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      50 years ago, Mauna Kea opened for astronomy. Controversy continues

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      Mauna Kea opened, Science News, August 1, 1970 —
      The new Mauna Kea Observatory of the University of Hawaii has been completed and dedication ceremonies have been held. Standing at an altitude of 13,780 feet on the island of Hawaii, the new observatory is the highest in the world. Its major instrument is an 88-inch reflecting telescope that cost $3 million to build.
      Update
      More than a dozen large telescopes now dot Mauna Kea, operated by a variety of organizations. Those telescopes have revolutionized astronomy, helping to reveal the accelerating expansion of the universe and evidence for the black hole at the center of the Milky Way. But the telescopes have long sparked controversy, as the dormant volcano is sacred to Native Hawaiians. Since 2014, protests have flared in response to the attempted construction of the Thirty Meter Telescope. Opponents have kept progress stalled by blocking the only access road to the site. Some scientists have spoken out against the telescope’s location. The Thirty Meter Telescope collaboration is considering the Canary Islands as a backup site. More

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      ‘The End of Everything’ explores the ways the universe could perish

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      The End of EverythingKatie MackScribner, $26
      Eventually, the universe will end. And it won’t be pretty.
      The universe is expanding at an accelerating clip, and that evolution, physicists expect, will lead the cosmos to a conclusion. Scientists don’t know quite what that end will look like, but they have plenty of ideas. In The End of Everything, theoretical astrophysicist Katie Mack provides a tour of the admittedly bleak possibilities. But far from being depressing, Mack’s account mixes a sense of reverence for the wonders of physics with an irreverent sense of humor and a disarming dose of candor.
      Some potential finales are violent: If the universe’s expansion were to reverse, the cosmos collapsing inward in a Big Crunch, extremely energetic swells of radiation would ignite the surfaces of stars, exploding them. Another version of the end is quieter but no less terrifying: The universe’s expansion could continue forever. That end, Mack writes, “like immortality, only sounds good until you really think about it.” Endless expansion would beget a state known as “heat death” — a barren universe that has reached a uniform temperature throughout (SN: 10/2/09). Stars will have burned out, and black holes will have evaporated until no organized structures exist. Nothing meaningful will happen anymore because energy can no longer flow from one place to another. In such a universe, time ceases to have meaning.
      Perhaps more merciful than the purgatory of heat death is the possibility of a Big Rip, in which the universe’s expansion accelerates faster and faster, until stars and planets are torn apart, molecules are shredded and the very fabric of space is ripped apart.

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      These potential endings are all many billions of years into the future — or perhaps much further off. But there’s also the possibility that the universe could end abruptly at any moment. That demise would not be a result of expansion or contraction, but due to a phenomenon called vacuum decay. If the universe turns out to be fundamentally unstable, a tiny bubble of the cosmos could convert to a more stable state. Then, the edge of that bubble would expand across the cosmos at the speed of light, obliterating anything in its path with no warning. In a passage a bit reminiscent of a Kurt Vonnegut story, Mack writes, “Maybe it’s for the best that you don’t see it coming.”
      Already known for her engaging Twitter personality, public lectures and popular science writing, Mack has well-honed scientific communication chops. Her evocative writing about some of the most violent processes in the universe, mixed with her obvious glee at the unfathomable grandness of it all, should both satisfy longtime physics fans and inspire younger generations of physicists.
      Reading Mack’s prose feels like learning physics from a brilliant, quirky friend. The book is sprinkled with plenty of informal quips: “I’m not going to sugarcoat this. The universe is frickin’ weird.” Readers will find themselves good-naturedly rolling their eyes at some of the goofy footnotes and nerdy pop-culture references. At the same time, the book delves deep into gritty physics details, thoroughly explaining important concepts like the cosmic microwave background — the oldest light in the universe — and tackling esoteric topics in theoretical physics. Throughout, Mack does an excellent job of recognizing where points of confusion might trip up a reader and offers clarity instead.
      Mack continues a long-standing tradition of playfulness among physicists. That’s how we got stuck with somewhat cheesy names for certain fundamental particles, such as “charm” and “strange” quarks, for example. But she also brings an emotional openness that is uncommon among scientists. Sometimes this is conveyed by declarations in all caps about how amazing the universe is. But other times, it comes when Mack makes herself vulnerable by leveling with the reader about how unnerving this topic is: “I’m trying not to get hung up on it … the end of this great experiment of existence. It’s the journey, I repeat to myself. It’s the journey.”
      Yes, this is a dark subject. Yes, the universe will end, and everything that has ever happened, from the tiniest of human kindnesses to the grandest of cosmic explosions, will one day be erased from the record. Mack struggles with what the inevitable demise of everything means for humankind. By contemplating the end times, we can refine our understanding of the universe, but we can’t change its fate.
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      The physics of solar flares could help scientists predict imminent outbursts

