Mariner Mars 1964, Mariners 3 and 4 to Mars: imaging plans? Options

[Phil Stooke]

I am currently working on a book about lunar exploration, but looking ahead to the next one, which will cover Mars. One question to which I think I have an answer - but I'd like to see what my fellow Mars enthusiasts think - is this:

Mariner 3 failed to leave Earth. But if it had flown successfully, what area on Mars would it have photographed?

My understanding is that there was no specific plan. The MM64 press kit, for instance, says nothing about image coverage for either Mariner 3 or Mariner 4. I believe that navigation to planetary distances was still so uncertain that the flight team could not predict at launch the sub-spacecraft point at closest approach - uncertainties included the exact time of the flyby, the distance and the point at which the spacecraft would pass through the target plane. These things would be known closer to the flyby but they weren't precisely predictable at launch, so Mariner 3 never got to the stage of having an imaging plan.

Am I right?

Phil

[Guest_BruceMoomaw_*]

Well, actually it DID leave Earth -- it just left Earth still wadded up inside its nose cone (and missed Mars by several tens of millions of km as a result of the added mass...)

I can confirm -- having followed the US space program pretty closely since the end of 1964 (in fact it was the 1964 Mariners that turned me into a fanatic on the subject) that there was no specific target planned for either of the 1964 Mariners. The 1969 Mariners were the first to have such a goal.

I imagine they would indeed have played it by ear for the 1964 Mariners, taking into account both the initial trajectory onto which the Mariners were injected by their Atlases and the serious needs of the non-imaging experiments (atmospheric occultation, flight through Mars' solar wind shock). I also know that any flyby distance between 3600 and 8600 miles was considered acceptable for the Mariners -- Mariner 4 was eventually targeted for a flyby at 5400 miles range, but due to a slight midcourse error actually flew at 6100 miles. As you say, their targeting accuracy at that time was not high enough that they could afford to get seriously persnickety over which part of Mars they tried to photograph with the two Mariners. It's likely that they would simply have assumed that, as long as the two Mariners photographed different parts of Mars, ANY two such different parts were acceptable. (I have, by the way, never been able to discover where the remaining 1969 Mars Mariner would have been retargeted had one of them failed early on.)

Personally, if Mariner 1 had succeeded, I wonder whether one of the 1962 Venus Mariners might have tried a radio occultation of Venus' atmosphere -- although Arvydas Kliore seems to have started pushing that experiment only in early 1964. I do know that this was actually the single highest-ranked experiment on Mariner 5.

And while we're taking a stroll through Space Memory Lane, remember that until Mariner 2 succeeded NASA was planning two more Mariner R spacecraft to be launched on Venus flybys in March 1964 -- with some science instrument modifications (improved microwave radiometer and magnetometer; ion chamber and cosmic dust detector replaced by the same UV photometer that got kicked off the 1964 Mars Mariners because it caused TV camera arcing, and finally DID get flown to Venus on Mariner 5. See "JPL Space Programs Summary #37-19".) I wonder if, had they been flown, THOSE Mariners would have carried out a radio occultation of Venus -- they even had high-gain antennas designed from the start to be smoothly tiltable, whereas Mariner 5's fixed high-gain dish had to add a feature allowing it to suddenly be tilted into a new fixed position to partially compensate for Venus' atmospheric refraction of its radio beam. (Since the 1964 Mariners had Earth sensors mounted on their high-gain dishes to aim and tilt them, they might even have been able to pull off another experiment very seriously considered but finally rejected for Mariner 5: an Earth sensor to measure the altitude of Venus' cloud tops.)

Coming soon, if you're all nice to me: a brief report on how NASA came within one month of trying to launch a VENUS ORBITER in June 1959 (although it definitely would have failed had it been launched). One of the most bizarre forgotten episodes of the early Space Age.

[JRehling]

I am currently working on a book about lunar exploration, but looking ahead to the next one, which will cover Mars.  One question to which I think I have an answer - but I'd like to see what my fellow Mars enthusiasts think - is this:

Mariner 3 failed to leave Earth.  But if it had flown successfully, what area on Mars would it have photographed?

