My understanding is that Oppy's tracks outside of Endurance had been almost completely obscured and filled in during the six months that Oppy spent inside the crater. It wasn't a matter of side dirt infalling -- the entire set of tracks simply disappeared at points, and were obviously degraded and filled in where they were still visible.

Of course, I didn't go walking around Endurance checking this out -- I got this from comments made on this board and by MER team members in the media. But from the images I saw, it appeared to be a valid observation.

-the other Doug

When I see meteors flash across the night sky, they always appear generally as streaks of white light. However, bright meteors leave a trail in the sky. It persists long enough that it's obviously not just an afterimage in the retina, it's a real, visible trail of plasma left in the wake of the meteor.

Whenever I see this phenomenon, the meteor trail looks greenish to me -- greenish with a slight yellowish tinge.

Does anyone know if that's the true color of the plasma, or is it a color trick played by the mind because of the retina's lack of color receptors at such a low light level? Or does it have more to do with the wavelengths of light scattered by the atmosphere -- maybe it's really a golden light, that gets scattered towards blue in its trip through the troposphere?

Anyone have any thoughts?

-the other Doug

I think the best reply to that is "we don't know." We've seen that rover tracks get partially filled in over the course of only a few months, so this cannot be a static scene. But we don't know, with enough certainty, the entire range of particle sizes of the particles that make up the drifts, and we don't know what the average and "special" rates of airborne particulate volume are. Finally, we don't seem to have really good models for how sand and dust particles move and flow over and around aerodynamic obstacles in the Martian atmosphere, and in the roughly 1/3G gravity of Mars.

My best guess is that these drifts evolve and change over fairly short time scales (tens of years), but that the whole system is near to a sort of static balance. It's not a closed system, since material is constantly being both removed and added from the local scene (the evaporite is slowly eroding and windborne dust is slowly being deposited). But I'd bet that these drifts are constantly being formed, deflated and re-formed in roughly the same sizes and orientations as time goes on.

So, specifically, any given drift you see on the ground right now may not have been there 10 or 20 years ago, but the overall scene looked very, very much the same as it does right now. And has for perhaps a billion years or more.

-the other Doug

Just a nomenclature issue, here...

While we all tend to call the windforms here at Meridiani "dunes," they're not really dunes. "Dune" specifies a given overall size (much larger than these) and particle size (usually much coarser particles, such as sand, than these things are made of).

These have also been called "ripples," which is more of a descriptive term than a geological term.

One guy on the MER team (I don't remember his name) has said that the most appropriate term for these landforms is "drifts." The form size and particle size are more similar to terrestrial snowdrifts than just about anything else. Also, the formation dynamics seem more akin to drifting than to duning.

I'm just as guilty of referring to them as dunes as anyone else. But from now on, I'm going to make an effort to refer to them as drifts.

-the other Doug

In regards lunar dust -- yes, on the lunar surface itself, dust grains adhere to other dust grains (and almost anything else) because of the electrical surface properties in vacuum. Apollo astronauts noted that dust tended to shed off of their suits a little bit after they pressurized the LM cabin.

However, that doesn't explain why a thin layer of lunar dust is so hard to get off of surfaces, even in pressurized environments or back on Earth, where it isn't in a vacuum.

That happens because lunar dust has never been wet, and was created in an airless environment. Rock dust on Earth (and on Mars, too, I bet) undergoes interactions with water and air that tend to round off little tiny crags and projections from the dust particles. That doesn't happen to rock dust particles on the Moon, so lunar dust gets tangled more easily in porous and semi-porous surfaces. And at that scale, most surfaces are porous or semi-porous.

-the other Doug

I've been saying for a while that the asteroids are the logical next place for humans to visit. They're pieces of the ancient accretion phase of the solar system and hence scientifically interesting, and they also offer literally tons of resources that make them far more exploitable than smelly, rusty, salty old Mars down there at the bottom of that gravity well.

Does anyone have the stat for the surface gravity of Ceres? If it's enough that you could firmly root heavy machinery without extreme measures, I think we may have a winner there...

