It may not be space debris, errant asteroids, supply shortages, thruster malfunctions or even the malevolent aliens envisioned in so many Hollywood films that thwart astronauts on any mission to Mars. It may be the ubiquitous galactic cosmic rays. Researchers said on Friday long-term exposure to these rays that permeate space may cause dementia-like cognitive impairments in astronauts during any future round-trip Mars journey, expected to take at least 2-1/2 years. In a NASA-funded study, mice exposed to highly energetic charged particles like those in galactic cosmic rays experienced declines in cognition and changes in the structure and integrity of brain nerve cells and the synapses where nerve impulses are sent and received. The irradiated particles in galactic cosmic rays, remnants of star explosions called supernovas, can penetrate spacecraft and astronauts' bodies. Earth itself is protected by its magnetosphere. University of California, Irvine radiation oncology professor Charles Limoli said "without a doubt" people would face the same issues as the mice.