I don't profess to know the answers. I am merely asking questions.

Unfortunately the hoax believer's stock in trade is the question. It's the questions that do the damage, and the hoax believers' implication that no suitable answers exist. We'll see how interested Mr. Brown is in answers to his questions.

In 1996 the Delta rocket DC-X toppled over on landing and exploded.

Yeah, but why did it tip over? It took off, flew around for a while, returned to its launch site, landed safely, and shut off its engine. It fell over because the landing leg collapsed. The supposedly difficult technology -- variable-thrust engines, guidance, attitude-control, navigation -- these were all solved successfully. Its downfall was caused by something so prosaic as an unsuitably strong weld.

Strange really, they managed to do this on the Moon twenty-seven years earlier.

Not really. The problem is much more difficult on earth, and much more difficult if the same propulsion system is to be used for launch and landing.

It's more difficult on earth because off-axis thrust introduces a stronger moment. To hover the same mass on earth as opposed to the moon requires six times more thrust. That means when the thrust goes off-axis, the moment is six times greater, inducing six times the rotation rate. This means the vehicle must respond much more quickly to shifts in the center of mass or other factors that move the thrust off-axis.

Fuel sloshes more slowly in lunar gravity than in earth gravity. Further, differences in moment to due fuel depletion are less pronounced in lunar gravity. This too makes it easier to control the rocket.

Then there are external factors such as wind. There are no wind gusts on the moon. There are on earth, and the guidance system must compensate for them. And on earth, the interaction of the exhaust with the nearby ground and the air produces a "ground effect" with all kinds of unpredictable air currents around the vehicle.

Finally, and perhaps most importantly, the LM was a piloted vehicle, operated by some of the finest pilots in the world who had trained extensively to fly it. The DC-X was unmanned, flown by automation and by remote.

If a rocket did manage to land and take off vertically, it would have blown huge amounts of dust and debris in all directions.

And it did. You can see it on the film. Did it blow all the dust and debris away? No, nor should it have, since the exhaust plume was somewhat limited in diameter.

However, when the Apollo 11 landed, Neil Armstrong famously left his footprint in undisturbed dust.

Interesting claim, since no photograph of the first footprint was taken. Hoax believers cannot support the claim that Armstrong's famous foot step took place in "undisturbed" dust. In fact, other Apollo 11 photographs -- and similar photos from other missions -- quite clearly substantiate a degree of disruption to the lunar surface from the DPS plume.

The 'sunlight' in this photograph is clearly coming from the right and flaring into the lens. How come there are shadows on the roof of the exit and why is Buzz Aldrin so well lit?

Backscatter, zero-phase effect, heiligenschein -- by any other name it's the same illumination phenomenon. He's a photographer and a physicist and can't figure this one out?

The 'sunlight' here is coming from low down on the right. What's lighting Buzz Aldrin's back-pack?

Low down on the right would be the sunlit portion of the lunar surface in which Armstrong is standing.

He should be in shadow, like the ground.

No. A convoluted surface can only very slightly illuminate itself by diffuse reflection. However, it can quite strongly illuminate objects some distance away from it.

On TV he bounced down the ladder with both feet together.

... until he reached the last step, where he's poised in this photo. Then in the EVA footage he raises his left leg, saying something along the lines of, "I'm going to keep my leg up like this." He holds his pose for a second or two, then jumps to the footpad. The photo and EVA footage are perfectly consistent. David Percy -- who is undoubtedly the source for this argument -- simply plays a clip from a few seconds earlier when Aldrin was higher up and claims that's the part that corresponds to this photo. Pure, inexcusable deception.

Sunlight generates parallel shadows.

But the shadows appear parallel only when seen from directly above, and only when cast on flat, planar surfaces.

Could this be a studio light about 30 feet away?

No, it can't. The LM's stride is wider than 30 feet, yet the same light is postulated to case the LM's shadow. Note that there are no illumination or shadow fall-offs, which would be inevitable with the lighting setup so described.

The Sun would generate a very small point of light in the helmet.