My understanding is that there was no specific plan.  The MM64 press kit, for instance, says nothing about image coverage for either Mariner 3 or Mariner 4.  I believe that navigation to planetary distances was still so uncertain that the flight team could not predict at launch the sub-spacecraft point at closest approach - uncertainties included the exact time of the flyby
[...]

If the time of the flyby was not well-constrained, then the answer falls out of that. An arrival a few hours earlier or later would have rotated any target terrain out of view.
It's an interesting question, however, since telescopic maps of Mars did exist, and there would therefore have been a basis for prioritizing targets, if the operational precision were there.

As a side note, we're now in a very similar circumstance with Pluto. Our telescopic (incl. occultation) maps of Pluto have roughly the detail that pre-Mariner maps of Mars did, and the single flyby we'll get will only provide best resolution at one longitude. (Although NH will certainly image the rest of Pluto in a way that Mariner 3/4 could not for Mars. Of course, Pluto won't receive any followups anytime soon.) It would be interesting to ask Alan Stern if any plutonian targets have thereby been prioritized. Have 40 years and a dozen+ solid worlds flown by given us any insights for Pluto-2006-launch that we lacked for 1964-Mars-launch??? The comparison is flawed, however: I think geometry regarding Charon's orbital position will actually drive the decision, rather than the visibility of specific Pluto terrain.

[Phil Stooke]

Thanks for these comments. And Bruce, I was inadvertently thinking of Mariner 8 not leaving Earth. You were right, of course.

Phil

[edstrick]

Mariner 4 had a scan platform, and one option they had was to do the flyby with the platform in a slow back-and-forth <I think> scanning mode, maybe within a limited angular range. The pre-encounter decision was, as I recall, to set the platform at an angle that gave a "best" single swath of pictures across the planet: Middle of the illuminated limb, approximately across the sub-spacecraft point, and on to the terminator.

Arrival date would have set the hemisphere covered. If Mariner 3 had not been enshrouded at launch, Mariner 4 would have launched earlier and imaged some other strip, possibly hitting more interesting features, possibly somewhat less.

[Bob Shaw]

Bruce:

"Tell us about it, Janet!"

A 1959 Venus Orbiter - betcha it wasanother NOTSNIK!

[peter59]

Mariner 4 had a scan platform, and one option they had was to do the flyby with the platform in a slow back-and-forth <I think> scanning mode, maybe within a limited angular range.  The pre-encounter decision was, as I recall, to set the platform at an angle that gave a "best" single swath of pictures across the planet: Middle of the illuminated limb, approximately across the sub-spacecraft point, and on to the terminator.

Arrival date would have set the hemisphere covered.  If Mariner 3 had not been enshrouded at launch, Mariner 4 would have launched earlier and imaged some other strip, possibly hitting more interesting features, possibly somewhat less.

You are right about Mariner 4 scan platform. A few hours before planetary encounter "NRT" (nonreal-time) power is turned on to energize the TV and scan platforms. The scan subsystem initiates a "search mode" of operation, causing the scan platform to oscillate such that the TV and scan sweep an arc approximately perpendicular to the motion of the spacecraft. When the planet comes in view of the scan subsystem sensor, the scan subsystem switches to a "tracking mode" in which the sensor and the platform is kept pointing at the planet.



More about Mariner 4 original telemetry data at Mariner 4 data analysis

[Guest_BruceMoomaw_*]

Yep. In fact, they decided some months before the flyby to orient the scan platform in advance such that it would already be aimed at the estimated center of Mars' disk as soon as the tracking system was turned on, thus eliminating any need to actually move the platform. (This, I think, is what is Ed Strick was really referring to when he mentioned a decision not to "do the flyby with the platform in a slow back-and-forth mode" -- that wouldn't have occurred during the actual photography session in any case.)

Now, the two 1969 Mariners had not only a 2-axis tiltable scan platform, but a Mars wide-angle sensor capable of keeping that platform pointed at the exact center of Mars' disk on both axes (not on just one axis and at just one time, like Mariner 4's wide-angle Mars sensor). This was to ensure good long-distance photos of Mars' whole disk by the narrow-angle camera during the few days before the flyby.

[edstrick]

Bruce.. that matches what I recall without digging into the archives. I expect the design was intended to be able to find the planet, even if command capability had been lost and the flyby geometry was poorly known.

