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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.

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  • $\begingroup$ Opportunity was a Delta II Heavy, so combining may not have been an option.\ $\endgroup$ – zeta-band May 20 at 18:39
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    $\begingroup$ Both the Atlas V and Delta IV had I think sufficient payload capability to do it, but both were new launchers at the time of the MER launches; NASA didn't use them until 2005 and 2006. Not sure of the exact pricing, but Delta II was a relatively inexpensive rocket. I'd guess risk-dilution was a big factor. $\endgroup$ – Russell Borogove May 20 at 20:48
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    $\begingroup$ Rocket A goes boom. If you've put both on rocket A, you're done. If you have a Rocket B, you've got a second shot. I'm sure this obviously isn't the only reason b/c Delta II has successfully launched more than 190 NASA missions and maintains a 98% success rate. $\endgroup$ – Magic Octopus Urn May 20 at 20:55
  • $\begingroup$ Perhaps you would want them to land far enough apart in time so that the ground crew could recover from the first landing (party or sadness), get some sleep, and then get ready for the second landing. Phasing them by a day and a half (to get them to their landing sites) might require a lot of delta-v, and I'm not sure how that would be done. $\endgroup$ – uhoh May 21 at 0:01
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The total mass of the rovers with their heat shield, landing stage, cruise stage, and all the other parts was about 1,063 kg each. They were launched into a heliocentric orbit (HCO) before letting the cruise stage take it to Mars. The Delta II payload capacity to HCO was 1,000 kg, so there would have been no room for a second rover to get it to that orbit. The shuttle wasn't available due to well, Columbia also that it couldn't get something to HCO. The Delta IV and Atlas V were too new to be chosen. For the reason why they didn't choose the Titan rocket? My guess is that it wasn't as dependable as the Delta II since it only successfully launched 12 of its 15 missions and its last launch was in 1989.

In conclusion, they launched separately because they were too heavy to launch at the same time. If they were launched today, they could possibly both be launched on a single Falcon 9 but sense they also wanted different landing sites they might still choose separate launches.

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