Only in a theoretically perfect helmet. There is no substance which generates a perfect reflection, and Lexan is far from being such a substance. Every reflector -- even putatively specular reflectors -- scatter light to some degree. The majority of the light rays reflect away in the predictable specular angle of departure (computed from the surface normal and the angle of incidence). But the scattered rays depart in a sort of cone whose axis is the angle of departure. So the farther away you are from the reflector, the larger the highlight appears to be.

Mr. Brown's JPEG is pretty small. Looking at a high-resolution image of the photo in question (AS11-40-5875) reveals the highlight to be perfectly consistent with a reflection from the sun.

Note the Lunar Module on the left. Can you see any disturbed dust around it?

The question is, would you see any disturbed dust around it? Most of the thrust effects are visible from above (i.e., the LM cockpit) or from a great distance such as in locators in the J-missions.

There should be a large 'star burst' pattern or even a crater generated by the blast of the rocket engine.

AS11-40-5918 and AS11-40-5921 clearly show the thermal and mechanical effects of the exhaust upon the lunar surface. Mr. Brown's assumption that some interaction would be visible is satisfied, but he will need to justify by computation his assertion that wide-scale "starbursts" or craters should be visible. He's handwaving, and for a physicist that's inexcusable.

Why is the horizon darker than the foreground?

Because the photo is basically up-sun. Under those conditions, the farther away an object (i.e., rock, dust grain) is from the photographer, the more of its shaded side and the less of its lighted side will be visible. The differential illumination caused by varying phase angle is more acute on the moon because the curvature is greater. The change in phase angle versus distance from the photographer is more acute on the moon.

Sunlight doesn't generate hot spots, but studio lights do.

Patently absurd. Sunlight focused by intentional or inadvertent reflectors most certainly produces variations in illumination, and even pools of pseudo-caustic reflections. The pool of light behind Aldrin is most likely the reflection of sunlight from the polished aluminum cover of the LM's aft equipment bay.

The rock in the background (top left) appears to be lit from the side. If the light is coming from behind, the rock should have more shadow area on it.

Apparent shadow direction can be drastically fooled by phase and distance. I have determined empirically that at phase angles of 15-30°, at a distance of 20 meters (hi, Mattias) the photographed shadow will appear to lie perpendicular to the line of sight. Only the transverse aspect of the shadow is significant at that distance.

These effects are readily observable on earth. The hoax belief here is based on simple inattention to everyday observation.

NASA claim that the camera used on the Moon was designed to be used at waist height.

No. The typical RCU-mounted camera is just under the chin.

The horizon in this photograph is at eye level, therefore the camera must have been at eye level.

This, of course, presumes a perfectly planar and horizontal surface, which is not the case.

Further, the endpoints of the reflection of the horizon in Aldrin's helmet bend downward, indicating that the line of sight is below Aldrin's eye level. The correspondence of the directly observed horizon to the reflected horizon is meaningless.

No sign of any blast damage on the surface of the Moon.

No, not in this picture either. But other pictures exist that do show the effect.

... a force of over 10,000 newtons would be required to slow the module and act against the Moon's gravity.

Yeah, that's about right. We estimate about 12,000 N.

This amount of force would not leave the surface undisturbed.

Apples and oranges. The thrust of the rocket is not the same as the pressure of the exhaust plume on the surface.

On Earth, we have not been able to land a rocket vertically.

False. Also, apples and oranges: it's a different problem.

If the rocket is even a few degrees off horizonal or the surface is uneven, the force of the engine will cause the rocket to tip over as it nears the surface.

... unless corrected by a digital autopilot.

Whilst the physics is slightly different in a vacuum ...

Granted, but the most drastic difference is the lesser gravity of the moon. Less gravity, less thrust, less moment, less error rate.

the force of the engine would have been reflected by the surface as the module landed, this would have made the landing very complicated.

The "force" of the engine would not have been "reflected". Such imprecise terminology from a highly-trained scientist?