-the other Doug

The standard Apollo CSM orbit was circular at about 100 km (about 70 statute miles). The so-called descent orbit, from which the LM began its descent near the low point, had an apocynthion of 100 km and a pericynthion of about 7 km (50,000 feet, or roughly 9 statute miles).

During the first two landings, only the LM went into the descent orbit, and it only stayed in that orbit for half a rev, until it started down to the surface. The CSM stayed up in the 100-km circular orbit. However, beginning with Apollo 13 (and actually implemented beginning with Apollo 14), to save LM fuel, the plan was changed to use the CSM's engine to place the entire CSM/LM combination into the descent orbit, starting with the LOI-2 burn. The burn that used to place the whole craft into a circular orbit was changed to place it into the descent orbit. And so, NASA started putting their Apollos into the descent orbit for more like a full 24 hours and not just an hour.

What happened? Well, on Apollo 14, not much -- the CSM circularized its orbit just before the LM came around to land, no problems. But on Apollo 15, the CSM/LM descended from a low point of 50,000 feet down to a low point of 33,000 feet while the crew slept the night before the landing -- all due to the Moon's lumpy gravity field. Since there were mountains on Apollo 15's approach path that reached up nearly 15,000 feet, this was dangerous, so the crew performed a "bail-out" burn early on landing day to raise the orbit's low point back up to 50,000 feet.

That happened because Apollo 15 flew directly over Serenitatis and Imbrium, two areas with localized positive gravity anomalies (mass concentrations, or "mascons"), which pulled its orbit down at the low point. Apollo 16, which overflew the gravitationally-blander equatorial regions, had no real problems. But Apollo 17, which was also to overfly Imbrium and Serenitatis, had its plans changed. The low point on its original descent orbit was only 80,000 feet, not the 50,000 used on previous missions, and the LM used its RCS jets to lower the pericynthion to 50,000 feet just half a rev prior to landing. This was partially to keep the orbit from degrading due to mascons, and partially because the Apollo 17 landing site was rather far to the east, and there was little time after acquiring the spacecraft to track it and ensure that its low point would not intersect the lunar surface. So they decided to risk only the 80,000-foot pericynthion for the entire spacecraft. As I recall, that orbit also decayed, though not as much as Apollo 15's did -- down to about 70,000 feet by the time landing rev came along.

So, depending on where the low point of an elliptical orbit overflies, a lunar orbit with a low point much under 20 or 30 km is stable only for a matter of days. However, Apollo 16, which had little problem with their orbits deforming badly over the course of a few days, left a small sub-satellite in orbit behind it, in the standard roughly circular 100-km orbit. And it crashed six weeks later. Therefore, low lunar orbits simply are not stable over time and must be constantly monitored and corrected, at an expense of a lot of fuel...

-the other Doug

It's worse than that, now. Here is what the small print at the top of Spirit's raw image page says right now:

58984 new images since 09/10/2005 02:12:39 PST

Yep -- it's now saying that 58,984 images are new.

Now, the pancam pages aren't all in orange boxes; the Sol 1 images aren't in orange. But the navcam pages all are orange.

I think that either the scripts just went defective, or else they just tried to install a "Sol1K" fix into the website and it didn't work very well...

-the other Doug

The problem with the Apollo 14 camera was in its electronics, although it was using a different vidicon coating than had been used on the three earlier color cameras developed for Apollo.

The first color camera was put together just barely in time to fly on Apollo 10. That same camera was re-flown on Apollo 11, although a back-up camera had been built by the time 11 flew. The first camera was used inside the CM only on 10 and 11.

After Apollo 11, the in-cabin camera used on 10 and 11 was re-packaged and mounted on the LM descent stage for Apollo 12. It was the camera that was burned out by being pointed into the sun. Apollo 12's CM camera was the back-up developed for Apollo 11.

The camera on the Apollo 13 LM was identical to the cameras previously flown in terms of vidicon coating and electronics, but it was never used. (Careful use of a lens cap was to protect it from direct sun-imaging burnouts.) The camera on the Apollo 14 LM was of the same design, but had a new and different type of vidicon coating that made it less susceptible to damage from direct sunlight (although they still used a lens cap).

