Joan Presky worries about dementia. Her mother lived with Alzheimer’s disease for 14 years, the last seven in a memory-care residence, and her maternal grandfather developed dementia, too.

“I’m 100 percent convinced that this is in my future,” said Ms. Presky, 70, a retired attorney in Thornton, Colo.

Last year, she spent almost a full day with a neuropsychologist, undergoing an extensive evaluation. The results indicated that her short-term memory was fine — which she found “shocking and comforting” — and that she tested average or above in every cognitive category but one.

She’s not reassured. “I saw what Alzheimer’s was like,” she said of her mother’s long decline. “The memory of what she went through is profound for me.”

The prospect of dementia, which encompasses Alzheimer’s disease and a number of other cognitive disorders, so frightens Americans that a recent study projecting steep increases in cases over the next three decades drew enormous public attention.

The researchers’ findings, published in January in Nature Medicine, even showed up as a joke on the Weekend Update segment of “Saturday Night Live.”

“Dementia is a devastating condition, and it’s very much related to the oldest ages,” said Dr. Josef Coresh, director of the Optimal Aging Institute at NYU Langone Health and the senior author of the study. “The globe is getting older.”

Now the findings are being challenged by other dementia researchers who say that while increases are coming, they will be far smaller than Dr. Coresh and his co-authors predicted.

Using data from about 15,000 Americans over age 55, collected at four research clinics around the country from 1987 through 2020, Dr. Coresh’s team projected a lifetime dementia risk much higher than previous studies had: 42 percent, though most of that risk didn’t emerge until after age 85.

The higher lifetime number probably reflected the study’s reliance on a more diverse sample than earlier researchers had used, Dr. Coresh said, and more dementia cases identified through in-depth questionnaires, regular phone calls, medical records and death certificates.

The researchers applied their risk calculations to the U.S. population and estimated that the number of people who would develop dementia each year would roughly double, to about a million by 2060, from 514,000 in 2020.

Eric Stallard, an actuary and co-director of the Biodemography of Aging Research Unit at Duke University, read the study and thought the team “seemed very competent at their analysis” of individual risk.

But when it came to the projection that cases would double, which assumed that the incidence of dementia would remain stable over the next 40 years, “I don’t believe it,” Mr. Stallard said.

“The notion that the number of people with dementia will double over the next 25, 30 or 35 years due to the aging of baby boomers is widespread, it’s pervasive — and it’s wrong,” he added.

He and two other Duke researchers recently published a commentary in JAMA pointing out that the age-specific prevalence of dementia in this country had steadily declined for 40 years.

“If your risks are lower than your parents’ risks and this trend continues, you won’t see the doubling or tripling of dementia that’s been projected,” said Dr. Murali Doraiswamy, director of the Neurocognitive Disorders Program at Duke and a co-author of the JAMA article.

To be clear, experts agree that the number of people with dementia will climb in coming decades, simply because the disorder rises so steeply with age and the number of older adults in the United States will increase.

But Mr. Stallard estimates that the increase will be more like 10 to 25 percent by 2050. “It will still be a significant challenge for the health system in the U.S.,” he said.

The Duke group relied on its own long-term study of people over age 65, with more than 21,000 respondents in 1984 and about 16,000 in 2004, plus later data from the national Health and Retirement Study and the National Health and Aging Trends Study.

Their analysis found that among 85- to 89-year-olds, for instance, the proportion with dementia was about 23 percent in the cohort born in 1905. In those born 10 years later, the figure had dropped to about 18 percent.

By the time Americans born in 1935 reached their late 80s, about 11 percent had dementia; the projection for those born from 1945 to 1949 is now about 8 percent.

To Dr. Coresh, whose primary interest was in individual risk, the assumption that past declines would continue at about the current rate “would be great, but is quite an optimistic, dramatic decrease,” he said in an email.

Yet in another longitudinal study of older adults in England and China, published in Nature Aging last year, “we also found these quite marked improvements in more recently born cohorts,” said the lead author, Dr. John Beard, a medical epidemiologist at the Mailman School of Public Health at Columbia University.

“You would expect the increase in the absolute numbers of people with dementia in the U.S. will be less than we feared,” Dr. Beard said.

What has led to the decrease in dementia, also seen in several European countries? Often cited explanations include rising education levels, reduced smoking and improved treatment for high blood pressure and high cholesterol.

The Lancet Commission on dementia, intervention and care has developed a list of 14 modifiable risk factors, including greater use of hearing aids and reduced air pollution, that could still lead to greater declines.

Yet the reverse could also happen. If earlier and more widespread testing increases the number of dementia diagnoses, or if the definition of dementia broadens, rates will increase, Dr. Doraiswamy noted. Increasing life expectancy would have the same effect.

Obesity and diabetes, more common in recent decades, could lead to more dementia, but much-touted new drugs that reduce them could blunt that trend — if people can get them.

