http://www.space.com/businesstechnol...ab_050105.html

Pete Theisinger (who said never to do a twin mission again):

"We have provided the program with estimates of what it would take to fly two. It’s up to my management to decide whether the risk/reward ratio is favorable for doing that," Theisinger said. "There is no technical or schedule reason why it’s not feasible."

I think you always must send two. EDL risks are too high for sending a single rover, representing half of the career lifespan of most of the people involved.
The article also mentions it would take a Delta-4 Heavy to launch the MSL rover. I'm not 100% sure, but I think they mean a Delta-4 Medium configuration.

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I agree. I would think most of the costs go into design and development - the cost for a second one has to be much less than the first. It seems that the risk/reward factor would strongly encourage redundancy. Maybe they shouldn't even stop with two....

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It's a risk tradeoff, though. Some problems are independent, but others are not.

A flaw that exists in both can wipe out both of them.

It would be nice if there was a significant time delay between the launches, but Mars doesn't wait around...

They were lucky that the memory problem didn't kill off Spirit, because if it had it is possible that it would have gotten Opportunity, too.

Out of curiousity, has a mission with two identical probes ever gone awry for both? Or even one of the two? Hmmm... I have trouble even recalling how many missions were even set up like this. Viking, Voyager, Pioneer ?

I can understand a cost basis for not wanting to send two, but not a risk management one. If a fatal design flaw exists, then building either 1 or 2 rovers yields 100% failure. Building two does help protect against environmental effects that might cause the failure of one particular model.

And lets not forget the fact that Opportunity, the second rover to land, has yielded the largest amount of water evidence. Even if everything goes correctly, and without fault, the benefit of having two different sites to study might make the case better than any other analysis.

I wonder now what the story of MER would have been if Spirit had been the only one sent.

They have found definite signs of water in the Columbia Hills, just as significant (I think) as the hematite, etc., at Meridiani. That would probably have made the headlines if Opportunity hadn't gotten in first.

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Don't get me wrong, I'm not saying that Spirit hasn't been a successful mission, I am very anxious to see what they make of the water evidence they are building up in the Columbia Hills. It does seem to be a more ellusive water story, not quite the slam dunk that Opportunity had with multiple lines adding up to the same conclusion, and Spirit wasn't able to accomplish its water story within its primary mission. Spirit now has evidence of salt coatings and Goethite, whereas Opportunity has bromine/chlorine ratio, crossbedding, the sulfur-salt rocks, vugs, and the concretions. But as I said, I think there is a tremendous value to having both there.

As was mentioned in part of the NOVA special last night, the Columbia Hills "watery" material could be significantly older than what Opportunity is seeing. With both producing data we can put together a more complete history of water over time on Mars.

The Russian Phobos mission pair weren't all that successful, although Phobos II did return some images and data before being lost. http://www.iki.rssi.ru/IPL/phobos.html

Pioneer 3/4: Lunar flyby missions, 3 was mis-aimed, 4 was successful
Mariner 1/2: Venus flyby mission, launch failure on 1, 2 was successful
Mariner 3/4: 3 failed (shroud didn't deploy), 4 took first close-up photos of Mars.
Mariner 6/7: Two successful Mars flybys
Pioneer 6-9: Solar orbit missions, all successful
Mariner 8/9: launch failure on 8, 9 became first spacecraft to orbit Mars
Pioneer 10/11: Jupiter flyby missions (11 also Saturn), both successful
Viking 1/2: Mars orbiter/landers, both successful
Voyager 1/2: Outer solar system probes, both successful

More info on JPL past missions here.
More info on Ames Pioneer missions here.

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I'd qualify Voyager 2 as outrageously successful. And for those of you who believe all eggs are safe in one basket, be mindful of the Galileo's hi-band antenna not deploying. Not a mission breaker, but certainly an issue they would hope to have avoided.

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Other probe pairs:

1960 Soviet Mars probes, both launch failures

Sputnik 7/Venera 1, first falied to leave orbit, other failed before encounter

1962 Venus window, three veneras failed to leave orbit

Venera 2/3 first failed before encounter, second successful flyby (entry probe failed)

Venera 4/Cosmos 167, 1st successful flyby and atmospheric entry, second failed to leave orbit

Venera 5/6 Both auccessful flybys and descent

Venera 7/Cosmos 359, successful flyby and descent, failed to depart orbit

Mars 2/3 Successful mars orbiters, descent probes failed

Venera 8/Cosmos 482, successful flyby and descent, failed to depart orbit

Mars 4/5 orbiters, first failed to enter orbit (flyby data), second successful

Mars 6/5, flyby orbiter missions, both successful flybys and probe failures

Venera 9/10, successful orbiters and landers

Venera 11/12, successful flybys and landers

Venera 13/14, successful flybys and landers

Vega 1/2 successful Venus flyby, Venus landing, venus balloon, comet Halley encounter

Venera 14/15 successful Venus orbiters

Jon

This is an interesting thought. I think that such a mission would have been covered as follows by the press(generalized over the whole year):
NASA sent Spirit to Mars, which has suffered a string of mechanical and software problems (i.e. wheel, brake, and software) and has not really found solid evidence of what it was looking for. Scientists were apparently wrong in thinking that Gusev was once an ancient lakebed (yes, yes, I know that's not supported by the evidence, but the media thinks that way), and Mars seems to have been drier than thought (media would also point out the lack of carbonates seen from orbit). But NASA's little rover keeps on going!