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      Space weather forecasting is a guessing game. Predictions of outbursts from the sun are typically based on the amount of activity observed on the sun’s roiling surface, without accounting for the specific processes behind the blasts.
      But a new technique could help predict the violent eruptions of radiation known as solar flares based on the physics behind them, researchers report in the July 31 Science. When applied to old data, the method anticipated several powerful flares, although it missed some as well.
      Radiation released in solar flares and associated eruptions of charged particles, or plasma,can be harmful. This space weather can disrupt radio communications, throw off satellites, take down power grids and endanger astronauts (SN: 9/11/17). More accurate forecasts could allow operators to switch off sensitive systems or otherwise make preparations to mitigate negative effects.
      Current prediction methods rely on tracking flare-linked phenomena such as large, complex sunspots — dark regions on the sun’s surface with powerful magnetic fields. But that leads to some false alarms.

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      In contrast, the new prediction method is rooted in the intricacies of how and when the sun’s tangled loops of magnetic fields rearrange themselves, in a process known as magnetic reconnection, releasing bursts of energy that mark solar flares.
      On the sun’s surface, magnetic fields can get gnarly. Magnetic field lines, imaginary contours that indicate the direction of the magnetic field at various locations, loop and cross over one another like well-mixed spaghetti. When those lines break and reconnect, a burst of energy is released, producing a flare. The details of how and under what conditions this happens have yet to be unraveled.
      In the new study, physicist Kanya Kusano from Nagoya University in Japan and colleagues propose that the largest flares result when two arcing magnetic field lines connect, forming an m-shaped loop, as a smaller loop forms close to the sun’s surface. This “double-arc instability” leads to more magnetic reconnection, and the m-shaped loop expands, unleashing energy.
      Using 11 years’ worth of data from NASA’s Solar Dynamics Observatory spacecraft, the researchers identified regions on the sun with high magnetic activity. For each region, the team determined whether conditions were ripe for a flare-inducing double-arc instability, and then aimed to predict the most powerful flares the sun produces, called X-class flares. The technique correctly predicted seven of nine flares that passed a threshold that the researchers chose, called X2, the second strength subdivision of the X-class.
      The successful predictions suggest that researchers may have identified the physical process that underlies some of the largest outbursts.
      “Prediction is a very good benchmark for how well we can understand nature,” Kusano says.
      The unsuccessful predictions are likewise illuminating: “Even if it fails, it tells us something,” says solar physicist Astrid Veronig of the University of Graz in Austria, who wrote a commentary on the result, also published in Science. The two flares that the technique missed had no associated ejection of plasma from the sun’s surface. “This kind of instability is maybe not a good way to explain these other flares,” Veronig says. They may instead have resulted from magnetic reconnection high above, instead of close to, the sun’s surface.
      The mechanism on which the researchers based their prediction “is really interesting and very insightful,” says solar physicist KD Leka of NorthWest Research Associates in Boulder, Colo. But, she notes, the method couldn’t predict how soon the flares will occur — whether the burst would come an hour or a day after the right conditions first occurred — and it didn’t identify slightly weaker X1 flares, or the next class down, known as M-class flares, which could still be damaging.
      “The mantra that I live by,” Leka says, “is any rule you think you’ve figured out about the sun, it’s going to figure out how to break it.” More

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      The Perseverance rover caps off a month of Mars launches

