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Suppose at the last minute, Houston and the Apollo 17 crew (let's assume that support extends all the way up to the White House) decided they should go for broke and attempt to reach Mars instead. The spacecraft has not been modified in any way; it's still entirely designed and loaded up for a moon mission. Where would they have wound up? (What systems would have failed first, and/or how long would they have stayed alive?)

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    $\begingroup$ why would all of these people who are well aware that those astronauts won't make it anywhere close to Mars make this decision $\endgroup$
    – Erin Anne
    Commented Mar 3 at 19:20
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    $\begingroup$ If they didn't modify it in any way, it would have flown the mission it was designed and programmed for. You're assuming everyone involved was insane. If what you're trying to ask is what kind of deep space missions could have been done with Apollo-ish systems, look up the Manned Venus Flyby. $\endgroup$ Commented Mar 3 at 19:44
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    $\begingroup$ They wouldnt make it to Mars, even if they burn each of the three stages to depletion. Then burn the LM descent engine and RCS to depletion. Swing past the Moon for gravity assist. Use up the LM's consumables, then detach the LM. Burn the service module engine to depletion. The capsule will now be in an elliptical orbit around the Earth with an apogee somewhat higher than the Moon's orbit. The CM consumables would run out a week or so later. Maybe someone will do the calculations to provide an actual answer to your hypothetical question, including correcting any of my wrong assumptions. $\endgroup$ Commented Mar 3 at 20:07
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    $\begingroup$ @StevePemberton The CSM alone has a shocking amount of delta-V available, and it's a relatively small step from moon intercept to Mars intercept; if you wait for the 1973 Mars launch window the CSM can burn for a Mars flyby without touching the LM at all. (I think if you leave the LM on the ground and maybe don't fully fuel the CSM, the Saturn V can get the CSM onto a flyby trajectory without using the CSM's engine at all.) But the CSM does die in a few weeks no matter what. $\endgroup$ Commented Mar 3 at 22:11
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    $\begingroup$ @StevePemberton The delta-v for Apollo's usual translunar injection is about 3200 m/s. The delta-v from LEO to Mars intercept is about 4300 m/s. The Apollo CSM can do about 2800 m/s. I think there'd be enough propellant for it to enter Mars orbit, though certainly not enough to return afterward. $\endgroup$ Commented Mar 4 at 6:41

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The Saturn V + Apollo stack would have had just about enough performance to do a Mars flyby mission in the appropriate launch window, for example if the launch was delayed to summer of 1973.

The delta-v (change in velocity) provided by the Saturn V's third stage for Apollo's usual translunar injection is about 3200 m/s. The delta-v needed to reach Mars from LEO is about 4300 m/s. The Apollo CSM, fully fueled, can do about 2800 m/s. I think there'd be enough propellant left over for it to brake into a stable Mars orbit, though certainly not enough to return afterward.

The one-way transit time from Earth to Mars, however, would have been at least 8 or 9 months. The Apollo spacecraft's fuel cells, which were its only source of power, wouldn't have lasted more than a few weeks. Neither would the astronauts' other supplies. Assuming they packed extra snacks and CO2 scrubbers, electricity would have been the limiting factor; once the fuel cells ran dry, they would have lost communication, air circulation, thermal management, and all control of the ship, dying in short order thereafter.

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First, let's assume that this decision was made because there was a launch window to Mars at the time that Apollo 17 was about to launch. Perhaps credible intel was received that another country, such as the Soviet Union, was about to launch a manned mission to Mars and America's leaders felt that it was vital that Apollo 17 beat them there. If we don't make this kind of assumption, the hypothetical scenario is hard to accept.

According to this answer, Maximum speed reachable by Saturn V, the first three stages of the Apollo rocket would had a delta-v of 17911.9 m/s.

If we assume that the delta-v to a 200km LEO orbit is 9.3 km/s, the injection delta-v is 7.82 km/s, and the post-injection delta-v of 2.07 km/s is achieved through aero-breaking, then the mission requires the rocket to have 9.3+7.82 = 17.12 km/s of delta-v. So, it should be possible to reach Mars - barely. Of course, the guidance computer would have to be reprogrammed with the new trajectory while in LEO.

