Christopher Williams

Medical physicist by training, selected in NASA’s 2021 class — the only physicist that year, and the rare astronaut who came out of a radiation-oncology lab rather than a flight deck. Christopher Williams brings a perspective to the Station that most crew members do not: he spent his pre-NASA career figuring out how to aim radiation precisely enough to kill cancer cells without harming the tissue around them.

Before NASA

Williams grew up in the United States and studied physics, eventually completing advanced training in medical physics — the speciality that bridges physics, medicine, and engineering to develop and operate radiation-therapy equipment. He worked as a medical physicist at Massachusetts General Hospital in Boston, one of the premier research hospitals in the country, where his work focused on cancer-treatment instrumentation and the dosimetry that ensures patients receive exactly the right amount of radiation in exactly the right place.

He came to NASA from that clinical research environment, which makes his path to the astronaut corps distinctive. Most NASA astronaut classes are dominated by military test pilots, aerospace engineers, and physicians. A medical physicist — someone whose expertise lies in the behaviour of ionising radiation and its interaction with biological tissue — is uncommon, and his skillset maps directly onto one of the Station’s most important ongoing research programmes.

Spaceflight career

Expedition 74 is Williams’s first spaceflight. He was selected in NASA’s 2021 astronaut class and completed the two-year Astronaut Candidate training programme, qualifying in spacewalk operations, robotics, Russian language, and Station systems. He launched aboard SpaceX Crew Dragon in December 2025 for a planned six-month stay.

This expedition

Williams serves as flight engineer with lead responsibility for the radiation-shielding study running this expedition. The research measures how different materials and configurations attenuate the cosmic rays and solar-particle events that penetrate the Station’s hull. The data is essential for designing the shielding on spacecraft that will carry crews to the Moon and Mars, where the protection of Earth’s magnetic field is no longer available.

His medical-physics background means he understands both the physics of the radiation environment and its biological consequences — how much exposure matters, what kinds of particles do the most damage, and where the thresholds sit between acceptable and dangerous. That combination of skills makes him unusually well placed to interpret the experimental results in context.

The person

Colleagues describe him as thoughtful, precise, and quietly funny — the kind of crew member who processes information carefully before speaking but has a dry wit when he does. He is an advocate for expanding the range of academic backgrounds represented in the astronaut corps and has spoken about how working in a cancer clinic shapes your sense of what counts as a high-stakes decision. Off duty he exercises, reads, and spends time in the Cupola.