The hearts of those who have traveled to space have become more spherical in shape, a new study says, a direct result of spending extended periods in zero-gravity or, at least, minimal-gravity environments.

Ultimately, the new research suggests the shift in heart shape likely makes long-term space visitors more prone to cardiac problems.

"The heart doesn't work as hard in space, which can cause a loss of muscle mass," the study's senior author, James Thomas, a cardiologist for the Sydell and Arnold Miller Family Heart & Vascular Institute at the Cleveland Clinic and lead ultrasound scientist for the National Aeronautics and Space Administration, said in a statement. "That can have serious consequences after the return to Earth, so we're looking into whether there are measures that can be taken to prevent or counteract that loss."

The researchers examined images of astronauts' hearts provided by ultrasound machines on the space station. According to a news report posited online by The Science Reporter, twelve astronauts provided images of their hearts to NASA scientists before, during and after spaceflight.

The study findings, presented at the American College of Cardiology's 63rd Annual Scientific Session, show that the human heart in space becomes more spherical by a factor of 9.4 percent, but then returns to its typical, elongated shape shortly after returning to Earth.

"The models predicted the changes we observed in the astronauts almost exactly. It gives us confidence that we can move ahead and start using these models for more clinically important applications on Earth, such as to predict what happens to the heart under different stresses," Thomas said.

Although scientists are still trying to determine the long-term health impacts of the shape alteration, they think that the more transition to a more spherical shape in space indicates the heart is performing less efficiently than it does on Earth.

The new heart findings will probably have an impact on how potential space travelers are selected and physically trained, now that several companies -- including the perhaps most recognizable Mars One --are hoping to establish a human presence on the Red Planet.

Mars One, which has already started selecting candidates to colonize Mars, starting in 2025, estimates the time it takes to journey to Mars will last between six to eight months -- since the orbits of both Mars and Earth around the sun are not perfectly circular.