This isn't time travel we're talking about. Simply seeing into the past. And the mirror thing would work, except it'd be useless for a while. Say you can send the mirror out 1 lightyear away in 2 years, so it travels at half the speed of light. Let's say you send it out in the year 2010. In the year 2012 it will stop, 1 lightyear away from us. We won't see anything from it for another year, so in the year 2013 we will be able to see stuff from 2011 (1 lightyear each way = 2 years difference). So we'll be seeing what we could have just recorded 1 year after we sent out the mirror.