The Sudbury Neutrino telescope in Canada. The 12-meter sphere contains 12,000 detectors that watch for the light from neutrinos that streak through the water filling the interior of the sphere. (Courtesy - Stanford Solar Center)
This image appears in the #59: Where did all the Neutrinos go? Technology Through Time article.
The largest, single, challenge for astronauts traveling to Mars will be to overcome exposure to solar storms and radiation.