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      President William Jefferson Clinton and First Lady Hillary Rodham Clinton with Eileen Collins in the Oval Office.Sharon Farmer and White House Photograph Office At that event, on March 5, 1998, the First Lady noted what a change it would be to have a female in the commander’s seat. Referencing Neil A. Armstrong’s first words on the Moon, Clinton proclaimed, “Collins will take one big step forward for women and one giant leap for humanity.” Collins, a military test pilot and shuttle astronaut, was about to break one of the last remaining barriers for women at NASA by being assigned a position previously filled by men only. Clinton went on to reflect on her own experience with the space agency when she explained how in 1962, at the age of 14, she had written to NASA and asked about the qualifications to become an astronaut. NASA responded that women were not being considered to fly space missions. “Well, times have certainly changed,” she said wryly.
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      Astronaut Eileen M. Collins looks over a checklist at the commander’s station on the forward flight deck of the space shuttle Columbia on July 23, 1999, the first day of the mission.  The most important event of this day was the deployment of the Chandra X-Ray Observatory.NASA By 1998, views about women’s roles had changed substantially, as demonstrated by the naming of the first female shuttle commander. The agency even commissioned a song for the occasion: “Beyond the Sky,” by singer-songwriter Judy Collins. NASA dedicated the historic mission’s launch to America’s female aviation pioneers from the Ninety-Nines—an international organization of women pilots—to the Women Airforce Service Pilots (WASPs), women who ferried aircraft for the military during World War II. Collins also extended an invitation to the women who had participated in Randy Lovelace’s Woman in Space Program, where women went through the same medical and psychological tests as the Mercury 7 astronauts; the press commonly refers to these women as the Mercury 13. (Commander Collins had thanked both the WASPs and the Mercury 13 for paving the way and inspiring her career in aviation and spaceflight in her White House speech.)
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      In a group interview with several of the WASPs in Florida, just before launch, Mary Anna “Marty” Martin Wyall explained why they came. “Eileen Collins was one of those women that has always looked at us as being her mentors, and we just think she’s great. That’s why we want to come see her blast off.” Betty Skelton Frankman expressed just how proud she was of Collins, and how NASA’s first female commander would be fulfilling her dream to fly in space. “In a way,” she said, “it’s like my dream come true.” In the ‘60s it was not possible for a woman to fly in space because none met the requirements as laid out by NASA. But by the end of the twentieth century, women had been in the Astronaut Office for 20 years, and opportunities for women had grown as women were selected as pilot astronauts. NASA named its second and only other female space shuttle commander, Pamela A. Melroy, to STS-120, and Peggy A. Whitson went on to command the International Space Station. Melroy and Whitson shook hands in space, when their missions coincided, for another historic first—two women commanding space missions at the same time.
      Twenty-five years ago, Eileen Collins’ command broke down barriers in human spaceflight. As the First Lady predicted, her selection led to other opportunities for women astronauts. More women continue to command spaceflight missions, including Expedition 65 Commander Shannon Walker and Expedition 68 Commander Samantha Cristoforetti. More importantly, Collins became a role model for young people interested in aviation, engineering, math, science, and technology. Her career demonstrated that there were no limits if you worked hard and pursued your passion.
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      The animation, exaggerated for clarity, illustrates how Earth’s rotation wobbles as the location of its spin axis, shown in orange, moves away from its geographic axis, which is shown in blue and represents the imaginary line between the planet’s geographic North and South poles.NASA’s Scientific Visualization Studio Analyzing polar motion across 12 decades, scientists attributed nearly all of the periodic oscillations in the axis’ position to changes in groundwater, ice sheets, glaciers, and sea levels. According to a paper published recently in Nature Geoscience, the mass variations during the 20th century mostly resulted from natural climate cycles.
      The same researchers teamed on a subsequent study that focused on day length. They found that, since 2000, days have been getting longer by about 1.33 milliseconds per 100 years, a faster pace than at any point in the prior century. The cause: the accelerated melting of glaciers and the Antarctic and Greenland ice sheets due to human-caused greenhouse emissions. Their results were published July 15 in Proceedings of the National Academy of Sciences.
