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How would spaceships made to go to the moon today be different from the Apollo ones?The big difference would be in automation and redundancy. The Apollo Command Module had a single computer — the Apollo Guidance Computer (AGC) — and its sole job was to do the navigation and engine burns. The computer had a few numeric displays and a numeric keyboard, and that was it. While the astronauts loved it, by today’s standards it’s hopelessly clunky. It also weighed a whopping 70 pounds.Everything else aboard the Command Module was done manually by the crew. There were no computerized systems for things like life support. communication, power generation, or anything else.The Lunar Module was pretty much the same — it also had an AGC for guidance and navigation, and it had a backup guidance computer, known as the Abort Guidance Systems (AGS). The AGS was even less powerful than the AGC. Everything else in the spacecraft was manual.The next US spacecraft — the Space Shuttle, first flown in 1981 — was very different. It contained five computers, each one able to control the spacecraft systems. During critical mission phases, each computer ran the entire set of guidance computations, and the computers compared results to make sure all the computers agreed on the calculations. It was the first spacecraft that had fully redundant computers.The only currently operational U.S. spacecraft — SpaceX’s Crew Dragon — is fully computerized and entirely autonomous. As long as nothing goes wrong, the crew has nothing to do except observe. The spacecraft is fully capable of flying the entire mission without assistance. It could theoretically fly with a completely untrained crew. Nonetheless, it has beautiful modern graphical displays to keep the crew fully informed about every aspect of the spacecraft’s operation:Modern spacecraft have more high-tech materials, more sophisticated engines, etc., but the big difference would be the computers. When Alan Shepard flew the first U.S. spacecraft — a Mercury capsule — in 1961, there was no computer of any kind onboard. Sixty years later we’re flying spacecraft that are so computerized that no crew action at all is required.(Above: Control panel of a Mercury spacecraft, 1961 - 1963)

It's said that our smartphones have more processing power than the Apollospacecraft. What apps would I need to build a spacecraft using my phone?Processing power is obviously important, but it’s not the crucial thing. What made the Apollo Guidance Computer (AGC) interesting is that it was connected to the spacecraft in a way that your phone never can be. It wasn’t so much a computer marvel as it was an engineering marvel.The AGC had hundreds of I/O ports with which it could “talk” to parts of the spacecraft. It knew its position and attitude in space by getting signals from the IMU (Inertial Measuring Unit), which contained gyroscopes and accelerometers. It got signals from the landing radar, the rendezvous radar, and from ground telemetry. It was connected to engines and thrusters, which enabled it to fire engines and (in the case of the LM descent engine) control engine throttles. It received inputs from various controls and switches, which enabled it to figure out what the astronauts wanted it to do.The spacecraft of the Apollo program — the CSM (Command and Service Module) and the LM (Lunar Module) — were the first “computerized” spacecraft. They were never intended to be flown directly by humans, but rather were intended to be flown by computers under the direction of humans. What made this possible was the fact that the computer was integrated into the spacecraft navigation and propulsion systems.(AGC in an Apollo Command Module)There’s a misconception that on Apollo 11 Neil Armstrong took over control of the Lunar Module from the computer and “manually” flew the LM to a landing. This is utterly false. No human ever directly flew a Lunar Module to or from the lunar surface. What Armstrong did was switch the computer to a mode (P66) in which he told the computer where he wanted it to go and the computer flew the LM as he asked. But always it was the AGC that was firing the engines, firing the thrusters, and doing the minute adjustments to make sure the LM remained upright. I don’t mean to take away from Armstrong’s amazing skill and decision-making abilities, but I want to make clear that the AGC was a crucial component in the process. This was true on all the lunar landings. Your iPhone just isn’t built to be integrated into hardware like this.One last point: Even if you had the engineers who could design an interface between an iPhone and a spacecraft, you would be absolutely insane to let an iPhone control a spacecraft. The MIT team that designed the AGC software spent millions of dollars testing the AGC software, because they knew lives depended on it working perfectly. The result was that it was almost impossible to crash an AGC. When the AGC became overloaded during the Apollo 11 landing, it displayed an alarm, reset itself, and within 2 seconds was back on the job, which was exactly what it was designed to do. iOS and iPhone apps simply are not tested to this level, nor are they built to be crash-proof. (How many of us have had our smartphones crash? Would you want that to happen while you’re landing on the Moon?) If things have to work, you’re far better off trusting your life to an AGC than an iPhone.

… Does that mean, theoretically, an iPhone could orchestrate a moon landing from take off to touchdown?Yes, it does.And that iPhone will cost you $30 million.Start by asking yourself this: Are you willing to trust your life to an iPhone? One glitch in the hardware, and you die. One bug in the software, and you die. One wrong calculation — and remember, this is rocket science — and you die. One wrong bit written to one wrong I/O channel, and you die.The thing about the Apollo Guidance Computer (AGC) wasn’t just that it was among the best technology available at the time — it was — but more importantly, the hardware and software were as bulletproof as it was possible to make them. Thousands of hours had been put into testing them, because everybody involved knew that if it didn’t work correctly, people were going to die.The guy who is selling that $5 Solitaire app in the App Store did not put that kind of testing into his app.Adding to the cost is the fact that the number of customers for the “app” is exactly one. Someone selling a typical app in the App Store hopes to sell to thousands of customers, and so can charge a small amount of money to each customer. By contrast, if you’re contracting for software for a lunar lander, it has to be written for that spacecraft. Custom software gets very expensive.You’re also going to have to develop all the hardware that interfaces the computer with the spacecraft. The Apollo astronauts didn’t just key in numbers into the AGC and get numbers back out — in addition to crew inputs, the AGC gathered navigational information from the gyroscopes and accelerometers and used that information to perform complicated maneuvers by firing thrusters and engines. If you’re going to use an iPhone, you have to develop all this hardware.As the other answers have pointed out, no one would ever approve an iPhone for mission-critical uses like this — iPhones are not built to withstand the physical environment of space. If you’re determined to use an iPhone, you’re going to have to pay Apple to develop a special radiation-hardened version of the iPhone, just for you. That’s going to require some big bucks.Finally, nobody would ever consider building a lunar lander today that only had a single computer. We did it during the Apollo program because the computers were big and heavy and we were willing to take risks that we wouldn’t take today. Today a space agency such as NASA would no doubt insist on several iPhones, all running the calculations and communicating with each other. If one iPhone fails, the other iPhones can take it offline and continue safely. So now you’ve got to develop custom software so that the iPhones can communicate and synchronize with each other. More custom software, more expense.And all of this custom software and hardware has to be tested, tested, tested. And it has to have complete, detailed requirements, specifications, and documentation (Thanks, George Koehler).So yes, an iPhone has the processing power to run an entire lunar mission, without a doubt. It’s just that there’s a lot more to a spacecraft computer than just processing power.