Daily exercise can keep astronauts' hearts strong enough for a mission to Mars, a new study has found.

Researchers tracked 13 astronauts on six-month missions aboard the International Space Station. The crew did 45 minutes to an hour of endurance and strength training each day.

In the first two weeks, the astronauts' hearts shrank slightly as their bodies adjusted to weightlessness. But as exercise routines settled in, the changes reversed.

After landing, each astronaut was tested under simulated Mars gravity. None showed a heart response more stressed than standing upright on Earth before the flight, the Circulation paper reported.

NASA cardiologist Benjamin Levine called the result "really reassuring" for Mars mission planners. The findings may also help treat gravity-intolerance conditions on Earth.

Crew members were trained to perform their own heart ultrasounds while in orbit. Future spacecraft will need exercise gear that protects the heart, muscles and bones together. The study followed the crew for six months in orbit.