Some of the biggest challenges NASA faces, if it is to meet its goal of sending humans to Mars, is making sure its astronauts are kept healthy and safe from harm.

Colorized scanning electron micrograph of a natural killer cell from a human donor. (NIAID via Flickr)

Colorized scanning electron micrograph of a natural killer cell from a human donor. (NIAID via Flickr)

Unlike current space flights, a round-trip mission to Mars could take up to 3 years.

Cosmic and solar radiation and a weakening of muscles and bones are among the health problems NASA has of sending humans into deep space.

A recent University of Arizona led study now expresses concern about the effects such a journey would have on an astronaut’s immune system.

The researchers studied the effects of space flight on natural killer or NK cells, a type of white blood cells that kills cancer and stops dormant viruses, in the body, from restarting.

They found weakened NK-cell functions in astronauts who were up in space six months or longer compared to those that hadn’t traveled to space.

The study called Spaceflight inhibits NK-cell function was recently published in the Journal of Applied Physiology.