Here's one for you, Emily:

I've heard that in our oblique view of M31, the near edge of the spiral is nearly two million light-years closer to us than the far edge of the spiral. The entirety of the M31 galaxy must rotate at least a quarter-turn (if not more than one complete turn) in two million years. So why do we see what appears to be a nice, contiguous, smoothly formed spiral pattern when the positional information received from the far end of the spiral is two million years older than that from its near end?

-the other Doug

Have y'all thought about where you'd want to fly this thing? Most of the U.S. has severe restrictions on flying any non-commercial, non-military vehicle above about 2,000 feet, and there are a lot of places where the restriction is lower than that, or where you can't fly anything. Most of this is due to airlanes, landing patterns, etc., but some has to do with laws against unlicensed surveillance.

I can't imagine Europe is a lot more open about this kind of thing -- if anything, Europe has more crowded skies than does the U.S.

And to add another inconvenient point -- even if you could launch from, say, the U.K., won't the camera pod come down several hundred miles from the launch point? There are a lot of things several hundred miles away from Britain that aren't very welcoming places for recovery of a pod -- the North Sea and the Alps are a couple that come to mind.

Just asking the obvious question I haven't seen raised yet...

-the other Doug

Oh, I wouldn't say that, Dan. I'm still planning on dropping the $$$ for Phil's book, when I can.

But yes, the site is engrossing. I just spent a half hour locating the LOIII images of the eventual Fra Mauro landing site. The landmarks are obvious once you find it. It was nice to see a lot more of the context of the general area (Imbrium ejecta field) and identify some of the things I had heard about but never clearly seen, such as what I'm sure was Star Crater (the original primary EVA goal for the site) and the lobate, layered, almost feathered-looking northern extension of Cone Ridge.

While that site was worthwhile visiting on any of the landings, I could almost have wished for a J mission there. I see a ton of interesting depositional landforms that would expose some pretty diverse rock types, I think.

Then again, the rocks from Fra Mauro were found to be breccias of primarily basaltic gross composition. Not only were the clasts in the breccias primarily basalt, the matrices of the breccias tended to be basaltic. So while the Fra Mauro formation, a huge, almost ropy splash of ejecta from the Imbrium impact, may look really interesting, it seems to have been created mostly from basaltic mare materials that were excavated and altered. Either that, or this portion of the Imbrium ejecta represents a large pocket of basaltic magmas that were excavated.

I'm not certain that even Imbrium excavated completely through the lunar crust and down into an upper mantle of basaltic magma -- but it's not unlikely that huge magma chambers could have formed in the Moon's ancient upper crust. Excavate one of these and you'll generate a fair amount of the kind of brecciated ejecta as we see at Fra Mauro. But this basaltic composition makes Fra Mauro entirely dissimilar to the true highlands formations visited by the three J missions (well, OK, two highlands-derived basin wall mountain locations and one true highlands location). Highlands materials have a relatively poor admixture of basaltic materials, with a dominance of noritic/troctolitic aluminum-rich rocks. That's why I tend to chafe a bit when Fra Mauro is defined as a "highlands" site. It's not -- it's an Imbrium ejecta site. It's morphologically and compositionally distinct from the lunar highlands.

-the other Doug

Many hours? Definitely. Wasted? Never!



-the other Doug

I'm not sanguine about the possibility that subsurface imaging radars would be useful for locating defunct spacecraft in Mars orbit. They're imaging radars, not tracking radars... the radiation pattern, reception characteristics and frequencies used are no doubt between somewhat and a lot different.

I would think that you could use the wider-angle cameras on the working Mars orbiters successfully in getting star track images that could identify craft in orbital motions. You'd have to devote some viewing time to it, but with enough observations you'd probably manage to catch the occasional piece of junk sharing Martian orbit... and the orbiters are almost always power-positive.

-the other Doug

Well -- I missed the launch (6:34 am is a little early for me these days, and that's when she lofted by my clock), but I just saw a quick replay of the launch at the beginning of the post-launch news conference, and I have to say, that thing heeled over to the left (from the camera angle I saw) pretty good before straightening out and angling to the right onto its correct trajectory. Took off like a bat out of hell, though...

Four issues were just mentioned -- the RCS switched itself from the primary to the secondary system, for reasons yet unknown; the RCS thrusters are running colder than anticipated, which is making the software controls lock them out, but reversion to hardware controls is keeping them running -- the fix is a minor re-set of the software's criteria values; the RCS brackets are running a little warmer than normal, but are cooling down; and there is a slight difference in electricity being generated between the two solar panels.

Those are the only issues that have been discussed.

-the other Doug

Unfortunately, it could be 30 to 50 years until we have another probe out that way (at least, another probe close enough to reliably image such an impact site), and without another camera on the scene at the time of impact, there is a pretty big issue as to whether we'd even be able to locate the crash site accurately enough to find the right crater later, much less 30 or more years later.