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The crew would need to survive for 240 days, which is 20 times longer than the actual Apollo 17 mission which was 12 days. Potentially this could be achieved by aggressively conserving power and rationing consumables.

As was done during Apollo 13, the crew could shut down the command module and live in the LEM until its power and supplies were used up, and then the LEM could be jettisoned and the astronauts could relocate to the command module for the rest of the journey.

Although, while perhaps not allowed by the framing of your question, potentially additional supplies could be tossed into the capsule right before launch. If an EVA was done to jettison everything non-essential to the last-minute Mars mission while still in orbit around Earth, then the delta-v lost due to the weight of the extra supplies could be made up during the Mars injection burn.

I think, however, that it is likely that the crew would do the math early in the mission and realize that they would need to draw straws. Two of them would have to sacrifice themselves so that there would be enough power, life support, and provisions for the third to survive to reach Mars - where they were likely to die anyway. There is a historical precedent for this...

The sinking of the Nantucket whaleship Essex on November 20, 1820 inspired Herman Melville to write Moby Dick. Melville’s story ends with Moby Dick sinking the Pequod, but for the Essex crew, their story began with the sinking. Far out in the Pacific Ocean, 2000 nautical miles west of South America, the sailors had just enough time to pack some provisions and load everyone into three whaleboats. They tried to sail east, but it took 95 days before they were rescued. Of the twenty crew members, only eight survived. And in order to survive, they had to resort to cannibalism. They also drew straws to decide which sailor would sacrifice his life to feed the others. (ref)

The next question is whether the Command Module would be capable of surviving reentry while aerobraking in Mars's atmosphere. Because the chutes are designed for Earth, they would not slow down the command module on Mars as much as they would on Earth. The remaining astronaut would experience a very hard - but conceivably survivable - landing.

From that point on it's hard to imagine a happy ending. Perhaps, if the intel was top-notch, our astronaut could have landed at the same landing site that the other country's mission was planning to land at. Then, there is a chance that arrangements could be made for the astronaut to join the crew of the better-planned mission and return to Earth with them.

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    $\begingroup$ This possibly over-estimates the Apollo parachutes on mars, certainly they appear to have been considered not automatically lethal but certainly dangerous for non water use on earth and Mars is notably lacking in ocean and with a far thinner atmosphere. In fact is a question if the main parachute would even deploy if it was pressure rather than software drive. $\endgroup$ Commented Mar 4 at 8:18
  • $\begingroup$ Looks like the automatic deployment sequence would probably fail on Mars but there was manual triggering if a live astronaut is in the capsule. Question then is if that actually makes a difference. scribd.com/document/49197880/… $\endgroup$ Commented Mar 4 at 8:23
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    $\begingroup$ Because the Apollo fuel cells are fed by cryogenic fluids, I think there are likely much more severe time limits on their use that couldn't be resolved by power rationing. The Apollo stack simply wasn't designed to be in space for that duration; the changes for Manned Venus Flyby in part address the power issue. $\endgroup$
    – Erin Anne
    Commented Mar 4 at 9:33
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    $\begingroup$ The OP posed an interesting technical question about the as flown Apollo Moon landing stack's capabilities, couched in a somewhat whimsical hypothetical scenario. This answer however stretches the whimsy beyond reason - Apollo capsule provides electrical, oxygen, CO2 removal, temperature, food and water for an astronaut for 2/3 year, parachute enables survivable landing on Mars, CM software reprogrammed on the fly to maneuver in the Mars atmosphere, delivering the capsule to a precise landing spot on Mars. And apparently the food problem is solved by drawing straws to decide who is on the menu $\endgroup$ Commented Mar 4 at 13:34
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    $\begingroup$ Huh, terminal velocity might be as low as ~45 m/s (~100 mph), but I don't know if the descent time frame is long enough to reach it. Our psychotic cannibal astronaut is going to have a lot of broken bones. $\endgroup$ Commented Mar 4 at 18:53

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