First Man into Space

U. S. Navy Cmdr. Charles "Chuck" Prescott (Thompson) is not sure if his brother, Lt. Dan Prescott (Edwards), is the right choice for piloting the rocket powered Y-13 to very high altitude. Capt. Ben Richards (Ayres) of the Air Force Space Command insists that Dan is their best pilot, even though when piloting the Y-12 into the ionosphere, he began experiencing flight difficulties. Upon landing, Dan broke flight regulations by going to see his girlfriend (Landi), rather than immediately filing his flight report. Despite these concerns, Capt. Richards insists that Dan pilot the Y-13 after a thorough check-out and briefing by Dr. Paul von Essen ([Jaffe).
The Y-13 takes off, and at 600,000 feet, Dan is supposed to level off and begin his descent. But he continues to climb, firing his rocket emergency boost for more speed. He climbs to 1,320,000 feet (250 miles) and suddenly loses control of the Y-13 while passing through a dense cloud of unknown material, which forces him to eject.
The New Mexico State Police later send a report that a Mexican farmer spotted a parachute, attached to some sort of aircraft, land near his farm, 10 miles south of Alvarado, New Mexico. Chief Wilson (Bill Nagy) meets with Cmdr. Prescott, showing him the wreckage. Tests later show that the automatic escape mechanism and braking chute operated perfectly. The tests also reveal an unknown rock-like material encased on the aircraft's hull; further testing shows this material is completely impervious to X-rays, infrared and ultraviolet light.
Later that night, a wheezing "creature" breaks into Alameda's New Mexico State Blood Bank, brutally murdering one of the blood bank's nurses; the thing then proceeds to drink vast quantities of blood. The next day, the headline of the Santa Fe Daily News reads "Terror Roams State" and tells of brutal and inhuman slaughtering of cattle on a farm right next to where the Y-13 crashed. Both the dead cattle and the blood bank nurse show similar jagged wounds. When Chuck and Chief Wilson examine the nurse's body, Chuck notices shiny specks around the wound, as well as on the blood bank door. They see the same specks on the necks of the dead cattle. Lying under one of them they find a piece of what looks like "a high-altitude oxygen lead" that is used in the Y-13.
Chuck suspects that the killings may have something to do with the crashed Y-13 and requests that Wilson send samples of the specks to Dr. von Essen at Aviation Medicine. The next day, test results show that they are particles of meteor dust that show no signs of structural damage, as would be expected from passage through atmosphere. Later, Dr. von Essen explains the metallurgical test results on the encrustation to Chuck: Wherever the covering occurs on the Y-13 hull, the metal is intact. In places not encrusted, the hull metal has been transformed into a brittle substance, like crumbling carbon, which is then easily reduced to powder. Chuck theorizes that this covering may be some sort of "cosmic protection".
Three more killings are reported. Chuck assumes that the same encrustation that protected the Y-13 hull also coated everything inside the cockpit. Which means that the creature behind the killings must be his brother Dan. Chuck theorizes that when the canopy burst, Dan's blood absorbed a high content of nitrogen as the protective encrustation quickly formed over his body, allowing him to survive. But with Dan's metabolism having been altered in space, his body and brain have now became starved of oxygen on Earth; he must now replace that oxygen by consuming any type of oxygen-enriched blood.
When Dan's encrusted helmet is found in a car with his latest victim, Chuck's theory is proven correct. Capt. Richards and Chief Wilson put in a call to Washington. Suddenly, the hulking, wheezing, encrusted creature that was once Dan crashes through a nearby window in their building.
Chuck realizes that his brother is finding it difficult to breathe. Dan then has Dr. von Essen open the high-altitude testing chamber while he taps into the building's public address system, warning everyone to stay out of the corridors. Chuck then instructs Dr. von Essen to relay directions over the system to Dan on how to find the high-altitude chamber. Dan follows the directions while Chuck follows behind.
Dan stumbles into the chamber. Chuck realizes his brother's hands are too badly deformed for him to operate the controls, forcing Chuck to enter the chamber to assist Dan. The chamber technician quickly increases the simulated altitude to 38,000 feet, enabling Dan to feel more comfortable. While Chuck breathes through an oxygen mask, Dan's humanity is slowly restored. His breathing is still labored, and he has no recollection of the events after he ejected from the Y-13. Dan then says, "I just had to be the first man into space". He then collapses completely, breathing his last.

Question: Who stumbles into the chamber?
Answer: Dan