Two interesting new Mars exploration documents Options

[Guest_BruceMoomaw_*]

(1) The Jan. 2005 "Geochemical News" ( http://geochemsoc.org/archives/gn/gn122.pdf , pg. 9-16) contains an interview with David Des Marais full of intriguing statements of his views on both Mars exploration in particular, and the difficulties of interpreting possible Precambrian fossil evidence in general.

(2) MEPAG's Mars Human Precursor Science Steering group -- which issued its report in mid-2005 -- had a subgroup devoted to determining the necessary measurements to allow safe landings and surface exploration by both unmanned and manned craft, and its report is at http://sirius.bu.edu/withers/pppp/original...resentation.ppt . It's particularly interesting for two reasons. First it summarizes the kinds of measurements that must be made by the atmosphere-oriented 2013 Mars Science and Telecom Orbiter for such purposes -- the biggest problem is wind measurements at various altitudes, which may require development of a Doppler lidar. Second, page 8 indicates that the MER landings were even a nearer thing than we thought:

"Spirit designed with range of atmospheric states for during EDL

• A week before entry TES observation of dust storm changed anticipated
atmosphere

• Based on TES, a new density vs altitude profile was created

• However, the reconstructed atmosphere, done post-flight, indicated a
significantly different density (reduced by 15% between 20-30 km) from TES
calculation, and was very close to the limit of system performance

• Also, steadily increasing oscillations of both Spirit and Opportunity before
parachute deployment nearly exceeded safe range (could get tangled chute).

• Oscillations due to either unexpected atmospheric turbulence (some unknown
aerodynamic instability) or mechanical instability of vehicle in fluid.

• Lesson: The atmospheric state is not well quantified, with both models and
NRT calculations yielding weather predictions with large intrinsic errors

• Lack of atmosphere information may affect vehicle design, possibly creating
unstable descent system

• There are still unexpected turbulent layers, and unexpected affects from large
atmospheric dust storms"
_____________________________

Obviously, we cannot keep running these risks as our landers get bigger and more expensive -- a better understanding of Martian atmospheric behavior and changes is crucial for the immediate future, not just the moderate future.

[djellison]

One of the squillion little projects I'm trying to do is to put together an animation of the MER entry up to the lander seperation event which would hopefully demonstrate the sort of occilation that talks about. Thanks for the headsup on the PPT - interesting reading. Mars really is a nasty planet to land on. Not Titan, not the Moon, just a nasty middle ground.

Doug

[Guest_BruceMoomaw_*]

While I was scooting around on the Web the other night trying to find something else, I also ran across a brief reference somewhere to the fact that MER-A would unquestionably have crashed due to the low upper air density, had it not been for the fact that (for some as-yet unknown reason) its parachute's braking ability also proved higher than expected! I'll see if I can track down that note again. Clearly we cannot go on just trusting to good luck where Mars landers are concerned.

[Guest_BruceMoomaw_*]

I've found it -- and it was a lot more than a brief reference. It was, in fact, the "MER Flight Operations Technical Consultation" ( http://nesc.larc.nasa.gov/admin/documents/...ition_Paper.pdf ), and the relevant material on the problems during MER-A's landing are spread over pg. 8-9, 25-34, and 40-43. Quoting the summary of the two main problems on pg. 8:

"1. The reconstruction of the atmospheric density profile was based on accelerometer measurements and the assumption that the aerodynamic characteristics of the aeroshell were nominal. The reconstructed density was lower prior to parachute deployment and higher after parachute deployment than had been predicted on the basis of the atmospheric model used. Apparently, as a consequence of the initial low-density encounter, parachute deployment time, triggered at a specified dynamic pressure of 725 Pa, was later in time and at a lower altitude (approximately 2-sigma) than expected. Although this reduced the time margins to complete descent and landing to a low level, as measured by the parachute deployment altitude, margin was regained because the parachute descended more slowly than expected. The cause of this fortuitous 'over-performance' of the parachute was not understood. Had relatively simple pressure and temperature sensors been included on the spacecraft for use during terminal descent and after landing, the physical situation actually encountered in situ during Spirit’s EDL would have been much clearer to the reconstruction team. The relative contributions of errors in the density profile model versus possible errors in drag coefficient could have been accurately assessed. As it was, the reconstruction team had to rely on assumptions and inferences in attempting to diagnose significant deviations from expected performance that had the potential for serious consequences for mission success.

"2. The aerodynamic performance of the entry vehicle did not conform to expectations from 175 thru 251 seconds after entry, just prior to parachute deployment. The angle-of-attack, while still well within safe margins, was higher than expected at flight conditions below Mach 6, with large-amplitude oscillations. Prior to entry there was an expectation of a dynamic instability when the vehicle decelerated to around Mach 3. The entry data showed that potentially this instability occurred as high as Mach 6. An instability occurring at a higher Mach number has a longer time to grow and hence can cause greater dispersions in attitude. The aerodynamic model used may be inadequate or incorrect, in the regime between Mach 6 and approximately Mach 3. The NESC representatives have emphasized that it is important for future planetary exploration missions to determine the cause of such instabilities. We are assured by the MER EDL Chief Engineer at JPL that this issue is under investigation at JPL. We do recommend that the results of the JPL study be published for the broader community to review, such as in an AIAA paper."

[djellison]

Also very interesting to this thread.....