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      NASA’s Perseverance rover took off at 7:50 a.m. EDT on July 30 from Cape Canaveral, Fla., and is now on its way to Mars with a suite of instruments designed to search for ancient life. The launch is the third this month of spacecraft en route to the Red Planet.
      This is the 22nd spacecraft NASA has aimed at Mars (16 of those missions were successful). But Perseverance will be the first mission to cache rock samples from the Red Planet for a future mission to bring back to Earth.
      It will also be the first NASA mission in more than 40 years to directly search for life on Mars. The rover will land in a region called Jezero crater (SN: 7/28/20). That crater was once an ancient lake bed, and scientists think its rocks and sediments could preserve signs of life, if life was ever there (SN: 7/29/20). The spacecraft will take video and audio recordings of its own landing as it touches down — another first for a NASA Mars mission.
      “This mission has more cameras on it than any we’ve ever sent before,” said Lori Glaze, director of NASA’s Planetary Science Division, on July 30 during a news conference. “It’s going to feel like we’re actually there, riding along with Perseverance on the way down.”
      Perseverance, shown here in an artist’s illustration, will seek signs that Mars once hosted alien life.JPL-Caltech/NASA
      Mars launches tend to come in clumps thanks to Mars’ and Earth’s orbits. The planets line up on the same side of the sun every two years, so scientists have narrow windows to launch for the most efficient trip. All three of this year’s missions will arrive in February 2021.
      The other missions launched in July represent firsts for their respective countries. The United Arab Emirates’ first interplanetary mission, which carries an orbiter called the Hope Probe, launched from Japan on July 19. Hope will measure Mars’ weather, from daily temperature changes to the significance of dust in the planet’s atmosphere (SN: 7/14/20).

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      Next up was China’s first Mars mission, Tianwen-1, which means “questions to heaven” and launched on July 23. China has previously sent spacecraft to orbit and land on the moon (SN: 1/3/19). And it is the first nation to send an orbiter, lander and rover all at once on its first attempt to reach Mars. “No planetary missions have ever been implemented in this way,” mission scientists wrote July 13 in Nature Astronomy. “If successful, it would signify a major technical breakthrough.”
      Tianwen-1’s lander and rover will touch down in Utopia Planitia in April 2021. Instruments on the rover and lander will test Mars’ soil composition and magnetic and gravitational fields and will probe Mars’ interior.
      Utopia Planitia is the same region where the first long-lived Mars lander, NASA’s Viking 1, touched down in 1976 (SN: 7/20/16). Viking was the first spacecraft to search for life on Mars, but its results were inconclusive. Perhaps with the rush of spacecraft this year, and the plans to bring red rocks home, scientists will finally learn whether Mars ever did — or does — host alien life. More

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      An Antarctic ice dome may offer the world’s clearest views of the night sky

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      An observatory in the heart of Antarctica could have the world’s clearest views of the night sky.
      If an optical telescope were built on a tower a few stories tall in the middle of the Antarctic Plateau, it could discern celestial features about half the size of those typically visible to other observatories, researchers report online July 29 in Nature. The observatory would achieve such sharp vision by peering above the atmosphere’s lowermost layer, known as the boundary layer, responsible for much of the undulating air that muddles telescope images (SN: 10/4/18).
      The thickness of Earth’s boundary layer varies across the globe. Near the equator, it can be hundreds of meters thick, limiting the vision of premier optical telescopes in places like the Canary Islands and Hawaii (SN: 10/14/19). Those telescopes usually cannot pick out celestial features smaller than 0.6 to 0.8 arc seconds — the apparent width of a human hair from about 20 meters away.
      “But in Antarctica, the boundary layer is really thin,” says Bin Ma, an astronomer at the Chinese Academy of Sciences in Beijing, “so it is possible to put a telescope above.”
      Ma and colleagues took the first-ever measurements of nighttime atmospheric blur from the highest point in East Antarctica, called Dome A. From April to August 2019, instruments on an 8-meter-tall tower at China’s Kunlun research station tracked how Earth’s atmospheric turbulence distorted incoming starlight. A nearby weather station also monitored atmospheric conditions, such as temperature and wind speed. Using these observations, researchers characterized the boundary layer at Dome A and its effect on telescope observations.
      From April to August 2019, instruments atop an 8-meter-tall tower at China’s Kunlun research station in East Antarctica observed how the local atmosphere distorted light from celestial objects.Zhaohui Shang
      The boundary layer was, on average, about 14 meters thick; as a result, the light sensors at the top of the 8-meter tower were completely free of boundary layer blur only about one-third of the time. But when these instruments were above the layer, atmospheric interference was so low that a telescope could pick out details on the sky 0.31 arc seconds across, on average. The best recorded atmospheric conditions would let a telescope see features as small as 0.13 arc seconds.
      “One-tenth of an arc second is extremely good,” says Marc Sarazin, an applied physicist at the European Southern Observatory in Munich who was not involved in the work. This is “really something you rarely achieve in Chile or on Mauna Kea” in Hawaii.
      Researchers have found similarly excellent visibility above the boundary layer at another spot on the Antarctic Plateau, known as Dome C. But the boundary layer there is around 30 meters thick — making it more difficult to build an observatory above it. An optical telescope planned for construction on a 15-meter tower at Kunlun could take advantage of Dome A’s stellar views above the boundary layer, Ma says. Such crisp telescope images could help astronomers study a range of celestial objects, from solar system bodies to distant galaxies.

