In reading this question (Why A=2 and B=1 in the call signs for Spirit and Opportunity?), it prompted me to wonder why Mars probes Spirit and Opportunity were launched separately, even though their launch times were very similar and destination essentially the same. Spirit launched on June 10th, 2003, and Opportunity launched 27 days later (July 7). I would think (as an observer on the outside) that it would be significantly less expensive to launch them both on the same launch from Earth rather than two completely separate launches.
Some theories I have that they were launched separately could have been:
- Combining them into one launch would have needed a larger more expensive launch rocket.
- A single launch rocket of the needed size for both probes couldn't be scheduled for optimal launch windows. (This was way before SpaceX, so I would guess there was a lot less flexibility in launch providers)
- Not wanting to risk both probes with one launch (basically spreading the launch catastrophe possibility across two launches rather than everything in one).
- Lack of control systems at the time to safely separate and control the two probes independently as they approached Mars.
- Their separate landing targets were not compatible with a single launch trajectory.
So, although I've been able to find lots of information on the launches themselves, I have not been able to uncover the reason(s) justifying the dual launches.