Samira Asgari had been preparing for the trip for months. She had just earned her Ph.D. from a Swiss university and was ready to start a postdoctoral fellowship at Brigham and Women’s Hospital in Boston, studying how a person’s genes affect our response to tuberculosis. But on Saturday morning, at Frankfurt Airport, she was intercepted by an American consulate, who stopped her from boarding her plane to Boston. “He said that it’s the U.S. government who issues the visa, and if they change their mind, the visa isn’t valid,” she says.

They had indeed changed their mind. On Friday, President Trump signed an executive order banning citizens of seven predominantly Muslim countries—Iraq, Iran, Syria, Somalia, Sudan, Libya, and Yemen—from entering America under any visa, for at least 120 days. Asgari, who is Iranian, was sent back to Switzerland. Having given up her apartment in anticipation of the move, she has nowhere to stay. To make matters worse, her luggage is missing.

“The shock wore off yesterday evening. Now there’s just extreme sadness, and a very strong feeling that I’ve been discriminated against,” she says. “Even in Iran, you have this picture of America as a dreamland. But for people like me, this isn’t the America we imagined.”

For years, Iran has led the U.S. State Department's list of state sponsors of terrorism, and so it is not unusual that Iranian scientists would face extra scrutiny from security officials, especially given concerns about nuclear proliferation in Iran. However, like the other countries affected by the ban, no immigrants from Iran have carried out terrorist attacks on U.S. soil between 1975 and the end of 2015. And given the blanket nature of the ban, it affects many scientists who have nothing to do with nuclear research.

Asgari is only one of hundreds of scientists who have been affected by Trump’s ban, which also applies to green-card holders who have permanent residence in the U.S., but have gone overseas for professional or personal reasons. That includes Ali Abdi, an Iranian Ph.D. student studying anthropology at Yale University. A few hours after taking part in the Women’s March in Washington, D.C., he left the U.S. to do ethnographic research in Afghanistan. He’s now stuck in Dubai, awaiting a visa from the Afghan consulate. If that falls through, he doubts he can return to the U.S. despite having a green card, and he rules out a return to Iran because of his record of civil-rights activism. “Let see how things unfold in the U.S.,” he says. “I am sure people around the globe will resist.”

Some already have. On Friday afternoon, thousands of academics, including a dozen Nobel laureates, signed a petition protesting Trump’s order. On Saturday, thousands of protesters filled airports in half-a-dozen major U.S. cities. On a Twitter account called @FreeSciNet, Jen Golbeck from the University of Maryland started building a network to support scientists who were blocked from re-entry, helping them with parked cars, untended pets, and more. The ACLU also sprang into action. In response to their petition, federal judge Ann Donnelly issued a nationwide stay, decreeing that anyone who had already arrived at U.S. airports with valid visas would be allowed to remain; three other judges followed suit. But these measures are temporary. They don’t undo the full executive order, which the Department of Homeland Security has said it will “continue to enforce.” And they don’t apply to future arrivals.

“Many talented friends of mine can’t come back to finish their degrees, simply because they went back to their hometowns to visit their parents,” says Saeed Mehraban, an Iranian Ph.D. student who is working on quantum computing at MIT, and is currently in Austin visiting his advisor. “I’m just taking a domestic flight from Texas to Boston, and I’m still scared they may do me harm.”

“Professional and personal lives are being destroyed,” says Josh Plotkin from the University of Pennsylvania. One of his postdoctoral fellows—an Iranian, and a legal permanent resident of the U.S.—was traveling abroad when Trump’s order was signed. “They are now separated from their spouse, and likely unable to attend faculty job interviews that are scheduled in the coming weeks. This postdoc was working on new ways to treat HIV/AIDS.”

Others who are in the country are effectively trapped. They can’t leave, lest they be denied re-entry. Indeed, many are being told to stay put by their institutions. On Friday afternoon, MIT sent an email to its international scholars advising them to “consider postponing any travel outside of the U.S.” until the executive orders had been clarified. Harvard University sent a similar email late Saturday, adding that since “the executive order also contemplates that additional countries could be added to the banned list … all foreign nationals should carefully assess whether it is worth the risk to travel outside the country.”

International travel is a major and inescapable part of modern science. Many scientists have foreign collaborators, which “substantially increases the pace of discovery and ideas,” says Plotkin. Researchers are expected to attend conferences abroad to share their work. Some have no choice but to fly to visit remote field sites, or unique paleontological digs, or sites of disease outbreaks, or one-of-a-kind facilities like telescopes and observatories.

