Your diagram shows the most common Hohmann transfer orbit, achieved by reducing the aphelion to coincide with the orbit of Venus (preferably when Venus itself is occupying that point along its own orbit).

There is another type of Hohmann trajectory for this type of mission. Instead of braking against the Earth's solar orbital velocity, the spacecraft thrusts at right angles to Earth's near-circular orbit (likely directly towards the Sun), creating a lopsided orbit with a perihelion at Venus and an aphelion well outside of Earth's orbit. The good thing is that the average orbital velocity of such an orbit is very nearly the same as the Earth's velocity, so you don't need to brake so much. The bad thing is that it takes even more energy to enter this transfer orbit, and your approach speed at Venus is somewhat higher, requiring more energy to brake you into orbit.

It gets you there a little faster than the brake-against-solar-orbit method, but it takes more energy.

-the other Doug

Another big downside, back during Apollo days, to using the hypergolics was they tended to be corrosive. You could keep them in aluminum tanks with no problems, but they ate away at gaskets and seals. You had to have all-metal seals from the fuel and oxidizer tanks into the feed lines, keeping the lines entirely clear of either propellant until you "armed" the vehicles for launch. At that point, you had roughly one month to launch the spacecraft before the propulsion systems were considered degraded. At that point, the fuels were drained and the seals and gaskets had to be replaced before they could fly. This was especially true of the LMs.

One of the biggest issues they had in adapting the Apollo CSM for SkyLab was to make the propellant systems "good" for at least three months after the main seals were cracked. The SPS and the RCS propellant systems were both upgraded with corrosion-resistant seals and gaskets. One of the results was a leak in the propellant lines in two of the RCS quads on the second SkyLab CSM -- the new corrosion-resistant seals and gaskets were apparently a little more prone to cracking than the older style, and sprung leaks.

One of the reasons why the Shuttle takes so much refurbishment is that it uses UDMH and nitrogen tetroxide in its OMS and RCS systems. After each flight, the gaskets and seals in the RCS and OMS propellant lines are cleaned and, often, replaced.

-the other Doug

Agreed!

"Greatest encounter... EVER!"



-the other Doug

Now, that's an icy moon! Most of these outer-planet moons with crusts made primarily of ice look more like rock to the untrained eye. They obviously have ground-up regoliths and such, and the whole thing gives the same impression as a rocky landscape.

But this looks like ice! Gleaming, very pure ice. The impact and tectonic features look glassy, and as if things have fractured along plane lines (as they ought to in ice).

This encounter is providing some very impressive images!

-the other Doug

The image you've linked to is, well... all I can do is quote Dave Scott, upon his arrival into lunar orbit:

"This is profound, let me tell ya."

Now THIS is what alien planets ought to look like!

-the other Doug

I agree -- though I can guarantee you that this "lump" does *not* have enough boulders on it to fill Galveston Bay.

-the other Doug

Possible, but unlikely. From above, the terrain around Victoria looks idenitcal to the plains where Oppy landed. And Victoria itself looks a lot like Endurance, just a lot larger.

And Endurance is pretty obviously an impact crater.

I think the difference between something like Erebus (which actually looks old and degraded) and Victoria or Endurance (which don't look nearly as old) is the state of the ground where the impactors struck. I think Victoria and Endurance were made well after the place dried out, and represent what happens when a crater is made in the "finished" evaporite layer (i.e., nothing has come along after those impacts to degrade them with the exception of wind). Whereas Erebus and other very degraded-looking craters were made back when the place was still wet, and further evaporite has been deposited on top of the impact features before the whole thing dried up and no more water was available to lay down more evaporite.

And, in response to Richard earlier, yes, those dark streaks *are* there around Victoria on earlier images. They're just not as pronounced. I'd bet it has something to do with sun angle and angle of incidience at the time each image was taken, as to the dark wind-blown dust streaks being more or less visible. And no, I doubt seriously they're made by dust devils -- there has been absolutely no sign of dust devils at Meridiani, and these streaks obsviously follow a pattern that shows they're formed by the aerodynamic disruption caused by the crater rim itself, so no need to conjure dust devils to explain them...