The real problem with the Apollo 14 camera was a bad component in the peak light control circuit. Instead of making the video signal balance to the peak lighting elements in the scene, it exaggerated peak lighting elements. The quality of the images from that camera when it was opened up all the way (biggest aperature), at the beginning of EVA-1 when it was looking into the shadow of the LM at the foot of the ladder, was actually quite good -- except that the sunlit surface beyond bloomed up and made the descending astronauts look like they had little tiny stick legs. But when Shepard came close to the camera as he took off the MESA blankets, the detail on his visor assembly and RCU was quite good.

Image quality was poor but acceptable when the camera was looking cross-sun. But when they set it up looking down-sun (during the ALSEP deployment from the SEQ bay and, later, during the setup of the ALSEP), the crew and their equipment were nothing more than big white blobs. It's a shame, since this was the only time the deployment of the ALSEP from the SEQ bay was televised or filmed, and the images are of extremely poor quality.

The glitch in the peak lighting circuit resolved itself somewhat by the beginning of the second EVA, and the images during EVA 2 were better. But since a vast majority of EVA 2 was spent with the camera pointed at the wrong segment of Cone Ridge to see any of the traverse, it really made no difference to the overall poor taste the Apollo 14 camera left in the public mouth.

I sort of wish they had gone to Plan B, which had been part of the planning on both Apollos 13 and 14. On both of those missions, the LM actually carried a back-up TV camera, a black-and-white camera identical to the unit used on the surface during Apollo 11. It was strapped to the side of the LM and covered with a thermal blanket. If the color camera was burned out on one of these missions, the crew was supposed to replace it with the B&W camera. I sometimes think the B&W camera would have given better pictures during EVA-1 than the color camera did.

On the J missions, of course, they used a much better color surface camera, capable of generating 525-line (NTSC-compatible) output. It still used the sequential field color wheel system, but it had a much sturdier vidicon coating (those cameras got pointed directly into the sun several times and never suffered a problem). Those cameras provided near-studio-quality video from the Moon during those missions -- a vast improvement over anything we saw during the first three landings.

-the other Doug

I was 13 years and nine months old when Apollo 11 landed on the Moon. I was 17 years and two months old when Apollo 17 landed. (Interestingly, I was 15 when Apollo 15 landed, 16 when Apollo 16 landed, and 17 when Apollo 17 landed.)

It was inconceivable to me that I might never, ever see human beings walking on the Moon again in my lifetime.

Now, the inconceivable is the most likely -- and it deeply depresses me. The best and most amazing thing we, as a race, have ever done happened when I was a teen-ager.

And now we don't do it anymore. And likely won't ever do it again in my lifetime.

As a lot of other people have said, I will believe we will return to the Moon when I see it. I've had my hopes raised too many times, just to be dashed against the rocks.

I guess you could say it's a sore subject with me...

-the other Doug

The problem is that the Moon is not perfectly spherical, its center of mass is not in the center of the physical body, and its gravitational field is notoriously lumpy.

A spacecraft in a 100km orbit requires orbital adjustments to avoid crashing once a month or so (such an orbiter, in such an orbit, with no propulsion system lasted about six weeks). At the altitudes you're speaking of, Richard, you'd have to adjust the orbit every few revs. You'd have to put *way* more fuel than spacecraft into lunar orbit if you wanted to try this. And if you want a low circular orbit, you have to settle for something no lower, on average, than about 20 km, because the Moon's off-spherical and mass-center status would literally crash a spacecraft in a 5km orbit after about half a rev.

Now, if you want to have a lunar orbiter in an elliptical orbit with a high apocynthion and a very, very low pericynthion, that's easier to do -- but you only get really close passes over a small, small fraction of the surface. And you can only stay in such an orbit for one or two revs before it crashes you.

Yes, it's a classic trade-off, but a very, very bad trade-off when it comes to the engineering of the task.

