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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 '19 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 '19 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 '19 at 20:55
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    $\begingroup$ 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 '19 at 0:01
  • $\begingroup$ From a complete amateur view , redundancy... as mentioned if one blew up on the pad it's bad. If they were both in one launch and that happened you'll lose everything. $\endgroup$ – AndyF Feb 12 '20 at 11:27
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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 reliable or cost-effective as the Delta II.

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 since they also wanted different landing sites they might still choose separate launches.

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  • $\begingroup$ Were there even any Titans left by 2003? $\endgroup$ – Vikki - formerly Sean Feb 17 at 22:32
  • $\begingroup$ @Sean The last Titan IV launched in 2005 (KH-11) but there were a lot of issues with it and I think they started phasing it out in the 90s. Similar to what ULA is currently doing with the Atlas and Delta rockets. $\endgroup$ – Seth Kurkowski Feb 18 at 23:43
  • $\begingroup$ @Sean Also, double-checked my facts on the number of successful Titan launches, only stated the number of Titan IVBs. It still had safety and reliability concerns and was not a cheap rocket to fly on. Edited the answer to reflect that. $\endgroup$ – Seth Kurkowski Feb 18 at 23:51
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From the earliest days of interplanetary probes, JPL has built and sent probes in multiples. While the practice is less common today, due to increased success rates & more expensive missions, it was common because the extra redundancy meant that, in the best case, you'd have twice the scientific returns, and in the case that one of the probes failed, at least one would complete the missions.

Examples:

  • Rangers 1 & 2
  • Rangers 3, 4, & 5
  • Pioneers 3 & 4
  • Mariners 1 & 2
  • Mariners 3 & 4
  • Pioneers 10 & 11
  • Surveyors 4 & 5
  • Vikings 1 & 2

I'm guessing MER (Spirit & Opportunity) were partially designed as a pair of vehicles for the same reason.

Now: to answer your question: the most common failure modes for interplanetary vehicles somehow involve the launch vehicle. Thus, putting the twin payloads on the same launch vehicle circumvents the entire purpose of having a twin mission--redundancy.

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