Another alleged irrefutable proof that the lunar missions were faked lies, according to conspiracy theorists, right under the belly of the Lunar Module. Looking at the photographs of the various landing sites, deniers triumphantly point out the complete absence of a blast crater under the nozzle of the descent engine (DPS - Descent Propulsion System). According to their logic, the devastating power of the scorching exhaust gases of a rocket engine, running flat out to brake and support the LEM during landing, should inevitably have dug a deep hole in the ground! Instead, the ground below looks inexplicably flat and free of dramatic upheaval, as if the spacecraft weighing several tons had been gently lowered from above by the ropes of a film-studio hoist. These claims, now a classic of conspiracy rhetoric, show a complete misunderstanding of the basics of space propulsion, of lunar geology and, above all, of fluid dynamics in the vacuum of space. To understand the origin of this deep-rooted visual prejudice, we need to go back to the early 1960s, when NASA presented to the public the preliminary artist's sketches of the Apollo program. In those early concept illustrations, artists had often drawn conspicuous smoking craters between the legs of the LM, assuming that the entire lunar surface was an ocean of soft dust tens of meters deep. The geological reality turned out to be very different and much more solid: under a very thin surface layer of dusty regolith (often only a few centimeters), the lunar ground consists of basalt rock that is extremely hard and compact. Digging a crater in a solid slab of volcanic rock takes much more than the momentary thrust of a rocket that is being shut down. The conspiracy theorists' second fundamental mistake concerns their assessment of the power delivered by the engine at the moment of touchdown. The LM descent engine was a masterpiece of engineering precisely because it was "throttleable", meaning that its thrust could be adjusted continuously as needed, exactly like pressing the accelerator pedal of a car.

During the descent toward the lunar surface, the LM lost speed thanks to the DPS engine, whose retro-thrust allowed the astronauts to approach the lunar surface...	... as they got close to the surface, the DPS engine was throttled to 25-30% of its rated power, allowing the LM to hover like a helicopter while looking for the best touchdown point...	... in this way the crew could avoid dangerous surface obstacles, such as boulders or craters, using the gimbal of the DPS engine, which allowed it to swivel by about 6 degrees along two perpendicular axes (pitch and roll), and the RCS directional thrusters...	... when the crew found a suitable landing spot, they reduced the power of the DPS engine further. This allowed the weight of the LM to prevail over the thrust so that it descended toward the surface. At contact the engine was throttled to 10% and then shut down before the legs touched the ground. The last two meters were covered in free fall.

During the first phase of the descent to leave lunar orbit, the engine did indeed burn at full power to brake the vehicle's enormous orbital speed. However, as the LM got closer to the ground and slowed down, the power was drastically reduced. In the last moments of flight, when the vehicle hovered like a helicopter a few meters above the surface so the commander could look for a landing spot clear of craters and boulders, the engine was delivering only 25-30% of its maximum thrust. Add to this the reduced lunar gravity. On the Moon, in fact, the Lunar Module did not weigh 15 tons as on Earth, but just over 2,500 kilograms. To support that weight and lower it gently in the vacuum took relatively modest thrust, completely incapable of shattering the rock below. For a comparison on Earth, military vertical take-off jets such as the Harrier routinely land on asphalt runways or aircraft carrier decks supporting a similar weight in six times the gravity, and yet their extremely powerful concentrated jets dig no hole under the aircraft!

But the most decisive factor in explaining the missing crater is, once again, the complete lack of atmosphere. On Earth, the pressure of the surrounding air compresses and "squeezes" a rocket's exhaust gases, confining them to a narrow, concentrated and devastating column of fire like the flame of an oxy-acetylene torch, capable of cutting into the ground. In the lunar vacuum, with no atmospheric pressure, this aerodynamic confinement does not exist. As soon as the exhaust gases pass the rim of the engine bell, they instantly expand in all directions, taking the shape of a very wide, very low-density fan. As they expand radially, the gas pressure per square centimeter drops dramatically and, according to the principles of thermodynamics, their temperature plunges. What hits the lunar surface in the last meters of the descent is not a concentrated laser beam, but a dispersed, radial puff that violently sweeps away the surface layer of dust, but has neither the mechanical density nor the thermal energy to dig into rock.