“None of this is inevitable,” said Dr. Gill Livingston, a psychiatrist at University College London who leads the Lancet Commission. “It depends on what we do.”

Public health policy makes a major difference, she noted, and, “The U.S. is in a time where policy is changing enormously.”

Dementia rates might rise, for example, “if people have less access to health care, so they are less likely to get their blood pressure treated and their high cholesterol treated,” Dr. Livingston said.

Slashed Medicaid coverage could lead to that result. So could a rollback of environmental policies, “if air pollution increases because of fossil fuels,” she added.

Already, dementia afflicts some American populations far more than others, researchers point out. Older women and Black people face greater risk, along with those who carry the APOE4 gene associated with Alzheimer’s disease.

Health disparities could mean that “affluent people will see lower rates of dementia” because of the new diabetes and obesity drugs, Dr. Doraiswamy said. “People who can’t afford them and whose conditions are not well-managed will see rates go up.”

The debate about how many older adults will develop dementia in coming decades, and about how individuals, families, government and the health care system should respond, will likely continue.

So will Ms. Presky’s fears.

For now, she enrolls in lifelong learning classes, takes walks and yoga classes despite orthopedic problems, listens to podcasts and reads a lot of history and fiction. She and her husband take in theater in New York and Phish concerts on the West Coast and will soon be heading to London and Paris.

Still, her advance directive contains many provisions about dementia. “I remain pessimistic,” she said, noting that her mother was diagnosed at 77. “I have seven more years before I meet her fate.”

NASA made a bet a few years ago that commercial companies could take scientific experiments to the moon on a lower budget than the agency could.

Last year, that was a bad bet. The first NASA-financed spacecraft missed the moon entirely. The second landed but fell over.

But this month, a robotic lander named Blue Ghost, built by Firefly Aerospace of Cedar Park, Texas, succeeded from start to finish.

On March 16, the mood at Firefly’s mission operations outside Austin was a mix of happy and melancholic. There was nothing more to worry about, nothing left to do — except watch the company’s spacecraft die.

A quarter-million miles away, the sun had already set on Mare Crisium, the lunar lava plain where Blue Ghost had collected scientific observations for two weeks.

For the solar-powered spacecraft, the hours remaining were numbered and few.

“I think the mood generally is pretty light,” Ray Allensworth, the spacecraft program director at Firefly, said that afternoon. “I think people are just excited and also just kind of relieved to see how well the mission went and just kind of taking a moment to enjoy the last few hours with the lander.”

Scientists with cargo on the other commercial moon missions had invested years of effort and ended up with little or nothing. Those NASA assigned to Blue Ghost are coming away with a cornucopia of new data to work with.

Robert Grimm, a scientist at the Southwest Research Institute in Boulder, Colo., who led one of the scientific payloads, acknowledged his good fortune. “Better than being a crater,” he said.

One of the NASA experiments had collected data just as Blue Ghost landed. Four cameras captured views from different angles of the exhaust of the spacecraft’s thrusters as they kicked up lunar dust and carved a small crater.

“This gives us the ability with these cameras to measure three-dimensional shapes,” said Paul Danehy, one of the scientists working on the project known as Stereo Cameras for Lunar Plume-Surface Studies, or SCALPSS.

Engineers want to understand those dynamics to prevent potential disasters when bigger and heavier spacecraft like SpaceX’s Starship land astronauts on the moon. If NASA sets up a lunar outpost, spacecraft will return to that site more than once. Rocks flying upward could knock out an engine on a descending spacecraft or damage nearby structures.

In early looks at the photographs, one of the surprises is that the exhaust plume from the thrusters started kicking lunar dust when Blue Ghost was still about 50 feet above the surface, higher than expected. The same camera system is to record the dust cloud from a much larger lander, the Blue Moon Mark 1, which Blue Origin, Jeff Bezos’ rocket company, plans to send to the moon later this year.

NASA not only wants to understand lunar dust, or regolith, but also how to get rid of it. The particles can be sharp and abrasive like shards of glass, posing a hazard to machinery and astronauts. An experiment on Blue Ghost called the Electrodynamic Dust Shield used electric fields to clean dust off surfaces.

Two experiments collected information that should cast light on the moon’s interior.

Dr. Grimm’s payload was the Lunar Magnetotelluric Sounder, the first of its kind deployed on the surface of another world.

To deploy, spring-loaded launchers flung four probes about the size of soup cans in four different directions. Connected by cables to the lander, the probes worked like supersized voltmeters. A second component, raised atop an eight-foot-high mast, measured magnetic fields.

Together, these readings reveal naturally occurring variations in electric and magnetic fields that tell how easily electric currents flow deep underground, and that tells something about what is down there. The conductivity of colder rocks, for example, is lower.