I'm still at a loss to understand why missions like Galileo and Cassini don't have "planet sensors" to help with fine targeting during close encounters. Cassini's lost full disk mosaics of moons just cause the targeting was say 50 or 100 km off and a big chunk of limb was clipped.

Mariner 69 was the "bridge" to everything later. Mariner 62 and 64 had "central computers and sequencers", but they weren't reprogrammable in flight. Mariner 69 had 128 or 256 command-words of storage. (Mariner 9 in 1971 had (I think) 512.)

With that added command capability, they were able to reprogram Mariner 7 to take a full tape recorder load of some 33 pictures instead of about 24 that Mariner 6 did, DESPITE having a Loss-of-Signal event and major spacecraft emergency only 5 days before, while Mariner 6 was doing it's close flyby. (They added more frames to the pass over the south polar cap and then let the tape run till it was full as it crossed the terminator over Hellas, trusting that real-time telemetry would get the nightside and night-limb data, as it had worked on Mariner 6.) Mariner 7's battery had overcharged, shorted, and *RUPTURED*, venting into the spacecraft, sending it into a roll, and giving a small propulsive nudge to the spacecraft from the vapors venting from the main body of the spacecraft!

[Guest_BruceMoomaw_*]

Yep, that's all correct. (Don't ask me how I remember exactly how many command words in memory Mariner Mars '71 had, when I have no idea how many any later spacecraft had...)

While the '69 Mariners were indeed a complete quantum leap upwards from the earlier Agena-launched Mariners in many ways, though, I think the most dramatic was the incredible leap in their communications rate from 8 bps for the earlier Mariners to fully 16,200 bits for the '69 Mariners. I couldn't believe that figure when I first read it in 1968 and thought it was a misprint -- especially since the earlier design for the "Mariner B" line of spacecraft on which Mariner 69 was patterened only had a bit rate of (I think) 256 bps. However, there had been very dramatic improvements in communcations technology since Mariner B was designed in 1962, and NASA decided to take full advantage of them.

Mariner 7's recovery from near-disaster was indeed very lucky -- although not as lucky as the incredible Perils of Pauline experiences of Mariner 10, when they practically had pieces fall off the spacecraft all the way to Mercury. (I suspect it was that experience that soured NASA on cost-control programs for some time.)

[tedstryk]

A planet sensor might be more trouble than it is worth in a system like the Saturnian system. For example, when studying one of the smaller moons, it might accidentally point the camera at Saturn, or even another moon. Also, it was originally hoped that the Mariner 4 track would touch the south polar cap, but it ended up missing. The planet sensor basically made sure that what happened to DS-1 at Braille did not happen (does anyone else find it ironic that imaging was unsuccessful at Braille?).

[Phil Stooke]

Ted - thanks for this... I'm not doubting you, but I would really like a source for the statement about the intention to image the south polar cap. I would be looking for more details about the plan, sufficient to plot the intended image coverage as well as the actual coverage, on a map.

Phil Stooke

[tedstryk]

I've got some documentation of this somewhere around here....might take a few days though....I need to figure out which box it is in, thanks to my recent move.

[edstrick]

Mariner 69 and it's mission was designed around a much slower communications link than they ended up using. ... some 512 bits/sec compared with 8 <at mars> for Mariner 4. The 16,200 bps data rate was an "engineering test" included on teh spacecraft. The original goal was to tape some 8 late far encounter full disk images and fill the rest of the tape with near encounter images.

Since the high-data-rate link worked flawlessly and clearly had plenty of margin at Mars, the mission ended up using it for all it was worth.

Similarly, Mariner Venus/Mercury 1973 <Mariner 10> was designed around a 16,200 or whatever datarate, and included an engineering-test data rate of 144,000 or some such data rate with the intent of using it for real time transmission of all data if it worked and taping the closest encounter frames on a Mariner 9 vintage tape for backup and multiple replay to get best quality data.

[edstrick]

Ted... I'd assume a planet sensor would only be "enabled" for final targeting adjustments when the target was already in view. Basically an auto-nav capability. They *** SHOULD *** have been able to do it using data from Cassini's wide angle <relatively> camera, but maybe the antique computer on Cassini can't do that much math. <sarcasm here>

And yeah... I though Braille was a bad-luck name choice even before DS-1 flew past.