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      To rehearse Perseverance’s mission, scientists pretended to be a Mars rover

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      Megan Barrington watched the sun rise over the rocky outcrop. When light struck at exactly the right angle, she mounted a gizmo that looked like eye exam equipment on a tripod and aimed it at the spot. The goal: gather evidence that this windswept wilderness once teemed with life, and then beam the information to her colleagues back home.
      Soon, a version of that setup (minus Barrington) will be deployed on Mars. The state-of-the-art, zoomable, multispectral camera is part of the toolkit on NASA’s Perseverance rover (SN: 7/28/20). “That instrument is going to allow me to look at the mineralogy of Mars at Jezero crater,” the rover’s landing spot, says Barrington, a planetary scientist at Cornell University.
      The rover is scheduled to launch to Mars on July 30. A February role-playing exercise in the Nevada desert by Barrington and six colleagues was a kind of dress rehearsal for the rover’s various instruments. Another 150 team members around the world played the “Earth” team during those two weeks, sending commands from remote mission control and receiving data as it would appear coming from the real rover.
      “We’re not just simulating a Mars mission,” says engineer Raymond Francis of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., who organized and led the trip. “We’re simulating a specific Mars mission by presenting data … to the people who designed the instrument that will take that data. So the standard is high not to look like clowns.”