-the other Doug

Yep -- those are basically the same fuel and oxidizer used by the Titan II. Storable propellants, non-cryogenic (thought perfect for ICBMs, until solid fuel motors struck everyone as even more storable and requiring less maintenance).

Of all the uses to which N204 and UDMH can be put, certainly launching manned spacecraft is one of the most benign. And I'd venture a guess that more of those fuels were used during Gemini and Apollo (for which the latter, of course, those same fuels were used on the CSM and the LM) than have ever been used testing and actually using nuclear-tipped weapons. Good thing, eh?

-the other Doug

I think it's ancient ejecta from the Erebus impact, eroded down to flat "paving stones." Possibly with further evaporite formation on top of jumbled ejecta blocks.

-the other Doug

I don't think so. The kind of computer graphics necessary to do such realistic re-creations of Apollo lunar surface operations simply didn't exist in 1969. They did exist in 1997, when Hanks' "From the Earth to the Moon" was produced, but were too expensive for a TV-miniseries, and so the effects on that former work, while good, were nowhere near as accurate as those in this latest effort.

I'm not just talking about needing to hook up a LOT of 1969-era computers -- I'm saying that the primary ability to manipulate images digitally did not exist then. Anywhere. The *only* way to do special effects back then was entirely optical, cutting and reprojecting images into composites. There wasn't a system available, anywhere in the world, that could digitally manipulate images to the extent necessary to even produce a single still frame from the new IMAX film.

Of course, if you're so steeped in self-doubt that you cannot allow yourself to believe the actual facts, then this fact, too, can be easily dismissed. I don't see my arguments changing the minds of any of those who believe in this conspiracy fantasy. But I also don't see this film swaying anyone with half a brain away from the side of factual rationalism, over to the "dark side"...

-the other Doug

Thanks! I knew most of the story; thanks for filling in the gaps!

-the other Doug

Not to mention the fact that Nifelheim and Terminus are both names of planets in classic and quite well-known science fiction novels. Besides, I really think Terminus ought to be a planet at the end of something -- it's the termination. If there was a way to identify a planet as occupying the very outer edge of the Solar System, I would vote for Terminus as an appropriate name. Otherwise, I'd save it.

-the other Doug

I know that the crew of Gemini 12 was supposed to observe a solar eclipse from orbit in November, 1966. My memory of the acytual event is fuzzy -- I *think* they were supposed to try to fly through the Moon's shadow and observe the eclipsed Sun from orbit, but this was abandoned when their Agena's Primary Propulsion System (PPS) showed signs of instability during orbital insertion and the crew was forbidden from using it. They needed that big engine to get into the right orbit to pass through the Moon's shadow.

However, I *think* they were able to observe and photograph the Moon's shadow on the face of the Earth.

It is possible that I have this wrong, that they were supposed to try to photograph the shadow upon the Earth but that the loss of the PPS meant that they weren't in the right place to do so. I just can't recall for certain which of these two scenarios was the one that played out.

-the other Doug

One thing to remember about Spirit and the tau of the atmosphere above it at any given time is that, up in the hills, Spirit is above the top of the local haze layer. This is very, very obvious when you look at images of the plains -- there is a definite haze layer that sits nearly on top of the surface, a layer that does not extend as far in height as the present height of Spirit.

Since this haze layer cannot be caused by humidity, it *must* be caused by dust particles entrained in the air. This is probably caused by the extreme thin-ness of the Martian atmosphere, surface-air heat exchange (the primary method of heating the air, as opposed to direct solar heating of the air itself), and the swiftness with which the air thins and cools as you rise above the surface.

I think perhaps the only reason we're seeing higher currents from Spirit's solar arrays now than at the beginning of the mission is that, unlike on Earth, rising only a few tens of meters above the plains has taken Spirit above the main haze layer and increased tau above what would be expected. And since we still have enough air above us to create about the same amount of diffuse light as we would see on the plains, we get the best of both worlds -- low tau and average diffusion.