More accurately, the plume would interact with the surface, and the deflection may produce a buffetting effect on the descending vehicle. This was anticipated. That's why the descent profile called for shutting down the engine while still some 1.8 meters from the surface.

The pattern of plume deflection is not as one would intuitively believe. The plume strikes the surface then departs radially at an inclined angle. It does not form a "cloud" nor does it produce a lingering "roiling" effect as exhaust clouds in an atmosphere do.

The British Hawker Harrier fighter jet can achieve vertical landings, but it requires four jet outputs for stability.

It does not require four jet outputs; the LLTV flew just fine on one. The Harrier provides four jets because it's not especially difficult to provide them in the Harrier design, and providing them greatly simplifies things for the pilot.

Modern helicopters, by analogy, provide automatic governors to keep the main rotor RPM constant as the collective pitch is varied. This means the pilot does not have to explicit manipulate the tail rotor or throttle to avoid yaw. In earlier helicopters the pilot had to use the throttle to keep the RPM constant as he adjusted collective pitch, or compensate with the tail rotor. This, of course, made early helicopters much more difficult to fly. But the addition of a convenience that is taken for granted today doesn't mean the task of flying a helicopter was impossible before then.

This is a common mistake in evaluating technology. Because something is provided, it's assumed to be necessary and the task impossible without it.

How come every lunar blast-off was perfect?

Because it had to be. Therefore an incredible amount of engineering was expended toward making sure that engine would fire, and that one of the two redundant guidance systems would function.

The dust being thrown up by the Apollo 16 Lunar Rover is forming clouds.

Not "clouds" in the sense of being suspended in air, but "clouds" in the sense of many particles at different points on different trajectories. Demonstrating this via a still photo is very misleading.

In a vacuum, the dust should be thrown back in a arc.

Only if every single particle is given exactly the same trajectory: angle of departure and initial velocity. Since that's next to impossible, each particle will follow its own precise trajectory and have departed at a different angle. This will look like a cloud even in a vacuum.

What is most apparent is the behavior of the dust when seen in motion. The dust is obviously dry, obviously composed of very small particles, and obviously following only ballistic (i.e., no aerodynamic effect) trajectories.

The notion that a handful of dust should form a "perfect" arc is ludicrous.

If this much dust was moved by the wheels of vehicle, how come the blast of a rocket appears to have moved no dust at all.

The simple difference between aerodynamic loading and direct mechanical loading. In Phil's book he proposes an experiment by which you blow on a handful of flour. The logical extension of that is to poke at the flour with your finger. You can blow on flour until you're dizzy without materially affecting the shape of your flour pile, but you can quite easily poke it into different shapes with your finger.

Kodak say that normal Ektachrome slide film was used.

No, this is untrue. The normal Ektachrome emulsion was used, but it was placed on the Estar base. A photographer should certainly know the difference between emulsion and base.

What happens to slide film if you try to use it below minus 20 degrees Celsius? It shatters!

The Estar base was specifically formulated for high-altitude (i.e., low temperature) aerial photography. It's good to about -70 C.

Radiation and UV doesn't do it a lot of good either!

Of course, and had it been subjected to significant sources of either, that might have been a problem.

Transparency film has to be exposed very accurately, an error of one stop will produce an unusable photograph.

Yes and no. While it has a poor exposure latitude, it is more amenable to "pushing" and "pulling" in the darkroom. Thus they provided the astronauts with film that could be salvaged by experts back on earth.

And not all the film was Ektachrome reversal film. The black and white stock was standard negative film.

How could they know what the exposure should be when nobody had ever been there before?

Because the sun is the sun, and "sunny 16" works anywhere there's sun.

There's only one way that NASA can clear this up.

This is awfully presumptuous. It implies the arguments in favor of the hoax are all but proven, when in fact they are seriously flawed. A better course of action to "clear this up" is for the hoax believers to actually know something about their arguments.

Send a probe to the Moon and take some pictures of the landing sites.

... which hoax believers will promptly dismiss conjecturally as more NASA fabrication.