Tracy Caldwell Dyson (born Tracy Ellen Caldwell; August 14, 1969) is an American chemist and NASA astronaut. She was a mission specialist on Space Shuttle Endeavour flight STS-118 in August 2007 and has participated in two long-duration missions to the International Space Station, Expedition 23 and 24 from April to September 2010 and Expedition 70 and 71 from March to September 2024. She has completed four spacewalks, logging nearly 24 hours of extravehicular activity.
Caldwell Dyson was born in Arcadia, California. She is the younger of two girls. In the early 1980s, she and her family moved to Beaumont, California, where her father worked as an electrician and where she attended junior high school and high school. Her recreational interests include running, weight training, hiking, softball, basketball, and auto repair and maintenance.
She attended California State University, Fullerton, where she competed on the CSUF Titans' track and field team as a sprinter and long jumper.
As an undergraduate researcher at California State University, Fullerton, she designed, constructed and implemented electronics and hardware associated with a laser-ionization, time-of-flight mass spectrometer for studying atmospherically relevant gas-phase chemistry. She also worked as a lab assistant in the university's research and instructional safety office, where she performed environmental monitoring of laboratories using hazardous chemicals and radioactive materials and calibrated survey instruments and helped process chemical and radioactive waste. During college and after it, she also worked as an electrician and inside wireman for her father's electrical contracting company, where she performed commercial and light industrial construction.
At the University of California, Davis, Caldwell Dyson taught general chemistry laboratory and began her graduate research. Her dissertation work focused on investigating molecular-level surface reactivity and kinetics of metal surfaces using electron spectroscopy, laser desorption, and Fourier transform mass spectrometry techniques. She also designed and built peripheral components for a variable temperature, ultra-high vacuum scanning tunneling microscopy system.
In 1997, Caldwell Dyson received the Camille and Henry Dreyfus Postdoctoral Fellowship in Environmental Science to study atmospheric chemistry at the University of California, Irvine. There, she investigated reactivity and kinetics of atmospherically relevant systems using atmospheric pressure ionization mass spectrometry, Fourier transform infrared and ultraviolet absorption spectroscopies. In addition, she developed methods of chemical ionization for spectral interpretation of trace compounds. Caldwell Dyson has published and presented her work in numerous papers at technical conferences and in scientific journals.
In June 1998, Caldwell Dyson was selected by NASA, and began NASA training two months later, in August 1998. Her astronaut candidate training included orientation briefings and tours, numerous scientific and technical briefings, intensive instruction in Shuttle and International Space Station (ISS) systems, physiological training, ground school to prepare for T-38 flight training, as well as learning water and wilderness survival techniques. Completion of this training and evaluation qualified her for flight assignment as a mission specialist.
In 1999, Caldwell Dyson was assigned to the Astronaut Office ISS Operations Branch as a Russian Crusader, participating in the testing and integration of Russian hardware and software products developed for ISS. In 2000, she was assigned prime crew support astronaut for the ISS Expedition 5 crew, serving as their representative on technical and operational issues throughout the training and on-orbit phase of their mission. During ISS Expeditions 4 through 6, Caldwell Dyson served as an ISS spacecraft communicator (CAPCOM) inside Mission Control. In 2003, she made a transition to the Astronaut Shuttle Operations Branch and was assigned to flight software verification in the Shuttle Avionics Integration Laboratory and worked supporting launch and landing operations at Kennedy Space Center, Florida. Caldwell Dyson also served as Lead CAPCOM for Expedition 11.
Between her second and third flights, Caldwell Dyson continued to work inside Houston’s Mission Control Center as CAPCOM for both space shuttle and space station operations, serving as the lead CAPCOM for various ISS missions, including the lead and development of the CAPCOM cadre for Boeing Starliner Mission Operations team. She was also the ground IV for US EVA 32, performed by Scott Kelly and Kjell Lindgren.
Caldwell Dyson initiated and led several projects to improve training and operations aboard the ISS, most notably developing the EVA Qualification training flow (EVQ) for astronaut candidates.
After her third trip to space, she served as the ground IV for US EVA 95, performed by Jessica Meir and Christopher Williams in March 2026. She also worked as one of the CAPCOM for the Artemis II mission, working during 3 separate shifts. During the summer 2026 with the European Space Agency, she participated in the CAVES training program with her NASA astronaut colleague, Ben Bailey and astronauts from other agencies John McFall, Ayu Yoneda and Rosemary Coogan.
As Tracy Dyson, she is the host of a series on NASA TV called StationLife, which focuses on facets of life aboard the International Space Station.
On March 21, 2017, Caldwell Dyson stood behind President Trump as he signed a bill for NASA to send humans to Mars in the 2030s and receive $19.5 billion in 2018 funding. Caldwell Dyson and fellow NASA astronaut Chris Cassidy presented Trump with an official flight jacket during the ceremony.
Caldwell Dyson was assigned to STS-118 on May 17, 2006. It was announced that she would serve as mission specialist 1 on the first flight of Space Shuttle Endeavour after the Columbia disaster.
On August 8, 2007, Caldwell Dyson lifted off for the first time on the 119th Space Shuttle flight, the 22nd flight to the station, and the 20th flight for Endeavour. During the mission, she successfully added another truss segment, a new gyroscope and external spare parts platform to the International Space Station. A new system that enables docked shuttles to draw electrical power from the station to extend visits to the outpost was activated successfully. A total of four spacewalks (EVAs) were performed by three crew members. Endeavour carried some 5,000 pounds of equipment and supplies to the station and returned to Earth with some 4,000 pounds of hardware and no-longer-needed equipment. Traveling 5.3 million miles in space, the STS-118 mission was completed in 12 days, 17 hours, 55 minutes and 34 seconds.
On day 7 of the flight of STS-118, Caldwell-Dyson celebrated her 38th birthday.
Caldwell Dyson was assigned for her second space flight on November 21, 2008. Her second space mission consisted of a six-month trip to the International Space Station.
Caldwell Dyson successfully lifted off on April 2, 2010, from the Baikonur spaceport on board the Soyuz TMA-18 as Flight Engineer 2 with Soyuz Commander Aleksandr Skvortsov and Flight Engineer 1 Mikhail Korniyenko. Following a two-day rendezvous and docking maneuver with the ISS, she joined Expedition 23 as a flight engineer and transferred on June 2, 2010, to Expedition 24 again as a flight engineer after the departure of the Soyuz TMA-17.