Currently the greatest hazard in Xenominer, Solar Radiation is generated by the nearby star. With no atmosphere to filter it, the radiation from the sun makes the outside world hazardous during the day.
Solar Radiation cannot directly hurt you, but over time you will get more and more irradiated. (See Local Radiation.) If left unchecked, this local radiation will begin to hurt you, taking huge chunks out of your health. Retreat into shelter when you can during the day. If you want to risk going out it's okay, but make sure the Local Radiation level doesn't get to about three quarters full.
The main indicators of Solar Radiation is a meter in the top left of the screen. There are two radials, one showing incoming radiation, and another showing total built up radiation. Make sure both are minimal, and you should be fine.
In more recent updates different suits have been added that are more damage and radiation resistant.