And, by the way -- Steve Squyres in his book "Roving Mars" gives a lot of detailed information about the solar cells. During the initial design phase, the number of strings of solar cells they could fit on the rovers was a critical factor, and Squyres gives a lot of detail on how much energy they expected to get out of their original configuration, how the solar panel guys goofed and could only get 30 strings (or less!) onto the vehicle instead of the 33 strings defined as the "absolute minimum" for meeting primary mission power requirements, and how each square centimeter of solar cell was going to affect the MERs' power budgets (and thereby, any chance of a mission extending past 90 days). The culmination of this story is how the designers ended up with the delta-wing-shaped array, which wasn't exactly how the arrays were originally designed...

-the other Doug

Now that it's been a while since the rovers landed, it's time to open a reminiscence thread.

What are your favorite memories of the MER missions thus far?

Bearing in mind that I have had NASA-TV on my cable system throughout most of the mission, here are a few of mine:

- The launches. The onboard camera from Spirit's launch was impressive, showing the Florida coast dwindling as the curvature of the Earth became more and more apparent. Opportunity's night launch gave less impressive onboard views, but the last-chance anxiety as Oppy finally launched, after several delays, near the end of its weeks-long launch window made her successful departure even more memorable.

- Spirit's landing. The pressure, the tension, the excitement as each phase successfully unfolded, the elation at the first bouncing signal, the sheer terror as signal was lost for many long minutes. The satisfaction of seeing a new place on Mars. I remember when the first navcam pans came in, I said to myself, "Yep, that's Mars, all right -- rock-strewn, wind-eroded, Mars." I remember thinking that Gusev actually looked considerably less rocky than the previous three landing sites, with a similar population of smaller rocks but not nearly so many large rocks as we had seen elsewhere.

- Six wheels in the dirt. As Spirit trundled off its lander and headed out, away from its landing point, I got this odd feeling. It seemed totally odd that we were leaving the lander, never to return. This has never happened in the course of American space exploration, ever. (I know the Lunokhods did it, but the Lunokhods' missions took place at such a remove, information-wise, from my world that they might as well never have happened.) It was a very funny feeling, leaving your touchdown point and never intending to return. It took some getting used to.

- Opportunity's landing. The pressure was no less, but since we knew the system worked, it seemed like less of a risk. And the absolute elation of the EDL team was electric -- I *loved* their victory lap at the post-landing press conference. And, I must admit, my first reactions to the first images from Oppy were very, very similar to what Steve Squyres reported. My first impression was "Wow, no rocks! That surface looks almost completely featureless! This is nothing like the other rock-strewn sites we've seen before!" And then I saw the outcrop, and said, out loud, "Oh. My. God. That's a layered outcrop of bedrock. A LAYERED OUTCROP!" (I then followed this with a string of totally flabbergasted expletives which, for the sake of the forum rules, I will omit from my discussion... *grin*...)

- The first Endurance rim pan. Just totally blew me away. What more do I need to say?

So, what are your most cherished memories of the MER missions?

-the other Doug

I wish they'd run The Sky at Night on BBC America! That's the only BBC feed we get here in the States (well, at least it's the only one on my cable system)...

-the other Doug

In this, Dan, we are in total agreement.

Everyone ought to go see this one.

-the other Doug

The dark floors of these craters seem quite "level," especially in relation to the very steep crater walls. And, in the highest resolution images, you can see that most of these dark units display cratering, in some cases down to the limits of resolution. The dark areas *seem* to be less heavily cratered at these smaller size ranges than the brighter inter-crater areas, but they do retain cratering morphologies. There are also some odd, almost viscous-looking morphologies in a few of the dark units.

If the dark units were just dark dust that's settled to the bottoms of craters in a relatively light-colored surface, you wouldn't expect them to be solid enough to retain a cratering record at such small scales.

This stuff just feels to me like stuff that seeped up into the craters from below, rather than stuff that filtered in from above. That's a gut feeling, and it will quite probably prove to be wrong, but that's what it looks like...

-the other Doug

Well -- I just saw "Magnificent Desolation" at the local IMAX theater.

OUTSTANDING!

There is a lot of 3D flummery and a lot of interesting "floating frame" presentation of 2D television transmissions, pictures and films.

But the 3D computer-gen sequences of the LM in flight, and the combination CGI and live action footage of the guys actually walking on the Moon -- those were incredible.

