NASA is asking scientists outside the agency to help solve a medical problem that astronauts may not even feel. On Sept. 25, the agency's Human Research Program posted a request for project plans and budget quotes for research on current, enhanced, or new approaches to monitor, reduce the risk of, or treat venous thrombosis, the formation of blood clots in veins, during spaceflight. Responses are due by 1 p.m. CT on Nov. 20, 2026.

The request comes as NASA's health officials evaluate crew risks for Artemis missions to the Moon and beyond, where a medical emergency cannot be handed off to a hospital within hours. It also follows years of ultrasound work on the International Space Station showing that weightlessness can slow, stall, or even reverse blood flow in the internal jugular veins of some crew members.

For most readers, the call changes nothing about daily life. It matters for the people who plan, fund and fly long missions, and the research it produces could add to what doctors know about clots in unusual places. The key point is that this is a request for proposals, not a new finding. NASA has not announced a new clot threat, a funded study, or a change in astronaut rules.

A Formal Call with a November Deadline

The NASA notice explains that venous clots are usually associated with the deep veins of the legs after surgery, injury or long periods of sitting on airplanes or lying in bed, while clots in the neck's jugular veins are much less common. In orbit, they can go unnoticed. If an astronaut develops a clot in the internal jugular vein during flight, the agency wrote, "there may be no symptoms, but the blood clot could be detected using ultrasound."

The program wants strategies for developing and validating ways to monitor, reduce the risk of, prevent or treat venous thrombosis during spaceflight and exploration missions, including preventive and treatment countermeasures. Responses may involve new prospective research, retrospective analysis of existing data, or both. NASA pointed applicants to a technical brief on venous thrombosis from its chief health and medical office and to past mission data available through the NASA Life Sciences Portal. The notice does not say how much money is available or how many projects may be selected.

Space Station Scans That Changed the Risk Picture

The concern traces to a 2019 study in JAMA Network Open that used ultrasound to examine the internal jugular vein in 11 space station crew members on long-duration missions. According to the study abstract, six of the 11 showed stagnant or reverse flow in the vein around their 50th day in flight. One crew member was found to have an occlusive clot, and a possible partial clot was identified in a second crew member only after a retrospective review. Lower body negative pressure, a device that draws fluid toward the legs, was associated with improved flow in 10 of 17 in-flight sessions, and the authors described it as a potentially promising countermeasure.

The astronaut with the complete clot was treated in orbit under guidance from doctors on the ground, according to a case report in the New England Journal of Medicine. The crew member started on enoxaparin, an injectable blood thinner available aboard the station, at a dose that was later reduced to stretch the supply until an oral blood thinner, apixaban, arrived on a supply spacecraft. Follow-up exams found a small amount of remaining clot 24 hours after landing and none 10 days after landing.

The underlying issue is gravity, or the lack of it. On Earth, gravity helps drain blood from the head through the jugular veins. In weightlessness, fluids shift toward the head, and those pressure differences largely disappear. A later parabolic flight study indexed on PubMed found that flow stasis can occur in the left jugular vein even during brief exposure to weightlessness, and the authors said this may increase clot risk during spaceflight of any duration.

Heart Opening Findings and Spacewalk Medicine

Three days before the research call, NASA's Office of the Chief Health and Medical Officer published recommendations from a working group it convened in August 2026 to review data on decompression sickness during spacewalks, in-flight clots and patent foramen ovale, or PFO, a small opening between the heart's upper chambers that remains in many adults.

The group's consensus was that a PFO is not considered a major risk factor for clot formation in space, and it recommended no changes to astronaut selection criteria regarding PFOs with respect to clots. It concluded that a small PFO does not pose a significant risk, but opinions were mixed on whether closing or screening out a large one would meaningfully reduce the chance that gas bubbles from a spacewalk could cross into arterial blood. The group did not recommend universal screening or exclusion, but said crew members who are assessed should be told their status and offered closure of a large PFO. It also called for clear guidance on aspirin and common pain relievers around spacewalks, since some of those drugs can mask decompression sickness symptoms.

The panel included Stephan Moll of the University of North Carolina School of Medicine, a coauthor of the NEJM case report on the in-flight clot, according to the report's PubMed record.

Small Numbers, Big Unknowns

The evidence base remains thin. The key flight study covered only 11 people, the treated clot involves a single patient, and much of the data are observational. Researchers have documented an association between weightlessness, altered jugular flow, and clotting, but they have not established how often clots actually form in space or which astronauts are most vulnerable.

Treatment is also harder off the planet. The NEJM case report noted that syringes are a limited commodity on the station and that drawing liquid from vials is a significant challenge in microgravity. Blood thinners also carry bleeding risks for crews who could be injured far from surgical care. Those trade-offs, between prevention and bleeding and between supplies and mass, are the kind of problem the new call is meant to address.

On Earth, blood that pools or stalls is also more likely to clot, which is why doctors encourage people to move around on long trips and after surgery. Swelling, pain or redness in an arm, leg or neck, or sudden shortness of breath or chest pain, can signal a clot and warrants prompt medical care.

Researchers interested in the NASA call can find submission instructions through the agency's posting, with questions directed to the Human Research Program's chief science office. NASA has not said when selections will be announced after the Nov. 20 deadline. The request confirms that jugular clots remain an open problem for deep-space medicine rather than a newly discovered danger, and the next milestone is which approaches the agency chooses to fund.

What Readers Want to Know

What is NASA asking researchers to do?

NASA's Human Research Program is requesting project plans and budget quotes for ways to monitor, reduce the risk of, prevent, or treat venous blood clots during spaceflight. Responses can involve new research, analysis of existing mission data, or both.

When is the deadline?

Responses are due by 1 p.m. CT on Nov. 20, 2026, according to NASA's posting.

Have astronauts actually developed blood clots in space?

Yes. A 2019 study of 11 space station crew members documented one occlusive clot in the internal jugular vein during flight and a possible partial clot in a second crew member identified later.

Why would weightlessness affect neck veins?

On Earth, gravity helps blood drain from the head. In weightlessness, fluids shift toward the head and normal drainage patterns change, which can slow or stall flow in the jugular veins of some crew members.

What did NASA's working group conclude about patent foramen ovale?

The group said a PFO is not considered a major risk factor for clot formation in space and recommended no changes to astronaut selection on that basis, though opinions were mixed about large PFOs and decompression sickness.

Does this research matter for people on Earth?

Possibly. The work focuses on astronauts, but better ways to detect and manage clots in unusual locations or with limited medical resources could inform care elsewhere. Anyone with clot warning signs should seek medical attention.