(go to 1:05:36 in the video).
(see footnotes below)
Ancient Aliens: “Arguably the most remarkable Mayan artifact ever found – the stone sarcophagus lid of King Pacal – has produced considerable controversy. Mainstream scholars believe the depiction of King Pacal on a journey to the underworld, but ancient astronaut theorists believe the king is portrayed at the seat of the controls of a space craft and have dubbed him the Palenque astronaut.”
AA: “He appears to be going into space. He is the original rocket man manipulating his spacecraft; going into space.”
AA: “We have maintained for a very long time that the depiction here is king Pacal sitting in some kind of spacecraft. He is at an angle like modern-day astronauts upon lift-off. He is manipulating some controls. He has some type of breathing apparatus or some type of a telescope in front of his face. His feet are on some type of a pedal. And you have something that looks like an exhaust – with flames.”
The sarcophagus lid of Pacal has been a centerpiece for the Ancient Astronaut theories since the beginning. Eric Von Daniken believes this to be one of his best pieces of evidence.
AA: “You see his upper hand – he is manipulating some controls. From the lower hand – he is turning something on. The heel of his left foot is on a kind of pedal and, outside the capsule, you see a linking flame. This is incredible. This is absolute proof of extraterrestrials.”
The theory rests on the idea that the Mayans were not depicting their usual symbols here but were trying to realistically depict a rocket with Pacal as its pilot.
I think the best thing I can do for you here is to clearly explain what Mayanists and other scientists who specialize in Mayan culture and artwork believe this scene is depicting.
As Ancient Aliens said, archeologists believe this scene is depicting the moment of Pascal’s death and his decent into the underworld. Let me show you why they believe that.
The most famous symbol in this picture is that of the “World Tree” which, if you believe Ancient Aliens, would make up the entire hull of the rocket. It’s hard to over emphasize the importance of the World Tree in Mayan mythology.
The idea was that the World Tree extended into the heavens with its branches, and its roots extended into the underworld. So it was a symbol of the bridge between the underworld, heaven and earth.
When it is depicted it almost always has the double headed “vision serpent” on its branches, just like it’s depicted on Pacal’s lid. The vision serpent was believed to live at the center of the world, thus it is depicted just above the underworld and just below the heavens.
In its top branches is the celestial bird which is seen a little more clearly in other World Tree depictions like this one, but on Pacal’s lid you can see its clearly depicting the same bird.
The celestial bird represented the heavens and thus was pictured on the top of the World Tree.
What Ancient Aliens says is the exhaust or fire from the rockets takeoff is the roots of the World Tree extending into the underworld. Which is not just typical for depictions of the World Tree, it’s pretty much a requirement.
In the underworld we see a picture of the Mayan sun monster which Pacal is riding into the underworld.
The idea was that every time the sun set it was actually travelling into the underworld where it would die, like everything else in the underworld. You can even see the bottom half of the suns face was a skeleton while the top half still had flesh and had not yet died. This too is a common theme showing the moment of transition.
So Pacal is hitching a ride on the sun into the underworld.
Even the so-called smoke is easily explained when you understand Mayan symbols.
In Mayan art whenever you see a so-called “traveler”- which is a person in transition from one world to the next – there must be something that is making that travel possible. Sometimes it is a twisted umbilical cord, but almost always it is a serpent, often a double headed serpent. In other words being in the mouths of a double headed serpent was a symbol of transition from one world to the next.
You can see that the so called smoke is actually the traditional serpent’s beard which appears in almost every depiction of a serpent in Mayan art.
Now that you have an idea about what the scholars believe about this, let’s look at some of Ancient Aliens’ specific claims.
AA: “He is manipulating some controls right here.”
They say his hands are manipulating some controls, but if you look closely his right hand isn’t touching anything at all, and the thing to its right is not connected to the tree anymore that any of the other floating design elements in the picture.
His left hand could be said to be manipulating controls more than his right, but you would have to say that all these marks on the tree are controls too which, in reality, is probably the bark of the World Tree, which was modeled after the Ceiba tree, which had a very unique bark and was usually depicted in some way or another in World Tree art.
The odd position of Pacal’s hands in this image is really what the fuss is all about. In the 1970s video refutation of the Ancient Astronaut theories the Mayan expert in that film made the point that the hands in Mayan art are often depicted in “delicate positions”
“The Maya liked to show hands in rather delicate gestures.”
“Such gestures are common in Mayan art. There are similar examples on the side of the slab.”
As far as the claim about his foot being on a type of pedal, I would have to say that if that is how aliens designed pedals, then we are far more advanced in pedal technology than they are because that may be the worst angle to put any kind foot pedal.
Plus, there are other reliefs of Mayans in the underworld on similar slabs, with their feet in similar positions, but without the rocket.
What about this so-called “breathing tube” for his nose?
Well, it doesn’t connect to anything and, if you look closely, thing it could potentially connect to is also represented on the other side of the lid and is clearly a stylistic element of the vision serpent.
What it actually is a nose piercing, particularly a bone.
“Here an earring extended from a pierced ear; a nose plug – which has the elements of death because it takes the form of a fleshless bone.”
Finally, consider the context that we find this image. It’s on a coffin lid, so the mainstream view is perfectly in line with that, and the whole concept was common in Mayan burials. It would have been a strange thing for them to all of a sudden abandon their usual symbolic artwork to depict a rocket on a coffin lid.
The fact is that the symbols on the sarcophagus lid are really consistently used. Often the symbols are even explained in the extensive Mayan writings. These symbols fit perfectly with what we know about their beliefs about the world and the afterlife. 
 Miller, Mary; and Karl Taube (1993). The Gods and Symbols of Ancient Mexico and the Maya. London: Thames and Hudson. ISBN 0-500-05068-6.
 DIANNA WILSON MOSLEY. “ANCIENT MAYA AFTERLIFE ICONOGRAPHY: TRAVELING BETWEEN WORLDS” (2000). http://etd.fcla.edu/CF/CFE0001258/WilsonMosley_Dianna_L_200608_MA.pdf.
 Francisco Cámara Riess. “PAKAL’S TOMB”, http://www.delange.org/PalenqueTomb/PalenqueTomb.htm.
 MOSLEY 2000
 MOSLEY (2000)