So I think it would have something like that: some praise for the resiliency of the rover, but an impression of something dogged by minor failures and in search of an apparently nonexistent goal. About the carbonates - Opportunity's discovery that the Martian sea was probably acidic and full of sulfur indicates, according to SkyTel, that that could account for the lack of carbonates. But had we not sent Opportunity, we wouldn't have known that! Opportunity's amazing success has really been the focal point of the mission for a lot of people.

What I'm trying to point out is that with Opportunity, there's definite evidence of an ancient sea, and a reason for the lack of carbonates. Had only Spirit been sent, it could have interpereted the lack of [obvious] water as meaning that the carbonates are only more evidence of absence of water. So I think that in some sense, Opporunity "saved the day" for MER.

That said, I personally like the idea of driving on a basalt plain, and I'm still interested in Gusev even if it turns out that the hills are volcanic. Opportunity has found evidence of habitable enivronments on Mars, so that goal is met. IMHO, it's good to explore other parts of Mars, because obviously not all of Mars was under a sea. Some parts of Mars are more like Gusev, and some are more like Meridiani. The lack of water doesn't make a site uninteresting or disappointing, in my opinion.

To stray slightly from the topic, has anyone been nearly as annoyed as I have with the pattern of news media coverage on the rovers? There was a huge bunch of stuff about Spirit's landing, and then it gradually plateaued down to a lower level. Then the file problem, which makes headlines just as big as the landing. Then we hear nothing for a week. Then comes Opportunity's landing, and there's a brief resurgence of interest, but not as much as there was when Spirit landing. Then the regular (i.e. not space.com, JPL, those sorts of sites) media forgets about the mission entirely except for (I saw all of these stories on the "regular" media online) Opportunity's heater problem, Spirit's wheel problem, and a little bit about the discovery of the Meridiani sea. I have not seen anything about either rover in the regular media for the last nine months, with the exception of Spirit's wheel problem. Nearly everyone that I tell that the rovers are working are amazed that they are still operational.

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There will be something when they finally conk out and if they find anything besides rocks, dirt and bunnys. For the average Joe what is being found now is mundane. Hardly news worthy for the media types. Besides they're too busy spreading fear and paranoia to care about Mars.

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Same experience over here. One of my friends reacted with: "Oh, wasn't that the thing that crashed on Mars?". The news coverage also focusses on the problems and funny 'car wash' events. On the other hand, how much science related news does pop up on the main stream media? I never, ever hear something about Hubble's science results. The only thing on TV lately was the 'landing' of Genesis. Even the programs on DiscoveryChannel focus on the technology and building of the mission, not on the discoveries.

It makes you realize that 99% of the people are simply not interested in (space) science. If they notice the missions at all they regard them as entertainment. Which Apollo missions do people remember? 11 and 13. Nobody knows they actually send a geologist up there to do some science.

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Had Spirit been the only rover I think the overal scientific results would have been disapointing. We would have found basalt - like the other sites. There would have been a few hints of water - goethite for example. The the idea of a wet Mars in the past would have taken a definite hit, as Gusev was seen pre-mission as the most like place to find evidence of a liquid history. Meridiani was more of a long shot, with the haematite being seen as having a good chance of being either hydrothermal of magmatic. So there are definite pluses of sending two rovers to wildly different sites, as well as redundancy. It is a good way of hedging our bets on targeting and of course doubles the return.

Jon

On the Nova program, one of the geologists stood up and defended Spirit and Gusev by saying, "This is working-class Mars - this is what most of Mars is like." It may not be sexy, but it is representative.

And my impression is that there are signs of water in the Columbia Hills just as conclusive as the findings at Meridiani. Steve Squyres said at one point, "On the plains, we could find no rocks with signs of water. In the Hills, we can't find rocks without signs of water."

The Washington Post and a few other sources I've seen have marked the first anniversary with articles.

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<<On the Nova program, one of the geologists stood up and defended Spirit and Gusev by saying, "This is working-class Mars - this is what most of Mars is like." It may not be sexy, but it is representative. >>

I cheered that comment myself. Gusev may not have quite the juicy water record of Meridiani, but the site definitely has its own charms. Not to mention that I think the Gusev site is more photogenic. Hills, valleys, craters and Gusev walls--good stuff!

A good point - Meridiani was striking at first because it's so different from any of the other landing sites, but the near-featurelessness becomes monotonous after a while.

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Yeah thats been my reaction. I nearly jumped up and down when I saw the flat plains of Meridiani.

Now its so... dull.