Not very unless they have a lot of protection. The amount of heat and light from the Sun goes up by a factor of 4 every time you half the distance to the Sun. So for instance the surface of Mercury is about 425 degrees Celcius during the day since its closeness to the Sun means the Sun is 7 x brighter than we feel at the Earth. Even in a spacesuit an astronaut would be very uncomfortable to say the least.

CHeers - David