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      Perseverance has the most demanding and ambitious to-do list of any rover yet: seek signs of past Martian life, prepare the way for future human missions and collect at least 20 samples of Martian rock for eventual return to Earth. And that’s just in its first two years. For contrast, Curiosity rover has drilled a few dozen holes over eight years on Mars, and didn’t store any of those samples for later (SN: 7/7/18, p. 8).
      The dress rehearsal in the desert will help ensure that when Perseverance lands on the Red Planet in February 2021, its handlers on Earth can get straight to the science.
      “We don’t want to get there and learn how to explore Mars while on Mars,” Francis says. “We want [team members] to be ready when the rover hits the ground.”
      Water marks the spot
      The first order of business was to find the right spot for the dry run. “We had to pick a site that kind of looked like Mars,” Francis says. “The parking lot would not do.” The team wanted the site to look as Mars-like as possible, no factories, footprints or foliage to break the illusion.
      An ideal site would have geology that echoed Jezero crater, which is thought to be the remnants of an ancient lakebed and river delta (SN: 11/19/18). It also had to be within a few hours’ drive of JPL, and not totally off the grid — the rover team slept in hotels, ate dinner in restaurants and had reliable Wi-Fi to send data to the Earth team every night.
      The final requirement was that it be someplace the Earth team hadn’t seen before. If mission control members recognized the site, they could bias their findings with what they already knew.
      Engineer and team leader Raymond Francis gets up close with the rocks to make a measurement.JPL-Caltech/NASA
      “Most of the popular Mars analogs are already well known to the Mars community,” Francis says. “So we had to be a little sneaky.”
      Previous exercises, in November 2017 and February 2019, were run in the Mojave Desert in California. For 2020, the rover team headed to Walker Lake in western Nevada. The lake’s water has been receding for a thousand years, so there are spots near the ancient shoreline where the present-day lake is invisible.
      Walker Lake’s rocks preserved a cornucopia of biological signals for the ground team to discover: fossilized fish bones and shells of tiny shrimplike crustaceans called ostracods, which are not expected on Mars; and microbial fossils called stromatolites, which could plausibly be found in Jezero crater (SN: 10/17/18).
      Toolkit
      Francis and his team brought handheld versions of almost all the rover’s instruments to gather whatever data the Earth team requested. They had a drill, handheld spectrometers, lasers, a ground-penetrating radar that they transported in a jogging stroller, plus several elaborate camera setups to represent the rover’s navigation, hazard avoidance and zoomable 3-D science cameras.
      Perhaps the most important piece of equipment was the broom used for sweeping away footprints. It became a running joke, Francis says: “We’ve got all this equipment, a multibillion-dollar mission, and it’s all hinging on this 99-cent broom.”
      Almost everything went smoothly. But a few days into the mission, Barrington’s zoomable camera had “a major malfunction,” she says. She framed her shot, and…. nothing happened. The camera wasn’t getting any power, she realized. “I took it apart and rewired many pieces, to no avail,” she says.
      She and her teammates finally realized one of the power adapters had completely blown. She had to drive two hours to the nearest city to get a new one.
      Of course, driving into town to get a new part won’t be an option on Mars. The real camera, called Mastcam-Z, has been through weeks of rigorous testing and calibration, and is probably up to the task. But “we all go into missions knowing that sometimes irreversible mistakes occur,” Barrington admits. “All we can do at that point is use the instruments to the fullest capacity of which they are capable of operating.”
      Planetary scientist Megan Barrington adjusts her instrument, a multispectral, zoomable camera standing in for Perseverance’s Mastcam-Z.JPL-Caltech/NASA
      Signs of life, big and small
      There was one major giveaway that the team was actually on Earth. “This is very much middle-of-nowhere desert, which is good,” Francis says. But the rover site was mere steps from a U.S. Department of Defense munitions facility, one of the largest in the world.
      “It was really something to behold,” Barrington says. “They had hundreds of bunkers lined up in rows as far as you could see…. All of that was one very crooked metal fence away from us.”
      More than once, military police showed up to check the team’s credentials. “I had to approach them and say, hello, people with the guns, I need you to stop walking now,” Francis says. “We’re running a Mars rover simulation and we don’t want you to put your footprints in this sand.”
      Despite Francis and colleagues’ best efforts, the bunkers showed up in a few photos. The ground team gamely ignored them, apart from a few jokes about SpaceX founder Elon Musk building a Martian city.
      By the end of the two-week exercise, the remote science team reviewing the data had noticed the ostracods and fishbones, and started exploring the stromatolites. “They were doing a good job of finding the biomarkers,” Francis says, who now has hope that “if Mars is hiding stromatolites, maybe we’ll see them.”
      Coming home to quarantine
      The field trip ended on February 27, just as awareness of the novel coronavirus SARS-CoV-2 was rising in the United States. By March 15, JPL told employees to work from home. “We only had a few days together before we were all on remote work,” Francis says.
      The pandemic has already contributed to the delay of the launch of the European and Russian ExoMars rover, which was also supposed to launch in July (SN: 3/12/20).  If Perseverance misses the late July to early August launch window, the rover can’t head to Mars until 2022.
      If the pandemic is still an issue by the time the rover lands in February, Francis doesn’t know what the team will do. “But,” he says, “the good news is the mission is designed for remote operations.” More

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      NASA’s Perseverance rover will seek signs of past life on Mars