      “The common thread between the two papers is that climate-related changes on Earth’s surface, whether human-caused or not, are strong drivers of the changes we’re seeing in the planet’s rotation,” said Surendra Adhikari, a co-author of both papers and a geophysicist at NASA’s Jet Propulsion Laboratory in Southern California.
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      The location of Earth’s spin axis moved about 30 feet (10 meters) between 1900 and 2023, as shown in this animation. A recent study found that about 90% of the periodic oscillations in polar motion could be explained by melting ice sheets and glaciers, diminishing groundwater, and sea level rise.NASA/JPL-Caltech Decades of Polar Motion
      In the earliest days, scientists tracked polar motion by measuring the apparent movement of stars. They later switched to very long baseline interferometry, which analyzes radio signals from quasars, or satellite laser ranging, which points lasers at satellites.
      Researchers have long surmised that polar motion results from a combination of processes in Earth’s interior and at the surface. Less clear was how much each process shifts the axis and what kind of effect each exerts — whether cyclical movements that repeat in periods from weeks to decades, or sustained drift over the course of centuries or millennia.
      For their paper, researchers used machine-learning algorithms to dissect the 120-year record. They found that 90% of recurring fluctuations between 1900 and 2018 could be explained by changes in groundwater, ice sheets, glaciers, and sea level. The remainder mostly resulted from Earth’s interior dynamics, like the wobble from the tilt of the inner core with respect to the bulk of the planet.
      The patterns of polar motion linked to surface mass shifts repeated a few times about every 25 years during the 20th century, suggesting to the researchers that they were largely due to natural climate variations. Past papers have drawn connections between more recent polar motion and human activities, including one authored by Adhikari that attributed a sudden eastward drift of the axis (starting around 2000) to faster melting of the Greenland and Antarctic ice sheets and groundwater depletion in Eurasia.
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      Longer Days
      For the second paper, the authors used satellite observations of mass change from the GRACE mission (short for Gravity Recovery and Climate Experiment) and its follow-on GRACE-FO, as well as previous mass-balance studies that analyzed the contributions of changes in groundwater, ice sheets, and glaciers to sea level rise in the 20th century to reconstruct changes in the length of days due to those factors from 1900 to 2018.
      Scientists have known through historical eclipse records that length of day has been growing for millennia. While almost imperceptible to humans, the lag must be accounted for because many modern technologies, including GPS, rely on precise timekeeping.
      In recent decades, the faster melting of ice sheets has shifted mass from the poles toward the equatorial ocean. This flattening causes Earth to decelerate and the day to lengthen, similar to when an ice skater lowers and spreads their arms to slow a spin.
      The authors noticed an uptick just after 2000 in how fast the day was lengthening, a change closely correlated with independent observations of the flattening. For the period from 2000 to 2018, the rate of length-of-day increase due to movement of ice and groundwater was 1.33 milliseconds per century — faster than at any period in the prior 100 years, when it varied from 0.3 to 1.0 milliseconds per century.
      The lengthening due to ice and groundwater changes could decelerate by 2100 under a climate scenario of severely reduced emissions, the researchers note. (Even if emissions were to stop today, previously released gases — particularly carbon dioxide — would linger for decades longer.)
      If emissions continue to rise, lengthening of day from climate change could reach as high as 2.62 milliseconds per century, overtaking the effect of the Moon’s pull on tides, which has been increasing Earth’s length of day by 2.4 milliseconds per century, on average. Called lunar tidal friction, the effect has been the primary cause of Earth’s day-length increase for billions for years.
      “In barely 100 years, human beings have altered the climate system to such a degree that we’re seeing the impact on the very way the planet spins,” Adhikari said.
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      Andrew Wang / Jane J. Lee
      Jet Propulsion Laboratory, Pasadena, Calif.
      626-379-6874 / 818-354-0307
      andrew.wang@jpl.nasa.gov / jane.j.lee@jpl.nasa.gov
      2024-101
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