How Does Apollo 11 Return to Earth?

The Saturn V rocket safely transported Apollo 11’s lunar module Eagle with Commander Michael Collins and Lunar Module Pilot Edwin “Buzz” Aldrin on board to the moon for their successful moon landing. Armstrong and Aldrin used Eagle’s ascent stage to rejoin Collins before departing lunar orbit and heading back towards Earth using Eagle’s ascent stage and engines on their return trip home.

1. The Spacecraft Arrives in Orbit

After a restful night’s rest, Apollo 11’s crew began preparations for its return to Earth. A few hours later, its lunar module’s ascent engine fired, propelling the spacecraft into orbit around the Moon; and hours afterward they fired the command module’s engines to set an appropriate trajectory for returning home.

During their 21-hour and 36-minute lunar module visit, astronauts collected lunar samples, took photographs, and deployed EASEP: Early Apollo Scientific Experiments Package which contained experiments such as seismometer for monitoring moonquakes; laser retroreflector to provide precise distance measurements between Earth and Moon; and device to collect solar wind samples.

When they reached the surface of the Moon, the astronauts reported back to a nervous Mission Control that Eagle had landed safely. Armstrong took his first step onto its surface and announced his famous words: “That’s one small step for man, one giant leap for mankind!” Aldrin followed shortly thereafter.

After more than two hours on the lunar surface, they reentered the LM and Armstrong deployed a television camera so that his landing could be broadcast live to Earth. He and Aldrin then collected piles of lunar material before Aldrin deployed EASEP experiments like seismometers for measuring moonquakes as well as laser reflectors that enabled precise distance measurements between Earth and Moon.

On the lunar surface, astronauts had spent over 20 hours without communication from the LM, so when they returned into it it felt like entering an entirely different universe. Now that they had reached uncharted territories they faced the daunting challenge of returning home.

Before they could begin their return journey home, the crew needed to wait until their orbital Apoapsis aligned with that of the moon, before waiting again until their lift vector matched up with Earth’s rotational axis and added nearly 200 miles onto their flight path.

2. The Spacecraft Separates

On the surface, Buzz Aldrin and Neil Armstrong should have found their return from the Moon simple. After all, they’d docked their lunar module with the Command Module, jettisoned it, performed an engine burn to enter Earth orbit, separated the Command and Service Modules once more and adjusted so their heat shield faced forward, before deploying their parachute at exactly the right moment – once safe inside Earth’s atmosphere. But in reality it proved anything but straightforward.

This step was key because it would decide if and when the astronauts re-entered at the right time and safely landed in the Pacific Ocean. If they landed too soon, it could mean having to fly all the way back out again from Moon – plus it was key as they ran low on fuel – they simply could not afford to waste any of it!

On July 24th, Armstrong and Aldrin successfully completed this critical mission. Once they had separated the LM from the CSM, Armstrong and Aldrin fired their ascent engine which produced 180 pounds of thrust for 435 seconds – enough to lift off of the lunar surface and bring its orbit within 100 miles of Moon orbit.

As soon as their Apoapsis (highest point in their orbit) had aligned with that of the moon, another engine burn was performed to bring their Periapsis back in line with that of its lowest point and resume their journey back towards Prograde.

Armstrong and Aldrin soon recovered their position aboard the CSM, jettisoned the LM, and prepared themselves for their homecoming.

Apollo 11 had gone through much to reach the Moon, and now they needed to make the return journey home. Quarantine would last 21 days as they ensured no “moon bugs” returned home with them. To successfully navigate this final leg of their journey would require engineering miracles as well as skill, determination and perseverance on all parts of this team of men.

3. The Spacecraft Fires Its Engines

Spacecraft fire their second stage engine for approximately six minutes to slow themselves enough to capture lunar gravity, as well as deploy solar arrays for transmitting and receiving data during its return to Earth. Cargo left behind includes soil samples, aluminum foil with solar wind particles collected from the Moon’s surface, film used in taking still and motion picture camera photographs on both camera types, as well as flags taken to the Moon.

As Eagle readies its return journey, Mission Control allows Armstrong and Aldrin to return home early by two hours. At 6:58 a.m., an RCS short course correction shot upward, placing Eagle 13 miles below that of the Command Service Module’s orbit. A series of maneuvers followed by another RCS burn were necessary before positioning Eagle within it again.

Once this was accomplished, they jettisoned the LM’s service engine and fired its ascent engine for reentry. Aldrin reported that their PGNS indicated they were at an altitude of 2,900 feet while radar readings indicated otherwise – leading them to be perplexed about this discrepancy but it seems not be cause any immediate issues.

Armstrong and Aldrin quickly checked their systems before beginning their first moonwalk, spending approximately two hours and 31 minutes outside of LM collecting data and deploying several instruments such as seismometer to measure moonquakes, laser retroreflector to measure distance to Moon and device to collect sample of lunar atmosphere.

The astronauts also took a three-by-five-foot United States flag, using a small mirror on the Lunar Module (LM), to extend it from its leg. Once on the surface they placed it alongside flags from 136 nations including those representing United States, DC, territories, territories under DC jurisdiction as well as some foreign nations like Russia. Aldrin then attached his TV camera’s long lens onto it and trained it on one of the legs on which was mounted a small plaque stating: “Here men from planet Earth first set foot upon Moon in July 1969 in peace for all mankind”. He read from it:

4. The Spacecraft Spits Out of the Earth’s Atmosphere

The astronauts have successfully dismounted from their lunar module, returned to Earth’s atmosphere, and safely splashed down into the ocean using an inflated parachute. Their mission has come to an end but one last thing must happen before they can return home: their spacecraft’s rocket engine must fire again – a risky maneuver with no safety net for protection.

Apollo 11 crew had already managed to avoid another crisis during their lunar landing by restarting a malfunctioning rocket engine, but now came a greater challenge: firing up the SPS engine on their Command/Service Module’s (CSM) back end in order to leave lunar orbit and return home. Although seemingly straightforward, plenty could go wrong here.

Armstrong and Aldrin were approaching the lunar surface when their computer warned that they were moving too quickly, due to lunar gravity varying irregularly and undocking from LM, combined with added speed when undocked from it. To add further anxiety, their computer also indicated they were flying too close downrange.

At this point, cold air which had previously cooled down the descent stage after its engines had been shut off had begun creeping back in and creating blockages in fuel lines, prompting alarm bells in Houston to go off. If they did not refire their engines immediately, the astronauts would enter Earth’s atmosphere unprotected against its scorching temperatures.

There was no other choice; they couldn’t delay igniting the SPS engine any longer; otherwise, their entire mission could be destroyed and all three men who made it safely to the Moon might never come home again – an extremely real possibility which had the potential to change lives irrevocably. This story originally appeared in Popular Mechanics June 2009 issue.

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