Mars Exploration Rover Entry Descent and Landing Trajectory Analysis
http://techreports.larc.nasa.gov/ltrs/PDF/...a-2004-5092.pdf

Mars Exploration Rover Terminal Descent Mission Modelling and Simulation
http://techreports.larc.nasa.gov/ltrs/PDF/...4-14sfmm-br.pdf

Doug

[Bob Shaw]

Bruce:

I wonder if MPL ever got near the ground in one bit at all, or if it became aerodynamically unstable during atmosphere entry and broke up? That might explain the threefold lack of communication...

Bob Shaw

[djellison]

When you say three-fold, do you means MPL and the two DS2's - as they were mounted to the cruise stage and deployed 18 seconds after cruise stage sep from the lander shell - so a common cause would have had to occur before cruise stage sep.

Perhaps high instability of the DS2 entry during the entry phase might have introduced a tumble or similar that caused the thing to present the ( I believe unprotected ) back of the entry capsule to re-entry and killed them there. The MPF capsule was the same proportions as the MPF and MER ones I believe, so one would expect it to have got thru entry, even if it were as rough a ride as we are told it was for MER.

Were such instabilities noted during MPF entry I wonder. (or, for that matter, V1 and V2)

Doug

[Bob Shaw]

When you say three-fold, do you means MPL and the two DS2's - as they were mounted to the cruise stage and deployed 18 seconds after cruise stage sep from the lander shell - so a common cause would have had to occur before cruise stage sep.

Doug

Doug:

Yes, of course, I wasn't thinking!

Three-for-one would have been *good*!

Bob Shaw

[Guest_BruceMoomaw_*]

We'll never know for sure whether such an accident might have killed it BEFORE the software flaw that would have caused its engines to shut off about 30 meters up -- but the latter would certainly have killed it in any case if nothing else did.

[djellison]

Hopefully - HiRISE will spot some or all of the EDL hardware to help nail down the failure to a specific time frame of the EDL sequnce.

i.e.

Found : Impact only
Failure during entry and before chute deployment

Found : Chute & Backshell and no heathsield
Entry survived, possible shredded chute / heatshield sep failure

Found: Chute & Backshell and Heatshield but no lander impact.
Failure of lander seperation from backshell

Found: Chute & Backshell, Heathsield and Lander impact
Failure of terminal descent (favoured failure mode after investigation)

Found, Chute, Backshell, Heatshield and a non impacted lander with deployment evidence
Failure of comms / systems onboard

Hopefully, the same will be true of Beagle 2 ( although with different criteria) - and given the results of MOC in finding MER hardware - as long as HiRISE can cover enough area (and even if they just 2x2 bin and jsut use the wider swath width ) they should be able to identify SOMETHING from MPL.

However - HiRISE obs of Pathfinder might be usefull - as I've not seen evidence of Heatshield or Backshell & Chute at the Pathfinder site from MOC, which is somewhat suprising given how obvious they were with MER.

Dust deposition remains an issue, but repeat MOC obs of Spirit and Opportunity might suggest that two years after landing, their hardware remains very visible.

Doug

[edstrick]

Entry hardware can readily be hidden by landing site topography.. note that Spirit couldn't see it till she got to the elevated rim of Bonneville.

I Believe the heatshield was suspected as a single bright pixel to the south east in Pathfinder data but it was not glaringly obvious.

Neither Viking saw entry hardware from their landed locations.

MRO will search for Polar Lander and Beagle and image the successful lander sites: MER, Pathfinder and Vikings. High science value in observations of these targets with "solid gold map-tacks" stuck in the map.

Forget finding the Mars 2 high-speed impact, Mars 3 landing and 6 low speed impact or landing sites except by sooner-or-later inevitable accident. Their landing coordinates are only known to about a degree at best and there's no ground truth imaging to refine the locations better.

[djellison]

Entry hardware can readily be hidden by landing site topography.. note that Spirit couldn't see it till she got to the elevated rim of Bonneville.

But MOC saw it weeks before that, from a topographically adventagous position (i.e. orbit)

Doug

[The Messenger]

Also very interesting to this thread.....

Mars Exploration Rover Entry Descent and Landing Trajectory Analysis
http://techreports.larc.nasa.gov/ltrs/PDF/...a-2004-5092.pdf

From this report:

"The reconstruction work is ongoing in order to gain a better understanding
of what transpired during the “Spirit” and “Opportunity” landings."

I have been wondering out loud if and when this reconstruction would be completed, and I got an answer, of sorts, from Paul Withers. Paul has submitted a detailed paper to Icarus on the atmospheric reconstructs from the MER descents, and he hopes that it will be published within six months. Unfortunately, since some of the initial EDL parameters have not been publicly released (Do to ITAR???), Paul was forced to make a number of assumptions that limit the accuracy of his assessment. He also stated that the accelerometers and other instruments used for engineering, but not scientific, purposes will not be included in the PDL. (GRRRR)

The good news is, he also plans to release the software he wrote/used to create the model, so we may have another toy to play with. (I wonder if it will let me tweak the Mars gravity and MoI the way I think they need to be

Paul's paper has words to the effect that NASA has spent a lot less time and effort trying to create a complete MER EDL profiles than prior missions, and that a complete workup is not likely in the future. This also contrasted with these other reports. Clearly much more must be understood before a manned flight could ever be attempted, and for heavens sake, let's make sure the next probe has enough sensors to give us definitive answers without resorting to lame assertions of gross parachute over/under performance. (We saw this in the Pathfinder & Viking EDL's.)