It's not the worst idea in the world, impacting a body onto Iapetus and then observing the effects over a period of time... but the observer needs to be there at the time of the impact, to triangulate the impact site and to observe the weathering effects from the beginning. To me, it makes no sense to crash Cassini there, making a crater in a difficult-to-locate position and that cannot be observed until/unless another Saturn orbiter comes along, two or three generations from now.

-the other Doug

Or, in other words...

"I find your lack of faith disturbing."



-the other Doug

That sounds like a good definition. Just remember, though, that in NASA-ese, you always capitalize Mission Success Criteria -- so that, if you don't meet those critera, everyone will know that You Have Failed...

-the other Doug

I will also point out that Mars is supposedly being pelted by meteors large enough to create secondaries at the rate of about one a month, according to an MGS study I read not too long ago.

Ejecta that lands on relatively flat ground will create secondary craters or just deposit clods of debris on generally flat surfaces. They'll cause landslides on steep enough slopes -- and small landslides create gullies.

I don't totally buy this theory, unfortunately, since it would statistically tend to create gullies with origin points of varying altitude up a given slope, and we tend to see gullies with consistent origin points along visible contacts in the slope walls. But it is one mechanism that undoubtedly does create gullies on Martian slopes, so it would be remiss not to recognize it.

-the other Doug

Thanks for the warm regards, Climber! However, I will somewhat diffidently point out that I'm the *other* Doug -- that doesn't render me unreal. (I do quite a good enough job of that all by myself, thanks... )

-the other, but nonetheless real, Doug

Well, yes -- orbital umsf began 50 years ago next week, to be sure. Sub-orbital umsf had been going on for years by that point, though.

I don't really remember Sputnik per se, though I seem to be able to recall there was a time pre-Sputnik and a time after. But I was one year and fifty weeks old, to the day, when Sputnik flew. I can't be expected to have clear memories from that age...

-the other Doug

A disposal orbit around Mars is possible because Mars doesn't have any large moons that could deform that orbit over the following decades. But the gas and ice giants have literally swarms of attendant moons, many of them quite large, which create gravitational resonances that aren't completely modeled or understood.

Any large orbit around Saturn carries the risk of a close encounter with Iapetus that could decelerate the orbiter back into the main moon system, with the possibility of impact into Titan or Enceladus. Add to that the gravitational resonances of the other moons, and you have a situation that you cannot guarantee will keep the orbiter from becoming a possible source of contamination.

To guarantee that Cassini will not impact Titan or Enceladus, you really do have to crash it into something. The most obvious and most effective body to use is Saturn itself.

Now, that doesn't mean that you can't skim it a few tens of km over the rings on its way towards its destruction...

-the other Doug

Pluto and Charon form a double-planet system, with the epicenter nearly outside of Pluto's surface. Is such a system gravitationally capable of maintaining a ring system for more than a few decades?

-the other Doug

A recent thread in the Saturn forum makes me wonder just how y'all read UMSF, and how much of it any one particular person follows.

I read every new post. I always have. I am beginning to think that this is not normal for most of the people who frequent this place.

When I come onto the site, I use the main board index as my entry point. The main index has new post indicators on it -- the blocks to the left of every forum name are light blue when there are no new messages in that forum, dark blue when there are new posts.

I start at the top, checking in on new "news" posts, then through the inner solar system (Mercury, Venus, Luna and Earth). Then I drop down to the bottom and work up, checking board maintenance, the miscellaneous categories, and the outer system (asteroids, Jupiter, Uranus and Neptune). I leave the Saturn fora last in this category, since they are the most interesting to me of the outer planet fora.

Finally, I go through the Mars fora, my absolute favorites here (sorry, Jason!). It's sort of like how some people eat a meal -- I go for the things I like the least first, saving the things I like most for last. When it comes to the Mars discussions, I save those so I can "finger-lick" them...

I find this satisfies me more than just going to a list of new posts and picking and choosing from there.

In this way, every day when I come in to check for updates, I end up reading all of the new posts. So yes -- I read them all.

So -- how do y'all read UMSF?

-the other Doug

Nope! Just a voracious reader, when it comes to topics in which I have an interest.

-the other Doug

Umm... yes.

-the other Doug

Let's see, now -- what do the Planetary Protection Protocols have to say about THAT idea, eh?

-the other Doug

So did Deke.

-the other Doug

Well -- it occurs to me that the first part of your post, Mike, bears on your final question.

If methane clathrates are common surface materials, and if they contain chemically energetic methane within a water ice matrix which is normally stable up to about 300K -- what happens when a meteor clobbers such a surface, generating temperatures far in excess of 300K?

