Active surface

Lore & Background

An active surface can try to compensate for many different types of errors. The first is gravity—this is simplest since previous measurements, or even a mathematical model, can be used to predict (and correct) any bending. More difficult is correction for wind and thermal errors, since these require measuring and correcting in real time. Some examples of active surfaces are the Green Bank Telescope, Large Millimeter Telescope, Sardinia Radio Telescope, and Noto Radio Observatory. The Chinese Five hundred meter Aperture Spherical Telescope uses an ambitious form of active surface, not only correcting errors, but applying deflections of up to 47 centimetres (19 in) in order to aim and focus the telescope.

Reader's Guide

The significance of an active surface lies in its ability to maintain high-frequency efficiency in large radio telescopes that would otherwise suffer from structural bending. The article notes that bending of a few millimetres dramatically reduces efficiency at higher frequencies where wavelengths are comparable to the distortion, with appreciable drop when deviation exceeds 1/10 of the wavelength. By using numerous small actuators to move surface panels relative to the underlying frame, an active surface compensates for gravity, wind, and thermal errors. The most ambitious example is the Chinese Five hundred meter Aperture Spherical Telescope, which not only corrects errors but applies deflections up to 47 centimetres to aim and focus. This technology extends the operational range of large telescopes to higher frequencies, enabling observations that would otherwise be impossible. The legacy of active surfaces is their role in making the world's largest radio telescopes scientifically viable at short wavelengths, as demonstrated by the listed observatories.

Did You Know?

• Large radio telescopes more than 10 m in diameter always bend during operation due to their weight.
• Efficiency drops appreciably when surface deviation exceeds 1/10 of the observed wavelength.
• The Chinese Five hundred meter Aperture Spherical Telescope uses deflections up to 47 cm to aim and focus.
• Wind and thermal errors require real-time measurement and correction, unlike gravity errors.

From Silverton to the Stars: An Unlikely Path to NASA

Pettit's journey to the cosmos began in Silverton, Oregon, where he was born in 1955 and raised with the discipline of an Eagle Scout—a trait that would serve him well through the rigors of spaceflight. His academic path was firmly rooted in the sciences: a bachelor's degree in chemical engineering from Oregon State University in 1978, followed by doctoral work at the University of Arizona, where he earned his Ph.D. in 1983. For the next twelve years, he pursued a career as a research scientist at Los Alamos National Laboratory. Yet his ambitions extended well beyond the lab bench. In 1991, he served as a junior advisor to the Synthesis Committee behind the Space Exploration Initiative's landmark report 'America at the Threshold,' which outlined a vision for sending humans to Mars. That year of visionary thinking foreshadowed his own trajectory. In 1996, NASA selected him as an astronaut candidate, and the rest became a remarkable chapter in human spaceflight history.

The Columbia Shadow and a Brutal Reentry

Pettit's maiden voyage to orbit was anything but routine. Selected as a last-minute replacement for Donald Thomas, who was grounded over medical concerns just weeks before launch, he flew as flight engineer on Expedition 6 aboard Space Shuttle Endeavour in November 2002. Once aboard the ISS, he performed two spacewalks to install external hardware, stepping in for a crewmate who was pulled from the task for health reasons. In his spare hours, he filmed 'Saturday Morning Science,' a lighthearted series exploring how fluids behave without gravity. The mission took an unexpected turn after the Columbia disaster in early 2003 grounded the shuttle fleet, extending the crew's stay by roughly two months. Their return came aboard the Russian Soyuz TMA-1, making them the first NASA astronauts to land in that capsule. A guidance malfunction forced a ballistic reentry that subjected the crew to more than eight Gs. Pettit, who had strapped a forty-four-pound pack of irreplaceable film to his chest, felt its weight balloon to nearly three hundred and fifty pounds, dislocating his shoulder. Officials later minimized the incident, reporting the crew in good condition.

Taming the Dragon: A First for Commercial Spaceflight

Perhaps no single moment in Pettit's career better captures the dawn of commercial spaceflight than the afternoon of May 25, 2012, when he took the controls of the Canadarm2 robotic arm to capture SpaceX's Dragon 1 capsule and berth it to the Harmony module. His radio call—'Houston, Station, we've got us a dragon by the tail'—became an instant classic. The maneuver marked the very first time a privately developed spacecraft had ever docked with the International Space Station, proving that companies outside the government could reliably resupply the orbiting laboratory. The following day, Pettit became the first human in the history of space exploration to step inside a commercially built and operated spacecraft while in orbit. He had launched on December 21, 2011, alongside Oleg Kononenko and André Kuipers, and during the Expedition 30/31 stay he also produced off-duty demonstrations on water thin films and Marangoni convection, continuing his tradition of making physics accessible to a global audience.

A Career That Defies the Clock

Few astronauts can claim a career spanning more than two decades of active flight, yet Pettit has done exactly that. With 590 cumulative days beyond Earth's atmosphere, three long-duration ISS expeditions, a Space Shuttle mission, and a six-week Antarctic expedition hunting meteorites, his résumé reads like a survey of nearly every frontier NASA has offered. At seventy-one, he remains the agency's oldest active astronaut and the third-oldest person ever to reach orbit, trailing only John Glenn and Larry Connor. His ingenuity extends to hardware as well: the Space Cup, a device he designed in microgravity, earned the first patent ever issued for an object invented in space. His astrophotography—thousands of star-trail images shared freely online, including the viral 'Lightning Bugs'—has made him a beloved figure in the space community. When he launched aboard Soyuz MS-26 in September 2024 for Expedition 71/72, completing 3,520 orbits over 220 days, it was a reminder that for Pettit, the clock simply does not apply.