For many Iranian students in the U.S., restrictions were already harsh. “When we get a visa, it’s usually a single-entry visa,” says Farshad Nasiri, who is studying for a Ph.D. in mechanical engineering at George Washington University. “If I were to leave the States and visit my family, I’d have to reapply for a visa and go through the whole process. I didn’t want to risk it so, for four years, I haven’t travelled. Even before Trump, it was already pretty rough; this will make it even more difficult.”

The new policies could also isolate American institutions from major sources of foreign talent. “The upshot is that, until further notice, science departments at American universities can no longer recruit Ph.D. students from Iran—a country that … has long been the source of some of our best talent,” wrote Scott Aronson (Mehraban’s supervisor), on his personal blog. “This will directly affect this year’s recruiting season, which is just now getting underway.”

Every scientist whom I contacted for this story had tales of colleagues who left and are being denied re-entry, friends who were applying for jobs in the U.S. and now reconsidering, departments that have lost prospective hires, international collaborators who were planning to travel to the U.S. for research but have been denied entry, and foreign academics who are planning to boycott American conferences. “It’s going to destabilize a lot of labs, faculty recruitments, contributions from conferences,” says Houra Merrikh from the University of Washington. “This will have a big impact at all levels in science.”

Merrikh is an immigrant herself. When she was three, her family fled the Iran-Iraq war and settled in Turkey. At 16, she moved to Texas with a green card, no family, and no money; she worked through several poor-paying jobs so she could pay for a college spot. Now a naturalized citizen of 14 years and a professor of microbiology, she studies the evolution of infectious microbes.

Of the seven countries singled out by Trump’s order, Iran has made especially rich contributions to American science, sending a steady stream of intellectual power westwards. “When people think of Iran, I think they think of Saudi Arabia or something, and it’s just a completely different place,” says Merrikh. “The culture emphasizes the importance of the sciences, and women are very much encouraged to be educated.”

Maryam Mirzakhani, for example, was born in Iran in 1977. Twenty-two years later, with two consecutive victories in a major mathematical competition under her belt, she moved to Harvard University to start work on a Ph.D. In 2014, at the age of 37, she was awarded the Fields Medal—the highest honor in mathematics, akin to its Nobel Prize. She was the first woman to win the medal since its inception in 1936.

Pardis Sabeti left Iran with her family just before the Iranian Revolution, when she was just 2 years old. They settled in Florida. Now a professor at Harvard, Sabeti helped to control the Ebola outbreak of 2014 by pioneering the use of genetic sequencing to track and monitor the virus. She continues to study the evolution of that virus and others, with a view to prevent future epidemics.

Firouz Naderi moved to the U.S. in 1964, at the age of 17, to study engineering at Iowa State University. Recently retired, Naderi served the U.S. government for 36 years at NASA’s Jet Propulsion Laboratory. There, he led the successful deployments of the Spirit and Opportunity rovers to Mars, a search for other Earth-like planets, and many other missions. He has received NASA’s highest honor—the Distinguished Service Medal—and he has an asteroid named after him. “We are Iranian-Americans and we have given a lot to this country—our country,” he recently wrote on Facebook.

“A very significant segment of the contributions to science comes from people born outside the country,” including 30 percent of American-based Nobel laureates, says Jan Vilcek from New York University School of Medicine. An immigrant himself, Vilcek set up a foundation that awards annual prizes to immigrants who have made extraordinary contributions to American society, in biomedical science and other fields. Two such prizes recently went to Pardis Sabeti and Houra Merrikh. “They represent the future of science in this country,” says Vilcek. “They show that by preventing people from Iran from coming to this country, we’re hurting our chances to excel in science. And contributions in science translate to economic gains.”

In 2015, after Iran reached a deal about its nuclear program with the U.S. (and five other world powers), many scientists from both nations took the chance to build even stronger collaborations. “I felt really optimistic,” says Maryam Ghadiri, a Ph.D. student at Purdue with an interest in science diplomacy. “A lot of effort, time, and expertise was spent bringing the two countries together. What the new administration did has undermined all of that.”

Trump’s executive order may stop the next Merrikhs, Mirzakhanis, Sabetis, and Naderis from realizing their ambitions—in America, at least. Take Azi Fattahi, an Iranian astrophysicist who studies the evolution of galaxies, and is working on a Ph.D. at the University of Victoria. A few days ago, she was set to give two talks at American universities, as she was mulling a job offer from the University of Michigan, and she had learned that MIT had shortlisted her for a postdoc position. All of those options have since evaporated. “There is excellent research being done in the U.S. but I won’t have the opportunity to even think about there now,” she says. “I have to go to Europe.”

“I came to America to do science, and I still have no other intention,” says Mehraban. “I have had many great friends and advisors, with whom I have been talking about life, religion, freedom and the foundations of the universe. I love the people of America. I don’t even hate Mr. Trump. I think he would feel differently if we have a cup of coffee sometime.”