How energetic does the liberated methane become under superheated conditions? And recall, you have a range of heating regimes as you increase distance from the impact point.

You'd get far different chemical reactions at the impact point (where kinetic and thermal reactions would dominate) than you would hundreds of meters, or tens of kilometers, away from the impact point. I can imagine a heat pulse being transmitted by Titan's think atmosphere in amazing fashions, increasing temperatures to a point where methane molecules are not vaporized into constituent gases but *are* explosively liberated from their quadrillions of icy prisons. The hydrogen and oxygen liberated by kinetic and thermal reactions at and close to the impact point would be available to react chemically with the methane, at least in some mixing zones.

Also, don't forget that the impact heating isn't the only heating you'll be dealing with when a meteor strikes. A meteor large enough to reach the surface will heat a slice of the atmosphere (I would have said "column of air" but meteors are more likely to enter obliquely than vertically), and some of that superheated air will be accelerated by the meteor and wash down onto the surface all around the impact site. It may not be a huge effect, but it could affect the impact effects with which it interacts.

I'm not saying there would be massive explosions from small impacts -- I *am* saying that we could see effects that build and shape landforms in ways we've never seen before. I'm tempted to think that the "rotten" terrain you've noticed, Mike, may be the result of these kinds of effects.

Just playing gedankenexperiment, here... but I think it's worth analyzing how a surface composed at least partially of methane clathrates would react to meteor impacts.

-the other Doug

Anyone have any charts handy telling us when we have Jupiter gravity assists to each of the outer planets? Jupiter orbits once every dozen years (roughly) -- that means we ought to have a gravity assist trajectory to each of Saturn, Uranus and Neptune every dozen years or so, right?

The order in which we send probes to the outer planets would seem to be dictated more by the availability of gravity assists than by our 'druthers, I think.

-the other Doug

I believe the most advanced processors being used in manned space flight today (on the laptops and integrated computer systems on the ISS, for example) are 80486's. We're not even into the Pentium era yet.

I could be wrong, of course... I haven't dug deeply into the latest news about ISS computers. I surely hope the Shuttles have upgraded their base computers from the PDP-8's they were originally fitted out with, though.

-the other Doug

Notice what else is being advertised -- they're taking applications for the position of Director, Flight Crew Operations.

So, Deke's old job is open. If you could stay in the position for another 12 years, you'd get the opportunity to select which people will be the *next* to set foot on the Moon... *grin*...

-the other Doug

After seeing some of the contrast-enhanced images from the trailing hemisphere, I have to say that my view has changed slightly. I still firmly believe that the dark surface on the leading edge is a mantling on top of a brighter ice substrate, but there is good evidence now that where this dark material has mantled the surface along the edges of the leading hemisphere, there has been deposition of bright ices on top of it. In places.

What is so odd to me is that neither hemisphere shows much in the way of cratering that exhumes a different-albedo substrate. There are occasional bright-rimmed craters on the leading hemisphere, but almost no dark-halo craters on the trailing one. And the bright-rimmed craters are just that -- bright-rimmed. There is little to no evidence of bright ejecta around them.

I'm having a hard time believing that the impact rate is so small that there have simply been few to no impacts of any size since the materials we see were emplaced on Iapetus' surface. But at the same time, I have a similarly hard time believing that the emplacement of these materials is anything but ancient -- particularly since some of the flow patterns hint at *aeolian* deposition/deflation, which is awfully difficult to explain on a currently airless body.

So we are left with a paradox.

-the other Doug

Point taken, Doug. I was just struck by the phrase "it has similar performance" when it currently has nothing. You have to have a vehicle in order for it to be able to perform... *smile*...

But yes, if Falcon 9 does work as advertised, it could well step in and fill the void that will be left by the Delta II's retirement. I'm still convinced that we need launch vehicles of the Atlas V / Delta IV class that don't cost $100-million-plus. MSL *could* be scaled down to be able to be launchable on a Delta II, but it would then be more of a MER-with-an-RTG than a larger, more capable vehicle.

Recall that the MERs pushed the Delta II capability right to the limits, and wouldn't have been easily flyable on that vehicle had the orbital dynamics not been so favorable in 2003. And with a Delta II, New Horizons would be arriving at Pluto sometime in the 2030's -- or would be carrying a single engineering camera, if that.

It's difficult to design a capable spacecraft when you have a $350 million budget and your launch vehicle is going to cost you nearly half that... especially when you really need a more powerful launcher than is available for less than $100 million. If y'all want to keep having to play mass-budget games that result in, for example, killing off the Raman spectrometer on the MERs or similar trade-offs, then I guess it's OK to try and find a Delta II replacement. But I'd rather see reliable LVs of the Delta IV / Atlas V class which can be purchased and flown for $50 million or less.

-the other Doug