The LM Descent Stage in the Grumman aerospace workshops, waiting to be covered with its external MLI protection. At the bottom you can see the bell of the DPS descent engine, complete with propellant tanks and hypergolic feed lines. Photo courtesy of NASA.	The DPS was an engine designed not to fail. It ran on hypergolic propellants that ignited spontaneously on contact, making it extremely reliable and at the same time technically simple. In its latest versions (J missions) it was 2.54 meters long, 1.6 meters in diameter at the base, weighed 179 kg and could produce a maximum vacuum thrust of 4,500 kg. Photo courtesy of Mr. Richard Kruse (https://historicspacecraft.com/)	The DPS engine of Apollo 11's LM "Eagle". In this detailed photo you can see the effect of the DPS engine on the lunar surface. The radial dispersion of the jet and the cracking of the basalt layer caused by the heat are clearly visible on the ground. Photo courtesy of NASA.
The DPS engine of Apollo 12's LM "Intrepid". The effect of the jet fanning out in the vacuum is clearly visible in the lunar regolith. Photo courtesy of NASA.	Another shot of the DPS engine of Apollo 11's LM "Eagle". The effect of the exhaust jet is visible on the ground, where its highly reactive combustion chemically "burned" the soil and stained it with nitrogen oxides. Photo courtesy of NASA.	Apollo 15's LM "Falcon" landed with one leg in a small crater that tilted it by about 11°. Because the rim of that crater was so close, the DPS engine bell struck the ground and was crumpled on one side. Photo courtesy of NASA.

The DPS engine of Apollo 14's LM "Antares". In this detailed photo you can see the effect of the DPS engine on the lunar surface, which appears swept clean down to the bare rock under the surface regolith. The radial dispersion of the jet and the cracking of the basalt layer caused by the heat are clearly visible on the ground. Photo courtesy of NASA.

Closely connected to this merciless logic of the vacuum is another recurring accusation: the mystery of the perfectly clean LM footpads... Critics wonder how it is possible that, landing on dusty ground with a rocket firing, the vehicle's wide gold footpads were not covered by a thick layer of debris. Science's answer to this demolishes the far-fetched idea of a film set on Earth in a single stroke! On Earth, in fact, the air is stirred up by the exhaust gases, creating convection currents and turbulence that lift the dust into swirling clouds; these clouds stay suspended for minutes before settling gently on everything below, the vehicle included. On the Moon, there is no air! Being in the vacuum of space, the engine gases swept the dust away only radially, firing it parallel to the ground at very high speed. The dust traveled straight and flat, never rising in billowing clouds (how could it?), falling back ballistically tens of meters away. In addition, from the LM's footpads hung contact probes about a meter and a half long. When the first of these probes brushed the ground, on the LM control console in front of the crew a pair of blue lights came on at the same time ("Contact Light"). At that signal, procedure required the astronauts to shut down the engine immediately, to prevent the gases from bouncing off the ground and dangerously entering the engine bell, making it explode. As a result, the LM covered the last meter and a half in free fall and, when the legs actually touched the lunar surface, absorbing the impact, the engine had already been off for more than a second. The radial sweeping had stopped and, with no atmosphere to keep particles suspended, the dust had already fallen back to the ground , so there was literally no dust cloud ready to fall from above onto the spacecraft's landing legs .