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      NASA’s next rover is a connoisseur of Martian rocks. The main job of the Perseverance rover, set to launch between July 20 and August 11, is to pick out rocks that might preserve signs of past life and store the samples for a future mission back to Earth.
      “We’re giving a gift to the future,” says planetary scientist Adrian Brown, who works at NASA Headquarters in Washington, D.C.
      Most of the rover’s seven sets of scientific instruments work in service of that goal, including zoomable cameras to pick out the best rocks from afar and lasers and spectrometers to identify a rock’s makeup. After the rover lands in February 2021, it’s capable of collecting and storing 20 samples within the first Martian year (about two Earth years). The NASA team plans to collect at least 30 samples over the whole mission, says planetary scientist Katie Stack Morgan of NASA’s Jet Propulsion Laboratory in Pasadena, Calif.
      Fortunately, Perseverance is headed to a spot that should be full of collection-worthy rocks. The landing site in Jezero crater, just north of the Martian equator, contains an ancient river delta that looks like it once carried water and silt into a long-lived lake.

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      “We can already predict which parts of that delta might give us the highest return for possible biosignatures,” Stack Morgan says. The crater has a “bathtub ring” of carbonates, minerals that settle in shallow, warm waters that are especially good at preserving signs of life. “That makes Jezero special,” she says.
      But Perseverance is more than a rock collector. The rover will probe the ground beneath its wheels, fly a helicopter, track the weather and test tech for turning Martian air into rocket fuel. Every part of the rover has a job to do.

      RIMFAX
      RIMFAX, or Radar Imager for Mars’ Subsurface Experiment, will use radio waves to probe the ground under the rover’s wheels. The instrument will take a measurement every 10 centimeters along the rover’s track and should be able to sense 10 meters deep, depending on what’s down there. The InSight lander, currently on Mars, has a seismometer that listens for Marsquakes, but a ground-penetrating radar to understand the Martian interior is a first.

      MOXIE
      Human explorers will need oxygen on Mars, but not just for breathing, says former astronaut Jeffrey Hoffman. “It’s for the rocket,” says Hoffman, now an engineer at MIT. To take off from the Martian surface and return home, astronauts will need liquid oxygen rocket fuel. Bringing all that fuel from Earth is not an option.
      To demonstrate how to make fuel from scratch, MOXIE, or Mars Oxygen In-Situ Resource Utilization Experiment, will pull carbon dioxide out of the Martian atmosphere and convert it to oxygen. MOXIE will produce about 10 grams of oxygen per hour, which is only about 0.5 percent of what’s needed to make enough fuel for a human mission over the 26 months between launch windows. But the effort will teach engineers on Earth how to scale up the technology.

      Mastcam-Z
      Set atop Perseverance’s neck, Mastcam-Z, the rover’s main set of eyes, can swivel 360 degrees laterally and 180 degrees up and down to view the surrounding landscape. Like its predecessor on the Curiosity rover, the camera will take color, 3-D and panoramic images to help scientists understand the terrain and the mineralogy of the surrounding rocks. Mastcam-Z can also zoom in on distant features — a first for a Mars rover.

      SuperCam
      How can Perseverance look for signs of ancient microbes in rocks too far away to touch? Enter SuperCam, a laser spectrometer mounted on the rover’s head. SuperCam will shoot rocks with a laser from more than seven meters away, vaporizing a tiny bit of the minerals. Researchers will then analyze the vapor to help figure out what the rocks are made of, without having to drive the rover down steep slopes or up rugged crags. The laser will also measure properties of the Martian atmosphere and dust to refine weather models.

      MEDA
      MEDA, or Mars Environmental Dynamics Analyzer, is the rover’s weather station. Six instruments distributed across the neck, body and interior will measure air temperature, air pressure, humidity, radiation and wind speed and direction. The tools will also analyze the physical characteristics of the all-important Mars dust. Scientists hope to use the information from these sensors to better predict Mars weather.

