The Japanese Hinode satellite, launched in 2006, is meant to study the sun from a sun-synchronous orbit. On December 13th, it got quite a show. Sunspot 930 has released an X-class solar flare: twice as large as the Earth, and sufficiently powerful to make the Aurora visible as far south as Arizona.
Such flares are one reason why it is dangerous to be heavily reliant upon satellites for either communication or navigation. During periods of extreme ionic disturbance, GPS receivers can give positions that are off by thousands of kilometres. The streams of highly energetic particles produced by such flares eventually reach the Earth and threaten both automated satellites and manned vehicles.
The radiation from solar flares is also one challenge involved in a possible manned mission to Mars; with the kind of timescales involved and the absence of the protection from Earth’s magnetic field, the danger posed by such radiation could be considerable.