Apollo 11's LM "Eagle" finally flies free in lunar orbit, ready to descend to the Sea of Tranquility for the historic landing. Before it left, however, procedures required the Command Module Pilot to check that the landing gear (the 4 gold legs) was properly deployed. This photo clearly shows the contact probes (red arrow) and the DPS engine (green arrow). Photo courtesy of NASA.	On the LM control console, in front of the crew's standing flight station, there was a blue light (red arrow). When it came on, it told the astronauts that one or more probes had touched the lunar surface (now 1.5 meters away) and that the engine had to be shut down promptly to prevent it from exploding. Photo courtesy of NASA.	Thanks to the early shutdown, the legs touched down when the engine was already off. Because of the vacuum of the lunar environment, the dust could not stay suspended and had all fallen back to the ground by the time of contact, leaving the footpads essentially clean.
When the Lunar Module touched down, the contact probes remained pressed against the ground, completely crushed by the mass of the LM, sometimes posing a tripping hazard for the crew. To prevent heat damage during the flight from the Earth to the Moon and during their stay on the surface, the legs were wrapped in gold MLI blankets. Photo courtesy of NASA.	In the lunar maria (the landing sites chosen for the first Apollo missions), the dust on the ground (regolith) was only a few centimeters thick. For this reason, the DPS exhaust jet partly uncovered the basalt surface underneath, so the legs sank only a little. Photo courtesy of NASA.	Not all landings were perfect. During the Apollo 14 mission, the crew could not completely cancel their lateral velocity, so one of the four legs ploughed the ground, getting covered in lunar regolith and tearing part of its MLI blanket. Photo courtesy of NASA.

A classic example , one anyone can check, that helps visualize this inescapable fluid-dynamic principle of gas expansion in a vacuum is the appearance of the exhaust plume of the Saturn V first stage. When the gigantic rocket lifted off the pad, the column of exhaust gas released by the 5 huge F-1 engines was kept narrow, focused and devastating precisely thanks to the strong atmospheric confinement provided by the pressure of the surrounding air at sea level. As the altitude increased, however, the pressure of the surrounding air dropped drastically and the exhaust plume began to look more and more like a broad fan. Approaching orbit, this unconstrained expansion became so extreme that it turned the gases into a gigantic, belching fireball that, seen from the ground, almost seemed to be consuming and swallowing the rocket itself. Precisely in order to survive the catastrophic backflow of heat caused by this monstrous scorching expansion of low-pressure gas, the first-stage engines were meticulously wrapped in thick, fireproof thermal blankets, while the entire base of the second stage was heavily armored with a dedicated heat shield, essential to withstand the heat generated by the five cryogenic J-2 engines firing in a vacuum. If physics forces a leviathan like the Saturn V to disperse its thrust in this way as atmospheric pressure falls, it is obvious that the tiny, throttled-down DPS engine of the LM could in no way maintain, with all due respect to the Moon-hoax theses, a cohesive, cutting flame capable of digging a hole in the hard lunar rock.

The F-1 engine photographed just before a test firing on the test stand at the Marshall Space Flight Center. The multilayer fireproof blanket called the "Aft Heat Shield" is clearly visible. This protection prevented engine failures at higher altitudes, when, because of the lower atmospheric pressure, the exhaust gases, no longer carried away by the wake turbulence, remained trapped in a fireball at the base of the rocket itself. Photo courtesy of NASA.	Detail of the heat shield at the base of the Saturn V second stage (S-II). It protected against the exhaust plume of the five J-2 engines. These engines fired at an altitude of over 70 km, where air pressure was already extremely low. Because of this, a fireball formed at the base of the stage that could have compromised the operation of the engines and their control electronics. Photo courtesy of NASA.

Everything said so far can therefore be summarized in a handy concluding slide...

In conclusion, the absence of a deep crater does not mean that the ground was left untouched at all! If you look at the high-resolution photos taken by the astronauts walking around the LM (photos that conspiracy theorists carefully avoid showing in full), you will see an obvious, spectacular "sunburst" pattern: the surface layer of light-colored dust has been swept away radially, exposing the darker, more compact ground below. In addition, the area directly under the nozzle often looks brownish or blackened. This is because the engine used hypergolic propellants (nitrogen tetroxide and Aerozine 50), whose highly reactive combustion chemically "burned" the soil and stained it with brown nitrogen oxides. Everything, from the lack of a deep excavation to the clean footpads and the chemical stains on the ground, fits perfectly and unforgivingly with the inflexible laws of nature, turning yet another alleged conspiracy anomaly into a magnificent lesson in aerospace engineering and vacuum physics.