      PIXL, SHERLOC and WATSON
      Geologists never go into the field without a hand lens. Likewise, Perseverance will be prepared with three arm-mounted magnifying instruments. PIXL, the Planetary Instrument for X-ray Lithochemistry, will have a camera that can resolve grains of Martian rock and dirt to scales smaller than a millimeter. It will also detect the chemical makeup of those rocks by zapping them with X-rays and measuring the wavelength of light the rocks emit in response. SHERLOC, or Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals, will take similar measurements using an ultraviolet laser. WATSON, the Wide Angle Topographic Sensor for Operations and Engineering, will take pictures with a resolution of 30 micro­meters to put the chemistry in context. The instruments will seek signs of ancient microbes preserved in Martian rocks and soil, and help scientists decide which rocks to store for a future mission to return to Earth.
      Ingenuity
      This helicopter will be a test case for future reconnaissance missions to help the rover see further on Mars.JPL-Caltech/NASA
      Perseverance will also carry a stowaway folded up origami-style in a protective shield the size of a pizza box: a helicopter called Ingenuity. At a smooth, flat spot, Ingenuity will drop to the ground and unfold, then take about five flights in 30 Martian days. These flights are mainly to show that the copter can get enough lift in the thin Martian atmosphere. If Ingenuity is successful, future helicopters might help run reconnaissance for rovers. “There’s always a question with the rover, what’s over that cliff? What’s over that rise?” says planetary scientist Briony Horgan of Purdue University in West Lafayette, Ind. “If you have a helicopter, you can see those things ahead of time.”

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      The star cluster closest to Earth is in its death throes

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      The closest cluster of stars to Earth is falling apart and will soon die, astronomers say.
      Using the Gaia spacecraft to measure velocities of stars in the Hyades cluster and those escaping from it, researchers have predicted the cluster’s demise. “We find that there’s only something like 30 million years left for the cluster to lose its mass completely,” says Semyeong Oh, an astronomer at the University of Cambridge.
      “Compared to the Hyades’ age, that’s very short,” she says. The star cluster, just 150 light-years away and visible to the naked eye in the constellation Taurus, formed about 680 million years ago from a large cloud of gas and dust in the Milky Way.
      Stellar gatherings such as the Hyades, known as open star clusters, are born with hundreds or thousands of stars that are held close to one another by their mutual gravitational pull. But numerous forces try to tear them apart: Supernova explosions from the most massive stars eject material that had been binding the cluster together; large clouds of gas pass near the cluster and yank stars out of it; the stars themselves interact with one another and jettison the least massive ones; and the gravitational pull of the whole Milky Way galaxy lures stars away too. As a result, open star clusters rarely reach their billionth birthday.

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      The Hyades has survived longer than many of its peers. But astronomers first saw signs of trouble there in 2018, when teams in Germany and Austria independently used the European Space Agency’s Gaia space observatory to find numerous stars that had escaped the cluster. These departing stars form two long tails on opposite sides of the Hyades — the first ever seen near an open star cluster. Each stellar tail stretches hundreds of light-years and dwarfs the cluster itself, which is about 65 light-years across.
      In the new work, posted July 6 at arXiv.org, Oh and Cambridge colleague N. Wyn Evans analyzed how the cluster has lost stars over its life. It was born with about 1,200 solar masses but now has only 300 solar masses left. In fact, the two tails of escapees possess more stars than does the cluster. And the more stars the cluster loses, the less gravity it has to hold on to its remaining members, which leads to the escape of additional stars, exacerbating the cluster’s predicament.

      Siegfried Röser, an astronomer at Heidelberg University in Germany who led one of the two teams that discovered the cluster’s tails, agrees that the Hyades is in its sunset years. But he worries that it’s too early to pin a precise date on the funeral. “That seems to be a little bit risky to say,” Röser says. Running a computer simulation with the stars’ masses, positions and velocities should better show what will happen in the future, he says.
      The main culprit behind the cluster’s coming demise, Oh says, is the Milky Way. Just as the moon causes tides on Earth, lifting the seas on both the side facing the moon and the side facing away, so the galaxy exerts tides on the Hyades: The Milky Way pulls stars out of the side of the cluster that faces the galactic center as well as the cluster’s far side.
      Even millions of years after the cluster disintegrates, its stars will continue to drift through space with similar positions and velocities, like parachutists jumping out of the same airplane. “It’s still probably going to be detectable as a coherent structure in position-velocity space,” Oh says, but the stars will be so spread out from one another that they will no longer constitute a star cluster. More