There is a full sequence of the LM Falcon landing at Hadley-Appenine. The point of view shifts many times, and in each case the geometry of the landing site, the scale of the LM, and even the interaction between the invisible DPS exhaust and the lunar surface, are all absolutely perfect.

There is a *very* nicely done sequence, using the CGI backgrounds they developed for the Descartes site, where they showed one team of astronauts on foot, then another toting the MET behind them, and then another driving up on a Rover. And then, for a short period, they phased all three teams into the view at once... very, very impressive!

One of the final sequences is a fictional look at what might have happened with a dual failure of the Rover and one of the PLSSes. With tongue firmly in cheek, executive producer and narrator Tom Hanks created his fictional lunar explorers, named -- wait for it -- Tom and Hank. Their Rover hits the rim of a sharp, fresh (and, because it is directly downsun of them, totally unseen) crater, the electronics boxes in the forward section of the Rover are dashed against the rocks, and the LMP's PLSS is damaged by the shock of the accident.

They're 3 kilometers from the LM. And because the LMP's PLSS is dead, the comm circuit is out. They're too far from the LM for the relay through the LM to work, and the LCRU on the Rover is busted. So they can't talk to each other or to Houston, and Houston has no data on either man. (At one point, you hear the PAO say in the background, "TELMU confirms that flags were reported just before contact was lost...")

The LMP turns on his OPS, they hook up the Buddy Secondary Life Support System (B/SLSS) hose, to feed cooling water from the CDR's PLSS into both suits, and they walk back to the LM. When they get in range of the LM, comm comes back through the LM circuit, but only the CDR can talk to Houston.

It's handled exactly how such an event would have played out if it had actually happened. Very, very well done.

And then there's the finale -- as Aitken Base wakes up for the lunar day...

I recommend this film very, very highly.

-the other Doug

MY




BRAIN




HURTS!!!!!


-the other Doug

Staying on this tangent for a moment, the problem I have with the current state of electronic publications their lack of portability. PDAs have such small screens that you have to constantly scroll through a very limited-line display. Notebooks and laptops are too big to carry around the same way you carry a book around. I mean, I like to read a book when riding on a bus, or lying in bed, or even -- dare I say it -- sitting in the lavatory, as it were. None of the current display devices are satisfactory.

What I really want is something the size of a book -- as small as a paperback, or as large as a trade paperback -- with one flat side that's mostly display screen. Simple-to-use buttons in the frame let you turn the viewer on, select from bookmarks, set display controls, turn the page, etc. The contents are stored on a memory stick that slides into a recessed slot, with no danger of it falling out or snagging on things. It even turns itself off if you wait more than 10 minutes to turn a page.

The page size would be the same as a regular printed book, and the type size would be similar. You could include pictures. The page composition would be similar to book page composition.

You could make them in several sizes, for regular books and for magazines. Imagine getting your Scientific American subscription as a download into a memory stick on your computer that then gets slipped into your magazine reader.

These viewers wouldn't be full-featured computers, they'd just be conveniently sized and shaped viewers, with only the "guts" needed to provide high-definition displays of pre-formatted text and image files. In book-sized (or magazine-sized) pages.

With such viewers, I could take electronic books anywhere I wanted to go. I could lie in bed reading. I could read them on trains and buses and airplanes. And I'd only have to take a single book-sized viewer and a small case of memory sticks to maintain a portable, personal library of *hundreds* of titles.

We have the technology, and the things wouldn't even have to be all that expensive.

Anyone have any good ideas why such a system has never even been suggested, much less tried out?

-the other Doug

To paraphrase Donald Rumsfeld, "You don't go to Saturn with the instruments you wish you had, you go to Saturn with the instruments you have."

-the other Doug

OK -- can we call the meter-high drifts that seem to block the path along the rim "The Knights Who Say Ni"?

Maybe not -- I mean, where the heck are we going to find a shrubbery around here???



-the other Doug

Could we *finally* be seeing in-place beds of lacustrine materials? If we see them anywhere, it's likely going to be at the summits of these hills, I guess...

-the other Doug

I dunno -- Monica? Lil' Bill?



-the other Doug