-the other Doug

I recall a story (that has been verified, I believe) that a member of the Grumman close-out crew at the Cape, when buttoning up the MESA equipment table in LM-5 prior to the launch of Apollo 11, wrote a short note (something like "Good luck and Godspeed") and his signature on the inside of the thermal blanket that covered the MESA's contents. When Armstrong pulled the blankets off, he saw the message, didn't say anything at the time, and didn't even mention it in the crew debriefing -- but he did tell his superiors quietly, and apparently that Grumman guy got sacked.

-the other Doug

If MSL does just get put off to 2013 and not canceled outright, perhaps this means that MTO can be re-instated with a program re-start sometime in 2009 or 2010, after most of the CEV development money will have been spent?

I *really* want that fat data pipe to make MSL a real telepresence explorer. If we're going to do this, we really should do it *right*!

-the other Doug

-the other Doug

So, it's relatively obvious that the largest of the main-belt asteroids (Ceres, Vesta, Pallas) are differentiated objects. My curiosity is whether or not these objects must have differentiated "in place," in other words, only in the context of their current mass, size and position.

Could they have once been part of a larger differentiated body that was destroyed by a massive impact? I guess we need a lot more data about the large asteroids, but I'm wondering how small a body can be and still undergo differentiation. When you consider that the Moon apparently is still predominantly made up of undifferentiated chondritic material, sandwiched between a once-molten core and a differentiated mantle and crust, we know that a Moon-sized body does not completely differentiate... and we also suspect that the Moon was formed by a giant impact which completely destroyed a Mars-sized body (which would have to have contained a lot of chondritic material for the Moon to contain a lot of it), so 1) the Mars-sized impactor must not have completely differentiated, and 2) lunar differentiation may not fit into a model of differentiation for primarily-accreted bodies.

Ceres lander/orbiter mission, anyone?

-the other Doug

Very nice, Doug! Great, great interview -- you structured the questions well, leading Steve through a narrative rather than just getting answers to individual questions.

I feel a great sense of affirmation at Steve's response to my question... And I must say, I *knew* some of those rocks Spirit was looking at up near Methuselah looked like welded ashflow tuffs! It's great to hear Steve say so, too.

So -- a new edition of the book after the MERs finally die, eh? So, we'll look forward to reading that sometime in 2012... *grin*...

-the other Doug

I have a new theory about the appearance of the rock in this RAT hole.

We're approaching Erebus, right? Erebus is a bigger crater than Victoria, although a lot older. In fact, it seems likely that Erebus was formed before the waters receded.

That would mean that Erebus ejecta would have been scattered on top of an existing evaporite layer, only to become embedded in later evaporite layers.

I think the dark cobbles we've been seeing, increasing in population density as we approach Erebus, are in fact the remnants of Erebus ejecta that have eroded out of the evaporite. I also think that the dark clasts visible in these MIs are small-grained ejecta particles that became embedded in the evaporite as it formed around it, during evaporite deposition eras that post-dated the Erebus impact.

And I think these materials may represent the sub-floor material that lies below the evaporite layer.

What do y'all think?

-the other Doug

Following up on the idea I just proposed about the evaporite near Erebus incorporating Erebus ejecta, it now seems clear that the dark clasts we saw in the RAT holes Oppy made recently (someone suggested they might be incompletely-generated concretions) may actually be sand- and pebble-sized grains of Erebus ejecta that were embedded in the evaporite. I bet we see a higher percentage of such clasts in the evaporite "floor" as we approach the rim of Erebus.

That would definitely, *strongly* suggest that Erebus was formed before the evaporite deposition process was complete. Since the Erebus rim materials seem to be made, at least in part, of evaporite, I'd think that there had to have been evaporite there when the crater formed -- so the process had started. But finding Erebus ejecta incorporated into more recent layers of evaporite would almost be proof that Erebus was flooded, altered and eroded by liquid water, and partially covered by more evaporite before the waters receded for good.

The Erebus impactor may actually have impacted into water... though, at this stage of degradation, I don't know how you'd establish that.

-the other Doug

We may not need to get all the way to Victoria to find examples of rocks ejected from below the evaporite layer.