Blue Ghost also deployed a pneumatic drill, using bursts of nitrogen gas to excavate dirt. A needle at the end of the instrument measured temperature and how easily heat flows through the material. Because of rocks in the way, the drill went down only about three feet, not the 10 feet that had been hoped.

In videos, “you can see the rocks flying out and sparks,” said Kris Zacny, vice president of exploration systems at Honeybee Robotics, which built the drill.

Still, three feet was deep enough for the scientific measurements, Dr. Zacny said. Data from the drill and the magnetotelluric sounder could both give hints about how the moon and other rocky worlds formed or why the near side of the moon looks so different from the far side.

“It’s really a basic question about lunar geology we’re trying to answer,” Dr. Grimm said.

Honeybee, which is part of Blue Origin, also built a second device called PlanetVac to demonstrate a simplified technology to collect samples. This device used compressed gas to stir up regolith into a small tornado and direct it into a container.

The technology will be used on a robotic Japanese space mission known as Martian Moons Exploration, which will bring back samples from Phobos, a moon of Mars.

“The fact that it worked on the moon gives us confidence that it should work on Phobos as well,” Dr. Zacny said.

Brian Walsh’s experiment on Blue Ghost did not look at the moon but back at Earth.

“It’s a really good vantage point,” said Dr. Walsh, a professor of mechanical engineering at Boston University.

Dr. Walsh is interested in the magnetic bubble that deflects solar wind particles around Earth. His telescope recorded X-rays emitted when high-speed particles from the sun slam into atoms in Earth’s upper atmosphere. The boundary between the Earth’s magnetic field and the solar wind is like two sumo wrestlers pushing against each other. The view from afar should help scientists tell whether that boundary shifts slowly or in sudden leaps.

That is important because it affects how well Earth’s magnetic field protects us from occasional gargantuan belches of charged particles that bombard the planet during solar storms.

“We’re trying to figure out how that gate opens and how energy spills through,” Dr. Walsh said.

Blue Ghost has already left a lasting impression.

Maria Banks said that as she left the mission operations center each night, she would look up at the moon hanging in the sky.

“Which would just basically stop me in my tracks every day,” Dr. Banks said. “I don’t think I’ll ever see the moon the same again, because for the rest of my life, Firefly’s lander and our instruments will be up there.”

A French scientist was prevented from entering the United States this month because of an opinion he expressed about the Trump administration’s policies on academic research, according to the French government.

Philippe Baptiste, France’s minister for higher education and research, described the move as worrying.

“Freedom of opinion, free research and academic freedom are values we will continue to proudly uphold,” Mr. Baptiste said in a statement. “I will defend the possibility for all French researchers to be faithful to them, in compliance with the law, wherever they may be in the world.”

Mr. Baptiste did not identify the scientist who was turned away but said that the academic was working for France’s publicly funded National Center for Scientific Research and had been traveling to a conference near Houston when border officials stopped him.

The U.S. authorities denied entry to the scientist and then deported him because his phone contained message exchanges with colleagues and friends in which he expressed his “personal opinion” on the Trump administration’s science policies, Mr. Baptiste said.

It was not immediately clear what led the border authorities to stop the scientist, why they examined the contents of his phone or what they found objectionable about the conversations.

Customs officers are allowed to search the cellphone, computer, camera or any other electronic device of any travelers crossing the border, according to U.S. Customs and Border Protection, although the agency says that such instances are rare. In 2024, less than 0.01 percent of arriving international travelers had their electronic devices searched, according to the agency.

Mr. Baptiste’s office declined to provide further details about the case. A spokesman for the American Embassy in Paris also declined to comment.

A spokeswoman for the National Center for Scientific Research said that the scientist who was turned back did not wish to speak to the media and declined to comment further.

The Agence France-Presse news agency reported earlier on the scientist’s refused entry to the United States.

Mr. Baptiste, the minister, has been particularly vocal over the past few weeks in denouncing threats to academic freedom in the United States, where funding cuts and layoffs by the Trump administration have targeted institutions of higher education, scientific research and the federal government’s own scientific work force.

Mr. Baptiste has urged French universities and research institutes to welcome researchers seeking to leave the United States.

“Europe must be there to protect research and welcome the talent that can contribute to its success,” Mr. Baptiste wrote on social media after meeting with his European counterparts in Warsaw on Wednesday to address “threats to free research in the United States.”

Jennifer Jones, the director of the Center for Science and Democracy at the Union of Concerned Scientists, an advocacy group in the United States, said she worried that cases like the one involving the French scientist would have a chilling effect on research collaboration across borders.

“My fear is that these are early cases with many more to follow,” Dr. Jones said. “I am hearing from my network that people are very concerned about any kind of international travel in either direction.”

“And that should worry all of us,” she added. If scientists limit their movements to conferences and other events designed to advance research, she said, “it is the public that is going to suffer.”

Ségolène Le Stradic contributed reporting.