In fact, fishers were 17 times more likely to catch fish, and caught nearly four times as much, when dolphins were present—regardless of how many fish were nearby. On the other side, besides their occasional snacks, the dolphins that cooperated gained a more dramatic advantage: a 13% higher survival rate over their non-cooperative peers. This difference may be related to illegal forms of fishing elsewhere in the lagoon that frequently cause dolphin entanglement. The researchers found that non-cooperative dolphins were much more likely to die as bycatch in a fishing net than their cooperative brethren.</p><p>“These dolphins and humans have developed a joint foraging culture that allows them both to do better,” Dalhousie University marine ecologist Boris Worm, who was not involved in the research, <a href="https://apnews.com/article/science-dolphins-and-porpoises-animals-queensland-brazil-47621bd822b462a91ba81bcf0c6ed4ee">told The Associated Press.</a></p><p>University of Bristol biologist Stephanie King, who studies dolphin communication and was also not involved, agreed. “This study clearly shows that both dolphins and humans are paying attention to each other’s behavior, and that dolphins provide a cue to when the nets should be cast,” King told The AP. “This is really incredible cooperative behavior."</p>
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<img alt="A man in dark waders stands boot-deep in water, pulling in fish nets, as two dolphin fins poke above the water in the distance" class="richtext-image full-width" src="https://www.pbs.org/wgbh/nova/media/images/2022-07739-3.width-990.jpg"/>
<figcaption><p>A fisher prepares his casting net to fish in cooperation with Lahille's bottlenose dolphins in Laguna, Brazil. Image Credit: Fábio Daura-Jorge, Universidade Federal de Santa Catarina</p>
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<p>Cooperation between dolphins and humans fishing with nets has occurred in communities from Mauritania to Myanmar, but over the centuries many of these relationships have died out. As overfishing and rising ocean temperatures cause a decline in the local mullet population, the researchers fear the Laguna partnership is also at risk. As mullet numbers drop, so do the benefits of cooperation, and the study authors calculate that this unique alliance may fade out in the next 40 to 60 years. “If things keep going the way they are right now, there will be a time when the interaction will no longer be of interest by at least one of the predators—the dolphins or the fishers," Daura-Jorge said in the statement. He and his colleagues predict that that trend will mean fewer cooperative dolphins, ultimately leading to more dolphin deaths. </p><p>As part of their study, the researchers used computer modeling to explore possible solutions. They concluded that the most effective response would be a combination of increased policing of the illegal techniques that lead to entanglement and economic incentives, such as premium prices, to encourage fishers to keep interacting with the dolphins, even if their fishing hauls decrease.</p><p>Stephanie King <a href="https://www.scientificamerican.com/article/a-famed-dolphin-human-fishing-team-up-is-in-danger-of-disappearing/">told Scientific American</a> she considered the study to be “of huge importance,” since it shows not just “how human-wildlife cooperation is coordinated and achieved but also how it can be conserved in the face of large-scale environmental and social challenges.”</p><p>And Cantor considers the study itself a form of intervention. “Getting people excited about these rare phenomena is a way to add more value to these kinds of cultural practices that seem to be fading away,” he told Scientific American. “Preserving cultural diversity has been shown to promote the preservation of biological diversity as well.”</p>
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<p data-block-key="mjqx1">A new study shows just what dolphins get out of cooperating with fishers in Brazil (besides lunch).</p>

Several stacks of white cylinders connected by blue tubes

The Claude "Bud" Lewis Carlsbad Desalination Plant near San Diego uses reverse osmosis filtration to provide 50 million gallons of desalinated seawater per day. ​Image Credit: Reed Kaestner, Getty Images

Why it's so hard to make salt water drinkable

<p>Water covers nearly three-quarters of Earth’s surface, yet only about 2.5% of it is fresh water. And the rising demand, coupled with climate change and drought, has put an increasing strain on the freshwater supply in some areas. So why not just remove the salt from–or desalinate–ocean water for human use? Well, it’s easier said than done. </p>
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<p>Seawater is on average 140 times saltier than drinking water. To efficiently remove those salts, desalination plants use specialized techniques that largely fall into two categories: thermal desalination and reverse osmosis. </p><p>Thermal desalination involves heating seawater to its boiling point and condensing the salt-free steam for drinking water. Though there’s a range of efficiency in both methods, thermal desalination requires roughly 10 times as much energy as reverse osmosis, says Emily Tow, a mechanical engineer at Olin College of Engineering who studies water desalination and reuse. </p><p>Most desalination plants today use reverse osmosis. In this process, seawater is pushed through a membrane to remove the salts. “The membranes are so fine that water can get through, but salts really can't,” Tow says, and the clean water that comes through is taken for drinking water. Although this method requires less energy than thermal desalination, it still uses quite a bit of electricity, about “four times what we would typically spend on our municipal water use, including wastewater treatment,” Tow says. </p><p>But for every gallon of seawater that plants process through reverse osmosis, only half is recovered as fresh water. That’s because the process has a built-in trade-off: As water is pushed through the membrane, the seawater on the starting side gets more concentrated, which makes it even harder to pass through the membrane. This, in turn, increases the energy needed to push the water through.</p>
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<p>The super-concentrated seawater that's left over, called brine, is a common byproduct of reverse osmosis plants. Once the filtration is done, plants typically pump the brine back into the ocean, far away from shore, while mixing it with seawater so it doesn’t create extra salty zones. Some researchers <a href="https://www.sciencedirect.com/science/article/pii/S0011916417307750">have pointed out</a> that having multiple plants in one area operating for an extended period could lead to an increase in ocean salinity and negatively affect the surrounding ecosystem. But with proper design and monitoring the effect should be minimal.</p><p>Although seawater is plentiful, removing the salt is an energy-intensive process that may only make sense in specific scenarios. “Our need for water as humans is never-ending, but our supply of fossil fuels is limited,” Tow says.</p><p>One way to bring down desalination energy costs is by coupling plants with <a href="https://www.pbs.org/wgbh/nova/article/solar-desalination/">renewable energy sources</a>. “Wouldn't it be great if anytime you build a new seawater plant you have to build enough renewable energy capacity to power that plant,” Tow says, “so that we're not just making things worse for the environment while we solve our short-term water problem.”</p>
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<p data-block-key="yvimw">Seawater might seem like an obvious solution to water scarcity, but it comes at a cost.</p>

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