Erebus is a larger crater than Victoria, albeit much, much older. And it may well have formed before the evaporite layer was complete (and thus its ejecta may have been covered with more evaporite, as well as having been altered by the standing water that must have covered it).

But we ought to expect to see fragments of Erebus ejecta embedded in some of the evaporite around Erebus. Some of it ought to have eroded out of the evaporite by now. It would look like, I don't know, cobbles of obviously different composition from the evaporite surrounding it, and also different from the basaltic dust and hematitic dust and concretions that make up the remainder of the ground mass.

In other words, I'm proposing that the dark cobbles we've been seeing in the interdunal areas are actually samples of the material that lies below the evaporite layer, that were ejected during the Erebus impact and have since been aqueously altered and embedded in evaporite. They've now been eroded out of the evaporite and are sitting out on the ground for Oppy to examine...

-the other Doug

Is it just me, or do most of the craters on Pandora seem to be in advanced states of erosional degradation?

Also, there is a highly interesting pattern of radial spokes within the crater roughly midway between "top" and "bottom" in thiese images, about a quarter of the width of the images to the right of the terminator.

-the other Doug

Not in the experiment title, no -- Apollo 17 carried two gravimeters, the Lunar Surface Gravimeter (LSG) and the Lunar Portable Gravimeter (LPG). The tidal reference may have been in the detailed description of the LSG, but it was not part of the instrument's name.

The LPG was the same type of instrument used by oil companies to find salt domes underneath otherwise flat land -- oil and gas are often entrained in salt domes. It detected negative anomalies on the slopes of the massifs and positive anomalies on the valley floor, indicating just how much more massive the basaltic valley fill is when compared to the massifs. IIRC, the anomalies were on the order of 10 to 30 milligals.

So, the LSG was an ultra-sensitive seismometer that hoped to use the entire mass of the Moon to detect gravity waves? Interesting... even if we now think that gravity waves would have been undetectable with such an instrument.

-the other Doug

Oh, that particular scientist-astronaut was well aware of the problem -- for one thing, when the PI found his instrument wouldn't uncage, he *insisted* that this particular scientist-astronaut must have deployed it improperly, must not have leveled it right. So Houston told him to go back and re-level the experiment -- three times. When told it would not uncage, Schmitt even kicked it, hard, and then re-leveled it again. It still did not uncage.

Schmitt was, indeed, *quite* angry that such a screw-up had cost precious lunar surface EVA time.

From what I understand, though, even with the main beam caged, the LSG actexd as a fair one-axis seismometer...

-the other Doug

Once again, I'm asking a question that I probably ought to just Google up for myself, but it does go along with the thread...

One of the experiments in the Apollo 17 ALSEP was the Lunar Surface Gravimeter. As I recall, it was designed to detect gravity waves. (It failed because it was balanced in 1G and was entirely out of balance, and hence useless, in 1/6G.)

Does anyone know what types of waves the LSG was designed to detect? Would it have been more in the LISA range or the LIGO range?

I guess I'm wondering what kinds of things we might have been gathering data on for more than 30 years if the instrument had just been designed properly...

-the other Doug

Out of curiosity, what did the Soviet RORSATs use in their nuclear reactors? Bearing in mind that they did scatter one of those all overa stretch of ground in Canada...

-the other Doug

The terrain all around Gusev looks remarkably different from orbit than it does from Spirit's vantage point, when it comes to albedo. Maybe it has something to do with angle of incidence, but all of those craters which seem to have extremely dark floors in the MOC images don't stand out as being nearly as dark in Spirit's images -- not even when Spirit has a good angle into the inside of craters (as it has from various vantage points on Husband Hill).

I know that when we got up on West Spur and were able to look back the way we came, there were two or three close craters which Spirit looked down into, and which had nothing like the darkness of floor that appeared in the MGS images. So, it's not just distant craters (whose floors might well be obscured from view), it's almost any crater we've looked down into.

The DIMES images from Spirit showed the crater floors as dark, too -- so I think it's more of an angle of incidence issue than a real difference between the cameras on Spirit and on MGS.

-the other Doug