We were discussing this in the chat room yesterday, and I thought it might make an interesting question. While missions are lasting longer today than they perhaps ever have, it seems like there are relatively few launches compared to the recent past. We could only come up with one mission in the window of SpaceFlightNow that is planned, March 4th InSight. I'm sure there are others, but it seems strange that NASA only has one interplanetary mission for the next many months.
The question is, is the rate of launch of missions over a short (Let's say 2 years) lower than the standard rate of launch of missions over, say, the last 10 years, and if so, why?